Syringomyelia

When congenital, the syrinx develops most frequently in the cervical and upper thoracic segments. Serial sections of pathologic material show that it arises as a diverticulum from the central canal of the spinal cord. It may then dissect into the posterior or anterior gray matter on one side or enlarge symmetrically into a large, fluid-filled cavity, which, in turn, causes transverse enlargement of the spinal cord. Anterior horn neurons and pain and temperature fibers crossing in the central gray matter are destroyed. Long tracts, first the pyramidal and later the posterior column, may be compressed. In some cases, the syrinx extends from the cervical area into the posterolateral medulla, producing syringobulbia. Lumbar extension is rare.

Pathogenesis. The pathogenesis of syringomyelia is poorly understood. Almost all patients with congenital syringomyelia have an associated type I Arnold-Chiari malformation, which may itself produce symptoms of medullary or upper cervical compression. Other associated developmental defects include basilar impression, the Dandy-Walker syndrome, and atresia of the foramen of Magendie. Some patients have hydrocephalus. It has been suggested that the presence of an Arnold-Chiari malformation means that flow of cerebrospinal fluid (CSF) from the fourth ventricle is diminished, and increased pressure forces fluid into the central canal, producing a gradually expanding syrinx. A communication between the fourth ventricle, central canal, and the syrinx is not always evident, but abnormalities in the hindbrain and CSF dynamics may still be present.

Some cavities appear to be caused by spinal cord trauma and may develop months or even years after injury. Spinal arachnoiditis and intramedullary tumors are also associated with syrinx formation, as occasionally are infectious or inflammatory states. In such circumstances, the secondary or acquired cavitation may occur in any part of the cord, depending on the site of the original pathology. A progressive neurologic deficit develops after a period of stability after the original injury.

Clinical Manifestations. Syringomyelia is rare, with a prevalence of 8 per 100,000 persons. The syrinx may be asymptomatic, being discovered incidentally on spinal cord imaging. Symptoms usually appear late in the second decade through the fifth. Average age at onset is 30 years. The disease progresses at a variable rate; some patients become quadriplegic in 10 years, while others have a more benign course with long periods of stabilization. By 20 years, however, probably 50% of patients have become wheelchair-bound.

The classic sign of cervical syringomyelia is dissociated anesthesia, or loss of pain and temperature sensation in a capelike distribution, with preservation of light touch sensation and proprioception. Fibers carrying pain and temperature sensation cross in the central gray matter near the central canal and then form the spinothalamic tract in the anterolateral portion of the spinal cord. These fibers are damaged near the central canal or in the posterior horn before they cross. Fibers carrying light touch and proprioception information do not cross and course rostrally in the posterior columns. Centrally generated pain is sometimes problematic and has a segmental distribution.

Trophic changes occur, and the appearance of Charcot’s joints is not uncommon. When the cavity expands into the anterior horn, atrophy and motor weakness become apparent. Fasciculations may be seen, and kyphoscoliosis resulting from paraspinal muscle weakness is common. Thus presentation is with a progressive central cord syndrome. As the lesion expands further, it compresses the corticospinal and spinothalamic tracts. Progressive spastic paraparesis with a sensory level then becomes apparent.

Diagnosis. Syringomyelia is readily diagnosed with the use of spinal MRI and CT myelography. The spinal cord may be expanded, and the intramedullary cavity and an Arnold-Chiari malformation may be seen.

Treatment. The cause of the syrinx dictates the appropriate treatment. In most cases, an Arnold-Chiari malformation is present, and decompression of the area with suboccipital craniectomy and upper cervical laminectomy may be sufficient. In patients with no cervicomedullary abnormality, syringostomy may be considered. A plastic tube is placed into the syrinx cavity to provide communication to the subarachnoid space. If the tube remains patent, the process may stabilize. Syringoperitoneal or syringopleural shunting may also be worthwhile. Symptomatic hydrocephalus is treated with ventriculoperitoneal shunting.

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