EOG Flashcards
What is an EOG?
Electrooculogram
Electrical response of the retina to changes in steady-state illumination
What does an EOG inform us of?
The electrooculogram (EOG) is a recording of the electrical responses in the retina which result from
changes in steady-state illumination and can inform us about the health of the retinal pigment
epithelium (RPE).
What type of potential do EOG’s use?
Standing trans-epithelial potential of ~ 10mV
(quite a large potential)
What does the dipole of the eye look like?
Front of eye is positive and back of eye is negative
If the retina isn’t working we won’t be able to record the change in the potential
How do we conduct an EOG?
Two LEDs in Ganzfeld, subtending an arc of 30°, illuminated alternately for 1 second and subject asked to track them. Alternate their fixation between the lights.
Recording electrodes are placed on the nasion and lateral canthus, with a reference electrode on the ear lobe
Signal size approx. 1 mV ( about 30 µV / ° )
Bandwidth of signal approx. 0.01 Hz - 30 Hz
Remember the cornea is positive!
Why do we do voluntary eye movements during EOG?
Standing potentials difficult to measure because of uncertainty concerning baseline position – e.g. electrode offset potentials
Therefore, signal of interest made to vary with time by voluntary eye movements
What is the signal size in EOGs?
Signal size approx. 1 mV ( about 30 µV / ° )
What is the bandwidth of EOGs?
Bandwidth of signal approx. 0.01 Hz - 30 Hz
What happens when we look left (in the LE) in an EOG?
Looking left the LE cornea positive as closest to temporal electrode but the retina is negative (back of eye) and is closer to the nasal part of the nose.
In an EOG what stimuli and response do we do?
Record response for 10 secs every 1 min (to avoid fatigue) for 15 mins during dark adaption
Amplitude ‘dark trough’ occurs after typically 12 mins
500 cd/m² steady illumination switched on
Recording continued until ‘light peak’ amplitude occurs, typically after ~ 10 mins
OR
Record response for 10 secs every 1 min (to avoid fatigue) for 16 mins under dark adaption conditions and 16 mins during light adaptation i.e. recording 10 seconds of alternating saccades every minute for 16 minutes in the dark, following 16 minutes with a background light switched on
What is the Arden Ratio in EOGs? What is it known as now?
“Arden ratio” is an old term and should be called the ‘light rise’.
Arden ratio =
Decreased ‘light peak’ / ‘dark trough’
A value of greater than 1.85 is regarded as normal.
Less than this = something wrong with pigment epithelium or the retina
When do we suspect something is wrong with pigment epithelium or the retina in an EOG?
When the Arden Ratio (light peak/dark trough) is less than 1.85
What are the clinical uses of an EOG?
- Best vitelliform macular dystrophy; v. subnormal (essential in the diagnosis)
- Acute quinine toxicity
- Retinitis Pigmentosa (rod/cone dystrophy)
but results parallel the ERG - AZOOR (supernormal)
- Adult vitelliform macular dystrophy (can be normal, but tends to be low-normal to slightly subnormal)
- Central retinal artery occlusion (flat, but ERG much more informative)
In what disorder is an EOG essential in?
Best vitelliform macular dystrophy
– v. subnormal
& pretty much essential for the diagnosis
How does AZOOR look in EOGs?
Supernormal