Arachnoid Web

Idiopathic syringomyelia is syringomyelia in the absence of abnormality detectable using standard diagnostic methods. Identifiable causes of syringomyelia include abnormalities at the level of the foramen magnum (e.g., Chiari malformation), spinal injury, previous spinal surgery, infection (e.g., discitis or meningitis), mass (extradural or intradural), disc herniation, or spinal deformity (e.g., tethered cord or myelomeningocele).

Arachnoid webs are now being recognized as a cause of "idiopathic" syringomyelia. Arachnoid webs are thought to be rare variants of arachnoid cysts and represent a focal web of arachnoid tissue obstructing the subarachnoid space. Various theories have been proposed for the mechanism of syrinx formation due to arachnoid webs and other obstructions, but none are really good enough to warrant committing to long-term memory.

Because arachnoid webs are usually not completely obstructive, they may be missed on CT myelography. Indirect CT myelographic and standard MRI findings include a relatively abrupt termination of a syrinx cavity and slight ventral displacement and slight change in caliber of the cord at the level of the termination of the syrinx. Heavily T2-weighted volumetric images in the sagittal plane, however, can show the actual web. CSF flow studies can also show the level of obstruction.

References

• Mauer UM, Freude G, Danz B, Kunz U. Cardiac-gated phase-contrast magnetic resonance imaging of cerebrospinal fluid flow in the diagnosis of idiopathic syringomyelia. Neurosurgery. 2008 Dec;63(6):1139-44.
• Sridharan A, Heilman CB. Transverse dorsal arachnoid web and syringomyelia: case report. Neurosurgery. 2009 Jul;65(1):E216-7.