Definition
- A thickened band of arachnoid over the dorsal aspect of the cord
Pathology
- Thickened band of arachnoid over the dorsal aspect of the cord → focal displacement of the cord anteriorly → altered CSF flow dynamics → Syringomyelia
- It is important then to check that the obstructing membrane is not the top of a more extensive, intradural cyst, most of which are located dorsal to the cord.
Imaging
- Hard to identify the arachnoid web
- Key feature
- Focal dorsal indentation
- Anterior displacement of the thoracic cord
Clinical presentation
- Episodic weakness and sensory symptoms, sometimes relieved by recumbency
- Hyperreflexia, spastic paraparesis, clonus and hypertonia
- Upper motor
- Pain
- Gait instability
- Post column
- High lesions may cause
- Headaches,
- Presumably because of reduction in the compliance normally provided by the spinal theca.
- No history of significant prior trauma or surgery
- In the absence of a history of trauma, meningitis, intraspinal haemorrhage, or prior surgery, such lesions are regarded as being idiopathic.
Differential diagnosis
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 the 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 is less focal, i.e. no-scalpel sign
Management
- Surgery
- Indication: progressive motor deterioration
- Excision is desirable, for isolated webs or short cysts. More extensive cysts may respond to fenestration manoeuvres.
- Neuropathic pains may, unfortunately, persist, despite an anatomically successful procedure. Lumbar puncture, by temporarily reducing the filling pressure outside the cyst, may act as a useful test as to whether the lesion is the cause of the patient’s symptoms.
- Alternatively, direct cyst aspiration may be performed.