What the EOG measures

The electro-oculogram (EOG) is the one test in routine use that reports on the function of the retinal pigment epithelium. The international standard describes it as "an electrophysiological test of the outer retina and retinal pigment epithelium (RPE) in which changes in the electrical potential across the RPE are recorded"[1].

Written out in full, the electrophysiology puts most readers off, so here is the working version.

The two facts you need first

To follow how the test works, two things have to be in place. Take them as given and memorize them.

First, across the retinal pigment epithelium: when the retina receives light, hyperpolarization and depolarization occur, and the result is that the vitreous side of the RPE becomes positive and the choroidal side negative.

Second, across the eye as a whole: the corneal side is positive and the optic nerve side is negative. The eye behaves like a dipole with the cornea at the positive pole.

This standing potential is what the EOG samples. It is not generated by the photoreceptors firing, the way an ERG is. It is a slow potential across the RPE, and the light and dark phases of the test move it up and down.

How the test is performed

Electrodes are placed on both sides of each eye, one nasal and one temporal. Take the temporal electrode as positive.

  • When the eye looks toward the temporal side, a positive deflection appears.
  • When the eye looks the other way, a negative deflection appears.
Hand-drawn diagram: nasal (N) and temporal (T) electrodes wired to a voltmeter across an eye drawn as a dipole. When the cornea turns toward the temporal electrode the trace steps positive; when it turns toward the nasal electrode the trace steps negative
Why the trace deflects. The cornea is the positive pole, so whichever electrode the eye turns toward reads positive. Top: eye straight ahead, no deflection. Middle: eye turned temporally, positive step. Bottom: eye turned nasally, negative step.

The patient is asked to look right, left, right, left at a steady rhythm until the trace is stable, and the average amplitude over roughly a minute is recorded and plotted.

That sequence is repeated through the two phases of the test. The standard uses 15 minutes of dark adaptation followed by 15 minutes of light adaptation[1].

Reading the result

Plot the averaged amplitudes and you get the curve the test is named for.

EOG curve: amplitude in microvolts against time through 15 minutes each of pre-adaptation, dark adaptation and light adaptation, with the baseline, the dark trough (DT) and the light peak (LP) marked
The EOG curve. Baseline under room light, the dark trough (DT) in the dark phase, and the light peak (LP) after the lights come up. FO and SO mark the fast and slow oscillations.

Recording starts under room light, and that amplitude is the baseline. With the lights off, the amplitude falls steadily and reaches its lowest point, the dark trough (DT), at 10 to 15 minutes[1]. Then the lights come up, the amplitude climbs, and reaches its maximum, the light peak (LP), 7 to 12 minutes after light onset[1].

The result is the ratio of those two numbers.

Read an EOG in this order
  1. Find the dark trough: the lowest amplitude during dark adaptation.
  2. Find the light peak: the highest amplitude during light adaptation.
  3. Divide LP by DT. That ratio is the result.

This LP:DT ratio is the number classically called the Arden ratio. The international standard has retired that name: it records that "the main clinical measure (the Arden ratio)" is "now termed the light peak:dark trough ratio"[1]. Both names refer to the same calculation, and you will meet both.

As a rule of thumb, "2 or above is normal" is fine to remember. Just keep in mind that the normal range is wide, 1.7 to 4.3[1], so a value near the edge deserves a second look rather than a verdict.

Which diseases show an abnormal EOG

Diseases that damage the retinal pigment epithelium over a wide area, retinitis pigmentosa for instance, reduce the ratio.

But that is not where the test earns its keep. When the RPE is damaged that widely, a fundus photograph or an OCT shows it at a glance, and you do not need a 30-minute recording to tell you so.

Where the EOG is genuinely useful is vitelliform macular dystrophy (Best disease) and autosomal recessive bestrophinopathy. In these, the abnormality is in bestrophin, which is involved in the RPE’s chloride handling and therefore in the depolarization that the EOG samples. So the EOG is abnormal.

One look-alike to keep apart is central areolar choroidal dystrophy (CACD). On the fundus it can resemble Best disease, but in many cases its EOG is normal. An abnormal EOG therefore points you toward Best disease rather than CACD.

Diseases with widespread RPE damage give an abnormal ERG as well. Best disease does not. In Best disease, "full-field ERGs are usually normal and the LP:DT ratio of the EOG abnormal"[1]. An abnormal EOG with a normal ERG is the signature, and the EOG is the test that finds it.

Two practical points

Look at how the test is performed and one limitation is obvious: the recording depends on the eye movements being normal. If the patient cannot make accurate, repeatable horizontal saccades, the amplitudes are not comparable and the ratio means nothing.

The same electrode arrangement is also used in a different role: recording eye movements themselves, for example to document the waveform of a nystagmus. That use has nothing to do with the LP:DT ratio, but it is why you may see "EOG" attached to a very different kind of trace.

Summary

  • The EOG measures the standing potential across the retinal pigment epithelium, not the photoreceptor response.
  • The result is the light peak divided by the dark trough: the LP:DT ratio, classically the Arden ratio. Roughly, 2 or above is fine; the normal range of 1.7 to 4.3[1] is wide, so do not call a borderline value on the number alone.
  • Widespread RPE disease lowers the ratio, but rarely needs this test to be diagnosed.
  • Best disease and autosomal recessive bestrophinopathy are where the EOG matters: abnormal EOG with a normal ERG.
  • An unreliable result usually means unreliable eye movements. Check the saccades before you trust the ratio.

References

[1] Constable PA, Bach M, Frishman LJ, Jeffrey BG, Robson AG; International Society for Clinical Electrophysiology of Vision. ISCEV Standard for clinical electro-oculography (2017 update). Documenta Ophthalmologica. 2017;134(1):1–9. PMID 28110380.