spinal arachnoid cyst

Spinal arachnoid cysts are relatively uncommon and may be either intradural (type III meningeal cyst) or extradural (type IA meningeal cyst).

This article specifically focuses on spinal arachnoid cysts. For a general discussion of arachnoid cysts, refer to the main article: arachnoid cyst.

Epidemiology

Spinal arachnoid cysts may present at any age. There is no gender predilection.

Clinical presentation

Most spinal arachnoid cysts are asymptomatic and are discovered incidentally. Clinical symptoms, if present, may include progressive spastic or flaccid paralysis, pain, weakness, numbness, and/or bladder/bowel incontinence. Symptoms may be exacerbated by postural changes and the Valsalva maneuver .

Pathology

As with any arachnoid cyst, spinal arachnoid cysts are CSF-filled sacs contained by the arachnoid mater. The degree of communication with the surrounding CSF space is variable with some cysts freely communicating and others not at all .

Etiology

They may be congenital or acquired. Secondary arachnoid cysts are usually due to trauma, hemorrhage, inflammation, surgery or iatrogenic injury like lumbar puncture, injection of anesthetic or contrast material within the subarachnoid space .

Location

Most primary intradural spinal arachnoid cysts are dorsal to the cord. They occur at the following locations:

  • thoracic: 80%
  • cervical: 15%
  • lumbar: 5%

Secondary spinal arachnoid cysts can be located anywhere, depending on where the insult has occurred (see "Etiology" above).

Associations

Radiographic features

CT

On CT myelography, the compressed cord is displaced anteriorly . Most arachnoid cysts eventually opacify with contrast, although the rate at which they do so is variable . In many instances, the cyst opacifies readily and as such, it may be challenging to diagnose with certainty. Early scanning is advisable, to 'catch' the cyst before it becomes isodense to CSF while considering the need for more delayed scanning if appropriate CSF mixing is not achieved. This may necessitate the introduction of contrast on the CT table rather than at fluoroscopy.

MRI

As the cysts follow the intensity of CSF and their walls are generally not visible, they may not be identified unless the cord is displaced.

  • T1: CSF intensity
  • T2: CSF intensity, may even be brighter than CSF since there is no signal loss from pulsation/flow
  • T1 C+ (Gd): no contrast enhancement
  • phase-contrast imaging: decreased CSF flow within the cyst
  • DWI: no evidence of restricted diffusion

Treatment and prognosis

Incidental asymptomatic cysts are managed conservatively. Symptomatic cysts can be treated with surgical excision. An important component of surgical treatment of epidural cysts includes closure of the arachnoid defect, if any, which could become a source of CSF leak.

Differential diagnosis

For intradural arachnoid cysts, several alternative diagnoses should be considered:

  • herniated ventral cord
    • focal cord deformity
    • phase-contrast MRI: absence of CSF flow ventral to the herniated cord and a normal CSF flow pattern dorsal to the cord
    • can be challenging to distinguish; myelography may be helpful
  • spinal epidermoid cyst
    • bright on DWI (as in the brain, spinal epidermoids are differentiated from arachnoid cysts by their brightness on DWI)
  • spinal dermoid cyst
    • most common location is the lumbar spine
    • imaging characteristics are variable but usually resemble fat
  • spinal endodermal cyst:
    • common in the lower cervical and upper dorsal spine ventrally
    • usually Indistinguishable from an arachnoid cyst on imaging
  • spinal hydatid cyst
    • most commonly extradural (intradural extramedullary lesions are extremely rare)
    • usually multiloculated
    • may show minimal enhancement
  • spinal cysticercosis
    • peripheral enhancement
    • usually multiloculated

Rarely, extradural cysts may be considered, although they are generally clearly not within the dura. They include: