Electrically Evoked Potentials Are Reduced Compared to Axon Numbers in Rhodopsin P347L Transgenic Rabbits With Severe Photoreceptor Degeneration

Purpose: To determine the relationship between the amplitudes of the electrically evoked potentials (EEPs) and the number of optic nerve axons at a late stage of retinal degeneration in rhodopsin P347L transgenic (Tg) rabbits, a model of retinitis pigmentosa. Methods: Six eyes of six wild-type (WT) (43.8 ± 7.5 months of age) and six eyes of six Tg (40.3 ± 2.6 months of age) rabbits were studied. The EEPs were elicited by 1 to 5 mA of transcorneal electrical stimulation. The first positive wave, the P1 component, was analyzed. After euthanasia, the number of axons in the optic nerve was counted. Results: The threshold current to elicit a P1 was significantly higher in Tg rabbits than WT rabbits. The amplitude of P1 elicited by 5 mA in Tg rabbits was about 24% of that in WT rabbits (P < 0.01). The number of axons in the optic nerve of Tg rabbits was reduced to about 59% of that of WT rabbits (P < 0.01). The correlation between the axon number and the amplitude of the P1 in Tg and WT rabbits was not significant. The mean ratio of the P1 amplitude/axon in Tg rabbits was decreased to 53% of that in WT rabbits (P < 0.05). Conclusions: The degree of reduction in the EEP in Tg rabbits is more severe than the reduction in the number of optic nerve axons. The use of transcorneal electrical stimulation to determine the suitable candidates for prosthesis at the end-stage of retinitis pigmentosa may underestimate the condition of the optic nerves.