[A case of a thoracic extradural arachnoid cyst presenting with slowly progressive muscle weakness in the right upper and lower limbs].

A 49-year-old woman developed slowly progressive muscle weakness of the right upper and lower limbs. Physical examination revealed exaggeration of deep tendon reflexes in bilateral lower extremities and a Th5-Th6 girdle sensation. Weakness in her right upper extremity suggested cervical or intracranial lesion. Neuroradiological studies detected no abnormalities in her cervical cord and cranium. So the symptoms and signs were similar to those of motor neuron disease except for the sensory disturbance. MRI study of thoracic cord demonstrated a thoracic extradural arachnoid cyst. After removal of the cyst, the patient's muscle weakness was prominently relieved. We postulate that the cyst stretched spinal cord and dura mater, which led to affection of her cervical cord. We propose weakness of an upper limb as a pseudo-localizing sign of a thoracic extradural arachnoid cyst.