PERG Flashcards
What is a PERG?
Pattern Electroretinogram
Electrical response of the retina to a reversing chequerboard
How is PERG recorded?
PERG is recorded using a counterphasing (reversing) chequerboard stimulus. The mean luminance remains constant typically being 50 cd/m^-2
Do we dilate the eyes in PERG?
Undilated pupils are required because contrast is the most important parameter and is highly dependent on focus (so no optical blur).
What can’t you have if you wanted to do a PERG?
No media opacity, nystagmus or uncorrected refractive error as this can fuck up the PERG.
Why do we fixate on the centre of a checkerboard in a PERG?
Fixate on centre of checkerboard so looking at macular and paramacular (not even to optic disc) opposed to whole retinal function.
What does Pattern ERG (PERG) measure?
The pattern electroretinogram (PERG) is an established clinical method of assessing macular and retinal ganglion cell (RGC) function. The PERG localises the retinal response by temporally modulating a patterned stimulus of constant mean luminance.
How is the amplitude of PERG compared to a full-field electroretinogram (ERG)?
The PERG is of relatively low amplitude compared with the full-field electroretinogram (ERG) e.g., typically 2 orders of magnitude smaller, and obtaining reliable results requires careful attention to technique, including stimulus and electrode quality as well as to sources of extraneous noise.
What size retinal response do PERGs give?
5µV (whereas ERG can be 200/300uV)
What is a P50 in a PERG recording?
Shows macula function
What is N95 in a PERG?
Retinal Ganglion Cell Function
The RGC contribution to P50 has been suggested to reflect activity close to the ganglion cell bodies, and contributions to N95 more localized to the optic nerve head.
What can a PERG differentiate between?
Macula Function (P50) & Retinal Ganglion Cell Function (N95)
Who is a PERG recording unsuitable for?
- Patients with nystagmus
- < 6 years (generally as need compliant patients due to it taking longer to record, 3-4 minutes)
When do we get responses on a PERG recording?
Transient response (i.e., a response that is complete before the next contrast reversal) every time the pattern reverses on the checkerboard.
What does a PERG waveform look like in the absence of dysfunction?
In the absence of dysfunction the PERG waveform typically has a small initial negative component with a peak time of approximately 35 ms (N35), followed by a much larger positive component with a peak time in the region of 50 ms (P50).
Approximately 70% of P50 is thought to arise in the RGCs cells and 30% from more distal retinal cells including the bipolar cells. The P50 component is followed by a larger negative component with a mean peak time in the region of 95 ms (N95).
What does a normal N95 look like in comparison to the P50?
N95 is larger than P50
What does Maculopathy look like in relation to the N95 and P50?
N95 concomitantly reduced with P50, P50 may be delayed
What does Optic Neuropathy look like in relation to the N95 and P50?
N95 smaller than P50
Macular fine but retinal ganglion cells reduced
What should you always do alongside a VEP?
Never do a VEP without a PERG as VEP response may be reduced because of an issue with the macular which is what the PERG measures
How is a PERG different to a full-field ERG?
Since the PERG (in contrast to the full-field ERG) is a focal response summed from the retinal area covered by the stimulus image, the P50 component can be used as a sensitive indicator of cone system function within the macular region of the retina and P50 reduction and/or delay can characterize macular cone system dysfunction.
The PERG may also complement the full-field ERG in distinguishing between generalized retinal dysfunction and macular cone dysfunction (or by identifying the severity of macular cone involvement).
Compared to mfERG, is PERG more or less spatially selective?
The PERG stimulus covers more or less the whole macular region, so not as spatially selective as the mfERG. It is quicker and easier to record, however.
How big is the retinal response in PERG?
The PERG is a tiny retinal response of 5 μV in amplitude. It differentiates macular / optic nerve disease.
