Factors affecting the use of multifocal electroretinography to monitor function in a primate model of glaucoma.

While elevated intraocular pressure (IOP) undoubtedly plays a crucial role in many glaucoma patients, vascular dysregulation and chronic regional ischemia are also thought to contribute to the pathophysiology of glaucoma. In an effort to critically evaluate hypotheses involving vascular abnormalities in glaucoma, Cioffi, Van Buskirk and co-workers have developed a model of optic neuropathy based on chronic regional ischemia. The multifocal electroretinogram (MERG) has previously been used to assess function in non-human primates with experimental glaucoma induced by high-IOP. In this study, the MERG was used to monitor function in macaque monkeys with experimental glaucoma induced by chronic anterior optic nerve ischemia. Initial recordings from experimental eyes, which were later documented histologically to have moderate axon loss, revealed little difference from recordings of control eyes. This suggested that many of the signal components in the macaque MERG, which are known (from other studies) to be eliminated by intravitreal injections of NMDA/TTX or by high-IOP experimental glaucoma, may also be affected by the choice of anesthetic agents and MERG recording parameters. Subsequent experiments were performed to specifically evaluate the effects of bipolar versus monopolar signal derivation, anesthetic agents, MERG stimulus design and spatial scale. The results demonstrate that successful measurement of inner retinal and optic nerve head MERG components, especially those which have been shown by other investigators to originate with ganglion cell spiking activity, will depend critically upon the choice of anesthetic agents and recording parameters. One of the most important parameters seems to be use of a monopolar signal derivation, with the contralateral cornea serving as the reference position.