Spinal Extradural Arachnoid Cysts: Clinical, Radiological, and Surgical Feature

James K. Liu, M.D.; Chad D. Cole, M.D.; Peter Kan, M.D.; Meic H. Schmidt, M.D.

Disclosures

Neurosurg Focus. 2007;22(2) 

In This Article

Mechanisms of Pathogenesis

The exact origin and pathogenesis of Type IA spinal extradural arachnoid cysts remain unknown. A congenital origin has been proposed for these cysts,[9] involving either congenital diverticula of the dura or herniation of arachnoid through a congenital dural defect (Fig. 1). The dural sleeve of the nerve root or the junction of the sleeve and thecal sac are the most common sites for these defects, although less commonly the dorsal midline of the thecal sac is involved.[1,2,3,4] The theory of a congenital origin is further supported by reports of a familial syndrome comprising multiple spinal arachnoid cysts, lower-extremity lymphedema (Milroy disease), and distichiasis (double rows of eyelashes).[1,2,15,23] A genetic component in the origins of spinal extradural arachnoid cysts is also suggested by their association in various cases with congenital pigmented nevi, diastematomyelia, multiple sclerosis, Marfan syndrome, neural tube defects, spinal dysraphism, and syringomyelia.[3,19] For instance, the loss of tissue elasticity and the decrease in tensile tissue strength associated with Marfan syndrome may be connected with dural ectasia in the development of these cysts.[26]

Cases of spinal arachnoid cysts that clearly do not have a congenital origin have been reported. The association of spinal arachnoid cysts with arachnoiditis, surgery, and trauma has led some authors to suggest that these cysts more likely arise from acquired dural defects.[21,27] Spiegel mannetal.[24] reported on a case in which hemosiderin-containing macrophages in the cyst wall caused spastic paraparesis 10 years after craniospinal injury.

As with the cyst's initial cause, its mechanism of enlargement leading to spinal cord compression has been a subject of debate. Active fluid secretion from the cyst wall, passive osmosis of water, and hydrostatic pressure of CSF have all been proposed as possible mechanisms for cyst enlargement.[11] Some authors have postulated that a ball-valve mechanism in the communicating pedicle is associated with pulsatile CSF dynamics and results in cyst expansion.[11,14,21] According to this theory,[14] intermittent surges of pressure in the subarachnoid space are communicated to the cyst, and fluid flows into the pouch (Fig. 2). When pressure decreases again, compression of the cyst pedicle inhibits fluid outflow. According to the Laplace law, the cyst body exerts a force on the neck sufficient to close the communication, because its radius and wall tension are greater. These factors then allow further enlargement with persistent CSF pulsations. This ball-valve mechanism has been observed intraoperatively by Rohrer et al.[21] Bone erosion of the spinal canal may imply the presence of a valve mechanism that is responsible for producing forces of CSF pressure greater than the normal hydrostatic forces within the cyst.[16]

Illustrations showing cyst expansion through a communicating dural defect. Upper: Intermittent surges of pressure in the subarachnoid space are communicated to the cyst, and flow occurs into the pouch. Expansion is due to pressure differences between the intrathecal CSF and the epidural space. Center: When pressure decreases again, egress of fluid is impeded by compression of the cyst pedicle. In accordance with the Laplace law, the body of the cyst exerts a force on the neck sufficient to close the communication, because its radius and wall tension are greater. These factors allow further enlargement with persistent CSF pulsations. Arrows indicate fluid surge. Lower: In the late stage, the thecal sac is compressed, and more fluid is driven into the valvelike communication, resulting in very large cysts that produce spinal cord compression. (Reprinted from McCrum C, Williams B: J Neurosurg 57: 849–852, 1982, with permission).

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