Clinical Presentation

The patient is a 72-year-old female with longstanding gait disturbance, spasticity, back pain, right hip pain, and lower extremity hyperreflexia. No bowel or bladder dysfunction. No thoracic sensory level. Based on clinical findings, obtained lumbar magnetic resonance (MR) study to rule out lumbar disc herniation, which was negative. Obtained additional thoracic and cervical MR studies because of hyperreflexia.

Imaging Presentation

Thoracic MR study revealed a fusiform T1 hypointense, T2 hyperintense fluid collection within the thecal sac displacing the spinal cord anteriorly from the C3 level to approximately the C6 level consistent with a posterior arachnoid cyst ( Figs. 9-1 and 9-2 ) .

Thoracic Arachnoid Cyst.

Sagittal T1-weighted MR image, A , and sagittal T2-weighted MR image, B , reveal widening of the CSF space posterior to the spinal cord from the mid T3 level to approximately the T6 level, representing an intradural arachnoid cyst ( arrows ). Note deformity of the posterior cord surface from T3 to T6 due to compression or cord atrophy. The upper margin of the cyst is probably at the mid-T3 level. The lower extent of the cyst is not defined.

Thoracic Arachnoid Cyst.

Same patient as in Figure 9-1 . Axial T2-weighted MR image shows flattening of the spinal cord posteriorly, likely by compression from the posterior intradural arachnoid cyst.

Discussion

Spinal arachnoid cysts are uncommon. They are intradural-extramedullary or extradural collections of cerebrospinal fluid (CSF)–like fluid, most commonly occurring in the thoracic spinal canal dorsal to the thoracic spinal cord. Intradural arachnoid cysts are less common than extradural arachnoid cysts ( Figs. 9-3 and 9-4 ) . Patients with arachnoid cysts usually present between the ages of 20 and 50, but these cysts can occur at any age and any spinal level ; 85% are located in the thoracic spinal canal, 15% in the cervical spinal canal ( Figs. 9-5 and 9-6 ) , and 5% in the lumbar spinal canal. The majority of spinal arachnoid cysts are located dorsal to the spinal cord (see Fig. 9-1 ).

Thoracic Arachnoid Cyst.

Sagittal T1-weighted MR image, A and contiguous sagittal T2-weighted images, B and C . Fusiform Tl hypointense and T2 hyperintense cyst ( short, thick arrows ) causes smooth indentation of the ventral cord surface and posterior displacement of the spinal cord ( long, thin arrows ) extending over three vertebral levels. It is difficult to determine whether or not this cyst is intradural or extradural in location based on these images.

Thoracic Arachnoid Cyst.

Same patient as in Figure 8-3 . Axial Post myelogram CT Image. Intrathecal contrast ( small arrows ) surrounds the relatively hypodense spinal cord. The cyst does not opacify with intrathecal contrast and is extradural in location, ventral to the thecal sac.

Cervical Arachnoid Cyst.

At C2 level cyst deforms and displaces the spinal cord posteriorly. The cyst is hypointense relative to the cord, but is slightly hyperintense relative to CSF on sagittal T1-weighted image, A . The cyst is slightly more intense than CSF on the T1-weighted image, A , and isointense relative to the cord on the sagittal proton density-weighted conventional spin echo image, B , obtained with echo time (TE) = 25 msec and recovery time (TR) = 2500 msec.

Cervical Arachnoid Cyst, C2 level.

Same patient as in Figure 8-5 . Axial T1-weighted MR image. The cyst is centered to the right of midline and indents the right anterior cord surface. The cyst is hypointense relative to the cord but slightly more intense than cerebrospinal fluid.

Primary (congenital) arachnoid cysts are almost always intradural and represent meningeal duplication cysts where the arachnoid layer, comprised of two layers of cuboidal epithelium, is split into two layers by the fluid collection, so the wall of the arachnoid cyst is lined by a single layer of cuboidal epithelium. Primary arachnoid cysts are almost always located dorsal to the cord. A hydrodynamic theory has been postulated regarding the formation of arachnoid cysts in which CSF pulsation is said to expand congenitally weakened areas in the arachnoid. Posterior intradural arachnoid cysts are believed to arise in the region of the septum posticum of Schwalbe. Secondary (acquired) arachnoid cysts (also called subarachnoid cysts ) may occur after trauma, hemorrhage, surgery, or infection (arachnoiditis), and are believed to be secondary to developing adhesions, which can result in loculation of a CSF collection. Secondary arachnoid cysts may be intradural or extradural.

Spinal arachnoid cysts are a type of meningeal cyst, which have been classified by Naybors and colleagues. Type 1 cysts are spinal extradural meningeal cysts without spinal nerve root fibers (see Fig. 9-2 ). Type 2 meningeal cysts are spinal extradural meningeal cysts with spinal nerve root fibers. Type 3 cysts are spinal intradural meningeal cysts (see Fig. 9-1 ). Type 2 meningeal cysts include the common Tarlov-type perineural cysts (spinal nerve root diverticula) that frequently occur within the spinal or sacral neural foramina.

Patients with arachnoid cysts can present with back pain or be myelopathic with hyperreflexia, paraparesis, paresthesias, or bladder/bowel incontinence. Radiculopathy can also occur. Ventral cysts are more likely to cause weakness and myelopathic signs. Some patients with arachnoid cysts are asymptomatic; the cysts are discovered incidentally on thoracic MR studies performed for other reasons. Back pain is the most common manifesting complaint, followed by sensory changes, urinary incontinence, and weakness. An intermittent waxing and waning clinical course is typical, which clinically may simulate multiple sclerosis. Postural changes can accentuate or alleviate the symptoms. Cyst enlargement is attributed to the presence of a ball-valve–type obstruction allowing CSF to flow into but not out of the cyst. Pain associated with arachnoid cysts may be accentuated during the Valsalva maneuver. If the arachnoid cyst is not removed, the adjacent spinal cord may become atrophic. Arachnoid cysts may be associated with scoliosis, syringohydromyelia and rarely spinal dysraphism (meningomyelocele or diastematomyelia). If an associated syringohydromyelia occurs, the syrinx typically extends above the level of the cyst. Arachnoid cysts have no malignant potential.