Diagnostic MRI in Dogs and Cats PDF

CHAPTER 7.8 EXTRAMEDULLARY CYST-LIKE 584 CONDITIONS OF THE SPINE...

CHAPTER 7.8 EXTRAMEDULLARY CYST-LIKE 584 CONDITIONS OF THE SPINE Wilfried Mai CONTENTS Meningeal cyst-like conditions.............................................................................................................................................................................584 Organization of the meninges..........................................................................................................................................................................584 Intradural arachnoid diverticulum....................................................................................................................................................................584 Extradural spinal perineurial (Tarlov) cysts......................................................................................................................................................588 Non-meningeal cyst-like conditions......................................................................................................................................................................588 Articular process joint cysts (synovial/ganglion cysts)....................................................................................................................................588 Ligamentous/discal cysts................................................................................................................................................................................590 References.............................................................................................................................................................................................................593 Extramedullary cyst-like spinal conditions are fluid-filled The space between the pia mater and the arachnoid is collections associated with the spinal structures outside of called the ‘subarachnoid space’ and contains the cerebro- the spinal cord, and they can be clinically silent or produce spinal fluid (CSF). clinical signs through compression of the spinal cord or Dorsal and ventral nerve roots travel through the sub- nerve roots.1 They have been referred to as ‘spinal cysts’ in arachnoid space covered by pia mater and pierce the dura the literature, although for the most part they do not cor- mater, taking a sleeve of arachnoid with them, before respond sensu stricto to a true cyst, which by definition is a joining a prolongation of the dura distal to the spinal ‘closed, epithelium-lined sac or capsule containing liquid, ganglion. The dural sheath of the spinal nerves fuses air or semisolid substance’. A number of the conditions with the epineurium at about the level of the interverte- referred to as ‘cysts’ in the literature do not correspond to bral foramen. true cysts, as they either are not completely enclosed or lack Within each spinal nerve, individual nerve fibers are an epithelial lining. covered by the endoneurium; fascicles of nerve fibers Extramedullary cyst-like lesions can be divided into are covered by the perineurium and fascicles of nerves meningeal and non-meningeal lesions. Non-meningeal are enclosed in the epineurium, a continuation of the cysts most commonly originate from the articular process dura mater. joints (synovial and ganglion cysts), while the existence of cysts of ligamentous or discal origin is controversial in dogs Intradural arachnoid diverticulum and cats. Meningoceles and dermoid sinuses, which also Spinal arachnoid ‘cysts’ are a rare cause of spinal form extramedullary fluid-filled structures adjacent to the cord compression in dogs and are very uncommon in spinal cord, are covered in Chapter 7.4. cats.1–25 They correspond to a focal dilation of the subarachnoid MENINGEAL CYST-LIKE CONDITIONS space filled with CSF, covered by dura mater (i.e., they are intradural) (Fig. 7.8.1). They are equivalent to the Organization of the meninges type III meningeal cysts reported in people.26 There are three concentric meningeal layers around the The terminology found in the literature regarding this spinal cord (Fig. 7.8.1). condition is inconsistent, and includes: ‘meningeal cysts’, The pia mater is the most internal layer and covers the ‘intra-arachnoid cysts’, ‘subarachnoid cysts’, ‘arachnoid surface of the spinal cord. cysts’, ‘leptomeningeal cysts’, and ‘spinal arachnoid The next layer is the arachnoid membrane and the pseudocysts’.1,2,4–14,16–20,22,24,25 outermost layer is the dura mater; these two layers are The term ‘cyst’ is inappropriate since these structures intimately attached to each other and the virtual space lack an epithelial lining, are typically not enclosed, and between them is called the ‘subdural space’. communicate freely with the anatomic subarachnoid  E x t r a m e du l l a ry Cys t -L i k e C on di t ions of t h e Spi n e 585 Arachnoid diverticulum Synovium Dura mater Epineurium Arachnoid Perineurium Pia mater Endoneurium NP Dorsal root ganglion AF Normal anatomy Cerebrospinal fluid Spinal cord, meninges and nerve sheaths Ganglion cyst Discal Tarlov cyst Synovial cyst cyst Non-meningeal cyst-like lesions Meningeal cyst-like lesions Fig. 7.8.1 Schematics showing the general organization of the meningeal layers and nerve sheaths and illustrating the location and origin of various extramedullary cyst-like structures seen in dogs and cats. Cyst-like conditions are divided into meningeal (arachnoid diverticulum and Tarlov [perineurial] cyst) and non-meningeal cysts (ganglion/synovial cysts and discal cysts). NP, nucleus pulposus; AF, annulus fibrosus. space. For these reasons, the term ‘intradural arachnoid Thoracolumbar diverticula are more common in small- diverticulum’ is more appropriate.1,15,27,28 and medium-breed dogs, with a predisposition in Pugs The pathophysiology of arachnoid diverticulum forma- and French Bulldogs.15 In these two breeds, concurrent tion is not clear, although most cases seem to be con- spinal conditions are more common, such as interver- genital and associated with an abnormal development of tebral disc disease or hemivertebrae.15 However, one the arachnoid membrane, such as duplicating or splitting recent study reported cervical arachnoid diverticula in during embryologic development, allowing slow expan- a series of seven related Pugs, with no obvious predis- sion through life and subsequent progressive compres- posing cause, suggesting that a hereditary component sion of the spinal cord.1,9,21,22 Occasionally, arachnoid is possible in a subset of dogs of this breed, and result- diverticula have been reported in dogs related to each ing in a different localization than typically seen in other,17,21 supporting congenital/developmental factors Pugs. 21 with a possible hereditary component. Arachnoid diverticula are more common in male dogs Arachnoid diverticula have also been reported in asso- overall.15 Median age at presentation in a recent meta- ciation with other conditions of the spine such as analysis was found to be 27 months,1 while a recent trauma, intervertebral disc herniation, hemivertebrae, large retrospective series reported a median and mean scoliosis, stenosis due to articular process hypertrophy, age at presentation of 36 and 46 months, respectively.15 arachnoiditis, atlantoaxial instability, and spinal dysra- Although previous studies suggested that dogs with cer- phism,4,7–9,15,19,22,24,29,30 although a causal relationship has vical diverticula tended to be younger,24 this study did not been established so far. not find a significant age difference in dogs with cervical Arachnoid diverticula most commonly develop in versus thoracolumbar diverticula.15 the cervical (C2-C3 and, less commonly, C5-C6) and Histopathologically, the wall of the diverticulum is com- thoracolumbar regions (T9-T13, more commonly posed of a normal to slightly thickened meningothelial at T13-L1).1,9,15,22,24 Lumbar diverticula are uncom- cell layer and is separated from the spinal cord by an mon. Cervical diverticula are more common in larger intact pia mater; leptomeningeal inflammatory infiltrates dogs with a predisposition in Rottweilers.1,9,15,21,22 are possible.9 586 CHAPTER 7.8 Most diverticula are located dorsally, while ventral and with the subarachnoid space is seen. This lesion is lateral locations are less common. Occasionally, diver- most commonly located dorsal to the spinal cord ticula encircling the spinal cord are observed and this (Figs. 7.8.2, 7.8.3), less commonly ventrally or later- presentation is more common in large-breed dogs and in ally (Fig. 7.8.4).15 the cervical region.15 The lesion most often extends over two vertebral The most common clinical signs include ataxia and hyper- bodies, although longer diverticula are occasionally metria, which may reflect the common dorsal position of seen.15 A single lesion is usually present, but multiple the diverticula: impairment of the dorsally located ascend- diverticula are possible. Multilobulated diverticula ing proprioceptive pathways may explain the ataxia and are occasionally seen, and are more common in the compression of the dorsolaterally located spinocerebellar cervical region.15 tracts may explain the hypermetria.15 Another relatively The signal intensity is similar to CSF, being T1 common clinical sign is urinary and fecal incontinence. hypointense and T2 hyperintense (Figs. 7.8.2–7.8.4). Fecal incontinence may be related to the dorsal compression In some cases, visualization of the lesion on a T2W series of sensory pathways important for conscious defecation.2 may be challenging, due to the similar hyperintense Although the condition is typically not painful,24 spinal signal of the fluid in the subarachnoid space and the hyperesthesia can be observed in 18%–24% of dogs.15,22 epidural fat, leading to false-negative diagnoses.27 The MRI is considered the modality of choice to image use of heavily T2W pulse sequences (‘T2-myelogram’) arachnoid diverticula because unlike other techniques such as single-shot fast spin echo (SS-FSE) or half-Fou- it allows assessment of the spinal cord parenchyma and rier acquisition single-shot turbo spin echo (HASTE) detection of comorbidities such as syringomyelia. can significantly increase the likelihood of identifying MRI features of the condition are listed below: arachnoid diverticula by highlighting the signal from On sagittal images, a discrete teardrop-shaped fluid-filled structures and suppressing the signal from lesion adjacent to the spinal cord and contiguous surrounding structures (Fig. 7.8.4).27 T13 (a) (b) (c) Fig. 7.8.2 Sagittal T2W (a) and transverse T2W (b–d) images at the level of T13 in an 8-year- old Pug with a 4-week history of ataxia and intermittent fecal incontinence. The transverse T2W images (b–d) are serial slices from caudal to cranial and show the progressive expansion of the dorsal subarachnoid space causing progressive dorsal compression of the spinal cord. The sagittal T2W image (a) shows the classic tear-drop shape of the cranial end of the arachnoid diverticulum (arrow). The T11-T12 and T12-T13 discs are degenerated with loss of signal and there is mild dorsal protrusion causing ventral cord compression. Cranial to the diverticulum, there is patchy hyperintensity in the central aspect of the spinal cord, likely (d) representing early syrinx formation and some gliosis (arrowhead). (1.5T MRI system)  E x t r a m e du l l a ry Cys t -L i k e C on di t ions of t h e Spi n e 587 * (a) (b) Fig. 7.8.3 Sagittal T2W (a), transverse T2W (b), and T1-FLAIR (c) images at the level of mid-C3 in a 9-year-old * Pug with hypermetria of the thoracic limbs. A dorsal arachnoid diverticulum is seen over C2-C3 (arrows) causing dorsal spinal cord (asterisks, b and c) compression. The diverticulum is markedly hyperintense on the T2W images and hypointense on T1-FLAIR, characteristic of a fluid-filled lesion. Caudal to the diverticulum, there is ill-defined patchy hyperintensity in the spinal cord parenchyma consistent with pre-syrinx stage (c) (arrowhead, a). (1.5T MRI system) (d) Fig. 7.8.4 Transverse T2W images at the level of T7 (a, b) and T9 (c) and sagittal T2-myelogram (SS-FSE slab, d) image in a 9-year-old Cocker Spaniel presented for paraparesis. There is a ventral and right-sided ventrolateral arachnoid diverticulum at T7 (arrows, a and b). This dog had a previous history of meningitis and this diverticulum may therefore be acquired and secondary to the (a) (b) (c) previous meningitis. The T2-myelogram image shows the focal expansion of the ventral subarachnoid space (arrowhead, d). At T9 there is a central hyperintensity in the spinal cord parenchyma consistent with a syrinx (dotted arrow, c). (1.5T MRI system) 588 CHAPTER 7.8 Although signal is typically suppressed on flu- traumatic etiology with mechanical injury to the nerve id-attenuated inversion recovery pulse sequences sheath leading to cyst formation. (T2-FLAIR), a lack of suppression on T2-FLAIR has MRI showed a well-marginated rounded, extradural, been reported.15 This may reflect that occasionally lateralized lesion associated with the region of the dor- the fluid in the diverticulum is of different chemical sal nerve root and the intervertebral foramen. The MR composition than CSF, as has been reported in intrac- signal was T1 hypointense, T2 hyperintense, and sup- ranial arachnoid cysts in people.31 pressed on T2-FLAIR, characteristic of a fluid-filled The spinal cord is compressed to various degrees at lesion (Fig. 7.8.5).33 In the dog with the lumbosacral the level of the lesion (Figs. 7.8.2–7.8.4). lesion, there was also thinning of the dorsal lamina of Concurrent syringomyelia is occasionally the sacrum at the level of the lesion, a finding that is also seen,5,8,14,18,22,24 and has been reported to improve reported in people32 (Fig. 7.8.5) and is attributed to pres- after surgical management of the diverticulum sure atrophy caused by slow progressive enlargement of (Figs. 7.8.2–7.8.4).18 the cyst, similar to what is seen with peripheral nerve In cases of acquired diverticula, additional abnor- sheath tumors. malities may be noted, such as chronic intervertebral disc herniation, or congenital abnormalities such as NON-MENINGEAL CYST-LIKE CONDITIONS hemivertebrae; their MRI signs are described in other chapters in this book. Spinal non-meningeal cyst-like conditions are degenera- tive intraspinal fluid-filled structures arising from the Extradural spinal perineurial (Tarlov) cysts vertebral articulations, ligamentous or discal structures Perineurial or perineural cysts, also called Tarlov cysts, of the vertebral column.1 Almost all such cystic struc- originate from the dorsal nerve root and ganglion sheath, tures reported thus far in veterinary medicine have been close to the point where the pia and arachnoid membrane associated with the articular process joints.1,3 differentiate into endoneurium and perineurium, respec- tively. Unlike arachnoid diverticula, they are therefore Articular process joint cysts extradural lesions filled with CSF and contain neural (synovial/ganglion cysts) structures pertaining to the nerve root and ganglion There are four articular processes on most vertebrae: the (Fig. 7.8.1).1,32,33 left and right cranial articular processes and the left and The etiology of these cysts is largely unknown, and pos- right caudal articular processes. They are located on the sible causes in people include trauma, genetic predisposi- vertebral arch at the junction between the pedicles and tion (e.g., collagen disorder), or inflammatory processes, lamina. The cranial articular processes of a given verte- causing focal weakness of the nerve sheath and allowing bra articulate with the caudal articular processes of the pulsatile and hydrostatic forces of CSF, along with a ball- vertebra located immediately cranially, thereby forming valve effect, to cause continued dilation of the cyst.32 In the paired articular process joints. The portion of the people they are common incidental findings on MRI of articular processes that is covered by articular cartilage the lumbosacral spine and are rarely symptomatic, often is called the facet; however, this term should not be used multiple, and more frequent in women.32 to designate the entire articular process or to describe Histopathologically, the cyst wall is formed of a fibro- the joint.34 Therefore, the term ‘articular facet joint’ is collagenous capsule. In theory, neural elements such incorrect. The articular process joints are synovial joints as nerve fibers or cell bodies should be identified for a and therefore contain synovial fluid and are lined by definitive diagnosis of Tarlov cyst, although these could synovium (Fig. 7.8.1). be missed if cyst wall resection was incomplete.1 Articular process joint cysts are extradural fluid-filled To date, these cysts have only been reported in two rounded or oval-shaped well-defined structures that female dogs (a 10-year-old German Shepherd Dog and develop as a result of degeneration of these joints.25,35,36 a 10-year-old Labrador Retriever).33 One was located at They are histologically divided into (Fig. 7.8.1): C5-C6, causing moderate right dorsolateral cord com- Synovial cysts, resulting from outpouching of syn- pression, and one was located at L7-S1, causing moderate ovial membrane through weakened capsular tissue, to marked right S1 nerve compression (Fig. 7.8.5). lined by synovium-like epithelial cells. Clinical signs included progressive neck pain and ataxia Ganglion cysts, which are the result of mucinous in the dog with a cervical cyst, and intermittent and pro- degeneration of periarticular connective tissue and do gressive non-weight-bearing lameness of the right pelvic not have a synovial lining. limb with radicular pain in the dog with a lumbosacral Clinically and imaging-wise, the two entities are cyst. indistinguishable and some have grouped them under In both dogs, there was adjacent intervertebral disc the terminology ‘juxtafacet cysts’,36 although the ter- degeneration and protrusion, which may support a minology ‘articular process cysts’ is preferable.  E x t r a m e du l l a ry Cys t -L i k e C on di t ions of t h e Spi n e 589 (a) (b) (c) (d) Fig. 7.8.5 Sagittal T2W (a), transverse T2W (b), sagittal T1W (c), and transverse T1W (d) images at the level of the cranial aspect of the sacrum in a 10-year-old female intact Labrador Retriever presented with a 2-month history of intermittent but progressive non-weight-bearing lameness of the right pelvic limb and pronounced radicular pain. There is degeneration and dorsal protrusion of the L7-S1 intervertebral disc (asterisks, a and c), thinning of the dorsal lamina of the sacrum on the right (dashed arrows, b and d), and an oval-shaped T2 hyperintense, T1 hypointense lesion (solid arrows) caudomedial to the right lumbosacral intervertebral foramen causing moderate to marked compression of the S1 nerve root. This was diagnosed as a perineurial (Tarlov) cyst. (1.5T MRI system; reproduced, with permission, from Liebel F-X, Platt S, Matiasek K et al. (2013). Diagnosis and management of perineurial (Tarlov) cysts in two dogs. Vet Rec 172(19):504.) In dogs, almost all reports of articular process cysts as spondylosis and osteoarthritis of the articular process have been synovial cysts,35–40 with only one report of a joints, which may reflect increased stress on these joints ganglion cyst in a German Shepherd Dog with multi- in these areas, leading to the formation of cysts. ple lesions at L6-L7 and L7-S1.41 Cysts in the mid- to caudal cervical area are more com- Most documented cases have been reported in the mid- monly seen in young giant-breed dogs and are often mul- to caudal cervical region,35,39 thoracolumbar junction tiple and bilateral. Commonly affected breeds include area,35,36,40 and lumbosacral junction area.36,37,41 There Mastiffs and Great Danes.35,39 In these dogs, there is are isolated reports of cysts in the cranial cervical spine an association with degenerative compressive osseous (C1-C3) in a Chihuahua, which were associated with proliferation from cervical spondylomyelopathy. The atlantoaxial instability, or articular process hypertro- degenerative changes at these joints with associated insta- phy similar to spondylomyelopathy in a Cavalier King bility may predispose to the development of cysts. Cystic Charles Spaniel.38,42 changes associated with the articular process joints are a Cysts in the thoracolumbar and lumbosacral area tend common feature reported on MRI in descriptive series of to occur mostly in older large-breed dogs. They are dogs with cervical spondylomyelopathy, affecting about often associated with spinal degenerative changes such 20% of patients.43,44 590 CHAPTER 7.8 Clinical signs associated with these conditions depend The signal of the lesions is similar to CSF in most cases on the location of the cysts, and vary from spinal pain and characteristic of fluid-filled lesions: hypointense to ataxia and tetra- or paraparesis, although in a number on T1W images, hyperintense on T2W images,36,38,41,42 of cases the clinical signs are not attributable to the cysts and suppressing on T2-FLAIR.38 only, but also to concurrent causes of cord compression In some cases, signal intensity can vary and become T1 such as disc herniation, articular process hypertrophy, or hyperintense if proteinaceous fluid or paramagnetic vertebral instability. blood breakdown products accumulate within the cyst.37 The MRI appearance of the lesions includes Faint rim-enhancement after gadolinium injection (Figs. 7.8.6–7.8.8): has been reported.37,38 Well-defined rounded extradural lesions associated with the articular process joints, causing variable Ligamentous/discal cysts degrees of compression of the nerve roots and/or Extradural intraspinal cysts can also rarely develop from spinal cord. The cysts can be present inside the verte- the ligaments and discal structures of the spine such as bral canal if they grow medially (Figs. 7.8.6, 7.8.7) or the dorsal longitudinal ligament, ligamentum flavum, or outside if they grow laterally (Fig. 7.8.8). annulus fibrosus.1 (a) (b) Fig. 7.8.6 Transverse T2W (a) and T1W (b) and sagittal T2W (c) images in a 9-year-old male Dachshund dog presented with severe neck pain. The patient had a disc extrusion at C2-C3 causing significant spinal cord compression (not shown). There is a left-sided articular process joint cyst at the level of C3-C4 (arrows). Ventrally at this site, there is mild intervertebral disc protrusion. The cyst is oval-shaped, well-marginated, T2-hyperintense, and T1-hypointense. It is causing mild left dorsolateral compression of the spinal cord. Enlargement and remodeling of the left articular processes is seen, consistent with osteoarthritis (arrowhead, b). (1.5T MRI (c) system)  E x t r a m e du l l a ry Cys t -L i k e C on di t ions of t h e Spi n e 591 (a) (b) Fig. 7.8.7 Transverse T2W (a) and T1W (b) images at the level of C6-C7 in a 6-year-old Rottweiler dog with cervical spondylomyelopathy. A well-marginated T2-hyperintense, T1-hypointense articular process cyst is seen on the right side (arrows), causing mild dorsolateral compression of the cord, which is also contributed to by hypertrophy of the articular processes causing lateral narrowing of the vertebral canal. (1.5T MRI system) So far, no intraspinal cyst of ligamentous origin has been reported in the veterinary literature. Although some authors reported a series of ventral intra- spinal cysts seemingly associated with the intervertebral disc and similar to ‘discal cysts’ seen in people,45,46 more recent studies and increasing evidence suggest that these structures were in fact due to acute extrusion of hydrated nucleus pulposus material (cf. Chapter 7.1), and do not correspond to the more chronic, degenerative entity seen in people.47–49 In people, discal cysts are ventral intraspinal fluid-filled structures with a wall made of dense fibrous connective tissue and communicating with the intervertebral disc through a ruptured annulus.1 They appear as ventral extradural lesions in the vertebral canal immediately adjacent to the intervertebral disc, and clinically mani- fest as chronic painful lumbar radiculopathy. In contrast, the condition initially deemed to repre- sent ‘discal cysts’ in dogs45,46 has a more acute presenta- Fig. 7.8.8 Transverse T2W image at L4-L5 in a 15-year-old tion, similar to that seen with acute disc herniation, is German Shepherd Dog. A small incidental articular process more common in the cervical region, and is typically joint cyst (arrow) is seen on the left side, bulging to the non-painful.47–49 The term ‘discal cyst’ therefore seems outside of the joint and not causing any significant spinal cord inappropriate in patients with this type of clinical pre- compression. (1.5T MRI system) sentation and MRI findings of an extradural fluid-filled lesion ventral to the cord and associated with the dorsal aspect of the disc. 592 CHAPTER 7.8 There is only one report in the veterinary literature of The lesion was somewhat bilobed in the transverse a condition that resembles the true degenerative dis- plane, T2 hyperintense with a hypointense rim, and cal cysts seen in people:50 a 6-year-old Rottweiler with T1 hypointense with mild rim enhancement after chronic episodic thoracolumbar pain and pelvic limb gadolinium injection. ataxia: On gradient echo T2*W images, the lesion was hyperin- On MRI, a ventral extradural compressive lesion was tense and there was a vertical linear hyperintensity inter- seen associated with the dorsal aspect of the T13-L1 rupting the dorsal aspect of the annulus fibrosus and intervertebral disc (Fig. 7.8.9). communicating with the cystic structure (Fig. 7.8.9). * * (a) (b) Fig. 7.8.9 Transverse T2W (a), T1W (b), and gradient- echo T2*W (c) images at the level of T13-L1 in a 6-year- old Rottweiler with chronic episodic thoracolumbar pain and pelvic limb ataxia. There is a ventral extradural lesion (solid arrows, a and b) that is T2 hyperintense with a thin * hypointense rim and T1 hypointense, causing compression of the spinal cord (asterisks). On the gradient-echo image (c), the lesion has an isointense to cerebrospinal fluid center and a hypointense rim; the T13-L1 annulus fibrosus is interrupted dorsally by a vertical hyperintensity located ventral to the extradural cystic lesion (dashed arrow). This was diagnosed as a degenerative intraspinal cyst, likely of discal origin. (1.5T MRI system; reproduced, with permission, from Penning VA, Benigni L, Steeves E et al. (2007). Imaging diagnosis – degenerative intraspinal cyst associated with an (c) intervertebral disc. Vet Radiol Ultrasound 48(5):424–7.)  E x t r a m e du l l a ry Cys t -L i k e C on di t ions of t h e Spi n e 593 At surgery, the cystic lesion was attached to the 20. Parker AJ, Smith CW (1974). Meningeal cyst in a dog. J Am annulus fibrosus with adhesions to the dura mater. 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