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Errors on ENG testing (under construction)

Timothy C. Hain, MD Page last modified: March 27, 2016

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:

The goal of this document is to illustrate some of the most common errors, and suggest solutions when available.

Global Errors:

tracking
Noisy ENG -- this is uninterpretable. Terrible VNG tracking produces an uninterpretable recording.

Paretic Eye fixation syndrome (see this link)

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.

calib error 1 calib error 2
The eye (read and green) does not reach the target (blue) Saccade velocities are very slow (this is a technical error)
Horizontal calibration error
calibration error
The horizontal calibration is wrong by a factor of two (see pursuit traces).

This is the most common calibration error for the Micromedical technology VENG system. Their calibration includes only a few refixations, and often patients miss the first one, resulting in this double-gain problem. Other brands of VENG systems do not have this problem because they use a more robust calibration process.

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:

 

Saccadic test errors

too fast
Calibration error results in eyes that go "too fast". Eye drops off of computer "strip" chart.

 

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

 

Spontaneous nystagmus test errors

Gaze evoked test errors

Pursuit test errors

Positional test errors

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

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.

Everybody makes mistakes from time to time. It is important to be able to catch them and recover.

Bad score
Here the operator allowed the software to "find" a response where a response cannot occur (it is too early).

 

Copyright August 3, 2016 , Timothy C. Hain, M.D. All rights reserved. Last saved on August 3, 2016