Scalloping Sacral Arachnoid Cyst as a Cause of Perianal Pain- A Case Report

INTRODUCTION: Scalloping sacral arachnoid cyst though a rare condition, should be suspected in cases of persistent perianal pain without any obvious urological or anorectal pathology. Such difficult cases justify ordering an M.R.I of spine as plain X-Rays and clinical examination may come out to be inconclusive. X-ray in later stages may show changes corresponding to scalloping of bone due pressure effect of cyst on surrounding tissue. Diagnosis may further be confirmed by doing contrast MRI which differentiates arachnoid cyst from other intradural and extradural pathologies. Though anatomically spinal arachnoid cysts are just an out pouching from the spinal meningeal sac or nerve root sheath they may be extradural or intradural in their location, communicating to main C.S.F column through their pedicle or an ostium leading to continuous enlargement in size. CASE REPORT: A 32 year old female was admitted under our spine unit with 1.5 year history of chronic pain, swelling and reduced sensation in perianal region. On examination she had tenderness and hypoesthesia over lower sacral region. The pain was continuous, dull aching in nature, not related to activity, localized over lower sacrum and perianal area. The neurological examination of her both lower limbs were unremarkable. Anal tone and anal reflex were normal. No sign of inflammation or tenderness was found over coccyx. Since the X-rays were inconclusive an MRI scan was done which showed a cystic lesion in the sacral area extending from S2 to S4 region with mechanical scalloping effect on the surrounding bone. The lesion had same intensity as C.S.F in both T1 &T2 weighted images. The treatment was done by way of surgical decompression with complete excision of cyst and obliteration of space by a posterior midline approach. Presently the patient is 1 year post operative and no sign of recurrence is there. CONCLUSION: Sacral arachnoid cysts should be considered as a differential diagnosis of perianal pain. Large symptomatic sacral cysts should be treated early with complete removal of the cyst including the cyst wall, to reduce the chances of recurrence. Complete decompression of the cyst cavity should be aimed at, but careful dissection of neural element is of highest importance.