We describe a case of a 50-year-old man with bilateral shoulder girdle weakness caused by anterior subdural hygroma secondary to a previous spontaneous CSF leak. The CSF leak occurred and resolved 16 years prior to presenting with a 6-year progressive painless, asymmetric proximal muscle weakness involving both upper extremities. Current examination reveals remarkably restricted atrophy and weakness in bilateral C5-6 muscles and absent biceps and brachioradialis reflexes. Neuroimaging shows a subdural CSF collection extending from C1 to L2 anteriorly causing thecal sac effacement at the C4 level and secondary Chiari deformity. The clinical picture demonstrates severe weakness in C5-6 muscles with sparing of all other myotomes. The acute clinical features as well as neuroimaging characteristics of spontaneous CSF leak are well known but the late effects are less described. The development of a subdural fluid collection secondary to a spinal fluid leak can cause damage to the anterior spinal cord years after the leak. The underlying pathophysiology of the motor neuron loss remains unclear but there appears to be a pressure effect localized to the C4-5 region. The possibility that intervention to prevent or treat the subdural CSF collection might have avoided the shoulder girdle weakness is considered.