Errors on ENG testing (under construction)

Timothy C. Hain, MD Page last modified: December 16, 2009

see also: Blunders

ENG (electronystagmography) testing is complex, and there are numerous errors that can easily arise. Unfortunately, these errors are very commonly encountered. In the author's view, this is due to a mixture of:

• Poorly designed equipment -- the ENG industry is immature and nearly all systems have serious software problems that requires considerable vigilance.
• Poorly trained testing personnel (usually audiologists). Here we favor institution of a certification process, but this is a whole other topic. A gigantic problem is that usually the overseer cannot observe what actually happened during the ENG test. Many operator errors can get "swept under the carpet"
• Inadaquete physician oversight. Often ENG's are not interpreted by anyone other than the technician who did the study. As most audiologists are untrained in neurology, this can lead to gigantic blunders. In our view, ENG's should be read by persons who have knowledge about how the brain affects eye movements.

The goal of this document is to illustrate some of the mos

t common errors, and suggest solutions when available.

Global Errors:

Noisy ENG -- this is uninterpretable.

• Noisy signal -- this is mainly a problem with ENG rather than VNG (see above).
• Drugged patient -- patients who come in on vestibular sedatives will produce decreased responses on testing.
• No data - -many labs omit to prove that their data is reliable. They omit the traces. This often means that they have something to hide. If you can't get a trace out of them, you often just have to repeat this uncomfortable test.

Calibration Errors

Calibration is the process of relating the signal coming from the recording apparatus to a known displacement of the eye. Calibration errors are very pernicious because they can make the entire test wrong (i.e. show too high or too low responses), and also they can also be easily hidden (i.e. operators neglect to provide an illustration of calibration.

The eye (read and green) does not reach the target (blue)	Saccade velocities are very slow (this is a technical error)

In essence, what happens during a calibration error, is that the operator asks the patient to look between two targets, and the patient does something else. This might occur because:

• Lack of cooperation
• Everything happened too fast
• Patient can't see the target -- a very common error that audiologists make when doing testing is to ignore assessing patient's vision. Of course, if the patient can't see the target, they can't be expected to follow it.
• Change in calibration between two different recordings

Saccadic test errors

• Blind patient
• Poor calibration
• Overly predictable protocol
• Blink artifact rejects saccadic nystagmus

• Head movement during testing

head movement during saccade testing produces a series backup saccades following the saccade with some drift (which is the VOR).

Spontaneous nystagmus test errors

• Patient with congenital nystagmus -- lack of appreciation that need to test each eye viewing
• Poor calibration
• Light leak
• Operator error - -ignores nystagmus

Gaze evoked test errors

• Too small a gaze angle
• Different gaze angle between right and left
• Poor calibration

Pursuit test errors

• Blind patient
• Poor calibration
• Lack of appreciation of congenital nystagmus

Positional test errors

Because of technician error and inability to record torsion, positional testing is best done at the bedside rather than with recordings.

• Wrong position (this is the main one)
• Too slow repositioning
• Too short recording

Caloric test errors

Caloric testing is usually the most important procedure, as well as the most uncomfortable procedure for the patient. Because of the discomfort factor, it is the place where technicians often "cut corners" -- they reduce the # of irrigations, they use air rather than water, etc.

• Wrong temperature
• Wrong stimulus (i.e. air rather than water)
• Too short irrigation
• Too short interval between irrigations
• Calibration error
• Lack of distraction
• Lack of ice calorics
• Too few irrigations