Correlation between macular volume and focal macular electroretinogram in patients with retinitis pigmentosa

PURPOSE. To determine whether a significant correlation exists between the morphology of the macula measured by optical coherence tomography (OCT) and the amplitude of focal macular electroretinograms (fmERGs) in patients with retinitis pigmentosa (RP). METHODS. fmERGs were recorded in 43 patients with RP and 43 age-similar normal subjects, with a 15 stimulus spot, 5.6 to 5.8 mm in diameter on the fundus. The sum of the volume of the neural retina in the central 6 mm (total macular volume) was measured with the OCT system. The length of the photoreceptor inner segment/outer segment junction (IS/OS line) in a 6-mm diameter macular area was also measured in the OCT images. RESULTS. There was a weak correlation between the total macular volume and the fmERG amplitudes (correlation coefficient, 0.46 for the a-wave and 0.54 for the b-wave). The fmERG amplitudes in the patients with RP with IS/OS line longer than 2 mm were significantly larger than those in patients with RP with IS/OS line shorter than 2 mm, but the correlations between these two factors were weak. One major reason for the low correlations between the macular morphology and fmERGs was that there were some patients with RP who had normal macular volume and long IS/OS line, but had severely reduced focal macular ERGs. CONCLUSIONS. Although the macular volume and length of the IS/OS line correlated weakly with the amplitude of the fmERGs, a preserved macular morphology does not necessarily guarantee normal-amplitude fmERGs in patients with RP.