Nystagmus is also a symptom of certain disorders affecting the inner ear, notably Menière's Disease. One of the diagnostic aids for Menière's Disease is to pour cool water into one of the patient's ears; the presence of prominent nystagmus is evidence towards the disease. A physician is shown performing this test on one of the astronauts slated to go land on the moon in a documentary series that ran on PBS (the name of the series eludes me at the moment) several years ago.