visual electrophysiology II

what is EOG - what does it measure (2)

• Measures the standing potential (integrity) of the retinal pigment epithelium (RPE)

• Standing potential is higher in daylight conditions than it is in dark conditions

how do we measure an EOG (3)

• Electrodes are used to monitor eye movements.

• Patients observes targets on left and right - 30 degree saccades - swap between L and R

• 20 minutes in both bright and dark conditions

explain why we ask the px to move their eyes from side to side (4)

•The EOG measures the potential that exists between the cornea and Bruch's membrane at the back of the eye (where the RPE is)

•The potential produces a dipole field with the cornea approximately 5 millivolts positive compared to the back of the eye, in a normally illuminated room

•By getting the eye to move from side to side we can measure the variation in the dipole

•Although the origin of the EOG is the pigment epithelium of the retina, the light rise of the potential requires both a normal pigment epithelium and normal mid-retinal function

what does a normal EOG look like (2)

• Microvolt level decreases to a minimum level during dark phase

• Amplitude Increases during light phase - Called “light rise”

describe how we quantify an EOG (4)

• using the Arden ratio

• Light peak amplitude /dark peak amplitude = Arden Ratio

• Normal range 1.7 up to 4.3 - whatever number you get means that the light peak amplitude is x times greater than the dark peak amplitude

• Abnormal result if below 1.7

describe the link between the Arden ratio and age

• as age increases - Arden ratio tends to decrease in both men and women

describe how best vitelliform macular dystrophy manifests in an EOG (5)

• Integrity of RPE in affected in Best’s - can see in bottom image there is no light rise of RE/LE

• Reduced light rise - not as steep as it should be

• Therefore we have a reduced Arden ratio

• ERGs appear normal in early stages

• is an egg yolk appearance of macula - rods, cones and pattern ERG not affected

state what other conditions than an EOG can give us information about (4)

–Retinitis pigmentosa

–Diffuse RPE disease

–Stargardt's diease

–Chloroquine retinal toxicity

describe how VEPs work - visual evoked potentials (3)

•Electrode is placed on occipital scalp adjacent to primary visual cortex - at the back of the head (unlike in EOG/ERG)

•A reference electrode is placed on a non-visual part of the brain.

•Stimuli - Pattern / Flash / checkerboard (which can be changed in size - helps us determine objective VA)

what does a normal flash VEP look like (3)

•Series of negative and positive inflections

•N1, P1 and N2 vary with age

•P2 found in almost all subjects ~120 ms

what does a normal pattern VEP look like (3)

•Negative Peak at 75 ms

•Large Positive peak at 100 ms

•Negative peak at 135 ms

explain the P100 peak and factors that affect it (5)

• Important factor as little variation between subjects over time or between eye

Factors that affect P100:

• Pattern size

• Contrast

• Luminanc

• Pupil miosis (size)

describe the uses of VEPs (4)

•Understanding the transmission Integrity of the visual pathway - E.g. Multiple Sclerosis

•Monocular stimulation - (one eye at a time and compare)

•Decussation anomalies - E.g Albinism

•Objective assessment of visual acuity - good for infants and px with functional vision loss (px with PTSD - everything is fine physically but something else has affected their vision)

describe the use of VEP in an Objective VA assessment (3)

• Can be used in cases of functional vision loss

• Pattern has been presented twice over 500 ms - over a number of different checkboard patterns

• This patient has good visual acuity - getting characteristic P100 and is = in both eyes - visual system is able to recognise the patterns at all these seconds of arc

describe use of VEP in Multiple sclerosis (4)

• Demyelination - optic neuritis and MS

• Compression - thyroid eye disease

• Transmission delays - MS slows the response - getting P100 in both eyes - but it is delayed in the LE (as LE is the one with optic neuritis)

• Pattern VEP latency changes

what is albinism and state the different types (4)

• Hypopigmentation - reduced pigment compared to normal levels

• Can either affect: Just the eyes - Ocular albinism

• Eyes and skin - Oculocutaneous albinism

• Ocular albinism more difficult to diagnose

describe tyrosinase-negative oculocutaneous albinism (4)

• Complete albinism

• Usually, AR inherited

• Incapable of synthesizing any melanin

• So px has White hair and very pale skin

what are the ocular signs of Tyrosinase-negative (5)

–VA usually <6/60 (due to foveal hypoplasia)

–Nystagmus – Pendular, Horizontal

–High refractive errors, strabismus, no stereopsis

–Choroidal vessels easily seen due to fundus lacking pigment (melanin)

–Optic Chiasm has increased crossing of nerve fibres - This can be detected using Visual Evoked Potentials

describe tyrosinase-positive oculocutaneous albinism (5)

• Incomplete albinism

• Usually, AR inherited

• Capable of synthesizing variable amounts melanin

• Patient hair may be white, yellow or red and pale skin at birth, darkens as they get older

• this type is more difficult to diagnose as it isn’t as obvious

what are the ocular signs of Tyrosinase-positive (2)

–VA impaired due to foveal hypoplasia.

–Iris - May be blue or dark brown with variable translucency - check with retro illumination

describe what ocular albinism is (2)

•Normal skin and hair.

••Usually X-linked inheritance but can be AR in some cases.

what are the ocular signs of albinism (6)

• Light iris and/or iris trans-illumination (retro-illumination of fundus)

• Foveal Hypoplasia

• Nystagmus

• Strabismus

• Amblyopia

• Increased decussation (crossing) of retinal nerve fibres

describe the set up for testing for albinism with VEP (2)

–A range of electrodes are placed across the occipital lobe (including reference electrode placed on non-visual part of brain)

–Lateral and centrally placed

VEP in a normal left + right eye

•One eye is stimulated at a time.

•The response from each hemisphere is measured

•The differences at subtracted

•Should be left with a fairly flat response - should be symmetrical

VEP RE + LE - albinism ()

• Stronger Contralateral Response - Earlier signal

• Weaker Ipsilateral Response - Later signal

• average is no longer flat between the 2 signals

• Occurs due to increased crossing of nerve fibres at optic chiasm.

crossed asymmetry - albinism (2)

• get a mirror image of the average responses together

• asymmetry between the 2 eyes

case history example

• Female, age 12, attended with brother

• Both with Pigmentary macular changes

• ERG and EOG - normal response

• Both children very fair and observed to have nystagmus

• Marked crossed asymmetry

• Probably due to albinism

summary table