An arachnoid cyst presenting as an intramedullary tumour.

A case of thoracic intradural extramedullary arachnoid cyst is presented in which an intramedullary low grade glioma was suspected preoperatively. The cyst was widely fenestrated and postoperatively, the patient experienced considerable improvement in her symptoms. As postoperative MRI studies also showed resolution of the intramedullary changes we regard the intramedullary changes as a result of the cyst, without the existence of primary medullary pathology. To our knowledge an arachnoid cyst, to date, has not been described as the cause of syringomyelia. As radiological findings can be misleading, extramedullary pathology, located more cranially, should be ruled out when treating cystic medullary changes.