Electronystagmography (ENG) 

Electronystagmography (ENG) is the most readily available test for assessing the vestibular system. Eye movements are recorded by means of the corneo-retinal potential by surface electrodes with the results printed on strip-chart recording paper or analyzed by a computer. A primary function of the ENG is to determine whether there is unilateral weakness or decreased caloric responses bilaterally. Each ear is irrigated separately with warm and cool stimulation, produced by either water or by air. The resulting nystagmus is analyzed manually or by computer to determine the slow phase velocity of the induced nystagmus. Peak slow phase velocity (SPV) resulting from the warm and cool stimulation of one ear is compared with that from the other ear. The most important finding during ENG is a significant reduction in the response on one side when compared with the other. A difference of > 20% to 25% in one ear, when compared with the other, is a clear indication of hypofunction in one peripheral vestibular apparatus. The ear with the weaker response is said to have a reduced vestibular response or unilateral weakness. A bilateral weakness is defined as an SPV of less than 8% to 10% for both warm and cool stimulation.

Typical ENG recordings are shown in Figure 5: Figure 5. Electronystagmogram (ENG). Typical bitemporal electrode recording using AC coupling. (A) Calibration: Upward sweep of trace indicates eye movement to right; decay in position of trace is due to AC drift. A DC-coupled recording, standard in some laboratories, would show maintenance of this position before the eye returns to midline. (B) Smooth pursuit tracking eye movement trace shows sinusoidal side-to-side movement interspersed with minor saccadic interruption. (C) Right ear cold caloric test demonstrating left-beating nystagmus. (D) Left ear cold caloric testing demonstrating right-beating nystagmus. (E) Right ear warm caloric testing demonstrating right-beating nystagmus. (F) Left ear warm caloric testing demonstrating left-beating nystagmus. (G) Optokinetic testing with tape moving to right demonstrating rightward-beating nystagmus. The electronystagmogram would be interpreted as showing minor reduction in right ear responses due to slightly reduced responses in right ear warm caloric testing. The asymmetry would be less then 30% and therefore not of clinical significance.

Equally important information gained from the use of the ENG includes: (a) documentation of spontaneous and induced nystagmus, (b) quantitation of fast eye movements, (c) smooth pursuit tracking, (d) optokinetic responses, and (e) gaze testing. These are briefly discussed below.

(a) Positional nystagmus induced by certain head movements may be documented by the ENG including the latency to onset. There is usually a delay in onset of the nystagmus or latency, with peripheral types of positional nystagmus. The ENG may document a primary position horizontal or vertical nystagmus. Vertical primary position nystagmus suggests central nervous system disease. One type of induced nystagmus is positional nystagmus provoked by certain head movements.

(b) The average speed of the fast eye movement is recorded. Slow saccades are an indicator of central nervous system disease such as degeneration in the brainstem.

(c) When smooth pursuit tracking eye movements is interrupted by a series of small saccades, it is known as cogwheel or saccadic pursuit, a non-specific abnormality and may be caused by drowsiness, drugs, or central nervous system disease.

(d) A major asymmetry in the optokinetic response is an indicator of unilateral parieto-occipital central nervous system dysfunction.

(e) Nystagmus produced during ocular excursions, in any direction, is known as gaze-evoked nystagmus. Gaze-evoked nystagmus can result from drugs such as sedatives or anticonvulsants, or from cerebellar and brainstem abnormality.

Rotational Testing 

The patient is rotated in a chair controlled by a computer with eye movements measured. Patients are rotated in the dark with eyes open while performing mental tasks assigned to distract them from mental imagery which may suppress eye movement. During a chair rotation to the right, the eyes move to the left and then recenter with a fast phase. Thus, the slow component (phase) is in the direction opposite the spin and the fast component of the resultant nystagmus is in the direction of the rotation. The fast components are eliminated by computer, and a slow phase is reconstructed and compared with the speed of the chair rotation. In this way, a gain (slow eye movement speed chair rotation speed) at different frequencies is obtained. Measurement is made of symmetry, which compares the response of the rotation in one direction with those in the opposite direction. Another measurement made during rotational testing is the time relationship between the slow eye movement and the slow movement of the chair. This difference is called the phase lag. Various phase lags are also plotted against the frequency of rotation of the chair. Therefore, both gain and phase plots are produced during rotational testing. Rotational testing provides little information about the site of the lesion as opposed to caloric testing in the ENG. However, it is quite beneficial in quantitating bilaterally reduced vestibular function such as occurs with ototoxicity. Rotational testing, therefore, is helpful in determining response patterns in patients with bilateral vestibular loss. A symmetric response of a person with a previous unilateral peripheral vestibular abnormality indicates vestibular compensation and abnormal phase-lag is a non-specific marker indicating some degree of prior peripheral vestibular abnormality.

Posturography

Posturography is a means of quantifying the Romberg test. Changes in body sway during Romberg testing with feet directly together, both with eyes open and eyes closed, are measured by means of a computer. Most recently, a dynamic posture platform has been introduced. The patient is surrounded by a movable visual field and stands on a posture platform that is mobile. By manipulating the visual field, visual cues that help maintain posture may be eliminated. Similarly, by moving the posture platform in response to movement of the feet attempts are made to remove proprioceptive cues. The test results in all conditions are reported and an interpretation is made based on which systems are defective. Posturography is a promising technology currently in use and under evaluation for assessment of balance disorders, and may be useful in rehabilitation.

Additional Diagnostic Tests 

Patients with undiagnosed vertigo should have metabolic screening tests including blood count, electrolytes, glucose and thyroid function testing. Many physicians involved in the evaluation of dizzy patients will also perform lipid screens for the presence of hypercholesterolemia or increased triglycerides. The laboratory investigation, like the physical examination, is directed particularly by the patient's history. If there is a history of presyncope or syncope, the patient must have a cardiac evaluation to include at least an electrocardiogram and rhythm strip. A more suggestive history would lead to a Holter 24-hour monitor or an event monitor, during which the patient wears a battery-powered apparatus that can be activated at times of symptoms. This device then records the cardiac rhythm. The presence of auditory symptoms requires a complete audiological evaluation as described below. Multiple or recurrent cranial neuropathy would lead to a variety of screening tests for collagen vascular disease or skull-based pathology or meningitic processes.

Neuroradiological Investigation 

In the past, the primary neuroradiological techniques for determining CNS abnormality and, in particular, cerebellopontine angle tumors included tomography of the temporal bone, computed tomographic scanning (CT), and posterior fossa myelography with air or other contrast material. Currently the high resolution obtainable on CT scanning has largely eliminated the need for tomography of the temporal bone. Magnetic resonance imaging (MRI) has largely supplanted CT scanning for cerebellopontine angle tumors. For general neurological screening, a CT scan, with and without contrast, is appropriate in patients suspected of having a CNS disorder on the basis of history or physical examination. The workup must include an MRI when there are persistent symptoms suggesting a CNS disorder. Some MRI scans are shown in the following three figures: