Influence of axial length of normal eyes on PERG.

The influence of the axial length (AL) of the eye on flash electroretinogram (ERG) responses has been well established in the literature, suggesting an association between ERG abnormalities with myopia (AL > 25 mm). The aim of our present study was to determine whether the AL of normal eyes can also influence the pattern electroretinogram (PERG) on normal subjects. Thirty-nine normal volunteers were subjected to PERG measurements following the standard set by the International Society for Clinical Electrophysiology of Vision (ISCEV). The AL of the eyeball was measured using a TOMEY ultrasonic A scanner. Each volunteer had a complete ophthalmic examination including visual acuity, refraction, intraocular pressure, visual field, colour vision, orthoptic assessment and retinal photographs and had a best corrected visual acuity of 6/9 or better. Only one eye from each of the 39 normal volunteers was included in the statistical analysis of the results. The normal volunteer group had a mean P50 amplitude of 3.8 +/- 1.1 SD microV. The range of AL was between 21.8 and 25.7 mm (mean = 23.8 +/- 1.0 SD mm). Overall findings obtained from this investigation indicate a significant correlation between the AL of normal eyes and the PERG P50 amplitude (Spearman rank correlation coefficient r = -0.413, p < 0.01). The correlation accounts for 17% of the variance observed in the 39 amplitude values. This confirms the current hypothesis that the PERG amplitude is inversely related to axial length and means that AL should be considered when interpreting PERG amplitudes.