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Spinal arachnoid cysts are rare, accounting for only 1%–2% of intradural spinal lesions. They may be primary congenital lesions or secondary to arachnoiditis after infection, trauma, haemorrhage or surgery. They are usually located in the thoracic region and may remain asymptomatic for years. 1 2 When associated with adhesive arachnoiditis, they can interfere with cerebrospinal fluid flow, progressively entrap nerve roots and occasionally cause abrupt spinal cord compression. Although their radiological appearance may seem benign, neurological deterioration can be sudden and severe. Early recognition of acute myelopathy is crucial, as timely decompression may reverse the neurological deficit.3 4 We present a case of acute non-traumatic myelopathy in which spinal imaging, together with clinical suspicion, proved crucial in identifying an uncommon but reversible cause.