We present a case of a 47-year-old man presenting with a 8-month history of sensory disturbance in his right leg. Clinical examination revealed brisk lower limb reflexes and impaired sensation of pain and temperature in the right lower limb with a T4 sensory level. Thoracic spine MRI confirmed ventral spinal cord herniation through the dura at T4 level. 5 years previously he was found to have had a spontaneous right sided subdural haematoma after presenting with 2 months of severe low pressure headache. Investigations at that time included imaging of the brain and cervical cord. The headaches and subdural haematoma resolved spontaneously with no surgical treatment. We propose that a spontaneous thoracic dural tear had caused the intercranial hypotension, low pressure headache, and subsequent subdural haematoma. The spinal cord at this level then sealed the defect and caused the resolution of the headaches. Continued CSF pressure fluctuations and gradients over time caused the cord to herniate slowly through the pre-existing dural defect eventually causing a clinically detectable myelopathy. Idiopathic cord herniation should be considered as a cause of mylopathy following spontaneous intracranial hypotension, as early surgical treatment can potentially prevent further progression of symptoms.