The author: Professor Yasser Metwally
INTRODUCTION
December 18, 2008 — The word syringomyelia means spinal cord cavitations. Lesions of the spinal canal and spinal cord of a varied nature can present as or in association with cysts or cavitations. 1
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Syringomyelia-Hydromyelia
Although cord cysts may be commonly associated with trauma or neoplasm, true syringomyelia and hydromyelia usually are found related to Chiari malformation or are idiopathic in origin. Hydromyelia is caused by congenital dilatation of the central canal. Syringomyelia represents a peculiar disposition of destructive lesion leading to longitudinal cavitation in the cord itself. It is rare in infancy, usually presenting in the second or third decade. 3 Macroscopically, the cervical cord is often swollen, filling the spinal canal. The syrinx is filled with clear or yellow fluid, is usually largest in the cervical region, but is often absent from the first cervical segment; it extends through the upper thoracic segments for a varying distance, rarely reaches the lumbosacral enlargement, typically extends transversely across the cord, passing beyond the central canal to involve the more posterior parts of the ventral horns and sometimes the posterior horns. If bilateral, the lesion may separate and rejoin at different levels; at any level it may reach the pial surface at the tips of the dorsal horns. Histologically, syrinxes may be lined by fibrous astrocytosis. There is a distinct interface between the normal cord and the syrinx itself, which often is well defined and smooth, but occasionally septae are detected. At one or more points, the ependymal-lined central canal is expanded and ruptured, permitting communication between the canal and syrinx. A thin layer of collagen covers part of the wall. Syrinxes may also be associated with tumors, and should be differentiated from neoplastic cysts. 5
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Figure 1. A, Transverse section of the spinal cord in a case of syringomyelia. Note the cavity located posteriorly to the spinal canal. B, A detail of the spinal canal and the wall of the cavity lined by gliosis (H&E stain; A, original magnification x 40; B, original magnification x 100). (Click to enlarge figure)
Intramedullary cysts may present within or adjacent to intramedullary cord tumors (astrocytomas, ependymomas, and hemangioblastomas) or less frequently in conjunction with extramedullary tumors. Tumor cysts generally are surrounded by enhancement, and their contents usually are more proteinaceous, representing a transudate rather than a collection of CSE MR imaging reflects the morphologic characteristic of the lesion; for instance, heterogeneity in ependymomas reflects a greater frequency of intratumoral hemorrhage, cystic necrosis, and calcification. Enlarged vessels help differentiate hemangioblastoma and AVM from astrocytoma or ependymoma. The cord is widened by the cyst and by the solid component of the tumor. 1 The MR imaging characteristics of cystic intradural extramedullary spinal cord tumors (cystic schwannomas, epidermoid cysts, and enterogenous cysts) have been reviewed. 1
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Figure 2. Reactive cavitations rostral and caudal to cervical astrocytomas(Click to enlarge figure)
When intramedullary cysts are not part of the tumor per se but form adjacent to the tumor, they have been termed benign reactive or syringomyelic cysts. It is important to recognize that these are histologically distinct from the tumor itself, and may form as a result of altered local CSF flow. Generally, cystic structures rostral or caudal to intraspinal gliomas expand the central canal, have smooth walls that lack associated contrast uptake, and cause symmetric expansion of the cord. A series of MR imaging examinations of syrinxes associated with intramedullary tumors, 3 however, showed that they resembled the idiopathic syringomyelia in some portions of the cavity; but at the tumor site, the cavities showed abrupt changes in diameters and position, and the surrounding spinal cord demonstrated an uneven thickness, an increased signal intensity on T2-weighted images, and pathologic contrast enhancement. 5
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Ependymal Cysts
Ependymal cysts are rare in the spinal cord and consist of intraparenchymal, often paraventricular, but also leptomeningeal cysts with a ciliated epithelial lining. 5
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Acquired Subarachnoid Cysts
Acquired subarachnoid cysts usually are a sequela of previous trauma, surgery, meningitis, epidural anesthesia, Pantopaque myelography, and inflammatory diseases of the spine, such as ankylosing spondylitis, all resulting in a chronic arachnoiditis that may in turn alter CSF flow dynamics, increasing the patients’ susceptibility to forming subarachnoid cysts or syrinx cavities. The walls of the cyst are thick and composed predominantly of fibrogranular tissue. 5
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Inflammatory Cysts
Inflammatory cysts may occur as a sequela of spinal osteomyelitis, epidural abscesses, and chronic arachnoiditis. Epidural abscesses are known to produce thrombosis of the blood vessels entering and exiting the cord. Thrombosis, in association with inflammation, may produce spinal cord ischemia, edema, infarction, or direct extension of infection. This in turn may lead to vacuolization and liquefaction of the affected cord, resulting in cyst formation. Specific parasitic infestations also may result in the formation of intramedullary cysts, particularly cysticercosis, which also may be associated with hydromyelic cavities. Cord compression may result from subarachnoid cysticercal cysts, which are generally more common than intramedullary cysts. 5
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Acquired Extradural Cysts
Acquired extradural cysts often are a result of degenerative changes of the spine, particularly at the lumbosacral region, followed by the cervical spine. Juxta-articular cysts, including ganglion and synovial cysts, may arise from any joint. Synovial cysts communicate with the facet joint and are lined with synovial tissue. Although uncommon, they may be a cause of spinal cord and nerve root compression. They appear as a cystic lesion, with or without calcification, extending anteromedially from the intra-articular facets, localized adjacent to or dorsal to the ligamentum flavum. The cyst wall is fibrous; there may be peripheral calcification and occasional hemosiderin deposition; the contents show transparent fluid that may have increased protein content or even be hemorrhagic. Ganglion cysts do not communicate with the facet, are not lined by synovium, and may contain myxoid material. 1,2,5
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Congenital Extramedullary Collections or Spinal Meningeal Cysts
Congenital extramedullary collections are diverticulations of the dural sac, nerve root sheaths, or arachnoid membrane. Malformations of the spinal cord include lesions associated with a dilation of the ventral subarachnoid space, and various degrees of protrusion through posterior spinal defects (dysraphic states), such as meningocele, meningomyeloceles, myelocystoceles, myeloceles, and lipomyelo(meningo)celes. Meningocele is characterized by herniation of highly vascular arachnoid or dura through a vertebral defect, with a covering of atrophic epidermis. It is most often lumbosacral. Although the cord remains within the spinal canal, it may show other anomalies such as hydromyelia, splitting, or tethering. Intradural cysts, more commonly known as arachnoid cysts or arachnoid diverticula, are found by splitting of the arachnoid membrane, which is reinforced by a thick layer of Collagen, to produce a thin-walled – cyst that occurs most often in the posterior arachnoid space. These cysts frequently are asymptomatic until they grow large enough to compress the spinal cord or nerve roots. Extradural arachnoid cysts are extradural outpouchings of arachnoid that communicate with the intraspinal subarachnoid space through a small defect in the dura. They are most common in the thoracic spine but also are reported to occur in the cervical, lumbar, and sacral spine, and can cause spinal cord compression if they enlarge. 2,5
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Endodermal Cysts
Also called enterogenous, neurenteric, bronchogenic or respiratory cysts, these lesions presumably arise from misplaced epithelium of the nasopharynx, respiratory tree, or intestinal tract, and may occur throughout the length of the spinal column, usually accompanied by abnormalities of the spine anterior to the lesion. Most are thin-walled sacs filled with gray-white viscous or mucoid material, usually intradural and extramedullary, although occasional lesions are intramedullary. They are lined by a simple columnar ciliated or goblet cell-containing epithelium. 5
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Spondylitic syringomyelia
Two types of spinal cord of cavitations are observed in cervico-dorsal spondylosis 4,5,6
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1- Pencil shaped necrosis: the cavitation is maximum at the level of C5,C6 spinal segments in the cervical region and at D11-D12 in the dorsal regions and extends one ore two segments above and/or below 2-Central gray matter cavitation: bilateral, symmetrical,rounded cavitations, localized in the bilateral anterior horns of spinal segments C5,C6 and at D11-D12 in the dorsal regions |
Pencil shaped necrosis and central gray matter cavitations, most probably represent spinal lacunar infarctions. The spinal cord atrophy observed in patients with spondylitic vascular myelopathy is very segmental and exclusively localized to the level of cervical enlargement between C4- C8 with maximum changes at C5,C6 segment in spondylitic cervical canal stenosis and D9-D12 in spinal canal stenosis due to dorsal spondylosis. The spinal cord above C4 and below C8 is often normal. The segmental spinal cord atrophy observed in patients with painless myelopathy due to spondylitic spinal canal stenosis is the result of long standing chronic ischaemia interrupted by recurrent thrombo-occlusive episodes.4,5
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Figure 3. Pencil-shaped necrosis (Click to enlarge figure)
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Figure 4. A, Central gray matter cavitations. B, pencil-shaped necrosis (Click to enlarge figure)
All the atrophic and cystic changes are exclusively limited to the spinal cord area between C4 and C7 in spinal stenosis due to cervical spondylosis and D9-D12 in spinal myelopathy due to dorsal spondylosis. The C5-C6 segments are most frequently involved in cervical spondylosis and the D11-D12 segments in dorsal spondylosis. The C5-C6 segments are the most vulnerable to vascular damage as they represent a watershed area with higher incidence of segmental arteriosclerosis.4
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Figure 5. In cervical spondylitic myelopathy, both cystic and atrophic changes are exclusively localized to the level of cervical enlargement with maximum changes at C5,C6 spinal segments (Click to enlarge figure)
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Figure 6. In cervical spondylitic myelopathy, both cystic [left image] and atrophic [right image] changes are exclusively localized to the level of cervical enlargement with maximum changes at C5,C6 spinal segments. (Click to enlarge figure)
The central gray matter cavitations demonstrated in cervico-dorsal painless spondylitic myelopathy represent lacunar infarctions involving the bilateral anterior horns. It results clinically in a purely LMN picture ( proximal muscles amyetrophy).4
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Figure 7. CT myelography showing central gray matter cavitations as bilateral symmetrical,well defined rounded zones of intramedullary contrast accumulations in the presumed anatomical areas of anterior horns [right image is a schematic representation] (Click to enlarge figure)
The association between spondylitic myelopathy and spinal cord atrophy and/or cavitation is well known. 4,5,6 The ischaemic aetiopathogenesis of myelopathy in this group of patients is further substantiated by the observation that relapses of myelopathy are intimately coupled temporally with rise of whole blood viscosity and thrombotic tendency. Relapses occurs more frequently in the summer time. Dehydration is more common in summer time, it results in contraction of the plasma volume and rise of the haematocrit value and subsequently blood viscosity. 4
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Figure 8. MRI T1,T2 images showing pencil-shaped necrosis (Click to enlarge figure)
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Posttraumatic syringomyelia
Posttraumatic syringomyelia is a term used to describe cystic cavities within the medullary substance of the spinal canal, commonly seen at, or extending cephalad or caudal from the level of traumatic injury. 5
A cyst is initially formed by liquefaction of the central haemorrhagic necrosis which occurs at the site of injury. Extension of the cyst takes place due to extension of necrosis into adjacent uninjured cord which occurs due to lysozomal digestion. Thus cavitation might extend several segments beyond the site of injury. Once a cavity is formed, fluid accumulates, extension subsequently takes place due to pressure waves induced by changes in intraspinal venous pressure.
Conversion of necrotic tissue into fluid filled cavity takes from three months to two years. The fluid in those cysts is different from CSF and contains high protein and cellular content which is compatible with an origin from liquefaction of necrotic cord tissues.5
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Figure 9. Posttraumatic syringomyelia (Click to enlarge figure)
Post traumatic cystic myelopathy is independent on the site or severity of injury. The development of progressive myelopathy in a previously neurologically stable post traumatic patient may be a catastrophic event. Ascending myelopathy in a quadriplegic patient may render a wheel-chair competent patient bed ridden or ventilator dependent. Post traumatic cystic myelopathy must be clinically suspected, radiologically confirmed, and surgically treated by cyst-shunt procedure if preservation of function is to be maintained. 5
References
1. Kochan JP, Quencer RM: Imaging of cystic and cavitary lesions of the spinal cord and canal. The value of MR and intraoperative sonography. Radiol Clin North Am 29:867,1991
2. Myles LM, Gupta N, Armstrong D, Rutka JT: Multiple extradural arachnoid cysts as a cause of spinal cord compression in a child. J Neurosurg (Spine 1) 91:116, 1999
3. Schubens P, Schbrner W, Hosten N, et al: Spinal cord cavities: Differential-diagnostic criteria in magnetic resonance imaging. Eur J Radiol 12:219,1991
4. Metwally MYM : Cervical spondylitic myelopathy, clinico-radiological approach,with correlation with the haemorheological pararameters and vascular risk factors. Ain-Shams medical journal, vol 46, number 4,5,6 , 721-750, 1995
5. Metwally MYM : Value of CT scan in the evaluation of spinal cord lesions. MD thesis, Ain Shams university, Cairo, Egypt (department of neurology) ,1991
6. Metwally MYM : Clearing the dilemma of myelopathy caused by degenerative dorsal disc disease, definition of the “spondylitic spinovascular disorders” as an emerging new concept. The Egyptian J Neurol, Psychiatr, Neurosurg, Vol 38 (1)205-232, 2002
