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Byron Li, Nalini Rangaswamy, Steven Sessums, Richard Ornberg, Ganesh Prasanna; Thinning of RNFL is correlated with declining of PhNR ERG amplitude in glaucomatous monkeys. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1979.
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To understand the structure-function relationship between retinal nerve fiber layer (RNFL) thickness and ERG in the non-human primate model of glaucoma.
Chronic ocular hypertension (OHT) of various durations (1.6-13 y) was induced in the right eye (OD) of 43 cynomolgus monkeys by argon laser trabeculoplasty. The left eye (OS) was untreated with normal intraocular pressure (IOP). An additional 25 naïve animals served as controls. The juxtapapillary RNFL thickness in the normal and experimental eyes was evaluated with Spectralis SD-OCT (Heidelberg Engineering, Germany). The photopic ERG responded to 8 ascending luminous intensities of brief-(≤ 5 msec) and long-(200 msec) durations red Ganzfeld flashes on a rod-suppressing blue-adapting background were recorded with Espion ERG System (Diagnosys, MA, USA).
In the 25 naïve 5-yo monkeys, there was no difference in RNFL thickness and ERG amplitudes between OD and OS. In 23 OHT 5-yo monkeys (OHT duration = 1.6 y), significant thinning of OD RNFL (-16 ± 5 µm, -11%, p < 0.05) was detected compared to the OS in the nasal-inferior (NI) section. This early structural change did not significantly alter ERG but a trend was observed in photopic negative response (PhNR). In a second group of 20 OHT monkeys (age = 6-17 y; OHT duration = 2-13 y), Global RNFL thickness of OD (82 ± 4 µm) was significantly reduced (-17%, p < 0.001) than contralateral control eyes (99 ± 2 µm). Only the ERG PhNR but not the a- and b-waves response was significantly reduced in the OHT eyes. Brief flash stimulation was more sensitive to detect PhNR amplitude changes than long duration flash. There was a significant negative correlation (R2=0.8304) between RNFL thickness and PhNR by comparing the difference of eyes (OS-OD) in each measure in the 43 OHT monkeys.
Initial thinning of RNFL was detected in the NI section of the eyes with 1.6 y of OHT. This early structural change did not alter ERG but a trend was observed in PhNR. A longer duration (7.5±3.7 yo) of IOP elevation significantly reduced RNFL thickness and PhNR in lasered monkey eyes with strong correlation. Progressive thinning of RNFL and reduction of PhNR amplitudes observed in monkey eyes with different duration of chronic OHT is likely indicative of progressive loss of retinal ganglion cells and is similar to the disease progression in POAG in humans.
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