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B. Edmunds, B. Fortune, B.V. Bui, G.A. Cioffi; The electroretinogram during acute intraocular pressure elevation in rats . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2128.
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Purpose: To characterize the effect of acute intra–ocular pressure (IOP) elevation on the rat full–field electroretinogram (ERG). Methods: IOP was manometrically elevated in one eye of anaesthetized (55:5:1 mg/kg ketamine:xylazine:acepromazine) adult Brown Norway rats for 90 mins. Animals were assigned to one of 9 pressure groups (n=6 each) ranging from 0–100 mmHg above resting IOP (ΔIOP). ERGs were recorded simultaneously from treated and control eyes beginning 75 minutes after IOP elevation. Brief, white, full–field flashes (–6.1 to 2.7 log cd.s/m2) were presented under fully dark–adapted conditions. Photopic ERGs were also recorded (1.2 to 2.7 log cd.s/m2) on a rod suppressing background (150 cd/m2). Maximal amplitudes for the following ERG components were derived: scotopic RmP3 (photoreceptors) and P2 Vmax (bipolar cells), positive and negative scotopic threshold response (pSTR and nSTR), scotopic and photopic oscillatory potentials (OPs) and photopic b–wave. A function relating ERG amplitude (treated/control, %) to ΔIOP was fitted with the Hill equation to establish the IOP sensitivity (S) for each ERG component. Results: Resting IOP was 12.1 ± 2.8 mmHg and mean arterial pressure was 97.6 ± 10.7 mmHg. Each ERG component showed a graded effect that was dependent on ΔIOP. No significant change in any of the ERG parameters were observed for ΔIOP of 0, 10 or 20 mmHg. The most sensitive effects of elevated IOP were systematic delays in the timing of the STR and photopic b–wave, observed between ΔIOP of 30 and 40 mmHg, without changes in any other ERG component. Comparisons of sensitivity (S) to ΔIOP revealed that the nSTR (54.2 mmHg) and the photopic OPs (55.2 mmHg) were most sensitive to acutely elevated IOP. ERG components arising from the middle layers of the retina were less sensitive to elevated IOP (photopic b–wave, 58.3 mmHg; scotopic P2 Vmax, 60.3 mmHg; scotopic OPs, 63.3 mmHg). The pSTR showed intermediate sensitivity to elevated IOP (63.1 mmHg). The least sensitive component was the rod photoreceptoral RmP3 (70.5 mmHg). Conclusions: ERG components generated in the proximal retina (nSTR) were affected at lower levels of IOP than those arising from the outer retina and below the ΔIOP needed to completely impede retinal perfusion.
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