Nystagmus and Electronystagmography

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Nystagmus, a rhythmic twitching of the eyes, is the only good physical clue that vertigo is taking place. It is not always obvious, so the patient or family members are not likely to report it. However, close physical examination during a spell of real vertigo should show it. Nystagmus is often horizontal, but it can also be rotary, or rarely, vertical. Certain positions may cause it, especially in benign paroxysmal positional vertigo, the most frequent cause of vertigo.

The physiologic explanation for nystagmus is complex. When something goes wrong in the vestibule, either from a disease process or an artificial stimulus, a message is sent to the oculomotor centers in the brainstem. In turn, these send an adaptive message to the eye muscles. A typical artificial stimulus would be spinning in place, as a child might do, sending the fluids of the semicircular canals into motion. Sudden cessation of the spinning will then result in classic vertigo with nystagmus. Similarly, a caloric stimulus, such as cold water in the ear, creates convection currents in the fluid of the lateral semicircular canal. In either case, the subject's neural pathways perceive a false sense of motion, and the vestibular-oculomotor connections cause the pupils to track sideways, with quick corrections in the opposite direction. This manifests itself as the "beating" nystagmus we

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observe, with slow motion of the pupils in one direction and fast "beats" in the other.

All sorts of disorders can create pathologic nystagmus, but the above-mentioned stimuli, like spinning or caloric stimulation, should produce nystagmus as a normal response. Also, be aware that when one examines a normal patient's ocular motions, transient fatiguable nystagmus is often seen on lateral gaze to either side.

Electronystagmography (ENG) has been available for years. It involves measurement of nystagmus by placing electrodes adjacent to the eyes and recording ocular motion under varying circumstances. As mentioned above, abnormal nystagmus may occur in disease states, but normal nystagmus should be present with caloric stimulation to each ear, with symmetrical responses. Experience with ENG testing over the years has accumulated a vast body of knowledge, and as a result, a fairly standard test battery has been developed. Certain findings may point to more central brainstem pathology, and others to peripheral inner ear disease. Often, there are specific patterns for specific diseases. In the ENG test procedure, the electrodes sensing eye motion are placed on either sides of both orbits, as well as above and below, to record nystagmus in either axis. The impulses received from these ocular motions are measured and recorded, and patterns of disease states can emerge and be described.

The ENG assesses ocular motions under several circumstances. The first is with the patient sitting still and looking straight ahead. Abnormal spontaneous nystagmus may be seen in this neutral position. Next, the subject watches certain visual stimuli without moving the head. Here, "tracking" abnormalities relating to optic-vestibular-ocular connections are assessed, picking up possible problems in the central brain and brain-stem, or even drug-induced disorders. The next set of tests involves moving the patient's head, or head and entire torso, into various positions to look for positional nystagmus. Here, certain disease states of the central pathways or peripheral inner ear may appear. The last test measures responses to caloric stimulation, using cold or warm air or water introduced into the ear canal. Body-temperature air or water elicits no convection currents in the semicircular canals, and thus no nystagmus. With cold or warm stimulation, normal ears will show symmetrical healthy nystagmus responses (although the subject has temporary vertigo). Absent or decreased responses in one or both ears, indicate pathology in the vestibular labyrinth(s).

When specialized and well-trained audiologists perform these tests, they can give us a great "read-out," even suggesting possible diagnoses. Figure 8.2 shows an example of an audiologist's ENG report on a patient with significant findings.

Patient:

Jane Doe

Birthdate:

12/20/57

Date of Evaluation:

05/27/02

Physician:

Dr. Menner

referral reason:

Mrs. Doe has continued to experience feelings of dizziness since March 2000. The sensation is heightened by movement or chance in head position. ENG administered on 4/24/00 revealed a significant right unilateral weakness.

Audiological test results today revealed a right moderate low-frequency loss.

Procedures:

Bithermal air calorics, saccade, tracking, optokinetic, gaze, and positional tests and Hallpike Maneuver.

Results:

—The caloric response of the right ear was 86% weaker than of the left ear.

—The patient readily supressed the induced caloric nystagmus with visual fixation.

—The results of the saccade, tracking, and optokinetic tests were within normals limits.

—There was no nystagmus.

—There was no nystagmus in the sitting, supine, head-right, head-left, right lateral, or left lateral positions.

—The results of the Hallpike Maneuver were negative, i.e., no nystagmus was elicited following either the head-down-left maneuver.

The right unilateral weakness is consistent with right peripheral vestibular pathology involving the right labyrinth or vestibular nerve

Fig. 8.2 Example of an ENG report.

(Source: Sheila Giovannini, Southern Tier Audiology, Elmira, N.Y.)

Posturography deserves brief mention. This is a newer form of balance assessment that evaluates spinal and peripheral mechanisms, as well as the vestibule. It is available at many centers and a database is building, but ENG is still the preferred test for vestibular assessment. Now, we will discuss clinical disease entities that cause vertigo.

Benign Paroxysmal Positional Vertigo 117

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