For those of you who have had a Videonystagmography (VNG) or Electronystagmography (ENG) exam, this is the part where you are asked to sit still and visually follow a series of moving lights, either projected on the wall, or moving along an LED light bar in front of you. Have you wondered what’s going on here?
There are really two things accomplished by these eye tracking tests. The first is that they serve as a neurologic screening test for cerebellar disorders. Simply stated (okay, over-simply stated) the cerebellum is the balance and coordination center of the brain. Gross abnormalities on these eye tracking exams might suggest that one’s balance or dizziness complaint is the result of a cerebellar disorder. The second thing accomplished is to identify any eye movement abnormalities that would impact the interpretation of later tests in the VNG battery that directly stimulate the inner ear. Most tests of inner ear function involve monitoring how the eyes respond to stimulation of the inner ear labyrinth.
Oculomotor tests include saccadic tracking, smooth pursuit tracking, and optokinetic tracking. The common bond of these tests is that each one evaluates eye movements that originate in the cerebellum, and abnormalities in these tests are considered signs of neurologic disease. Although certain patterns of abnormality found on oculomotor tests can be suggestive of specific cerebellar lesions, the examiner is cautioned to avoid over-interpreting these results. There are a number of possibilities other than cerebellar dysfunction for poor oculomotor performance. The patient’s mental state, fatigue, and a variety of medications can affect oculomotor performance.
Oculomotor tests performed with computerized ENG analysis software allow sensitive and documented evaluation. The results of oculomotor tests, when considered with the patient’s history and age, can help the health care provider (HCP) in deciding whether neurologic examination or neuroimaging is needed.
More on this next week.