Chronic neurologic deterioration caused by a missed thoracic intradural extramedullary arachnoid cyst in a 74-year-old patient: a case report

Spinal intradural arachnoid cysts (SIACs) are rare but potentially disabling lesions that can cause chronic and progressive neurological deterioration. These lesions often present with varying degrees of neurological deficits and, in advanced cases, compressive myelopathy. Thoracic SIACs are particularly challenging to diagnose due to subtle imaging features and frequent artifacts, which can lead to delayed or missed recognition. We report the case of a 74-year-old woman who experienced 2 years of progressive gait disturbance and bilateral lower limb weakness despite repeated evaluations and spinal magnetic resonance imaging (MRI) showing no abnormalities. Upon referral, thoracic spine MRI identified an intradural extramedullary mass at the T4–T6 levels. The mass was surgically excised via laminectomy, and it was histopathologically confirmed to be an arachnoid cyst. Postoperatively, the patient’s neurological function improved significantly; she regained the ability to walk and achieved a better thoracic Japanese Orthopedic Association score. This case underscores the importance of maintaining a high index of suspicion for thoracic SIACs in older patients with unexplained, progressive neurological symptoms and highlights surgical resection as an effective treatment to restore function and prevent long-term disability.