Spontaneous Spinal Arachnoiditis Ossificans: A Case Report and Review of Diagnostic Pitfalls and Surgical Management

Background Arachnoiditis ossificans (AO) is a rare neurological disorder marked by heterotopic ossification in the spinal arachnoid membrane, causing stenosis and neural compression. Spontaneous AO, lacking triggers like trauma or infection, is exceptionally rare (< 10 global cases), posing diagnostic challenges. Case Presentation A 36-year-old female presented with two years of progressive thoracic pain, lower limb weakness, and autonomic dysfunction, culminating in suspected acute cauda equina syndrome or rapidly progressive myelopathy. Initial MRI suggested arachnoid cysts, but contrast-enhanced imaging with CISS sequences defined multiloculated intradural cysts (T4–S2/3), with maximal prominence between T5-T7. Rapid neurological deterioration prompted emergency surgical decompression. Thoracic/lumbar laminectomies exposed calcified arachnoid plaques adherent to the dura; microsurgical debridement relieved neural compression and restored cerebrospinal fluid (CSF) flow. Histopathology confirmed benign ossified arachnoid cyst with lamellar bone. Postoperatively, interestingly, she achieved full neurological recovery, including migraine and bowel dysfunction resolution, highlighting the broad neural impact of AO, sustained at six months follow-up despite demonstration of residual cystic fluid on postoperative imaging. Conclusion This case highlights the diagnostic complexity of spontaneous AO: the limited specificity of MRI risks delays, while CT remains gold-standard for ossification detection. Early surgical decompression is critical, aligning with literature showing 60% neurological improvement. Conservative therapies offer limited benefit in advanced disease. Multidisciplinary care and lifelong surveillance are vital, given the progressive nature of AO and recurrence risks. Spontaneous AO should be suspected in unexplained myelopathy with systemic neurological decline, emphasizing timely intervention to mitigate irreversible damage.