Myasthenia gravis: an unrecognized cause of head drop in Parkinson's disease.

Head drop is characterized by marked anterior flexion of the cervical spine, caused by weakness of the neck extensors or by increased tone of the flexor muscles. We report a woman with Parkinson's disease and head drop not due to cervical dystonia (a common cause of antecollis in parkinsonisms). Clinical, radiological, and neurophysiological features together with responsiveness to anticholinesterases and plasma exchanges indicated the possibility of a concomitant myasthenia gravis.