

## Electroretinogram
<ul>
  <li>
    A measurement of the retina&rsquo;s electrical mass response to a light stimulus (a summation of
    photoreceptor and bipolar cell activity)
  </li>
  <li>Affected by:</li>
  <ul>
    <li>Adaptive state of the eye eg photopic (cone ERG) vs scotopic (rod ERG)</li>
    <li>Type of stimulus</li>
  </ul>
  <li>Electrode placed in contact with cornea and reference electrode on forehead</li>
  <li>
    The ERG is <span className="font-semibold">unaffected by small localised lesions</span> 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
  </li>
  <li>
    Does <span className="font-semibold">not </span>measure VA
  </li>
</ul>
<h4>Waveforms</h4>
<ul>
  <li>Two main waves in response:</li>
  <ul>
    <li>
      Negative A wave, arising from the <span className="font-semibold">photoreceptors</span>
    </li>
    <ul>
      <li>A1 originates in cones</li>
      <li>A2 originates in rods</li>
    </ul>
    <li>
      Positive B wave, caused by bipolars but magnified by Muller cells (which act as a sink for
      potassium ions released by depolarising bipolar cells):{' '}
      <span className="font-semibold">surrogate for bipolar cell function</span>
    </li>
    <li>
      Oscillatory potentials are superimposed on b wave and increase with stimulus intensity.
      Represent amacrine cell function.
    </li>
    <li>
      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
    </li>
    <li>
      D wave: only seen when using stimulus of long duration which is then stopped (ie. cessation of
      constant illumination)
    </li>
  </ul>
  <li>Implicit time: time from stimulus to peak of b wave</li>
  <li>
    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.&nbsp;
  </li>
  <ul>
    <li>
      Pupil size and media opacity will affect the intensity of light stimuli entering the eye
    </li>
  </ul>
  <li>
    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).
  </li>
  <ul>
    <li>Rod system is more sensitive across the whole spectrum compared to cones.</li>
  </ul>
  <li>
    Frequency of stimulation: the critical fusion frequency (CFF) represents the maximum frequency
    that can be perceived as flickering
  </li>
  <ul>
    <li>
      Highest CFF for rod vision is 15-28Hz (hence a higher frequency flicker will only elicit a
      cone response)
    </li>
    <li>Highest CFF for cone vision is 50Hz</li>
  </ul>
  <li>
    Bright white flash (standard stimulus): both a and b wave amplitudes are maximal in scotopic
    (dark adapted) conditions
  </li>
  <li>Dim white or blue flash: response is generated only by rods</li>
  <li>
    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)
  </li>
</ul>
<h4>Examples</h4>
<ul>
  <li>
    Reduced b wave (with preserved a wave implying normal photoreceptors but abnormal{' '}
    <span className="font-semibold">bipolar cells</span>) ie. an &ldquo;<span className="font-semibold">electronegative response</span>&rdquo;,
    occurs in many conditions including
  </li>
  <ul>
    <li>Inner retinal process</li>
    <ul>
      <li>CRAO/CRVO</li>
      <li>Toxicity: iron (siderosis), quinine, methanol</li>
      <li>Juvenile X-linked retinoschisis</li>
    </ul>
    <li>Bipolar cell or photoreceptor-bipolar synapse process</li>
    <ul>
      <li>Melanoma associated retinopathy</li>
      <li>Congenital stationary night blindness</li>
      <li>Autosomal dominant neovascular inflammatory vitreoretinopathy</li>
    </ul>
    <li>Miscellaneous&nbsp;</li>
    <ul>
      <li>Duchenne/Becker muscular dystrophies</li>
      <li>Birdshot chorioretinopathy</li>
    </ul>
  </ul>
  <li>
    Reduced or absent photopic response: cone dystrophies (which show a normal or subnormal but
    present scotopic ERG)
  </li>
  <ul>
    <li>Achromatopsia&nbsp;</li>
    <li>Cone dystrophies</li>
  </ul>
  <li>No response (extinguished):</li>
  <ul>
    <li>Batten&rsquo;s disease</li>
    <li>Leber&rsquo;s congenital amaurosis</li>
    <li>Rod-cone dystrophies like RP</li>
    <li>Total RD</li>
    <li>Cancer associated retinopathy</li>
    <li>Advanced siderosis</li>
  </ul>
  <li>Increased a wave: albinism</li>
  <li>Reduced oscillatory potentials: vigabatrin toxicity</li>
</ul>
<HotTopic>
  <p>
    Siderosis (eg. retained intraocular iron foreign body): increasingly negative a wave
    (supranormal) followed (in late stage) by decreasing b wave amplitude. Advanced siderosis leads
    to extinguished ERG.
  </p>
</HotTopic>

## Pattern electroretinogram
<ul>
  <li>
    Retinal electrical response to a patterned stimulus and represents{' '}
    <span className="font-semibold">central retinal and ganglion cell function</span>
  </li>
  <li>Objective assessment of macular function</li>
  <li>
    Reference electrode must be place on ipsilateral temple or outer canthus to avoid interference
    from cortical-evoked potential
  </li>
  <li>
    Stimulus is usually a <span className="font-semibold">reversing chequerboard</span>
  </li>
</ul>
<h4>Indications</h4>
<ul>
  <li>Macular dystrophies eg. Stargardt&rsquo;s</li>
  <li>Differentiating macular and optic nerve dysfunction</li>
  <li>Assessment of unexplained visual loss</li>
</ul>
<h4>Waveform</h4>
<ul>
  <li>
    Three responses (N=negative and P=positive. The subscript refers to the timing in miliseconds of
    the response&rsquo;s peak):
  </li>
  <ul>
    <li>Negative N35 wave</li>
    <li>Positive P50 wave (macular photoreceptors)</li>
    <li>Second negative N95 wave (ganglion cell layer)</li>
  </ul>
  <li>
    Therefore the P50 component is abnormal in macular disease (but a full field ERG would also be
    needed to exclude a generalised retinal dysfunction)
  </li>
  <li>Abnormal N95 component suggests optic nerve/ganglion cell disease</li>
  <ul>
    <li>
      The PERG will only be abnormal in optic neuropathy if there is sufficient damage to cause
      retrograde degeneration of the ganglion cells
    </li>
    <li>
      Eg. optic nerve demyelination, ischaemic optic neuropathy, tobacco-alcohol amblyopia,
      compressive lesions, LHON and dominant optic atrophy
    </li>
  </ul>
</ul>

## Multifocal ERG
<ul>
  <li>Useful to detect outer retinal disease&nbsp;</li>
  <li>Provides a topographic representation that can be mapped to patient&rsquo;s visual field</li>
</ul>
<h4>Waveform</h4>
<ul>
  <li>N1: initial negative deflection</li>
  <li>P1: positive peak</li>
</ul>

## Electro-oculogram
<ul>
  <li>
    Measures the &lsquo;corneoretinal standing potential&rsquo;: ie. the electrical mass response to
    the eye, but primarily reflecting the metabolic activity of the RPE
  </li>
  <li>The cornea is positively charged compared to the RPE</li>
  <li>The resting potential of the eye is: 60mV</li>
  <li>Uses fixed excursion lateral eye movements under conditions of varying luminance</li>
  <li>
    Electrodes are placed at the outer and inner canthi during dark adaptation and followed by
    bright light
  </li>
</ul>
<h4>Waveform&nbsp;</h4>
<ul>
  <li>
    The &lsquo;dark trough&rsquo; is measured when the eyes make saccadic movements in the dark
  </li>
  <li>The &lsquo;light peak&rsquo; is measured when the move against a steady light background</li>
  <li>
    The amplitude should increase markedly during light adaptation compared to a trough in the dark
  </li>
  <li>
    The ratio of the light peak to dark trough is known as the Arden ratio and is normally expressed
    as a percentage
  </li>
  <li>Arden is usually &gt;180% in normal eyes (&gt;1.8). &lt;1.4 is considered abnormal</li>
  <li>
    The EOG reflects<span className="font-semibold"> retinal pigment epithelium (and outer retina) activity</span>
  </li>
  <li>
    Results tend to correlate with the ERG (except in{' '}
    <span className="font-semibold">Best&rsquo;s vitelliform macular dystrophy </span>where the ERG is normal and the EOG
    light peak is much reduced)
  </li>
</ul>
<HotTopic>
  <p>
    Patients with Best&rsquo;s and carriers of Best&rsquo;s disease both show a diminished Arden
    ratio
  </p>
</HotTopic>
<ul>
  <li>EOG can be used to distinguish local from diffuse retinal disease</li>
  <li>Note: requires pupillary dilatation</li>
</ul>

## Visual evoked potential
<ul>
  <li>
    Measurement of occipital cortical electrical response to retinal stimulation via flash or
    pattern stimuli (pattern is preferred as it has better inter-subject reliability)
  </li>
  <ul>
    <li>Reflects the activity of the whole visual pathway</li>
    <li>The wave pattern of flash VEP shows great variability between people</li>
    <li>The pattern VEP arises from the V1 area of the cortex</li>
    <li>The flash VEP arises from the V2 area of the cortex</li>
  </ul>
  <li>
    Primarily represents optic nerve function and useful to quantify function between the retina and
    the cortex
  </li>
</ul>
<Note>
  <p>VEP cannot localise pathology</p>
</Note>
<ul>
  <li>
    <span className="font-semibold">Can approximate to visual acuity</span>: loss of amplitude approximates to reduced VA.
    may provide indication of visual function in infants
  </li>
  <li>Electrodes placed over occipital scalp areas</li>
  <li>
    Pupils must not be dilated as accommodation is lost, and refractive error must be corrected
    (except for flash VEPs)
  </li>
  <ul>
    <li>
      Flash VEPs do not require as much cooperation from the patient and so can be useful in
      children or unconscious patients
    </li>
  </ul>
</ul>
<h4>Waveform&nbsp;</h4>
<ul>
  <li>N75</li>
  <li>
    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
  </li>
  <li>N135</li>
  <li>P100 latency is the most useful measurement but amplitude is also useful</li>
  <ul>
    <li>The amplitude approximates to visual acuity</li>
  </ul>
  <li>
    Monocular stimuli evoke responses from both cortices representing the degree of chiasmal
    crossover
  </li>
  <li>Any abnormalities of the visual pathways or visual cortex can affect the VEP</li>
</ul>
<h4>Examples&nbsp;</h4>
<ul>
  <li>
    Multiple sclerosis: delayed P100 component in affected eye (presentation is usually unilateral
    therefore asymmetric retrobulbar neuritis).
  </li>
  <ul>
    <li>Over time both nerves tend to be involved and amplitudes are diminished</li>
  </ul>
  <li>
    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
  </li>
  <li>
    Tumours: eg in patients with NF1. VEPs may show prolongation and eventually diminished
    amplitudes then obliteration
  </li>
  <li>Quantification and monitoring of progression in patients with atrophic/pale optic discs</li>
  <li>
    Drug toxicities: slow P100 components eg ethambutol, amiodarone, methanol, carbon monoxide,
    isoniazid, linezolid, sildenafil, infliximab
  </li>
</ul>
<HotTopic>
  <p>
    Albinism: enhanced chiasmal cross-over (a larger proportion of retinal ganglion cell fibres
    cross at the chiasm in albino patients). Stimulating the left eye produces predominantly right
    occipital lobe response
  </p>
</HotTopic>
<ul>
  <li>Amblyopia: flash VEP is normal but pattern VEP is abnormal</li>
</ul>
<Note>
  <p>
    VEP abnormalities are symptomatic but not diagnostic. Retinal disease alone will significantly
    affect VEP and so they cannot distinguish optic neuropathy from retinal disorders
  </p>
</Note>


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

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