Dorsal thoracic arachnoid web
Dorsal thoracic arachnoid web is a cause of focal thoracic cord distortion with resulting neurological dysfunction.
Due to the limited number of reported cases the incidence of this condition may well be under-recognised. The cases reported have a variety of signs and symptoms attributed to the band, including 1:
- episodic weakness and sensory symptoms, sometimes relieved by recumbency
- hyper-reflexia, spastic paraparesis, clonus and hypertonia
- gait instability
In most cases there is no history of prior significant trauma or surgery 1.
This condition is due to the presence of thickened band of arachnoid over the dorsal aspect of the cord. This results in focal displacement of the cord anteriorly and is often associated with syringomyelia, believed to be due to altered CSF flow dynamics due to the aforementioned web 1.
Although direct visualisation of the web is beyond routing imaging able to visualise the thoracic cord (CT, CT myelography and MRI, the key feature which implies the diagnosis is anterior displacement of the thoracic cord. This is best routinely imaged on MRI.
The thoracic cord appears focally displaced anteriorly, with widening of the dorsal CSF space. The outline of this enlarged CSF space on sagittal imaging has been likened to the silhouette of a surgical scalpel, and has been termed the scalpel sign 1.
The thoracic cord above and/or below the band often demonstrates high T2 signal sometimes with a defined syrinx.
Treatment and prognosis
Provided the diagnosis is suspected, neurosurgical intervention with resection of the band can be curative 1.
ventral cord herniation
- cord pulled rather than pushed forward
- no space between cord and ventral theca (this may also be true of both arachnoid cysts and dorsal arachnoid webs however)
- focal distortion at point of herniation
- herniation may be visible
dorsal spinal arachnoid cyst
- appearances are very similar but cyst can be demonstrated on myelography (usually fill with contrast slower than the rest of the subarachnoid space)
- distortion of the cord less focal i.e. no scalpel sign
Other causes of intramedullary cysts/abnormal signal should also be considered and contrast is important to exclude an intramedullary mass (e.g. ependymoma).