Electronystagmography is an important test done to evaluate patients with vertigo, dizziness and balance problems. The study of eye movements provides a lot of information about the functioning of the brain and the balance system. ENG is used to second eye movements in different conditions and with different tests. By analyzing these movements, we are able to understand the area affected by the disorder.
ENG detects eye movements by virtue of a change in the corneo – retinal potential during these movements. The changes in potential are picked up by surface electrodes placed near the eyes of the patient. Findings of ENG must be correlated with the history given by the patient and other neuro – ontological tests to identify the cause of vertigo or dizziness.
Since the introduction of Videonystagmography (VNG) ENG has lost its pride of position. VNG uses goggles fitted with infrared cameras along with a computer & software. These cameras provide high-resolution images of the eye movements which can be analyzed with much higher precision than ENG.
The ENG and VNG are used to record oculomotor and vestibular tests. A comparison of their features is given in the table below. The table below shows the comparison of ENG & VNG.
|Uses electrodes to record corneo – retinal potential.||Direct recording of eye movements with infrared cameras|
|The sensitivity of recording is 2°(less than VNG) due to distantly placed electrodes.||Sensitivity of recording is 0.1°This makes it more superior.|
|Able to record horizontal eye movements. Vertical eye movement recording is not very accurate. Cannot record torsion.||Able to record eye movements in horizontal and vertical planes with high precision and graphically present the nystagmus with slow wave velocity and a number of beats. Torsional eye movements are well presented on video.|
|Artifacts due to blinking electromagnetic & electrical disturbances and EEG activity of the brain can interfere with interpretation of results.||Not affected by any extrinsic factors.|
|The detailed finding of oculomotor tests not possible.||Each oculomotor test like gaze, saccades, smooth pursuit, skew deviation and optokinetic stimulation can be studied in detail. (Description is given below)|
|Detection of posterior canal BPPV not possible in many cases as a torsional element not detected.||Accurate diagnosis of posterior canal BPPV with detection of up beating nystagmus with a torsional component is possible.|
|Reviewing and replaying tests not possible.||Able to replay tests, visualize in slow motion and review videos.|
|As ENG is dependent on Corneo – retinal potential, it is less useful in blind patients.||As VNG directly records eye movements, it is not affected by any eye condition.|
Tests done on ENG &VNG examination
ENG/VNG is used to do various oculomotor and vestibular tests. A brief description of tests
- Test for spontaneous nystagmus – Nystagmus is the jerky oscillatory movement of the eyes caused by disturbances in the balance system. In many cases, this nystagmus cannot be accurately diagnosed by naked eye examination. VNG examination allows high resolution magnified imaging of eye movements.
- Test for gaze – This test checks the ability to hold gaze at a point. Patients with balance disorders may have problems in holding their eyes steady on a fixed point.
- Saccades – This test checks the ability to move the eyes and appropriately focus on another object of interest. 3 parameters are measured.
- Latency the time it takes the eyes to move from the time of the command.
- The velocity of eye movement from one point to another.
- Precision or accuracy of being able to focus on random points. Problems in these 3 features point to different areas of the brain and help in deciding the area or disease affecting the patient.
- Smooth pursuit – A light moving like a pendulum is presented to be followed by the patient with eye movements without moving the head. The ability to track it with the required velocity and precision is measured.
- Positional & positioning tests – The patient is made to lie down and taken to different positions to see if these cause any vertigo or dizziness. The eyes are tracked during this test. BPPV is a common condition causing vertigo which can be diagnosed by this test. The canal and side affected by BPPV need to be identified so that the correct repositioning manoeuvre (E.g. Eply’s, Barbeque, Semont’s etc.) may be done to cure the patient. Only VNG can help in this diagnosis.
- Other tests done – include skew deviation, hyperventilation, vibration, valsalve and optokinetic stimulation.
- Thus VNG is a very important test which should be done in all patients with vertigo and dizziness. ( click here to learn more)