Thoracic Arachnoid Cyst Made Symptomatic by Demyelination.

Spinal arachnoid cysts are uncommon distinct pouches of cerebrospinal fluid (CSF) or CSF-like fluid found adjacent to normal CSF spaces commonly extradural and rarely intradural. They are usually asymptomatic and discovered incidentally. We present a patient with rapid upper motor neuron neurologic deterioration over the course of 1 week. Findings on magnetic resonance imaging revealed various central nervous system demyelination lesions and thoracic arachnoid cyst with cord compression. This acute presentation, in the absence of trauma, is not in favor of the natural history of the intradural thoracic arachnoid cysts. The patient's sensory, gait, and lower limb neurologic deficits improved after surgery with residual mild but stable upper limb deficits. Our patient likely had a static compensated cyst made symptomatic by demyelination as an additional central nervous system lesion.