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Prashant K. Parekh, Brandon Bosse, Olga A. Shif, Lori M. Ventura, Vittorio Porciatti; Metabolic Adaptation of the Steady-state PERG in Normal Controls and Early Glaucoma Patients. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5479.
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The onset of high-contrast patterned stimuli reversing at ~ 8 Hz induces rapid blood flow increase in retinal vessels  and slow changes of the steady-state PERG amplitude [2,3], implying adaptive regulation of retinal ganglion cell (RGC) function in response to changed metabolic requirements. We tested the hypothesis that adaptive PERG changes of normal controls (NC) differed from those of glaucoma suspects (GS) and patients with early manifest glaucoma (EMG).
Subjects were 28 NC, 46 GS (Humphrey visual field MD -0.9 ±1.8 dB), and 24 EMG (MD -1.6 ±1.9 dB). All subjects had corrected visual acuity of ≥ 20/20. The age range was comparable in all groups. The PERG was recorded from both eyes in response to contrast-reversing (8.13 Hz) gratings (1.6 cyc/deg, 100% contrast) for 4 minutes from the stimulus onset in successive samples of 12 sec each [2,3]. Amplitude values as a function of time were fitted with a Lowess smoothing function to retrieve the Initial (IA) and Final (FA) amplitudes, and calculate the corresponding adaptive change [AC= (FA-IA)/IA*100]. Standard PERGs were also obtained by averaging samples over the entire recording period. For statistical analysis, responses of the two eyes were averaged.
The range of IAs was similar in all groups (0.25 - 1.5 µV). On average, IAs in the lower quartile tended to an enhancement with time, whereas IAs in the upper quartiles tended to a decline (ACs ranged from -40% to +60%). Multiple regression analysis using AC as dependent variable and IA, Age (years), and Diagnosis (NC=1,GS=2,EMG=3) as relevant explanatory variables, revealed that AC could be entirely predicted from the explanatory variable IA (P<0.001). Age (P=0.8) and Diagnosis (P=0.9) did not influence the model: AC = 26.409 - (42.516 * IA) - (0.040 * Age) - (0.155 * Diagnosis). Standard PERG amplitudes were smaller in GS (P=0.04) and EMG (P<0.001) compared to controls.
While standard PERG amplitude was reduced in GS and EMG patients compared to NC, adaptive PERG changes depended on IA only and were remarkably similar in all groups. This may signify that neurovascular/neurometabolic coupling in the inner retina is normal in early glaucoma. It remains to be tested whether adaptive PERG changes are altered at later stages of glaucoma or in other diseases involving inner retina/optic nerve.References.  Riva et al, Prog Ret Eye Res 2005,  Porciatti et al, IOVS 2005,  Porciatti & Ventura Vision Res 2009
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