Multifocal, pattern and full field electroretinograms in cats with unilateral optic nerve section

Abstract

Aim: To look for a subcomponent of the mFERG generated at the optic nerve head and increasing in latency with distance from it. To compare multifocal electroretinogram (mFERG, mPERG) changes to those in full field ERGs and transient and steady state pattern and focal ERGs (PERGs, FERGs) in cats with total unilateral optic nerve section.

Method: We recorded multifocal flash ERGs (mFERGs) at three levels of intensity and multifocal pattern ERGs (mPERGs) within 61 equal areas after total unilateral optic nerve section in five long term (> 18 month) survival cats, as part of a long term serial study of full field flash and pattern ERG changes to many stimuli, in a larger population. Cats were anaesthetised with Ketamine/Xylazine and wore Henkes electrodes with 6mm artificial pupils. Intact retinal circulation was verified by fluorescein angiography and optic nerve section by retinal photography and histology. We compared the mean and mean summed multifocal responses, from the normal and denervated eyes. We also compared the mean interocular difference around the area centralis and as a function of distance from the optic nerve head, across the horizontal meridian for the mFERG to the most intense stimulus. The degree of change was compared to that in other types of ERG, in the larger set of cats.

Results: mFERGs were similar across cats. Response density was flat with no prominence at the area centralis. Average summed mFERGs were similar in the normal and denervated eye. In the interocular differences a component near OP2 was reduced in the first kernel to the most intense stimulus, near OP1 and OP3 in the second kernel and locally, there was a hint of a component near OP2, which varied in latency in a ring around the disk and at the area centralis. Nevertheless, no component could be seen, varying in latency with distance from the optic nerve head, across the horizontal meridian. No mPERG was recordable in these conditions. Full field PERGs and FERGs were very reduced. Full field flash ERG amplitude changes were small (4-20%) and slower to appear than PERG changes. Degree of ERG reduction and correlation with PERG losses was greatest for the mesopic OPs, low for the scotopic tests (STR, ERG and OPs) and near zero for the mesopic ERG. The mFERG and mesopic ERG both lost OPs without overall amplitudes.

Conclusions: The cat mFERG does not have a component, varying in latency with distance from the optic nerve head. The only change was qualitiatively similar to that in the light adapted ERG. With intense stimuli there were local changes around the time of OP2 near the area centralis which might be explained by local variations in ganglion cell density.