Electrodiagnostics

Electroretinogram

• A measurement of the retina’s electrical mass response to a light stimulus (a summation of photoreceptor and bipolar cell activity)

• Affected by:
• Adaptive state of the eye eg photopic (cone ERG) vs scotopic (rod ERG)
• Type of stimulus
• Electrode placed in contact with cornea and reference electrode on forehead

• The ERG is unaffected by small localised lesions and will therefore be normal in disease confined to the macula and will also not detect disease of the ganglion cell layer or optic nerve

• Does not measure VA

Waveforms

• Two main waves in response:

• Negative A wave, arising from the photoreceptors

• A1 originates in cones
• A2 originates in rods

• Positive B wave, caused by bipolars but magnified by Muller cells (which act as a sink for potassium ions released by depolarising bipolar cells): surrogate for bipolar cell function

• Oscillatory potentials are superimposed on b wave and increase with stimulus intensity. Represent amacrine cell function.

• C wave (occasionally present): slow positive wave generated by RPE but also depends on photoreceptor integrity. Can be used to represent the function of these two structures and their interactions

• D wave: only seen when using stimulus of long duration which is then stopped (ie. cessation of constant illumination)

• Implicit time: time from stimulus to peak of b wave

• In a constant subject, increasing the intensity of the stimulus will first cause increase in the b wave amplitude followed by development of the preceding a wave which also increases in size and both waves become faster. 

• Pupil size and media opacity will affect the intensity of light stimuli entering the eye

• Colour of stimulus: rod responses peak at the blue-green region of spectrum while cones responses vary with types of cones but average peak is at orange light (high end).

• Rod system is more sensitive across the whole spectrum compared to cones.

• Frequency of stimulation: the critical fusion frequency (CFF) represents the maximum frequency that can be perceived as flickering

• Highest CFF for rod vision is 15-28Hz (hence a higher frequency flicker will only elicit a cone response)

• Highest CFF for cone vision is 50Hz

• Bright white flash (standard stimulus): both a and b wave amplitudes are maximal in scotopic (dark adapted) conditions

• Dim white or blue flash: response is generated only by rods

• Bright background (saturates rods) or high frequency flickers elicit a pure cone response (since rods have poor temporal resolution): low amplitudes but very fast kinetics (time to peak is 30-32ms)

Examples

• Reduced b wave (with preserved a wave implying normal photoreceptors but abnormal bipolar cells) ie. an “electronegative response”, occurs in many conditions including

• Inner retinal process
• CRAO/CRVO
• Toxicity: iron (siderosis), quinine, methanol
• Juvenile X-linked retinoschisis
• Bipolar cell or photoreceptor-bipolar synapse process
• Melanoma associated retinopathy
• Congenital stationary night blindness
• Autosomal dominant neovascular inflammatory vitreoretinopathy
• Miscellaneous 
• Duchenne/Becker muscular dystrophies
• Birdshot chorioretinopathy

• Reduced or absent photopic response: cone dystrophies (which show a normal or subnormal but present scotopic ERG)

• Achromatopsia 
• Cone dystrophies
• No response (extinguished):
• Batten’s disease
• Leber’s congenital amaurosis
• Rod-cone dystrophies like RP
• Total RD
• Cancer associated retinopathy
• Advanced siderosis
• Increased a wave: albinism
• Reduced oscillatory potentials: vigabatrin toxicity

Pattern electroretinogram

• Retinal electrical response to a patterned stimulus and represents central retinal and ganglion cell function

• Objective assessment of macular function

• Reference electrode must be place on ipsilateral temple or outer canthus to avoid interference from cortical-evoked potential

• Stimulus is usually a reversing chequerboard

Indications

• Macular dystrophies eg. Stargardt’s
• Differentiating macular and optic nerve dysfunction
• Assessment of unexplained visual loss

Waveform

• Three responses (N=negative and P=positive. The subscript refers to the timing in miliseconds of the response’s peak):

• Negative N35 wave
• Positive P50 wave (macular photoreceptors)
• Second negative N95 wave (ganglion cell layer)

• Therefore the P50 component is abnormal in macular disease (but a full field ERG would also be needed to exclude a generalised retinal dysfunction)

• Abnormal N95 component suggests optic nerve/ganglion cell disease

• The PERG will only be abnormal in optic neuropathy if there is sufficient damage to cause retrograde degeneration of the ganglion cells

• Eg. optic nerve demyelination, ischaemic optic neuropathy, tobacco-alcohol amblyopia, compressive lesions, LHON and dominant optic atrophy

Multifocal ERG

• Useful to detect outer retinal disease 
• Provides a topographic representation that can be mapped to patient’s visual field

Waveform

• N1: initial negative deflection
• P1: positive peak

Electro-oculogram

• Measures the ‘corneoretinal standing potential’: ie. the electrical mass response to the eye, but primarily reflecting the metabolic activity of the RPE

• The cornea is positively charged compared to the RPE
• The resting potential of the eye is: 60mV
• Uses fixed excursion lateral eye movements under conditions of varying luminance

• Electrodes are placed at the outer and inner canthi during dark adaptation and followed by bright light

Waveform 

• The ‘dark trough’ is measured when the eyes make saccadic movements in the dark

• The ‘light peak’ is measured when the move against a steady light background

• The amplitude should increase markedly during light adaptation compared to a trough in the dark

• The ratio of the light peak to dark trough is known as the Arden ratio and is normally expressed as a percentage

• Arden is usually >180% in normal eyes (>1.8). <1.4 is considered abnormal

• The EOG reflects retinal pigment epithelium (and outer retina) activity

• Results tend to correlate with the ERG (except in Best’s vitelliform macular dystrophy where the ERG is normal and the EOG light peak is much reduced)

• EOG can be used to distinguish local from diffuse retinal disease
• Note: requires pupillary dilatation

Visual evoked potential

• Measurement of occipital cortical electrical response to retinal stimulation via flash or pattern stimuli (pattern is preferred as it has better inter-subject reliability)

• Reflects the activity of the whole visual pathway
• The wave pattern of flash VEP shows great variability between people
• The pattern VEP arises from the V1 area of the cortex
• The flash VEP arises from the V2 area of the cortex

• Primarily represents optic nerve function and useful to quantify function between the retina and the cortex

• Can approximate to visual acuity: loss of amplitude approximates to reduced VA. may provide indication of visual function in infants

• Electrodes placed over occipital scalp areas

• Pupils must not be dilated as accommodation is lost, and refractive error must be corrected (except for flash VEPs)

• Flash VEPs do not require as much cooperation from the patient and so can be useful in children or unconscious patients

Waveform 

• N75

• P100: this is the major component and is very reliable between subjects and generally stable from age 5 to 60 (time to peak only slows 1ms across this age gap). The p100 is predominantly a macular response

• N135
• P100 latency is the most useful measurement but amplitude is also useful
• The amplitude approximates to visual acuity

• Monocular stimuli evoke responses from both cortices representing the degree of chiasmal crossover

• Any abnormalities of the visual pathways or visual cortex can affect the VEP

Examples 

• Multiple sclerosis: delayed P100 component in affected eye (presentation is usually unilateral therefore asymmetric retrobulbar neuritis).

• Over time both nerves tend to be involved and amplitudes are diminished

• Trauma: visual pathway compression may initially lead to no VEP response being found but as inflammation subsides it may return so progress can be monitored

• Tumours: eg in patients with NF1. VEPs may show prolongation and eventually diminished amplitudes then obliteration

• Quantification and monitoring of progression in patients with atrophic/pale optic discs

• Drug toxicities: slow P100 components eg ethambutol, amiodarone, methanol, carbon monoxide, isoniazid, linezolid, sildenafil, infliximab

• Amblyopia: flash VEP is normal but pattern VEP is abnormal