Syringomyelia

Syringomyelia is a chronic disorder characterized pathologically by the presence of long cavities, surrounded by gliosis, situated in the central part of the spinal cord and sometimes extending up into the medulla (syringobulbia). The term syringomyelia was first used by Ollivier in 1824.

Pathology

The typical pathological changes are most frequently found in the lower cervical and upper thoracic regions of the cord. Extension to the medulla is common. Thoracolumbar and lumbosacral syringomyelia is rare and is usually due to a true hydromyelia associated with developmental anomalies, although ascending cavitation following traumatic transverse lesions of the cord, in association with cord tumours, is also seen.

The affected region of the cord may be enlarged, mainly in the transverse plane. In rare cases, the enlargement is sufficient to cause erosion of the bones of the spinal canal, or at least widening of its antero-posterior diameter.

The expanding cavity and surrounding gliosis, affecting the less-resistant grey matter more severely than the dense white matter, at least in the first instance, invade the anterior horns of the grey matter, thus causing atrophy of anterior horn cells, and degeneration of their axons in the ventral roots and peripheral nerves.

Extension to the brainstem (syringobulbia) usually occurs first in the postero-lateral medulla near the spinal nucleus of the trigeminal nerve and the nucleus ambiguus, so that the earliest signs of brainstem dysfunction are usually due to the involvement of such nuclei.

Compression of the long ascending and descending tracts of the cord or brainstem occurs rather later, giving secondary degeneration, most marked first in the corticospinal tracts, later in the spinothalamic tracts, and later still in the posterior columns. Haemorrhage into a syringomyelic cavity constitutes one uncommon form of haematomyelia.

Aetiology and pathogenesis

Gardner (1965) was among the first to stress the relationship of the condition to congenital anomalies and other lesions in the neighbourhood of the foramen magnum, including the Chiari type I anomaly (congenital extension of the cerebellar tonsils below the foramen magnum), craniovertebral developmental abnormalities with or without occult hydrocephalus, and basal arachnoiditis.

Gardner suggested that abnormalities of this type (as well as the Dandy–Walker syndrome of closure of the foramina of Magendie and Luschka) prevented, perhaps intermittently, the egress of CSF from the fourth ventricle into the subarachnoid space, with the result that pressure waves of fluid were forced down into the central canal of the cord which thus became dilated (hydromyelia).

This view is now generally accepted, although opinions differ as to the exact nature of the hydrodynamic mechanisms involved. The fact that a syringomyelic cavity is sometimes found to lie alongside an apparently normal spinal canal can be accounted for by the fact that, with dilatation of the canal, its ependymal lining quickly disappears and diverticula may form which dissect downwards (or sometimes upwards) alongside the canal in the central grey matter. In several large series of cases, the Chiari type I anomaly has been the most common congenital anomaly to be found, but basal arachnoiditis, developing as a sequel to previous trauma, subarachnoid haemorrhage, or meningitis, or without evident cause, accounts for about a quarter of these cases.

Primary cerebellar ectopia can be present without causing syringomyelia but with other neurological signs, such as hydrocephalus, paraparesis, or a cerebellar syndrome. True communicating syringomyelia has also been described as a complication of midbrain glioma.

In non-communicating syringomyelia, by contrast (Barnett et al. 1973), the condition is more often due to, or associated with, spinal injury, with or without paraplegia, spinal arachnoiditis, or spinal tumour. In these cases, the cavity may develop in the thoracic or lumbar cord first; indeed, except in cases of spina bifida (with which hydromyelia may be associated), the discovery of a lumbar syrinx in a patient without a history of injury should always raise the possibility of a spinal glioma or ependymoma, although intramedullary metastases or extramedullary tumours are less common associations. In cases of traumatic paraplegia or arachnoiditis, the cavities usually ascend from the site of the lesion, but in upper cervical lesions downwards cavitation is sometimes found. The cavitation has been attributed to a combination of factors, including venous obstruction, exudation of protein, and ischaemia, and oedema may be another factor.

Brewis et al. (1966) found the prevalence of syringomyelia to be 8.4 per 100 000. The pathological condition is probably more common, since the widespread availability of MRI has identified that some individuals can have asymptomatic syrinxes; these are usually small in size. The condition has been described in more than one member of a family and other congenital malformations, including spina bifida, have been found in families containing affected members. It is more common in males than in females and symptoms can appear at any age between 10 and 60 years, but usually do so between the ages of 25 and 40.

Syringobulbia

The medulla may be involved by upward extension from the cord or may be the primary site of the disorder, when the onset of symptoms may be sudden or gradual. Trigeminal pain, vertigo, facial, palatal, or laryngeal palsy, or wasting of the tongue may each be the presenting symptom. The physical signs of syringobulbia are described above.

Imaging

MRI is the diagnostic investigation of choice; its widespread availability has enabled a rapid and non-invasive diagnosis, which is often made earlier than in the past. T1-weighted sagittal and axial spin-echo images reveal the low-signal central cavity in the spinal cord, the longitudinal extent of which is highly variable. Cord expansion is usually apparent. When the syrinx is associated with a Chiari malformation, the latter is also readily demonstrated on sagittal T1-weighted images which include the level of the foramen magnum. Where the differential diagnosis includes intrinsic spinal cord tumour, gadolinium enhancement may identify enhancing tumour tissue (enhancement is not seen in syrinxes). Myelography, followed by immediate and delayed CT scanning, the previous ‘gold standard’ diagnostic investigation, has now been rendered obsolete, except where MRI is contraindicated or unavailable.

Associated abnormalities

Many developmental and other abnormalities have been described in association with syringomyelia, occurring either in affected individuals or in members of their families. In 1926, Bremer drew attention to the following: deformities of the sternum, kyphoscoliosis, a difference in the size of the breasts, increase in the ratio between arm and body length, acrocyanosis of the hands, curved fingers, enuresis, and anomalies of the hair and ears. Common abnormalities which may be added to this list include cervical rib, spinal bifida, basilar impression of the skull, fusion of cervical vertebrae (the Klippel–Feil syndrome) with shortening of the neck, and other craniovertebral anomalies, hydrocephalus, and pes cavus (Barnett et al. 1973). Light brown pigmentation, either in spots or diffuse sheets, often in a segmental distribution, is occasionally seen, especially on the shoulders.

Prognosis

The course of syringomyelia, if untreated, is progressive, though progress is frequently slow, and prolonged arrest may occur, sometimes lasting for many years. A sudden intensification of symptoms may occur following coughing, straining, or minor trauma, or be produced by haemorrhage into a syringomyelic cavity, and exceptionally distension of the spinal cord may become so marked as to produce a complete transverse lesion, leading to paraplegia. These events, however, are exceptional, and sufferers often live for many years, death occurring either from bulbar paralysis, leading to bronchopneumonia, or from some independent disease.

Treatment

Symptomatic treatment for pain and spasticity may be required. Protection of analgesic areas and early treatment of cutaneous lesions in order to promote healing are also essential. In non-communicating cases secondary to spinal tumour or arachnoiditis, laminectomy with partial or complete removal of the causal tumour, decompression, the drainage of arachnoidal cysts or the tumour itself, or the division of fibrous bands tethering the cord have all been helpful in some cases. When ascending cavitation follows a complete traumatic transverse lesion of the cord, the process may be arrested by total excision of a segment of spinal cord at and just above the level of the injury (Barnett et al. 1973). With incomplete post-traumatic myelopathy and syrinx, syringostomy (i.e. shunting the cavity) may relieve pain.

In cases with an associated Chiari malformation, decompression of the foramen magnum and upper cervical cord is sometimes performed. The results are variable (Stevens et al. 1993); there may be relief of head and neck pain, or sometimes reduction in long-tract signs, but less often is there a beneficial effect on the segmental sensory and motor deficits. The response may be better if surgery is performed earlier in the course of the disease. The value of syringostomy in cases of communicating syringomyelia is uncertain. This procedure may also be considered in patients with isolated syringomyelia of uncertain cause, where there is progressive pain or neurological deficit.



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