Diagnosis of retinal dystrophies: ERG and other clinical techniques

Question 1

What is a clinical electroretinogram?

Answer 1

Clinical tool for differential diagnosis of retinal hereditary diseases
ERG can detect abnormality at any stage of the disease, including in
asymptomatic patients without visible fundus abnormalities.
Allows functional assessment in an objective way
ERG is recorded by stimulating the retina with light
Recorded at the cornea
Requires stimulus, electrodes and amplifier
Currently ERG is the only test available to assess the functional viability of the retina in vivo
It allows to differentiate diseases of the outer retina to the inner retina by studying the various components of the ERG waveforms.

Question 2

What are the indications for an ERG to be done?

Answer 2

 Inherited retinal diseases
 Acquired retinal degenerative conditions
 Unexplained visual loss
 Vascular diseases of the retina
 Infant with questionable vision
 Toxic and nutritional eye disease

Question 3

How does ERG work?

Answer 3

Two currents (A and B):
A: flows through a route
remaining entirely within
retina
B: leaves retina through
vitreous and anterior
ocular tissue, then returns
to the retina via sclera,
choroid and RPE

Question 4

How is it recorded?

Answer 4

Recording electrode
Contacts cornea or bulbar conjunctiva
Includes contact lens, fibre or foil electrodes

Reference electrode
Skin near outer canthus
Mastoid, earlobe or forehead
locations may result in contamination
of the ERG from cortical potentials or
the fellow eye (ISCEV guidelines).

Ground electrode
Forehead or earlobe

Question 5

What are the different types of ERG?

Answer 5

 Full field ERG (Flash & Flicker ERG)
 Pattern ERG (PERG)
 Multifocal ERG (mfERG)

Question 6

What is the full field flash ERG?

Answer 6

A-wave: the first negative wave
which reflects the general
physiological health of the
photoreceptors in the outer retina
B-wave: large positive wave,
reflects the health of the inner
layers of the retina, including ON-
bipolar cells and muller cells
Oscillatory potentials: small but
high frequency oscillations on
ascending limb of b wave.
Generated by amacrine cells in
INL

Question 7

What is the full field ERG protocol?

Answer 7

Pupils should be maximally
dilated.
Full-field (ganzfeld) stimulation
used to provide uniform
luminance over entire visual field.
20 mins of dark adaptation before
recording dark-adapted ERGs.
10 mins of light adaptation before
recording light-adapted ERGs.
After dark adaptation, weak flashes should be presented before stronger flash
Request the patient to fixate and not move eyes. Ocular movements can change the positions of electrodes, can cause blockage of light by eyelids or electrode and may induce electrical artifacts.

Question 8

What is the pattern ERG?

Answer 8

 The PERG is a retinal
response evoked by a contrast-reversing
pattern, usually a black and white checkerboard
 Provides an objective
measure of central retinal function
 Provides information about macular and retinal ganglion cell function.

Question 9

What is the pattern ERG protocol?

Answer 9

Stable head position is important
Should be recorded without pupil dilation to maximize retinal image quality
Fixation mark in centre of screen is essential
Binocular recording is recommended as it is generally more stable and reduces exam time

Question 10

Why are pattern ERGs used?

Answer 10

Clinically, PERGs can be used in patients with abnormal pattern Visual Evoked Potential (VEPs) to establish if:
1. Central retinal disorder is present.
2. Differentiate between retinal and optic nerve dysfunction as a cause for
the abnormality.

Question 11

How do you read PERGs?

Answer 11

The amplitude and implicit
time of the P50, amplitude of
N95 are important.
Useful for differentiating
between optic nerve (ON)
dysfunction and macular dysfunction.
ON dysfunction: normal P50, abnormal N95.
Macular dysfunction: abnormal P50.

Question 12

What is the multifocal ERG?

Answer 12

• A limitation of the full-field ERG is
  that the recording is a massed
  potential from the whole retina
  and hence cannot detect localised
  damage- macular degeneration.
• Unless 20% or more of the retina
  is affected, the ERGs are usually
  normal.
  The display appears to flicker as each hexagon goes through a pseudo-
  random sequence of black and white presentations.
  Provides topographic measure of retinal electrophysiological activity.
  The produces multiple ERG recordings reflecting the retinal response at
  each of the corresponding stimulated areas- it is not a direct physiological
  response but mathematically extrapolated response.

Question 13

What is the mfERG protocol?

Answer 13

 Pupils should be maximally dilated
 Good fixation is essential
 Pre-exposure to light: the patient should be in ordinary
room illumination before testing
 Room illumination: Moderate or dim room lights should
be on and ideally should produce illumination close to
that of the stimulus screen.
 Ambient lighting should be same for all recordings
 Total recording time of at least 4 min for 61 element
arrays, or 8 min for 103 element arrays.
 Standard recording done monocularly

Question 14

Structure vs Function?

Answer 14

OCT Imaging is an important test to determine structural viability of the retina.
Provides detail information along with ERG
which tells the functional viability of the retinal
Clinical diagnosis and management would be based on both structural and functional changes.