Electromyography & Neuromuscular Disorders PDF

SECTION VIII Clinical Disorders PART I Common Mononeuropathies Radial Neuropathy...

SECTION VIII Clinical Disorders PART I Common Mononeuropathies Radial Neuropathy 24 In the electromyography (EMG) laboratory, the radial & nerve is studied less frequently than the median and ulnar nerves and their respective well-­known lesions. Neverthe- & less, entrapment of the radial nerve does occur, often affect- 8SSHUPLGGOHORZHUWUXQNV ing the main radial nerve either in the upper arm or axilla. & Isolated lesions of its terminal divisions in the forearm, the & posterior interosseous, and superficial radial sensory nerves, 3RVWHULRUFRUG also occur. Although radial motor nerve conduction studies 7 are technically demanding, the electrophysiologic evaluation 5DGLDOQHUYH of radial neuropathy usually is able to localize the lesion, assess the underlying pathophysiology, and provide useful information regarding severity and subsequent prognosis. In addition, similar to other entrapment neuropathies, neuro- muscular ultrasound is often very useful in adding specific anatomic information regarding the location and etiology of a radial neuropathy. ANATOMY The radial nerve receives innervation from all three trunks of the brachial plexus and, correspondingly, a contribu- tion from each of the C5–T1 nerve roots (Figs. 24.1 and Fig. 24.1 Anatomy of the radial nerve. The radial nerve receives 24.2). After each trunk divides into an anterior and poste- innervation from all three trunks of the brachial plexus and, cor- rior division, the posterior divisions from all three trunks respondingly, a contribution from each of the C5–T1 nerve roots. unite to form the posterior cord. The posterior cord gives off (Adapted with permission from Haymaker W, Woodhall B. Peripheral the axillary, thoracodorsal, and subscapular nerves before Nerve Injuries. Philadelphia, PA: WB Saunders; 1953.) becoming the radial nerve. In the high arm, the radial nerve first gives off the posterior cutaneous nerve of the arm, the then given off to the brachioradialis and the long head of lower lateral cutaneous nerve of the arm, and the posterior the extensor carpi radialis. The radial nerve then enters cutaneous nerve of the forearm (Fig. 24.3), followed by the radial tunnel, which is the space formed posteriorly by muscular branches to the three heads of the triceps bra- the distal humerus and radiocapitellar joint, the brachialis chii (medial, long, and lateral) and the anconeus. In some muscle medially, the brachioradialis muscle anteriorly, and patients, there is evidence that the long head of the triceps the extensor carpi radialis brevis muscle laterally. The radial may be supplied either by the axillary nerve or the posterior tunnel is approximately 5 cm in length and runs between cord directly. The anconeus is a small muscle in the proxi- the area where the radial nerve pierces the lateral intermus- mal forearm that effectively is an extension of the medial cular septum to where the deep motor branch enters the head of the triceps brachii. After giving off these muscu- proximal edge of the supinator.a Next, 3–4 cm distal to the lar branches, the radial nerve wraps around the posterior lateral epicondyle, the radial nerve bifurcates into two sepa- humerus in the spiral groove. The posterior cutaneous nerve rate nerves: one superficial and the other deep. The super- of the forearm accompanies the radial nerve through the spi- ficial branch, known as the superficial radial sensory nerve, ral groove and remains in the posterior compartment of the descends distally under the brachioradialis in the forearm arm before becoming subcutaneous approximately 6–7 cm and eventually moves subcutaneous over the radial bone to directly proximal to the lateral epicondyle. Descending into supply sensation over the lateral dorsum of the hand as well the region of the elbow, the main radial nerve then pierces the lateral intermuscular septum to run between the bra- aSome consider the radial tunnel to continue to where the posterior chialis and brachioradialis muscles. Muscular branches are interosseous nerve leaves the distal border of the supinator. 417 418 SECTION VIII   Clinical Disorders & 5DGLDOQHUYH 3RVWHULRUFXWDQHRXVQHUYHRIDUP 3RVWHULRUFXWDQHRXV QHUYHRIWKHDUP /RZHUODWHUDOFXWDQHRXVQHUYHRIDUP /RZHUODWHUDOFXWDQHRXV 3RVWHULRUFXWDQHRXVQHUYH QHUYHRIWKHDUP RIIRUHDUP 7ULFHSV 'HHSUDGLDOPRWRUQHUYH 7ULFHSVDQGDQFRQHXV 3RVWHULRUFXWDQHRXV %UDFKLRUDGLDOLV QHUYHRIWKHIRUHDUP ([WHQVRUFDUSLUDGLDOLVORQJXV ([WHQVRUFDUSLUDGLDOLVEUHYLV 6XSHUILFLDOUDGLDO VHQVRU\QHUYH 6XSLQDWRU 3RVWHULRU ([WHQVRUGLJLWRUXPFRPPXQLV LQWHURVVHRXV $QWHULRU 3RVWHULRU QHUYH ([WHQVRUGLJLWLTXLQWL Fig. 24.3 Sensory territories supplied by the radial nerve. (Adapted with permission from Haymaker W, Woodhall B. Peripheral Nerve ([WHQVRUFDUSLXOQDULV Injuries. Philadelphia, PA: WB Saunders; 1953.) $EGXFWRUSROOLFLVORQJXV ([WHQVRUSROOLFLVORQJXVDQGEUHYLV 6XSHUILFLDOUDGLDO ([WHQVRULQGLFLVSURSULXV VHQVRU\QHUYH 'RUVDOGLJLWDOQHUYHV Fig. 24.2 Anatomy of the radial nerve. The radial nerve is derived from the posterior cord of the brachial plexus. In the high arm, the radial nerve first gives off the posterior cutaneous nerve of the arm, the lower lateral cutaneous nerve of the arm, and the posterior cu- taneous nerve of the forearm, followed by muscular branches to the triceps brachii and anconeus. The radial nerve then wraps around the humerus, descending into the region of the elbow where muscular branches are given to the brachioradialis and long head of the exten- sor carpi radialis. The nerve then bifurcates into the superficial radial Fig. 24.4 Sensory territory of the superficial radial sensory sensory and deep motor branch of the radial nerve. The deep motor nerve. The superficial radial sensory nerve supplies sensation over branch supplies the extensor carpi radialis brevis (in most cases) and the lateral dorsum of the hand, as well as part of the thumb and dor- the supinator muscle before continuing on as the posterior interosse- sal proximal phalanges of the index, middle, and ring fingers. ous nerve. The posterior interosseous nerve supplies the remainder of the wrist and finger extensors, as well as the abductor pollicis longus. (Adapted with permission from Haymaker W, Woodhall B. Peripheral Nerve Injuries. Philadelphia, PA: WB Saunders; 1953.) as part of the thumb and the dorsal proximal phalanges of the index, middle, and ring fingers (Fig. 24.4). Distally, the nerve is quite superficial, running over the tendon to the extensor pollicis longus, where it can easily be palpated (Fig. 24.5). The deep branch, known as the deep radial motor branch, first supplies the extensor carpi radialis brevis and the supinator muscles before it enters the supina- tor muscle under the Arcade of Frohse (Fig. 24.6). The Arcade of Frohse is the proximal border of the supina- tor and in some individuals is quite tendinous. After the Fig. 24.5 Superficial radial sensory nerve. The superficial radial nerve enters the supinator, it is known as the posterior nerve runs distally in the forearm over the radial bone to supply sensation over the lateral dorsum of the hand, as well as part of the interosseous nerve, which then supplies the remaining thumb and the dorsal proximal phalanges of the index, middle, and extensors of the wrist, thumb, and fingers (extensor ring fingers. It runs over the extensor tendons to the thumb (arrows), digitorum communis, extensor carpi ulnaris, abductor where it can easily be palpated.  Chapter 24 Radial Neuropathy 419 6XSLQDWRU %LFHSV (&5/ (&5% %5 Humerus 5DGLXV Radial nerve +XPHUXV 8OQD $UFDGHRI)URKVH Fig. 24.7 Anatomy and nomenclature of the radial nerve around the elbow. As the main radial nerve enters the region of the elbow (purple), it supplies the brachioradialis (BR) and extensor carpi radialis longus (ECRL) Deep radial motor muscles. It then divides into a superficial radial sensory branch (green) nerve and a deep radial motor branch (yellow). The deep radial motor branch Arcade of Frohse typically innervates the extensor carpi radialis brevis (ECRB) and supinator Supinator muscle muscles before entering into the substance of the supinator muscle at the Arcade of Frohse. Past the Arcade of Frohse, the continuation of the deep Superficial radial radial motor branch is known as the posterior interosseous nerve (blue). Extensor carpi sensory nerve radialis brevis muscle However, please note that some anatomic texts define the posterior inter- osseous nerve as originating at the bifurcation of the main radial nerve and thus use the terms deep radial motor branch and posterior interosseous Ulna nerve interchangeably. If this definition is used, then both the ECRB and the supinator muscle would both be supplied by the posterior interos- seous nerve. (Adapted with permission from Thomas SJ, Yakin DE, Parry BR, et al. The anatomical relationship between the posterior interosseous nerve and the supinator muscle. J Hand Surg Am 25. 2000;(5):936–941.) Radius I n some individuals, the main radial nerve will also supply a third muscle, the extensor carpi radialis brevis Fig. 24.6 Anatomy of the radial nerve at the elbow. Distal to the el- muscle.b bow, the radial nerve bifurcates into the superficial radial sensory and deep radial motor branch. The deep radial motor branch enters the The Bifurcation Near the Elbow supinator muscle under the Arcade of Frohse, where it is then known The main radial nerve always bifurcates into superficial as the posterior interosseous nerve, which supplies the remaining extensors of the wrist, thumb, and fingers. (Adapted with permission and deep branches just distal to the elbow. from Wilbourn AJ. Electrodiagnosis with entrapment neuropathies. AAEM plenary session I: entrapment neuropathies. Charleston, South Superficial Branch Carolina; 1992.) The superficial branch continues as a pure cutaneous sensory branch (the superficial radial sensory branch). pollicis longus, extensor indicis proprius [EIP], extensor However, in a small number of individuals, there is an pollicis longus, and extensor pollicis brevis). Although anatomic variation wherein the superficial branch near the posterior interosseous nerve is thought of as a pure its origin will supply one muscle, the extensor carpi motor nerve (supplying no cutaneous sensation), it does radialis brevis.b contain sensory fibers that supply deep sensation to the interosseous membrane and joints between the radial and Deep Branch ulna bones. The deep radial motor branch first supplies the extensor carpi radialis brevis muscle in some Nomenclature of the Branches of the Radial individuals.b Nerve Near the Elbow It then supplies one or more branches to the supinator One of the more confusing aspects of radial nerve anatomy muscle before entering the supinator muscle proper. is the inconsistency regarding the nomenclature of the The deep radial motor branch then runs under the branches of the radial nerve near the elbow used in various Arcade of Frohse (the proximal border of the supinator) anatomic texts and clinical reports (Fig. 24.7). The follow- and through the supinator muscle. ing points should help the electromyographer when dealing After leaving the supinator muscle, branches are given with potential lesions of the radial nerve in this area: off that supply the extensor muscles to the thumb and fingers as well as the abductor pollicis longus and Radial Nerve Between the Spiral Groove and the extensor carpi ulnaris. The inconsistency in the nomen- Bifurcation Near the Elbow clature regarding these nerve branches involves where Distal to the spiral groove but before the elbow, the main radial nerve always supplies two muscles: the bra- bThus, the innervation to the extensor carpi radialis brevis has several chioradialis and the extensor carpi radialis longus (also normal variations: from the main radial nerve, the superficial radial known as the long head of the extensor carpi radialis). nerve, and the deep radial motor branch of the radial nerve. 420 SECTION VIII   Clinical Disorders the posterior interosseous nerve begins and whether the posterior interosseous nerve and the deep radial motor branch are one and the same nerve: %LFHSV In some textbooks and many clinical reports, the %UDFKLDOLV entire deep radial motor branch is known as the 6SLUDO +XPHUXV posterior interosseous nerve, with the two names used JURRYH 5DGLDOQHUYH interchangeably. Thus, using this anatomic definition, a complete posterior interosseous neuropathy (PIN) 7ULFHSV would include the supinator and the extensor carpi radialis brevis muscles, as well as the extensors to the 5DGLDO 6XEFXWDQHRXV thumb and fingers, and the abductor pollicis longus QHUYH WLVVXH &URVVVHFWLRQ and extensor carpi ulnaris. In most anatomic texts, however, only the seg- $QWHULRU ment of the deep branch between the bifurcation of the main radial nerve at the elbow to where the /DWHUDO 0HGLDO nerve enters the supinator muscle at the Arcade of 3RVWHULRUKXPHUXV 3RVWHULRU Frohse is known as the deep radial motor branch. Fig. 24.8 Radial nerve and the spiral groove. The most common The posterior interosseous nerve is then the continu- radial neuropathy occurs at the spiral groove on the posterior side of ation of the deep radial motor branch after it enters the humerus. Here, the nerve lies juxtaposed to bone and is suscepti- the supinator. In the remainder of this text, we will ble to external compression. use this latter anatomic definition. Thus, with this anatomic definition, a complete PIN would spare sensation over the lateral dorsum of the hand, part of the the supinator and the extensor carpi radialis brevis thumb, and the dorsal proximal phalanges of the index, muscles. As the most common entrapment site of middle, and ring fingers. the posterior interosseous nerve is at the Arcade of In isolated radial neuropathy at the spiral groove, median-­ Frohse, the use of this anatomic convention fits the and ulnar-­innervated muscles are normal. However, tested common clinical syndromes most appropriately as in a wrist drop and finger drop posture, finger abduction well. may appear weak, giving the mistaken impression of ulnar nerve dysfunction. To prevent this error, one should test the patient’s finger abduction (ulnar-­innervated function) with CLINICAL the fingers and wrist passively extended to a neutral wrist Radial neuropathies can be divided into those caused by position. This often can be accomplished by placing the lesions at the spiral groove, lesions in the axilla, and isolated hand on a flat surface. lesions of the posterior interosseous and superficial radial sensory nerves. These lesions usually can be differentiated by clinical findings. Radial Neuropathy in the Axilla Radial neuropathy may occur in the axilla from prolonged compression. For instance, this is often seen in patients on Radial Neuropathy at the Spiral Groove crutches who use them inappropriately, applying prolonged The most common radial neuropathy occurs at the spiral pressure to the axilla. The clinical deficit is similar to that groove. Here, the nerve lies juxtaposed to the humerus seen in radial neuropathy at the spiral groove, with the and is quite susceptible to compression, especially fol- notable exception of additional weakness of arm extension lowing prolonged immobilization (Fig. 24.8). One of the (triceps brachii) and sensory disturbance extending into the times this characteristically occurs is when a person has posterior forearm and arm (posterior cutaneous nerves of draped an arm over a chair or bench during a deep sleep the forearm and arm). Radial neuropathy in the axilla is dif- or while intoxicated (‘Saturday night palsy’). The subse- ferentiated from even more proximal posterior cord lesions quent prolonged immobility results in compression and by normal strength of the deltoid (axillary nerve) and latis- demyelination of the radial nerve. Other cases may occur simus dorsi (thoracodorsal nerve). after strenuous muscular effort, fracture of the humerus, or infarction from vasculitis. Clinically, marked wrist drop and finger drop develop (due to weakness of the EIP, Posterior Interosseous Neuropathy extensor digitorum communis, extensor carpi ulnaris, and PIN clinically resembles entrapment of the radial nerve at long head of the extensor carpi radialis), along with mild the spiral groove at first glance. In both conditions, patients weakness of supination (due to weakness of the supinator present with wrist drop and finger drop with sparing of muscle) and elbow flexion (due to weakness of the bra- elbow extension. However, with closer inspection, several chioradialis). Notably, elbow extension (triceps brachii) is important differences easily separate the two. In PIN, there spared. Sensory disturbance is present in the distribution is sparing of radial-­innervated muscles above the takeoff of of the superficial radial sensory nerve, consisting of altered the posterior interosseous nerve (i.e., brachioradialis, long  Chapter 24 Radial Neuropathy 421 and short heads of the extensor carpi radialis, triceps). bands, watches, or bracelets may result in compression of Thus, a patient with PIN still may be able to extend the the superficial radial nerve. Handcuffs, especially when wrist, but weakly, with a radial deviation. This is due to the excessively tight, also characteristically result in a super- relative preservation of the extensor carpi radialis longus ficial radial neuropathy. Because the superficial radial and brevis that arise proximal to the posterior interosseous sensory nerve is purely sensory, no weakness develops. A nerve, with a weak extensor carpi ulnaris. In addition, of characteristic patch of altered sensation develops over the course, are the sensory findings. In PIN, there is no cutane- lateral dorsum of the hand, part of the thumb, and the ous sensory loss. However, there may be pain in the forearm dorsal proximal phalanges of the index, middle, and ring from involvement of the deep sensory fibers of the posterior fingers. interosseous nerve that supply the interosseous membrane and joint capsules. Five potential sites of compression of the deep radial DIFFERENTIAL DIAGNOSIS motor branch/posterior interosseous nerve have been The differential diagnosis of wrist drop, aside from a radial reported. These include (1) the medial proximal edge of neuropathy at the spiral groove, axilla, and PIN, includes the extensor carpi radialis brevis muscle; (2) the fibrous tis- unusual presentations of C7 radiculopathy, brachial plexus sue anterior to the radiocapitellar joint between the brachia- lesions, and central causes (Box 24.1). Because most mus- lis and brachioradialis muscles; (3) the “Leash of Henry” cles that extend the wrist and fingers are innervated by the (recurrent radial vessels that fan over the deep motor C7 nerve root, C7 radiculopathy may rarely present solely branch proximal to the supinator; (4) the Arcade of Frohse; with a wrist drop and finger drop, with relative sparing of and (5) the distal edge of the supinator muscle. PIN most non-­radial C7-­innervated muscles. However, several key often occurs as an entrapment neuropathy under the ten- clinical features help differentiate a C7 radiculopathy from dinous Arcade of Frohse. Rarely, other mass lesions (e.g., a radial neuropathy, PIN, brachial plexopathy, or central ganglion cysts, tumors) result in PIN. lesion (Table 24.1). Radial neuropathy at the spiral groove or axilla should result in weakness of the brachioradialis, a C5– Radial Tunnel Syndrome C6-­innervated muscle, which should not be weak in a lesion This is one of the more controversial and disputed nerve of the C7 nerve root. On the other hand, radial neuropathy entrapment syndromes. In radial tunnel syndrome, patients at the spiral groove and PIN should spare the triceps, which are reported to have isolated pain and tenderness in the exten- would be expected to be weak in a C7 radiculopathy. If a sor forearm, not unlike persistent tennis elbow, thought to C7 radiculopathy is severe enough to cause muscle weak- result from compression of the posterior interosseous nerve ness, other non-­radial C7-­innervated muscles also should near its origin. However, as opposed to patients with a true be weak (e.g., pronator teres, flexor carpi radialis), leading PIN (see previous discussion), these patients typically have to weakness of arm pronation and wrist flexion. However, no objective neurologic signs on examination and accordingly in rare situations, non-­radial C7-­innervated muscles may be have normal EDX studies. They are said to have increased relatively spared, making the clinical differentiation quite pain with maneuvers that contract the extensor carpi radialis difficult. or the supinator (e.g., resisted extension of the middle finger Although lesions of the posterior cord of the brachial or resisted supination, respectively). However, there is little plexus result in weakness of radial-­innervated muscles, the compelling evidence that this chronic pain syndrome is caused deltoid (axillary nerve) and latissimus dorsi (thoracodorsal by any nerve entrapment in most patients. Nevertheless, this nerve) should also be weak. Central lesions may result in a syndrome is important to know of, as it is not unusual for a wrist drop and finger drop. The typical upper motor neuron patient to be referred to the EMG laboratory for evaluation of posture results in flexion of the wrist and fingers, which in “radial tunnel syndrome.” In such cases, the focus of the EDX the acute phase or when the lesion is mild may superficially is to look for any objective evidence of a PIN, although in the resemble a radial neuropathy. Central lesions are identified absence of any weakness or other neurologic signs, the EDX by increased muscle tone and deep tendon reflexes (unless study is almost always normal. Nevertheless, follow-­up with an acute), slowness of movement, associated findings in the ultrasound study of the deep motor branch of the radial nerve/ lower face and leg, and altered sensation beyond the radial posterior interosseous nerve can be useful to exclude any struc- distribution. tural abnormalities of this nerve (see later). Superficial Radial Sensory Neuropathy The superficial radial sensory nerve is derived from the Box 24.1 Wrist Drop: Possible Anatomic Localizations main radial nerve in the region of the elbow. In the dis- Posterior interosseous nerve tal third of the forearm, it runs subcutaneously next to Radial nerve at the spiral groove the radius. Its superficial location next to bone makes it Radial nerve in the axilla Posterior cord of the brachial plexus extremely susceptible to compression, a syndrome coined C7 root “Cheiralgia Paresthetica,” which translates from the Greek Central nervous system as cheir + algos, meaning pain in the hand. Tight-­fitting 422 SECTION VIII   Clinical Disorders ELECTROPHYSIOLOGIC EVALUATION potential radial neuropathy, assess its location and sever- In the evaluation of a patient with a wrist drop, the role ity, and, by defining the underlying pathophysiology, of nerve conduction studies and EMG is to identify a establish a prognosis (Table 24.2). Table 24.1 Clinical Differentiating Factors in Wrist Drop. Posterior Interosseous Radial Nerve: Radial Nerve: Neuropathy Spiral Groove Axilla Posterior Cord C7 Wrist drop or finger drop X X X X X Radial deviation on wrist extension X Weakness of supination (mild) X X X Weakness of elbow flexion (mild) X X X Diminished brachioradialis tendon reflex X X X Weakness of elbow extension X X X Diminished triceps tendon reflex X X X Weakness of shoulder abduction X Sensory loss in lateral dorsal hand X X X X (equivocal) Sensory loss in posterior arm or forearm X X X (equivocal) Weakness of wrist flexion X X, May be present. Table 24.2 Electromyographic and Nerve Conduction Abnormalities Localizing the Lesion Site in Wrist Drop. Posterior Interosseous Radial Nerve: Radial Nerve: Posterior Neuropathy Spiral Groove Axilla Cord C7 EMG Findings Extensor indicis proprius X X X X X Extensor digitorum communis X X X X X Extensor carpi ulnaris X X X X X Extensor carpi radialis-­long head X X X X Brachioradialis X X X Supinator X X X Anconeus X X X Triceps X X X Deltoid X Latissimus dorsi X X Flexor carpi radialis, pronator teres X Cervical paraspinal muscles X Nerve Conduction Study Findings Abnormal radial SNAP (if axonal) X X X Low radial CMAP (if axonal) X X X X X Conduction block at spiral groove X (if demyelinating) Conduction block between forearm X and elbow (if demyelinating) X, May be abnormal; CMAP, compound muscle action potential; EMG, electromyography; SNAP, sensory nerve action potential.  Chapter 24 Radial Neuropathy 423 Box 24.2 Recommended Nerve Conduction Study Protocol for Radial Neuropathy Routine studies: 1. Radial motor study recording extensor indicis proprius, *URXQG stimulating the forearm, elbow, below spiral groove, and above spiral groove; bilateral studies 2. Ulnar motor study recording the abductor digiti minimi, &DWKRGH * stimulating the wrist, below groove, and above groove in * the flexed elbow position 3. Median motor study recording the abductor pollicis brevis, stimulating the wrist and antecubital fossa 4. Median and ulnar F responses 5. Superficial radial sensory study recording over the extensor tendons to thumb, stimulating the forearm; bilateral studies 6. Ulnar sensory study recording digit 5, stimulating the wrist 7. Median sensory study recording digit 2 or 3, stimulating the wrist The following patterns may result: Fig. 24.9 Radial motor study. The active electrode is placed over the P osterior interosseous neuropathy (axonal loss lesion): Normal extensor indicis proprius, 2 cm proximal to the ulnar styloid, with the ref- superficial radial SNAP, low-­amplitude distal radial CMAP. erence electrode over the ulnar styloid. The radial nerve can be stimulated P osterior interosseous neuropathy (demyelinating lesion): in the forearm, at the elbow, and below and above the spiral groove. Normal superficial radial SNAP, normal-­amplitude distal radial CMAP with motor conduction block between the 3–5 days, when wallerian degeneration for motor fibers has forearm and elbow. occurred. In fact, the best way to assess the degree of axonal P osterior interosseous neuropathy (mixed axonal loss and loss is to compare the CMAP amplitudes between the involved demyelinating lesion): Normal superficial radial SNAP, low-­ amplitude distal radial CMAP with motor conduction block side and the contralateral side. between the forearm and elbow. Several significant technical points must be considered R adial neuropathy at the spiral groove (axonal loss lesion): when performing radial motor studies. First, placement of the Reduced superficial radial SNAP, low-­amplitude distal active recording electrode over the EIP almost always results radial CMAP. No conduction block across the spiral groove. in a CMAP with an initial positive deflection. This occurs R adial neuropathy at the spiral groove (demyelinating lesion): because volume-­conducted potentials from other nearby radial-­ Normal superficial radial SNAP, normal amplitude distal radial CMAP with conduction block across the spiral groove. innervated muscles (e.g., extensor pollicis brevis and longus) R adial neuropathy at the spiral groove (mixed axonal loss contaminate the CMAP response, resulting in an initial positive and demyelinating lesion): Reduced superficial radial SNAP, deflection. Second, it may be difficult to make accurate surface low-­amplitude distal radial CMAP with conduction block distance measurements. Because the radial nerve winds around across the spiral groove. the humerus and takes a somewhat circuitous course through R adial neuropathy at the axilla (axonal loss lesion): Reduced superficial radial SNAP, low-­amplitude distal radial CMAP. the arm, surface distance measurements often are inaccurate. R adial neuropathy at the axilla (demyelinating lesion): Normal Measuring distance with obstetric calipers, especially between superficial radial SNAP, normal-­amplitude distal radial CMAP the elbow and arm, reduces some of this error. However, the with normal motor study to above spiral groove. combination of difficulty measuring the true nerve length and S uperficial radial sensory neuropathy: Reduced superficial the initial positive deflection CMAP can lead to considerable radial SNAP, normal radial motor study. potential inaccuracies in measuring true conduction velocities. CMAP, Compound muscle action potential; SNAP, sensory nerve action Radial conduction velocities sometimes are calculated as facti- potential. tiously fast (>75 m/s). The value of performing radial motor studies usually lies not in the measurement of conduction veloc- ities but in looking for a focal conduction block between the proximal and distal sites and determining the relative CMAP Nerve Conduction Studies amplitude to assess axonal loss (Fig. 24.10). The most important nerve conduction study in assessing a wrist In cases of radial neuropathy at the spiral groove, CMAPs drop is the radial motor study (Box 24.2). A radial compound recorded with stimulation at the forearm, elbow, and below muscle action potential (CMAP) can be recorded over the EIP the spiral groove can be completely normal if the lesion is muscle, placing the active electrode two fingerbreadths proxi- purely demyelinating. However, stimulation above the spiral mal to the ulnar styloid with a reference electrode placed over groove will result in electrophysiologic evidence of a conduc- the ulnar styloid (Fig. 24.9). The radial nerve can be stimulated tion block, i.e., a marked decrease of amplitude and area. The in the forearm, at the elbow (in the groove between the biceps relative drop in distal to proximal CMAP amplitude will give and brachioradialis muscles), and below and above the spiral some indication of the proportion of fibers blocked. groove. The normal CMAP recorded from the EIP typically Rarely, in cases of PIN, there may be conduction block is 2–5 mV. Comparing the CMAP amplitude to that on the between the forearm and elbow sites. However, most cases contralateral asymptomatic side is always important. Any axo- of PIN are pure axonal loss lesions (akin to ulnar neuropa- nal loss will result in a decreased distal CMAP amplitude after thy at the elbow), and no conduction block is demonstrable. 424 SECTION VIII   Clinical Disorders P9 PV )RUHDUP )RUHDUP (OERZ (OERZ %HO6S*URRYH %HO6S*URRYH $E6S*URRYH $E6S*URRYH Fig. 24.10 Radial motor studies for radial neuropathy at the spiral groove. Left, Symptomatic arm. Right, Contralateral asymptomatic arm. Recording extensor indicis proprius and stimulating the forearm, elbow, below spiral groove, and above spiral groove. Note the marked drop in amplitude and area across the spiral groove on the left (conduction block) and the symmetric distal compound motor action potential amplitudes from side to side. Taken together, these findings imply a predominantly demyelinating lesion at the spiral groove. —9 PV &DWKRGH * * Fig. 24.12 Radial sensory nerve action potential. The radial sensory nerve action potential is easy to record and typically has a triphasic morphology. It is expected to be normal in all posterior interosseous neuropathy lesions, as well as in other higher radial neuropathies that are purely demyelinating. Box 24.3 Causes of Wrist Drop and a Normal *URXQG Superficial Radial Sensory Nerve Action Potential Posterior interosseous neuropathy Fig. 24.11 Radial sensory study. The superficial radial sensory nerve Demyelinating radial neuropathy at the spiral groove or axilla is easy to palpate over the extensor tendons. The active electrode is C7 radiculopathy placed over the nerve with the reference electrode placed 3–4 cm Central nervous system lesion distally. The superficial radial nerve is stimulated 10 cm proximal to Hyperacute axonal loss injury of the main radial nerve (