Myasthenia gravis as a cause of head drop in Parkinson disease.

Head drop is characterized by marked anterior flexion of the cervical spine. As a result, the affected patient presents with the head tilted forward and the chin resting on the chest. We report a 75-year-old male patient with parkinsonism and head drop caused by isolated myasthenic weakness of the neck extensor muscles. Our case is the second report of isolated head drop as a presenting symptom of myasthenia gravis in a patient with parkinsonism.