Electromyography and Neuromuscular Disorders 4th Ed - Ulnar Neuropathy at the Elbow (PDF)

SECTION VIII Clinical Disorders PART I Common Mononeuropathies 22 Ulnar Neuropathy at the Elbow Ulnar neuropathy at the elbow (UNE) is second only to to EDX studies in the evaluation of UNE...

SECTION VIII Clinical Disorders PART I Common Mononeuropathies 22 Ulnar Neuropathy at the Elbow Ulnar neuropathy at the elbow (UNE) is second only to to EDX studies in the evaluation of UNE, as it is particularly median nerve entrapment at the wrist (i.e., carpal tun- good at visualizing the ulnar nerve throughout its course in nel syndrome [CTS]) as the most common entrapment the upper extremity. neuropathy affecting the upper extremity. In contrast to CTS, localizing the site of the lesion by electrodiagnostic (EDX) studies often is much more difficult in patients with ANATOMY ulnar neuropathy. Indeed, the diagnosis of a nonlocalizable The ulnar nerve is essentially derived from the C8 and T1 ulnar neuropathy is not infrequently the best that can be roots (Fig. 22.1), although some anatomic dissections have accomplished in the electromyography (EMG) laboratory. also demonstrated a minor component from C7. Accord- Although the elbow is the most common site of compres- ingly, nearly all ulnar fibers travel through the lower trunk of sion, the ulnar nerve is susceptible to entrapment at other the brachial plexus and then continue into the medial cord. sites, especially at the wrist. In addition, lesions of the lower The terminal extension of the medial cord becomes the ulnar brachial plexus or C8–T1 roots may result in symptoms nerve. The medial brachial and medial antebrachial cutane- similar to UNE. It is the role of the electromyographer to ous sensory nerves and a large contribution to the median identify the ulnar nerve lesion, localize it as accurately as nerve are derived from the medial cord as well. As the ulnar possible, and exclude other disorders that may mimic it. nerve descends through the medial arm, it does so without Neuromuscular ultrasound is an extremely useful adjunct giving off any muscular branches. The ulnar nerve pierces      8OQDUQHUYH 0HGEUDFKLDOFXWDQHRXVQHUYH 0HGDQWHEUDFKLDOFXWDQHRXVQHUYH &XWDQHRXVGLVWULEXWLRQ )OH[RUFDUSLXOQDULV )OH[RUGLJLWRUXPSURIXQGXV $GGXFWRUSROOLFLV )OH[RUSROOLFLVEUHYLV 3DOPDULVEUHYLV $QWHULRU 3RVWHULRU Fig. 22.1 Ulnar nerve anatomy. The ulnar nerve, along with the medial brachial and medial antebrachial cutaneous nerves, is derived from the medial cord of $EGXFWRU the brachial plexus. Inset, Cutaneous distributions of the 2SSRQHQV 'LJLWLPLQLPL ulnar, medial antebrachial, and medial brachial cutane- )OH[RU ous nerves. (Reprinted with permission from Haymaker 7KLUGDQGIRXUWKOXPEULFDOV W, Woodhall B. Peripheral Nerve Injuries. Philadelphia, PA: WB Saunders; 1953.) 3DOPDUDQGGRUVDOLQWHURVVHL 372  Chapter 22 Ulnar Neuropathy at the Elbow 373 the medial intermuscular septum in the mid-­arm and then between the medial epicondyle and the olecranon process. passes through the arcade of Struthers, which is composed Just distal to the groove is the HUA (cubital tunnel). of deep fascia, muscle fibers from the medial head of the Studies have shown that the distance from the medial triceps, and the internal brachial ligament. The ulnar nerve epicondyle to the cubital tunnel distally varies between 3 then travels medially and distally toward the elbow. and 20 mm in cadaver dissections and from 0 to 22 mm At the elbow, the nerve enters the ulnar groove formed in surgical specimens. This variation underscores the impor- between the medial epicondyle and the olecranon process. tance of stimulating the below-­elbow site at least 3 cm distal Slightly distal to the groove in the proximal forearm, the to the elbow in routine ulnar motor studies, to ensure that ulnar nerve travels under the tendinous arch of the two the stimulation is distal to the cubital tunnel, a common site heads of the flexor carpi ulnaris (FCU) muscle, known as of entrapment. In the cubital tunnel, the ulnar nerve then the humeral-­ ulnar aponeurosis (HUA) or cubital tunnel. continues under the FCU to exit between the deep fascia Muscular branches to the FCU and the medial division separating the FCU and FDP. The location of this exit from (fourth and fifth digits) of the flexor digitorum profundus the cubital tunnel varies from 3 to 7 cm distal to the ulnar (FDP) are then given off. groove, according to cadaver studies. The muscular branch The nerve then descends through the medial forearm, to the FCU usually arises distal to the cubital tunnel in giving off no further muscular branches until after the wrist. 93%–95% of cadaver dissections, and always follows the Five to eight centimeters proximal to the wrist, the dorsal same course as the main ulnar nerve. ulnar cutaneous sensory branch exits to supply sensation to the dorsal medial hand and the dorsal fifth and medial fourth digits. At the level of the ulnar styloid, the palmar ETIOLOGY cutaneous sensory branch originates to supply sensation to UNE usually occurs as a result of chronic mechanical com- the proximal medial palm. pression or stretch, either at the groove or at the cubital tun- The nerve next enters the medial wrist through Guyon’s nel. Although rare cases of ulnar neuropathy at the groove canal to supply sensation to the volar fifth and medial fourth are caused by ganglia, tumors, fibrous bands, or accessory digits and muscular innervation to the hypothenar muscles, muscles, most are caused by external compression and the palmar and dorsal interossei, the third and fourth lum- repeated trauma. Elbow fracture, often sustained years ear- bricals, and two muscles in the thenar eminence, the adduc- lier, and subsequent arthritic change of the elbow joint may tor pollicis, and the deep head of the flexor pollicis brevis. result in so-­called tardy ulnar palsy. In addition, chronic minor trauma and compression (including leaning on the elbow) can either exacerbate or cause ulnar neuropathy at DETAILED ANATOMY AT THE the groove. Ulnar neuropathy at the groove also is common ELBOW in patients who have been immobilized because of surgery or As the nerve approaches the ulnar groove, it becomes quite who sustain compression during anesthesia or coma. More superficial (Fig. 22.2). The ulnar nerve normally runs in the controversial is the possibility that repeated subluxation of groove formed by the medial epicondyle of the humerus the ulnar nerve out of the groove (during elbow flexion) also and the olecranon process of the ulna. In some individuals, leads to ulnar neuropathy. fully flexing the elbow may allow the ulnar nerve to sublux Distal to the groove is the cubital tunnel, the other major out of the groove medially over the medial epicondyle. In a site of compression of the ulnar nerve in the region of the small number of individuals, a dense fibrotendinous band or elbow. Although some use the term cubital tunnel syn- an accessory epitrochlearis muscle (or both) may be present drome to refer to all lesions of the ulnar nerve around the elbow, it more properly denotes compression of the ulnar nerve under the HUA. Some individuals have congenitally tight cubital tunnels that predispose them to compression. 8OQDU Repeated and persistent flexion stretches the ulnar nerve QHUYH and increases the pressure in the cubital tunnel, leading to 5DGLXV subsequent ulnar neuropathy. 0HGLDO HSLFRQG\OH CLINICAL &XELWDO UNE caused by compression at the groove or at the cubital WXQQHO tunnel may present in a similar manner. In contrast to CTS, 2OHFUDQRQ in which sensory symptoms predominate, motor symp- toms are more common in ulnar neuropathy, especially in )OH[RUFDUSLXOQDULV chronic cases. In some patients, insidious motor loss may Fig. 22.2 Detailed ulnar nerve anatomy at the elbow. Entrapment occur without sensory symptoms, particularly in those with of the ulnar nerve occurs both at the groove (between the medial slowly worsening mechanical compression. Because most of epicondyle and the olecranon) or distally at the cubital tunnel. the intrinsic hand muscles are ulnar innervated, weakness (Reprinted with permission from Kincaid JC. AAEE minimonograph no. 31: the electrodiagnosis of ulnar neuropathy at the elbow. Muscle of these muscles leads to loss of dexterity and decreased Nerve. 1988;11:1005.) grip and pinch strength. These are often the complaints 374 SECTION VIII   Clinical Disorders that bring the patient to medical attention. There may be atrophy of both the hypothenar and thenar eminences (the ulnar-­innervated adductor pollicis and deep head of the flexor pollicis brevis are in the thenar eminence). However, thumb abduction is spared (median and radial innervated). In moderate or advanced cases, examination often shows the classic hand postures that occur with ulnar muscle weakness. The most recognized is the Benediction posture (Fig. 22.3). The ring and little fingers are clawed, with the metacarpophalangeal joints hyperextended and the proxi- mal and distal interphalangeal joints flexed (from third and fourth lumbrical weakness), while the fingers and thumb are held slightly abducted (from interossei and adductor pollicis weakness). The Wartenberg’s sign is recognized as a passively abducted little finger due to weakness of the third palmar interosseous muscle (Fig. 22.4). The clinical corre- late to this sign is that patients report getting the little finger caught when trying to put their hand in their pocket. The Froment’s sign occurs when the patient attempts to pinch an object or a piece of paper (Fig. 22.5). To compensate for Fig. 22.4 Wartenberg’s sign. The sign results from difficulty adduct- intrinsic ulnar hand weakness, the long flexors to the thumb ing the fifth digit because of preferential weakness of the third palmar interosseous muscle. In the photo, the patient was asked to hold her and index finger (median innervated) are used, creating a fingers together. Note that the patient’s left fifth finger is held abducted. flexed thumb and index finger posture. Examination of the patient’s grip often reveals it to be abnormal. Weakness of the ulnar-­innervated FDP will result in the inability to flex the joints of the ring and little fingers. This often can be demonstrated just by having the patient make a fist (Fig. 22.6). Patients with UNE may not be able to flex the distal fourth and fifth fingers completely when making a grip; in contrast, the median-­innervated second and third distal digits flex normally. In UNE, sensory disturbance, when present, involves the volar and dorsal fifth and medial fourth digits and the medial hand (Fig. 22.7). The sensory disturbance does not extend proximally much beyond the wrist crease. Sensory involvement extending into the medial forearm implies a higher lesion in the plexus or nerve roots (i.e., this is the territory of the medial antebrachial cutaneous sensory nerve, which arises directly from the medial cord of the brachial plexus). Another important skin territory to check is the dorsal medial hand. Sensory abnormalities here are important because they indicate that the dorsal ulnar cuta- neous sensory nerve territory also is involved. This finding excludes an ulnar neuropathy at the wrist as the dorsal ulnar cutaneous sensory nerve arises proximal to the wrist. Pain, when present, may localize to the elbow or radiate down to the medial forearm and wrist. Paresthesias may be reproduced by placing the elbow in a flexed position or by applying pressure to the groove behind the medial epicondyle. The ulnar nerve may be palpably enlarged and tender. The nerve may also be palpably taut with decreased mobility, espe- cially in patients with ulnar neuropathy at the cubital tunnel. DIFFERENTIAL DIAGNOSIS The differential diagnosis in a patient suspected of having UNE (Table 22.1) principally includes C8–T1 radiculopa- Fig. 22.3 Benediction posture. The deformity results from a combi- thy, lower trunk or medial cord brachial plexopathy, and nation of finger adduction weakness (interossei) and clawing of digits ulnar neuropathy at the wrist. Very rare cases of ulnar nerve 4 and 5 (extension at the metacarpophalangeal joints and flexion of the distal and proximal interphalangeal joints from weakness of the entrapment in the proximal arm and more distally in the third and fourth lumbricals). forearm have also been reported.  Chapter 22 Ulnar Neuropathy at the Elbow 375 1 3 2 Fig. 22.7 Sensory loss in ulnar neuropathy. The ulnar nerve contains three sensory branches: (1) ulnar digital sensory branches supply the volar fifth and medial fourth fingers; (2) palmar cutaneous branch supplies the proximal volar medial hand, arising 1–2 cm proximal to the wrist; and (3) dorsal ulnar cutaneous sensory branch arises 5–7 cm proximal to the wrist and supplies the dorsal medial hand and the dorsal medial fourth and fifth fingers. Lesions at the elbow may be associated with abnormalities in all three territories; lesions at the wrist never involve the dorsal ulnar territory (3) or the proximal volar ulnar palm (2). A cervical radiculopathy at the C8–T1 level, although seen less frequently than radiculopathy at the C6 and C7 root levels (which are more commonly affected in cervical disc disease or spondylosis), may be difficult to differenti- ate clinically from ulnar neuropathy. Neck pain and radia- tion into the arm, sensory disturbance extending into the forearm, and weakness involving the median-­and radial-­ Fig. 22.5 Froment’s sign. Top, Normally, to pinch a piece of paper, innervated C8–T1 muscles are the major differentiating the finger pads of the thumb and index finger are brought together features. Of course, weakness often is minimal and sensory by the action of the ulnar-­innervated adductor pollicis and first dorsal interosseous, respectively. Bottom, In ulnar neuropathy, weakness of loss often vague in radiculopathy, making the differentiation these muscles results in a characteristic posture, known as the Froment’s between a mild C8–T1 radiculopathy and an ulnar neuropa- sign. To compensate, the median-­innervated flexor pollicis longus and thy demanding, if based on clinical findings alone. flexor digitorum profundus (digit 2) have to contract, resulting in marked Lower trunk/medial cord brachial plexopathies are flexion of the interphalangeal joints of the thumb and index finger. uncommon. Entrapment of the lower trunk by a fibrous band or hypertrophied muscle results in neurogenic thoracic outlet syndrome (see Chapter 33). Lower trunk plexopa- thies may also result from infiltration by neoplasm, prior radiation, or a self-­limited inflammatory process (e.g., neu- ralgic amyotrophy). Like C8–T1 radiculopathy, lower trunk plexopathies may demonstrate weakness of non-­ ulnar-­ innervated C8–T1 muscles and sensory disturbance that extends into the medial forearm. Other than in the region of the elbow, entrapment of the ulnar nerve in the arm or forearm is rare. In the arm proper, entrapment under the arcade of Struthers has been reported. In the forearm, infrequent cases of ulnar neuropathy occur at the exit of the cubital tunnel. The entrapping structure is the deep fascia between the FCU and FDP. Unusual cases of ulnar neuropathy in the distal forearm have also been reported due Fig. 22.6 Weakness of ulnar flexor digitorum profundus. In ulnar to a fibrovascular band supplying blood to a hypertrophied neuropathy at the elbow, making a fist may result in the inability to FCU muscle. Clinical differentiation of these unusual cases completely flex the distal phalanx of the fourth and fifth digits due from typical UNE is difficult. In these situations, neuromus- to weakness of the flexor digitorum profundus to digits 4 and 5. cular ultrasound is particularly helpful (see later). Before The median-­innervated flexor digitorum profundus to digits 2 and 3 is normal (affected hand shown is the right hand—left side of the the advent of neuromuscular ultrasound, these conditions photo). were usually discovered either by careful electrophysiologic 376 SECTION VIII   Clinical Disorders Table 22.1 Clinical Differentiating Factors in Suspected Ulnar Neuropathy. UNWa UNE Medial Cord Lower Trunk C8–T1 Weakness of the interossei X X X X X Weakness of the hypothenar muscles X X X X X Weakness of the third and fourth lumbricals X X X X X Weakness of distal finger flexion of the little and ring fingers X X X X Weakness of thumb abduction X X X Weakness of thumb flexion X X X Weakness of index finger extension X X Sensory loss of the volar medial hand, volar little finger, and X X X X X volar medial ring finger Sensory loss of the dorsal medial hand, dorsal little finger, and X X X X dorsal medial ring finger Sensory loss of the medial forearm X X X Tinel’s sign at the elbow X Neck pain X    X, May be present; UNE, ulnar neuropathy across the elbow; UNW, ulnar neuropathy at the wrist. aAssumes both motor and sensory branches are involved; some cases of UNW may spare the hypothenar muscles and/or the sensory branch (for details, see Chapter 23). examination, at the time of surgery, or at the time of a second may manifest as slowing of conduction velocity or conduc- surgery after a failed ulnar surgery at the elbow. tion block between proximal and distal stimulation sites (Fig. 22.8). As for focal slowing, one needs to consider how much slowing is abnormal. In general, conduction velocities ELECTROPHYSIOLOGIC of more proximal nerve segments are the same as, or more EVALUATION often faster than those of, distal segments. This is due to Like other mononeuropathies, the goal of nerve conduction a combination of (1) larger nerve fiber diameter and less studies and EMG is to demonstrate abnormalities that are tapering of the nerve more proximally (the reason that con- limited to one nerve, in this case the ulnar nerve. Although duction velocities are faster in the upper compared to the in most cases the lesion is at the elbow, entrapment at the lower extremity) and (2) warmer temperatures in the prox- wrist, at the medial cord or lower trunk of the brachial imal limb compared to the distal limb. In ulnar motor nerve plexus, or at the C8–T1 nerve roots can mimic an UNE conduction studies, however, this relationship may not hold clinically. Patterns of nerve conduction and EMG abnor- true unless the position of the elbow is controlled. malities often can be used to differentiate these possibili- ties (Table 22.2). If the ulnar nerve lesion is demyelinating, Differential Slowing: Flexed Versus Extended Elbow nerve conduction studies may demonstrate conduction Conduction Techniques velocity slowing, conduction block, or both at the lesion One of the more complicating factors in ulnar conduction site. Unfortunately, in many cases of UNE, the pathophysi- studies is the position of the elbow and its effect on the cal- ology is that of axonal loss, and nerve conduction studies culated conduction velocity across the elbow. It has been well demonstrate only a nonlocalizable ulnar neuropathy. The established in many studies that the position of the elbow dur- needle EMG study, if abnormal, can then be used to local- ing ulnar conduction studies strongly influences the calculated ize the lesion only to at or proximal to the takeoff to the conduction velocity. Ulnar conduction studies performed in most proximal muscle affected on EMG. Because there are the extended (i.e., straight) elbow position often show arti- no ulnar-­innervated muscles above the elbow, the electro- factual slowing of conduction velocity across the elbow due physiologic impression often is one of an ulnar neuropathy to underestimation of the true nerve length (Fig. 22.9). This at or proximal to the FCU muscle (the most proximal ulnar-­ is because in the extended elbow position, the ulnar nerve is innervated muscle). slack with some redundancy, which underestimates its true length. In normal subjects, this results in ulnar conduction velocities being slower in the across-­the-­elbow segment than Nerve Conduction Studies in the segment above or below it, if the study is performed The goal of nerve conduction studies in patients with UNE with the elbow in the extended position. Autopsy studies have is to demonstrate, when possible, focal demyelination confirmed that the length of the ulnar nerve across the elbow across the elbow (Box 22.1). Focal demyelinating lesions is measured more accurately with the elbow flexed (i.e., bent).  Chapter 22 Ulnar Neuropathy at the Elbow 377 Table 22.2 Electromyographic and Nerve Conduction Study Abnormalities Localizing the Lesion Site in Ulnar Neuropathy. UNW UNE Medial Cord Lower Trunk C8–T1 Electromyographic Findings First dorsal interosseous X X X X X Abductor digiti minimi X X X X X Flexor digitorum profundus (digits 4, 5) X X X X Flexor carpi ulnaris X X X X Abductor pollicis brevis X X X Flexor pollicis longus X X X Extensor indicis proprius X X Cervical paraspinal muscles X Nerve Conduction Study Findings Abnormal ulnar digit 5 SNAP (if axonal) X X X X Abnormal dorsal ulnar cutaneous SNAP (if axonal) X X X Abnormal medial antebrachial cutaneous SNAP (if axonal) X X Low ulnar CMAP (if axonal) X X X X X Low median CMAP (if axonal) X X X Conduction block/slowing of ulnar nerve across elbow X (if demyelinating)    Note: The table indicates classic patterns; other patterns may be seen (see text for details). X, May be abnormal; CMAP, compound muscle action potential; SNAP, sensory nerve action potential; UNE, ulnar neuropathy across the elbow; UNW, ulnar neuropathy at the wrist. Box 22.1 Recommended Nerve Conduction Study Protocol for Ulnar Neuropathy at the Elbow Routine studies: Ulnar neuropathy at the elbow with pure demyelinating 1. Ulnar motor study recording the abductor digiti minimi, features: stimulating the wrist, below elbow, and above elbow in the Normal distal ulnar SNAP and CMAP amplitudes and flexed elbow position (note: the optimal site for stimulating latencies at the below-­elbow site is 3 cm distal to the medial Unequivocal evidence of demyelination at the elbow epicondyle) (conduction block and/or slowing >10–11 m/s across the 2. Median motor study recording the abductor pollicis brevis, elbow compared with the forearm segment, in the flexed stimulating the wrist and antecubital fossa elbow position) 3. Median and ulnar F responses Nonlocalizable ulnar neuropathy (axonal features alone): 4. Ulnar sensory response, recording digit 5, stimulating the Low ulnar SNAP wrist Normal or low-­amplitude CMAP with normal or slightly 5. Median sensory response, recording digit 2 or 3, stimulating prolonged distal latency the wrist No focal slowing or conduction block across the elbow 6. Radial sensory response, recording snuffbox, stimulating the If the ulnar neuropathy is nonlocalizable, the following studies lateral forearm should be considered: The following patterns may result: Repeat motor studies recording the first dorsal interosseous Ulnar neuropathy at the elbow with demyelinating and axonal Inching studies across the elbow features: Sensory or mixed nerve studies across the elbow Low ulnar SNAP Recording the dorsal ulnar cutaneous SNAP (bilateral Normal or low-­amplitude ulnar CMAP with normal or studies) (remember that the dorsal ulnar cutaneous SNAP slightly prolonged distal latency can be normal in some patients with ulnar neuropathy Unequivocal evidence of demyelination at the elbow across the elbow). (conduction block and/or slowing >10–11 m/s across the Recording the medial antebrachial cutaneous SNAP elbow compared with the forearm segment, in the flexed (bilateral studies) if sensory loss extends above the wrist elbow position) on clinical examination or there is a suggestion of lower CMAP, Compound muscle action potential; SNAP, sensory nerve action potential. brachial plexus lesion by history 378 SECTION VIII   Clinical Disorders P9' PV' $PS P9 :ULVW PV $PS P9 %HORZHOERZ PV $PS P9 $ERYHHOERZ Fig. 22.8 Focal slowing and conduction block at the elbow. The ulnar compound muscle action potential amplitude is normal at the wrist and below the elbow. Stimulation above the elbow results in a marked drop in amplitude and focal slowing between the above-­elbow and below-­ elbow sites (40 m/s) compared to the forearm segment (60 m/s). These are the electrophysiologic markers of focal demyelination, which allow for definitive localization of ulnar neuropathy at the elbow. 0HDVXUHGGLVWDQFH FP 0HDVXUHGGLVWDQFH FP 7UXHGLVWDQFH FP 7UXHGLVWDQFH FP &DOFXODWHG&9 PV &DOFXODWHG&9 PV Fig. 22.9 Extended versus straight elbow technique and measurement error. Left, Ulnar conduction studies performed in the extended elbow position often show artifactual slowing of conduction velocity across the elbow due to underestimation of the true nerve length. In the extended elbow position, the ulnar nerve is slack, with some redundancy. Right, With the elbow flexed, the true length of the ulnar nerve is more accurate- ly measured, and the resultant conduction velocity is more valid. To prevent this error, the flexed elbow is the preferred position when studying the ulnar nerve. CV, Conduction velocity. In several studies of normal controls, the mean differ- avoid erroneously diagnosing UNE in the normal popula- ential slowing comparing the across-­the-­elbow conduction tion. An arbitrary cutoff value of 10 m/s differential slow- velocity to forearm conduction velocity in the flexed elbow ing between the forearm and across elbow segments, in the position (90–135 degrees) was 0 m/s, with an upper limit extended elbow position, will result in many false-­positive of normal of 10–11 m/s. In contrast, in the extended elbow diagnoses of UNE. A patient with sensory loss in the little position, mean slowing was 10–11 m/s, with an upper limit finger from a C8 radiculopathy would not be pleased to of normal in the range of 25–30 m/s (to reemphasize, in undergo ulnar nerve surgery simply based on a conduction normal controls!). This extent of factitious conduction velocity slowing of 10 m/s across the elbow compared to the velocity slowing across the elbow, in the extended elbow forearm segment, if tested in the extended elbow position position, is poorly appreciated in some EMG laboratories. (as this is a normal finding in the extended elbow position). Some laboratories arbitrarily use a value of 10 m/s differ- Similar considerations apply to the absolute conduction ential slowing across the elbow, in the extended elbow posi- velocity across the elbow in normal controls. The lower limit tion, to localize an ulnar neuropathy to the elbow. However, of normal for absolute conduction velocity across the elbow is appreciation of the large range of variability in normal sub- 38 m/s in the extended elbow position but never drops below jects, with the elbow in an extended position, is crucial to 49 m/s in the flexed elbow position. Some have found that the  Chapter 22 Ulnar Neuropathy at the Elbow 379 absolute conduction velocity across the elbow is a better mea- are rare reports of very proximal MGAs wherein the drop sure than differential conduction velocity slowing for detect- in amplitude and area occurs between the below-­elbow and ing abnormalities in patients with ulnar neuropathy. Although above-­elbow stimulation sites—that is, across the elbow. absolute conduction velocity across the elbow may be consid- Thus, in all cases of ulnar conduction block across the elbow, ered a sensitive indicator of ulnar neuropathy, it does not local- it is prudent also to check for an MGA (by stimulating the ize the lesion. In any patient with significant axonal loss and median nerve at the wrist and antecubital fossa, and record- dropout of the largest conducting fibers, conduction velocity ing the ulnar muscle) (see more later in the Nerve Conduc- will decrease across all nerve segments. An ulnar conduction tion Pitfalls section). velocity across the elbow segment of 40 m/s has little local- izing value if the forearm conduction velocity is also 40 m/s. Short Segment Incremental Studies (“Inching”) In studies comparing the relative usefulness of the flexed In a technique similar to that used for CTS, short segment versus extended elbow position in demonstrating focal incremental studies (SSISs), also known as “inching,” can be slowing across the elbow, in those patients who had local- performed effectively on the ulnar nerve across the elbow izing electrophysiology, the flexed elbow position has been to try to localize the lesion, looking for an abrupt change in found to be more sensitive than the extended position. The either latency or amplitude. The technique is performed as difference in the yield between the flexed and extended follows:    positions likely is related to the greater range and variabil- 1. Either the abductor digiti minimi (ADM) or first dorsal ity found in normal subjects for differential and absolute interosseous (FDI) muscle is recorded. A mark is first conduction velocities across the elbow when tested in the placed halfway between the medial epicondyle and the extended elbow position, leading to lower cutoff values. olecranon to mark the ulnar groove. The location of the Thus the flexed elbow position is considered the pre- ulnar nerve is then mapped out. This process is basically ferred technique when performing ulnar nerve conduction identical to that of ensuring that the stimulator is studies across the elbow. However, the flexed elbow posi- directly over the nerve, as described in Chapter 3. This is tion is more demanding in terms of measuring the curved accomplished by using a submaximal current (10%–25% anatomic course of the ulnar nerve around the elbow. In supramaximal) and stimulating medial to and lateral to addition, the flexed position has the drawback of undercall- the suspected nerve location in successive sites across ing patients with UNE and subluxable ulnar nerves, which the elbow. Several locations are tested sequentially from might lead to an overestimation of the true nerve length the below-­elbow to above-­elbow sites. At each site, the (see later). Nevertheless, it is far better to undercall UNE in location that gives the highest CMAP amplitude is the one this uncommon patient group, using the flexed technique, that is closest to the nerve and is marked with a marker than to erroneously diagnose UNE in normal patients, using pen. A line is then drawn across the elbow “connecting the extended position, with inappropriate low cutoff values. all the dots” to mark exactly where the nerve lies. Conduction Block 2. The spot between the medial epicondyle and olecranon is marked as the “zero” point along the line that was drawn In addition to focal slowing, the other electrophysiologic across the elbow and denotes the spot adjacent to the marker of demyelination is conduction block (Fig. 22.8). medial epicondyle. Next, 1-­cm increments are carefully There is some controversy regarding how much the ampli- marked off, along the line that was drawn, from 4 cm tude or area must drop between distal and proximal sites below the “zero” point (medial epicondyle) to 4 or 6 cm to be considered conduction block (see Chapter 3). Ulnar above. motor conduction studies in normal subjects have shown 3. The ulnar nerve is stimulated supramaximally at each a maximum drop in compound muscle action potential location at successive 1-­cm intervals from below to above (CMAP) amplitude of 10% comparing below and above the medial epicondyle (Fig. 22.10). elbow and 20%–25% comparing wrist and above-­ elbow    sites. Accordingly, any drop in amplitude of more than 10% Any abrupt increase in latency or drop in amplitude between below and above the elbow, especially if associated between successive stimulation sites implies focal demy- with a very small change in stimulating electrode position elination. In normal individuals, the latency between two (see the following section) or an abrupt drop in conduction successive 1-­cm stimulation sites usually is 0.1–0.3 ms velocity, likely represents true demyelination and is of local- and rarely 0.4 ms (Fig. 22.11). Any greater latency shift izing value. (i.e., ≥0.5 ms) suggests focal slowing and demyelination The other issue that must be considered in the proper (Fig. 22.12). The inching technique is very sensitive but interpretation of a conduction block is to not confuse a technically demanding. Any error in measurement is mag- Martin-­ Gruber anastomosis (MGA) with a conduction nified when such short distances are used. The technique block. Almost always, an MGA is recognized on routine has the advantage of potentially being able to directly ulnar motor nerve conduction studies as a drop in amplitude locate the lesion either at the groove or at the cubital and area between the wrist and below-­elbow stimulation tunnel. This may be of more than just academic inter- sites (i.e., mimicking a conduction block in the forearm). est, because it may be of some help in deciding the best The site of the MGA is typically between 3 and 10 cm dis- surgical technique to use (e.g., a lesion of the cubital tun- tal to the medial epicondyle, a location that is not thought nel may be best handled by a simple release rather than a to interfere with EDX evaluation of UNE. However, there transposition). 380 SECTION VIII   Clinical Disorders P9' PV' ±FP  ±FP  ±FP ±FP  0(       FP      0(  FP FP  FP  FP  Fig. 22.10 Short segment incremental studies. To perform these studies, a mark is first placed halfway between the medial epicondyle (ME) and the olecranon to mark the ulnar groove (the red circles are over the ME and olecranon). The location of the ulnar nerve is then mapped using a submaximal current and stimulating from the below-­elbow to above-­elbow sites, stimulating medial to and lateral to the suspected nerve location in successive sites across the elbow. P9' PV' The location that gives the highest compound muscle action potential amplitude is the one that is closest to the nerve. Several locations are tested from the below-­elbow to above-­elbow sites to mark exactly where the nerve lies. Then, 1-­cm increments are carefully marked off from 4 cm below the elbow to 4–6 cm above. The ulnar nerve is then stimulated supramaximally at each location at successive 1 cm inter- vals from below to above the elbow looking for any abrupt change in latency or drop in amplitude. Fig. 22.11 Inching around the elbow—normal. Top, Ten successive Recording the First Dorsal Interosseous traces in 1-­cm increments from 4 cm below the medial epicondyle In entrapment neuropathies, it is well known that nerve to 5 cm above. Bottom, Superimposed traces. The numbers on the fibers to certain muscles may be preferentially affected right are the latency differences in milliseconds between succes- whereas others are preferentially spared. Within a nerve, sive traces. Note: In normals, the amplitude of the waveform stays bundles of nerve fibers to different muscles run in separate constant, and the latency difference between successive traces is 0.1–0.3 ms. fascicles separated by connective tissue. External compres- sion may preferentially affect the fibers within the fascicle nearest to the compression, thereby preferentially affecting antidromically or orthodromically from the fifth digit using the muscle that those fibers innervate (Fig. 22.13). Thus, the wrist, below-­elbow, and above-­elbow sites for either recording from different muscles sometimes can increase stimulating or recording, respectively. Likewise, a mixed the yield of demonstrating either focal slowing or con- nerve potential can be recorded below and above the elbow, duction block. Some studies have shown that recording stimulating the mixed nerve at the wrist. the FDI may be a slightly more sensitive technique than Although these studies may have increased sensitivity recording the ADM in UNE. When recording the FDI, it for identifying UNE, they involve significant technical is best to place the active electrode over the muscle belly difficulties. Both sensory and mixed nerve potentials dra- and the reference over the metacarpophalangeal joint of the matically decrease in amplitude when recorded at greater thumb (Fig. 22.14). If the reference electrode is placed on distances because of the normal phenomena of tempo- the metacarpophalangeal joint of the index finger, an initial ral dispersion and phase cancellation (Fig. 22.16). For positive deflection will often be seen, which complicates instance, a normal antidromic ulnar sensory nerve action latency measurements (Fig. 22.15). potential (SNAP) amplitude may be 20 μV when stimu- lated at the wrist; however, stimulating at the below-­elbow Mixed and Sensory Nerve Conductions site, the amplitude may fall to 5 μV, and above the elbow, Mixed and sensory ulnar nerve conductions across the to 2 μV. In patients with ulnar neuropathy, these potentials elbow may increase the yield of identifying focal slowing in are often low if axonal loss has occurred. In such cases, the patients with UNE. Sensory conductions can be performed potential at the below-­elbow and above-­elbow sites may  Chapter 22 Ulnar Neuropathy at the Elbow 381 P9' PV' ±FP +\SRWKHQDU +\SRWKHQDU  )', ±FP &6 )', &6  &6 )'3 &6 )'3 ±FP &6 &6 &6 &6 )&8 &6 &6 )&8  0( &6 &6 &6 &6 / /  / / FP  FP  8OQDUQHUYH ([WHUQDOFRPSUHVVLRQ FP Fig. 22.13 Fascicular involvement and sparing in entrapment neuropathies. Within a nerve, bundles of nerve fibers to different muscles run in separate fascicles separated by connective tissue. External compression may preferentially affect the fibers within the P9' PV' fascicle nearest to the compression, thereby preferentially affecting the muscle that those fibers innervate. Thus, recording from different muscles sometimes can increase the yield of demonstrating either focal slowing or conduction block. In this example, the fibers that supply the first dorsal interosseous (FDI) run in a fascicle closest to the site of compression. Accordingly, studying the FDI may have a higher yield of demonstrating abnormalities. CS, Cutaneous sensory; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; 4L, fourth Fig. 22.12 Inching around the elbow—cubital tunnel syndrome. lumbrical; 3L, third lumbrical. Top, Seven successive traces in 1-­cm increments from 3 cm below the medial epicondyle (ME) to 3 cm above. Bottom, Superimposed traces. The numbers on the right are the latency differences in milliseconds between successive traces. Note: Between the 1-­cm below-­elbow site and the ME, there is an abrupt change in latency (1.8 ms) and a drop in amplitude. In this case, inching studies not only have confirmed an ulnar neuropathy at the elbow but have precisely located the lesion at the cubital tunnel. be very low or absent. Averaging is frequently required, * and identification of the onset latency of these very small * potentials is often difficult. These techniques may be best suited for mild cases of UNE, in which the distal sensory and mixed nerve potentials are of relatively normal ampli- tude. It is important to emphasize that when performing Fig. 22.14 Recording the first dorsal interosseous muscle. The active these studies, one is looking for focal slowing across the electrode (G1) is placed over the muscle belly, and the reference elec- trode (G2) is placed over the metacarpophalangeal joint of the thumb. elbow and not for conduction block. Because of the normal Recording the first dorsal interosseous is helpful in ulnar neuropathy at drop in amplitude of sensory and mixed nerve fibers over the wrist and in some cases of ulnar neuropathy at the elbow. distance, separating conduction block from normal tem- poral dispersion and phase cancellation may be very diffi- potential can be recorded using a small stimulating current cult (unless the stimulation sites are very close, such as the (e.g., 5–15 mA). A normal antidromic response is greater 1-­cm increments used in inching across the carpal tunnel). than 8 μV, but, as in other uncommonly performed sensory nerve conduction studies, comparison with the contralateral Dorsal Ulnar Cutaneous Sensory Study asymptomatic side frequently is helpful. Any potential that Recording the dorsal ulnar cutaneous sensory nerve can be is less than 50% that of the contralateral asymptomatic side a useful technique to perform in patients with suspected likely is abnormal.a ulnar neuropathy. The dorsal ulnar cutaneous sensory nerve arises 5–8 cm proximal to the wrist to supply sensa- a Remember that, very rarely, there is an anomalous innervation wherein tion over the dorsal medial hand, as well as the dorsal fifth the superficial radial sensory nerve supplies the entire dorsum of the and medial fourth digits. The dorsal ulnar SNAP can be hand, including the usual territory of the dorsal ulnar cutaneous sensory recorded by placing recording electrodes in the dorsal web nerve. Thus, in cases where the dorsal ulnar cutaneous sensory response is absent, it is prudent to stimulate the superficial radial sensory nerve space between the fifth and fourth digits and stimulating along the lateral radius with the recording electrodes in place for the 8–10 cm proximally just below the ulnar styloid with the dorsal ulnar cutaneous sensory study to ensure that this very rare anom- hand placed in a pronated position (Fig. 22.17). Usually, the alous innervation is not present. 382 SECTION VIII   Clinical Disorders P9' PV' 'LJLWXOQDU61$3 —9 PV :ULVW 5HIHUHQFHHOHFWURGH %DVHRIWKHWKXPE 5HIHUHQFHHOHFWURGH %HORZHOERZ %DVHRIWKHLQGH[ILQJHU 5HFRUGLQJWKH)', Fig. 22.15 First dorsal interosseous (FDI) muscle—compound mus- cle action potential morphology and placement of the r­ eference electrode. When recording the FDI, it is best to place the active electrode over the muscle belly and the reference electrode over the $ERYHHOERZ metacarpophalangeal joint of the thumb. If the reference electrode is placed on the metacarpophalangeal joint of the index finger, an ini- tial positive deflection often will be seen, which complicates latency Fig. 22.16 Ulnar sensory conduction studies: normal responses. measurements. An antidromic ulnar sensory nerve action potential (SNAP) can be recorded over the fifth digit, stimulating the wrist and below and above the elbow. Normal sensory responses decrease markedly With knowledge of the anatomy of both the routine in amplitude and area, whereas their duration increases, at more digit 5 ulnar SNAP and the dorsal ulnar SNAP, one can proximal stimulation sites because of the normal processes of tem- predict the expested patterns of abnormalities in lesions of poral dispersion and phase cancellation. For this reason, proximal demyelinating lesions in sensory fibers can be detected only by the ulnar nerve at the elbow and at the wrist (Fig. 22.18). conduction velocity slowing and not by drop in amplitude or area. In patients with UNE, one would expect both SNAPs to be abnormal, provided there has been axonal loss. If the lesion is pure demyelination at the elbow, both distal sen- sory responses will be normal. Conversely, the presence of a normal dorsal ulnar SNAP with an abnormal digit 5 ulnar sensory response suggests that the lesion is at the wrist (the dorsal ulnar branch arises proximal to the wrist). However, this latter pattern does not completely exclude the possibil- ity of UNE. In some patients with definite UNE and axonal *URXQG loss, the dorsal ulnar cutaneous SNAP can be spared. This finding is thought to be due to preferential fascicular spar- &DWKRGH ing of the dorsal ulnar cutaneous sensory fibers at the elbow. Studies of the microanatomy of the fascicle that forms the * dorsal ulnar cutaneous sensory branch have shown that it commonly separates from the main ulnar trunk above the elbow and effectively travels as a separate nerve next to the ulnar nerve in the forearm. Therefore, care must be taken in * interpreting the findings of an abnormal digit 5 ulnar SNAP and a normal dorsal ulnar cutaneous SNAP. This pattern has limitations as a diagnostic marker and cannot be used alone to reliably localize the site of the lesion to the wrist. Elec- trophysiologic measurement of the dorsal ulnar SNAP is Fig. 22.17 Dorsal ulnar cutaneous sensory study. With the hand in useful, but only in those cases where it is abnormal, imply- a pronated position, recording electrodes are placed in the dorsal web ing localization of the lesion proximal to the wrist. To sum- space between the fourth and fifth fingers. The stimulation site is just marize, although an abnormal dorsal ulnar cutaneous SNAP below the ulnar styloid, 8–10 cm proximal to the recording electrodes.  Chapter 22 Ulnar Neuropathy at the Elbow 383 —9 PV 1RUPDO 8OQDU QHXURSDWK\ DWWKHHOERZ 8OQDU QHXURSDWK\ DWWKHZULVW 8OQDU'LJLW61$3 'RUVDO8OQDU&XWDQHRXV61$3 $ % Fig. 22.18 Ulnar sensory response patterns. The typical patterns of the routine ulnar digit 5 (A) and the dorsal ulnar cutaneous sensory nerve action potential (SNAP) responses (B) in ulnar neuropathy at the elbow and at the wrist. Both assume lesions that involve axonal loss and that there is no anomalous innervation of the dorsal ulnar cutaneous sensory nerve. However, significant exceptions to these classic patterns may occur: if (1) the ulnar neuropathy at the elbow is purely demyelinating or (2) there is an ulnar neuropathy at the wrist that spares the sensory branch. In both of these situations, both ulnar SNAPs may be normal. In addition, some cases of axonal loss ulnar neuropathy at the elbow may spare the dorsal ulnar cutaneous sensory response and thus display the pattern common to ulnar neuropathy at the wrist, sparing the dorsal ulnar cutaneous SNAP. indicates that the lesion is proximal to the wrist, the converse required at the below-­elbow site, and supramaximal is not necessarily true. stimulation may be difficult to achieve. 3. The distance from the below-­elbow site to the above-­ Nerve Conduction Study Pitfalls elbow site ideally should be 10 cm. If shorter distances When performing routine ulnar nerve conduction studies, are used, slight errors in measurement may create large one must keep in mind several important technical factors: differences in calculated conduction velocities. If longer    distances are used, a longer length of normal nerve may 1. When stimulating at the below-­elbow site, the stimulator dilute any conduction velocity slowing across the elbow, must be located 3 cm distal to the groove to ensure that yielding normal or equivocal results. the stimulation point is distal to the cubital tunnel. 4. The examiner should be cautious of any apparent 2. Care must be taken not to stimulate too distally at the conduction block between the wrist and below-­elbow sites below-­elbow site. The mistaken impression of UNE (Fig. 22.19). Although there are very rare ulnar lesions in may occur if the below-­elbow stimulation site is too the forearm (e.g., exit of the cubital tunnel, fibrovascular distal and the patient has a coexistent high MGA (see bands in the forearm), this finding usually indicates an Chapter 7). In this situation, one could see a drop MGA, which is a normal finding. In these situations, in amplitude between the below-­elbow and above-­ stimulating the median nerve at the wrist and antecubital elbow stimulation sites. Thus, the optimal position to fossa, while recording the ulnar muscle (ADM or FDI), is stimulate the below-­elbow site is 3 cm distal to the essential to demonstrate that an anastomosis is present. medial epicondyle, not less, not more. In addition, as 5. As noted previously, an MGA is usually recognized the nerve runs deep under the FCU, higher current is on routine ulnar motor nerve conduction studies as 384 SECTION VIII   Clinical Disorders a drop in amplitude and area between the wrist and 2 cm to be distal to the cubital tunnel and less than 4 below-­elbow stimulation sites. However, there are rare cm to avoid the nerve being so deep that it is difficult reports of a very proximal MGA wherein the drop in to stimulate. Although almost all MGAs occur in the amplitude and area occurs between the below-­elbow and forearm and do not interfere with EDX determination above-­elbow stimulation sites (i.e., across the elbow), of conduction block at the elbow, some reports have mimicking a conduction block across the elbow. The found an MGA to be as proximal as 3 cm distal to optimal site for the below-­elbow stimulation site is 3 cm the medial epicondyle (Fig. 22.20). Thus, in this very distal to the medial epicondyle. It should be more than unusual situation, one could confuse an MGA (a normal finding) with an ulnar conduction block across the elbow. This underscores that the correct site to stimulate the P9 ulnar nerve below the elbow is 3 cm distal to the medial PV epicondyle. In all cases where an ulnar conduction block is found across the elbow, it is prudent to check for an :ULVW P9 MGA (by stimulating the median nerve at the wrist and antecubital fossa, and recording the ulnar muscle). 6. Rarely, flexing the elbow causes the ulnar nerve to sublux %HORZHOERZ P9 out of the groove medially over the medial epicondyle. Recurrent subluxation of the ulnar nerve has been a suggested cause of repetitive ulnar trauma and UNE, $ERYHHOERZ P9 although the incidence of subluxation is similar between patients with UNE and healthy controls. Thus, in a Fig. 22.19 Martin-­Gruber anastomosis mimicking conduction block situation where a patient does have UNE and a subluxed of the ulnar nerve in the forearm. The most common cause of drop nerve when in the flexed elbow position, the measured in amplitude between the wrist and below-­elbow sites during routine ulnar motor conduction studies is not true conduction block but a distance around the groove will actually overestimate Martin-­Gruber anastomosis. Conduction block of the ulnar nerve the length of the nerve, resulting in a spuriously fast in the forearm should never be diagnosed unless the median nerve conduction velocity across the elbow (see Fig. 22.21). has been stimulated at the wrist and antecubital fossa to exclude a In one study of normal individuals with a subluxed Martin-­Gruber anastomosis. ulnar nerve at the elbow, the average change in distance P9 PV :ULVW %HORZHOERZ %HORZHOERZ $ERYHHOERZ $ERYHHOERZ 0HGLDQZULVW 0HGLDQZULVW 0HGLDQHOERZ 0HGLDQHOERZ Fig. 22.20 Martin-­Gruber anastomosis (MGA): “conduction block” patterns in the forearm and across the elbow. Left, Recording from the first dorsal interosseous (FDI), there is a marked drop in amplitude between the wrist and below-­elbow stimulation sites, suggesting a conduction block in the forearm. However, stimulating the median nerve at the wrist and antecubital fossa (elbow) while recording from the FDI confirmed an MGA, as a much larger response was present at the antecubital fossa than at the wrist. This is the common presentation of an MGA during routine ulnar motor conduction studies recording from the FDI. Right, Recording from the FDI, there is a marked drop in amplitude between the below-­elbow and above-­elbow stimulation sites, which was interpreted as a conduction block across the elbow (i.e., ulnar neuropathy at the elbow). However, stimulating the median nerve at the wrist and antecubital fossa (elbow) while recording from the FDI also demonstrated an MGA in this case. By short segment “inching studies,” the site of the MGA was found to be 3 cm distal to the retrocondylar groove. This finding of an MGA mimicking an ulnar conduction block across the elbow is exceedingly rare. (With permission from Leis AA, Stetkarova I, Wells KJ. Martin-­Gruber anastomosis with anomalous superficial radial innervation to ulnar dorsum of hand: a pitfall when common variants coexist. Muscle Nerve. 2010;41:313–317.)  Chapter 22 Ulnar Neuropathy at the Elbow 385 across the elbow segment was overestimated by 1.6 cm suggested based on clinical history, then sampling the cervi- (range, 0.6–2.5 cm), which equated to an overestimated cal paraspinal muscles is also indicated. ulnar conduction velocity across the elbow an average Of interest, the FCU is either normal or minimally of 7.9 m/s (range, 3.0–13.0 m/s). One can see how this affected in many surgically proven cases of UNE. Overall, could result in a false-­negative study in a person with involvement of the FCU correlates with the severity of UNE, if the conduction velocity across the elbow were the ulnar neuropathy both clinically and electrically. The overestimated and did not meet the cutoff criteria for finding of FCU involvement is slightly more common in absolute and differential focal slowing across the elbow. lesions at the groove than at the cubital tunnel. Although It is important to emphasize that a subluxed ulnar nerve sparing of the FCU was classically thought to be due to interferes with determination of the true conduction the muscular branch to the FCU arising proximal to the velocity across the elbow, but not conduction block groove, cadaver anatomic studies have found this not to be across the elbow. These cases emphasize the value of the case. The true etiology of FCU sparing likely is related SSISs (“inching”) in suspected UNE, as this technique to either differential fascicular susceptibility (i.e., differ- maps out the exact location of the ulnar nerve and is able ent fascicles are involved or spared depending on their to detect the precise area of slowing. position within the nerve trunk in relationship to the com- pression site) or the “dying back” concept of nerve lesions (i.e., the most distal muscles are maximally affected and Electromyographic Approach the proximal ones are relatively preserved). This EMG The strategy in the EMG examination of UNE is directed pattern of an abnormal FDI and FDP to digits 4 and 5, toward identifying denervation or reinnervation limited to but a spared FCU, is important to recognize in patients the ulnar-­innervated muscles of the hand and forearm (Box with UNE. 22.2). Useful muscles to check are the FDI, FDP (to digit Unfortunately, there is no ulnar-­innervated muscle above 4 or 5), and FCU. Of the intrinsic ulnar hand muscles, test- the elbow. If all ulnar-­innervated muscles, including the ing of the FDI is usually tolerated best by patients. Test- FCU, are abnormal and the nerve conduction studies do ing of the ADM often is perceived as more painful, similar not identify any focal slowing or conduction block around to the abductor pollicis brevis (APB). Median-­and radial-­ the elbow, the best electrophysiologic diagnosis that can innervated C8 muscles are screened to rule out evidence be reached is one of an ulnar neuropathy that can only be of a C8 radiculopathy or lower trunk brachial plexopathy. localized at or proximal to the takeoff to the FCU muscle. Useful muscles to check are the APB, flexor pollicis longus, Although most such cases will in fact be cases of UNE, the and extensor indicis proprius. If a cervical radiculopathy is electrophysiologic examination cannot exclude an unusual ulnar neuropathy in the proximal arm or a lower brachial plexopathy that selectively involves ulnar fibers. In this situation, neuromuscular ultrasound is particularly helpful (see the following section). Examination of the medial ante- brachial cutaneous sensory nerve, which comes directly off the medial cord of the brachial plexus, may help identify a lower brachial plexus lesion. ULTRASOUND CORRELATIONS Neuromuscular ultrasound is an extremely useful com- plement to EDX studies in the evaluation of UNE. It is especially helpful in those cases where the EDX study is unable to localize the ulnar neuropathy. Ultrasound is able to visualize the ulnar nerve throughout its course in the upper extremity, from the wrist to the axilla. The nerve is fairly superficial at all locations and is easily imaged with a high-­frequency probe. Not only can ultrasound local- ize the site of the ulnar neuropathy when it occurs at the Fig. 22.21 Ulnar nerve location at the elbow. In the vast majority elbow, but it can also be precise in differentiating between of individuals, the ulnar nerve runs in a curved pattern (green line) a lesion at the retrocondylar groove versus the cubital tun- through the ulnar groove between the medial epicondyle (upper black dot) and the olecranon (lower black dot). However, in some nel proper. Since ultrasound can visualize the entire course individuals, when the elbow is flexed, the ulnar nerve will sublux out of the ulnar nerve, it is also useful because of its ability to of the groove medially over the medial epicondyle (red line). In these exclude lesions at other locations along the ulnar nerve that cases, the measured distance (green line) will overestimate the true can mimic UNE. distance (red line), which then results in an overestimate of the ulnar conduction velocity across the elbow (i.e., the conduction velocity Like other entrapment neuropathies, ultrasound can will be spuriously fast). Accordingly, in some cases of true ulnar neu- yield key diagnostic information about nerve entrapment ropathy at the elbow, this could result in a false-­negative study. and structural lesions affecting the ulnar nerve (Table 22.3). 386 SECTION VIII   Clinical Disorders Box 22.2 Recommended Electromyographic Protocol Table 22.3 Ultrasound Assessments of the Ulnar Nerve. for Ulnar Neuropathy at the Elbow Cross-­sectional area Routine muscles: Distal wrist adjacent to the pisiform bone (Guyon’s canal) 1. Ulnar muscle distal to the wrist (first dorsal interosseous or Mid-­forearm (where the ulnar nerve and artery separate) abductor digiti minimi) Cubital tunnel 2. Ulnar muscles in the forearm (FDP 5 and flexor carpi ulnaris) Retrocondylar groove If any of the ulnar muscles are abnormal, test the following Mid-­arm additional muscles: Swelling ratio 1. At least two non-­ulnar lower trunk/C8–T1-­innervated Elbow (either cubital tunnel or groove)/mid-­forearm muscles (e.g., abductor pollicis brevis, flexor pollicis Elbow (either cubital tunnel or groove)/mid-­arm longus, extensor indicis proprius) to exclude a lower Echogenicity brachial plexopathy, polyneuropathy, or C8–T1 Mobility radiculopathy Subluxation/dislocation at the groove with elbow flexion 2. C8 and T1 paraspinals Ulnar nerve ± triceps Special considerations: Vascularity If the ulnar neuropathy is superimposed on another Anomalies condition (e.g., polyneuropathy, plexopathy, radiculopathy), Accessory epitrochlearis muscle at the groove a more detailed electromyographic examination will be Accessory abductor digiti minimi muscle at the wrist (see required. Chapter 23) The abductor digiti minimi frequently is painful and more Presence of structural lesions difficult for some patients to tolerate than the first dorsal Ganglion cyst at the elbow interosseous. Bone spurs at the elbow In ulnar neuropathy at the elbow, the flexor carpi ulnaris Neuroma may be spared even when the FDP 5 is abnormal. Intraneural and extraneural scar If no evidence of ulnar neuropathy is found on nerve Tumor conduction studies, electromyographic study should focus Schwannoma on evaluation for lower trunk brachial plexopathy or C8–T1 Neurofibroma radiculopathy if clinically indicated. Fibrolipoma FDP, Flexor digitorum profundus. the forearm. The ulnar nerve CSA is next measured at this location in the forearm. The probe is then moved proximally Ultrasound also has the unique advantage of being able to while following the ulnar nerve as it runs first through the examine the ulnar nerve during movement, looking for sub- cubital tunnel and then through the retro-­condylar groove. luxation or frank dislocation of the nerve out of the groove. We prefer to visualize the nerve in the retrocondylar groove In some cases, not only will the ulnar nerve sublux out the with the elbow in the extended position, with the arm supi- groove but part of the triceps muscles will do so as well (Fig. nated. A generous amount of gel is needed when the probe 22.22). The patient may note the snap of the ulnar nerve is placed on a line connecting the medial epicondyle and the coming out of the groove during certain movements and, in olecranon. In this position, two bony shadows are seen: the the case of the ulnar nerve and the triceps, a double snap. large medial epicondyle on one side and the olecranon on the When visualizing the ulnar nerve, the patient lies supine other (Fig. 22.24). The ulnar nerve runs in between, often on the examining table, with the arm initially extended. The very close to the posterior border of the medial epicondyle. probe is first placed in short axis over the median nerve at In this location, the normal ulnar nerve is often hypoechoic. the distal wrist crease at the standard starting median nerve The shape can be oval or round, but sometimes has a two-­ location. Once the median nerve is well identified, the lobed configuration. At the retrocondylar groove, the CSA probe is slowly moved toward the ulnar side of the wrist is measured while assuring the probe is perpendicular to the looking for a prominent hypoechoic structure, which is the nerve. In addition, at this location, very gentle probe pressure ulnar artery. This can be confirmed by color or power Dop- is maintained while keeping the nerve in view as one passively pler. Once the ulnar artery is identified, the probe is very flexes the elbow. One looks to see if the ulnar nerve subluxes slowly moved distally. The pisiform bone will then appear over the medial epicondyle, and in some cases may frankly on the ulnar side of the wrist. It is easily identified by its dislocate to the other side of the medial epicondyle. Rarely, well-­demarcated bone shadow. Between the ulnar artery and part of the triceps muscle behind the ulnar nerve may dislo- the pisiform bone is the ulnar nerve (Fig. 22.23). The ulnar cate as well. One can repeat this maneuver several times to nerve cross-­sectional area (CSA) can then be measured at the determine if the nerve subluxes over the medial epicondyle. wrist while inspecting for any nearby structural lesions. Next, Recurrent subluxation and dislocation are purported causes the probe is slowly moved proximally over the forearm while of UNE. Presumably, the repetitive friction over the medial following the ulnar nerve. The ulnar nerve and ulnar artery epicondyle results in injury to the ulnar nerve. accompany each other. They run under the tendon to the At the elbow, one also inspects for other structural FCU, which then becomes muscular. At approximately the lesions, especially ganglion cysts and anomalous muscles. mid-­forearm level, the nerve and artery separate. The ulnar There are several structural abnormalities to specifi- artery runs laterally to eventually join the median nerve in cally look for when assessing the ulnar nerve at the elbow.