Syringomyelia is the development of a fluid-filled cavity or syrinx within the spinal cord. Hydromyelia is a dilatation of the central canal by cerebrospinal fluid (CSF) and may be included within the definition of syringomyelia. The following are types of syringomyelia.
Syringomyelia with fourth ventricle communication
About 10% of syringomyelia cases are of this type. This communication can be observed on MRI. In some cases, a blockage of CSF circulation occurs. A shunt operation may be the best therapeutic option for these patients.
Syringomyelia due to blockage of CSF circulation (without fourth ventricular communication)
Representing at least 50% of all cases, this is the most common type of syringomyelia. Obstruction of CSF circulation from the basal posterior fossa to the caudal space may cause syringomyelia of this type. The most common example is Arnold-Chiari malformation, which is also associated with communicating syringomyelia. Other causes include the following:
Basal arachnoiditis (postinfectious, inflammatory, postirradiation, blood in subarachnoid space) 
Basilar impression or invagination
Pathological masses (arachnoid cysts, rheumatoid arthritis pannus, occipital encephalocele, tumors)
Syringomyelia due to spinal cord injury
Fewer than 10% of syringomyelia cases are of this type. Mechanisms of injury include (1) spinal trauma, (2) radiation necrosis, (3) hemorrhage from aneurysm rupture or arteriovenous malformation or in a tumor bed, (4) infection (spinal abscess, human immunodeficiency virus, transverse myelitis), and (5) cavitation following ischemic injury or degenerative disease.
Syringomyelia and spinal dysraphism
Spinal dysraphism may cause syringomyelia through a variety of mechanisms, including those mentioned under the previous three categories. Identification and treatment of associated dysraphism has the greatest impact on arresting progression of syringomyelia.
Syringomyelia due to intramedullary tumors
Fluid accumulation is usually caused by secretion from neoplastic cells or hemorrhage. The tumors most often associated with syringomyelia are ependymoma and hemangioblastoma. Extramedullary intradural and extradural tumors are considered separately under the second category because the mechanism of syrinx formation is blockage of the CSF pathway.
Idiopathic syringomyelia has an unknown cause and cannot be classified under any of the previous categories.  Surgical decompression can help in some patients with remarkable neurologic deficit.
Although many mechanisms for syrinx formation have been postulated, the exact pathogenesis is still unknown. Frequently cited theories are those of Gardner, William, and Oldfield.
Gardner's hydrodynamic theory
This theory proposes that syringomyelia results from a "water hammer"-like transmission of pulsatile CSF pressure via a communication between the fourth ventricle and the central canal of the spinal cord through the obex. A blockage of the foramen of Magendie initiates this process. 
This theory proposes that syrinx development, particularly in patients with Chiari malformation, follows a differential between intracranial pressure and spinal pressure caused by a valvelike action at the foramen magnum.  The increase in subarachnoid fluid pressure from increased venous pressure during coughing or Valsalva maneuvers is localized to the intracranial compartment.
The hindbrain malformation prevents the increased CSF pressure from dissipating caudally. During Valsalva, a progressive increase in cisterna magna pressure occurs simultaneously with a decrease in spinal subarachnoid pressure. This craniospinal pressure gradient draws CSF caudally into the syrinx.
Downward movement of the cerebellar tonsils during systole can be visualized with dynamic MRI. This oscillation creates a piston effect in the spinal subarachnoid space that acts on the surface of the spinal cord and forces CSF through the perivascular and interstitial spaces into the syrinx raising intramedullary pressure. Signs and symptoms of neurological dysfunction that appear with distension of the syrinx are due to compression of long tracts, neurons, and microcirculation. Symptoms referable to raised intramedullary pressure are potentially reversible by syrinx decompression. 
The intramedullary pulse pressure theory
The here-proposed intramedullary pulse pressure theory instead suggests that syringomyelia is caused by increased pulse pressure in the spinal cord and that the syrinx consists of extracellular fluid. A new principle is introduced implying that the distending force in the production of syringomyelia is a relative increase in pulse pressure in the spinal cord compared to that in the nearby subarachnoid space. The formation of a syrinx then occurs by the accumulation of extracellular fluid in the distended cord.
Etiology of syringomyelia often is associated with craniovertebral junction abnormalities.
Bony abnormalities include the following:
Small posterior fossa
Platybasia and basilar invagination
Assimilation of the atlas
Soft-tissue masses of abnormal nature include the following:
Tumors (eg, meningioma at foramen magnum)
Neural tissue abnormalities include the following:
Cerebellar tonsils and vermis herniation
Membranous abnormalities include the following:
Arachnoid cysts  , rhombic roof, or vascularized membranes
Posthemorrhagic or postinflammatory membranes
Other etiologies not associated with craniovertebral abnormalities may include the following:
Arachnoid scarring related to spinal trauma
Arachnoid scarring related to meningeal inflammation
Arachnoid scarring related to surgical trauma
Subarachnoid space stenosis due to spinal neoplasm or vascular malformation
Subarachnoid space stenosis, with possible scarring, related to disk and osteophytic disease
Estimated prevalence of the disease is about 8.4 cases per 100,000 people and occurs more frequently in men than in women. The disease usually appears in the third or fourth decade of life, with a mean age of onset of 30 years. Rarely, syringomyelia may develop in childhood or late adulthood.
Prognosis depends on the underlying cause, the magnitude of neurological dysfunction, and the location and extension of the syrinx.
Patients presenting with moderate or severe neurological deficits fare much worse than those patients with mild deficits. Patients with central cord syndrome have poor response to treatment.
Natural history of syringomyelia still is not well understood. Although older studies had suggested that 20% of patients died at an average age of 47 years, mortality rates are likely lower in today's patients as a result of surgical interventions and better treatment of complications associated with significant paresis, such as pulmonary embolism. 
Myelopathy is the most serious consequence of syringomyelia. The following are the seven grade classifications of disability from myelopathy according to the Modified Nurick Classification.
Grade 0 - No root signs or symptoms
Grade I - Root signs or symptoms; no evidence of cord involvement
Grade II - Signs of cord involvement; normal gait
Grade III - Mild gait abnormality; able to be employed
Grade IV - Gait abnormality prevents employment
Grade V - Able to ambulate only with assistance
Grade VI - Chairbound or bedridden
Complications due to myelopathy include the following:
Paraplegia or quadriplegia
Bowel and urinary dysfunction
Assessing treatment results is difficult because of the rarity of syringomyelia, variability of presentation and natural history, and the relatively short follow-up in most studies.
In one study, half of all patients with syringomyelia were in clinically stable condition for several years.
Although an older study had suggested that 20% of patients died at an average of 47 years, mortality rates are likely lower in today's patients as a result of surgical interventions and better treatment of complications associated with significant paresis, such as pulmonary embolism.
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