Spinal arachnoid cysts are mostly intradural. Spinal extradural arachnoid cyst (SEAC) is very rare condition accounting for only 1% cases of spinal tumors. [1,2,3] SEAC is mostly found in males in their second to fifth decades. Most common location of SEAC is thoracic spine

Spinal arachnoid cysts are mostly intradural. Spinal extradural arachnoid cyst (SEAC) is very rare condition accounting for only 1

SEAC is a rare cause of compressive myelopathy, which accounts for only 1% of all spinal tumors [1, 2, and 3]. It is more common in males in their 20s [3]. Thoracic spine is most common location [1, 2, and 3].SEAC results from herniation of arachnoid membrane through the tiny dural defect [1, 2]. These cysts communicate with subarachnoid space through which CSF accumulates [1, 6]. Rarely, the cyst does not have any communication with subarachnoid space12.  Etiology of these dural defects still remains unclear. These defects can be congenital or acquired [1, 2, 4]. Acquired causes can be trauma, infection or inflammation [1, 2, 4]. It may be associated with dural ectasia, Marfan syndrome [1]. In this condition, organization of collagen is defective, which results in decreased tensile strength of ligamentous structures and other supporting tissues [8]. Dural stretching can lead to dural thinning to such an extent that it becomes ectatic and may be deficient in areas [8]. Although there is still debate in determining the etiology of SEACs, the theory of congenital dural defect is widely accepted [8]. Dural defect is often present near the nerve root sleeves. Proposed reason is that tension across the movable dural sac and relatively fixed roots can predispose such dural defects2.  Outpouching of arachnoid takes place through these small defects [1, 2]. These herniations become enlarge with time during exercise or Valsalva [1, 6]. It explains the symptomatic fluctuation during exercise and Valsalva maneuvers. Based on this, pulsatile CSF flow dynamic theory was proposed by many authors to explain enlargement of cyst [1, 2, 9].  These defects may act like valve as defects are small and arachnoid herniates beyond their margins. Rootlets may also get trapped and it again act like a valve2. As enlargement continues, a SEAC can aggravate spinal cord compression or nerve root compression, which leads to myelopathy or radiculopathy [2, 3, 4]. Nabors et al. classified in to 3 categories [10].

1. Type 1: Extradural cyst without nerve tissue.

1A-Extradural meningeal cyst, 1B- Sacral meningocele.

1. Type 2: Extradural cyst containing nerve tissue.
2. Type 3: Spinal cyst.

MRI is the most useful tool to diagnose a SEAC [3] Radiological studies report that a SEAC appears similar to cerebrospinal fluid [2, 3, 7] A CT myelography is also a useful diagnostic tool as it can more reliably detect the anatomical location of the cyst [2] In addition, a myelogram and CT myelography can help locate the Dural defective site [2, 4]. CSF flow MRI can identify the pulsating turbulent flow void of a defective site [1].
Symptomatic cyst needs surgical treatment [1, 2, and 7]. There is a consensus among surgeons to repair the dural defect in the treatment of a SEAC. However, there is still disagreement regarding the treatment of the cyst [2, 3, 4].  Diverse surgical techniques have been described and complete microsurgical resection of SEACs with meticulous repair of dural defect has been advocated as treatment of choice for SEACs [2, 9]. Long segment laminectomy and complete excision of cyst may be associated with complications like bleeding, post-operative kyphotic deformity and instability [1, 3, 4, 7, 8]. Alternative surgical techniques have been described to avoid these complications. Payer et al described selective interlaminar fenestration at communication site with repair of dural tear. Communication site was identified as flow void seen on preoperative cine MRI [9]This technique has an advantage of minimal laminotomy but requires very precise localization of Dural defect that may not be possible every time. Woo et al described laminectomy at defect site, penetration of cyst and Dural repair [11]. But they had to extent their laminectomy because of spatial limitation or not finding dural defect. Won Choi et al described tailored laminectomy, fenestration and closure of dural defect [1]. Javier Quillo-Olvera et al did evacuatory puncture of cyst and concluded that if the patient has mild symptoms, clinical observation is recommended [13].

SEACs are rare cause of compressive myelopathy but should be kept in mind as they respond very well after surgery. Various surgical techniques have been described. Meticulous repair of dural defect is necessary to prevent recurrence.

■ The authors have no financial conflicts of interest.