Assessment of macular function by multifocal electroretinography following epiretinal membrane surgery with indocyanine green-assisted internal limiting membrane peeling

Abstract

Dear Editor, Recently, Lai et al. [1] investigated the toxicity of indocyanin green (ICG) dye in patients who underwent ICG-assisted internal limiting membrane (ILM) peeling. Their study included 13 eyes of 13 patients; seven and six patients were given 0.5 mg/dl and 1.25 mg/dl, respectively. The authors evaluated N1 and P1 peak latency and amplitudes 3 months and 6 months after the surgery. They reported that the 1.25 mg/dl ICG group showed significant reductions in N1 and P1 response amplitudes at 3 months compared with baseline. There was no significant difference at 6 months in either group. Multifocal electroretinogram (mfERG) is a new electrophysiological technique that provides topographic information of the retinal function. It is primarily a response of the cone photoreceptors. However with this technique, the function of a relatively small retinal area can be recorded by commercial units as used by Lai et al. (VERIS 4.8 system, 25° on either side of the fixation). MfERG waveforms (when simulating the recording parameters of traditional full-field ERG) were shown to be little ERGs [2]. The response amplitudes in mfERG are rather small when compared with those of full-field electroretinograms (ERGs). This requires technically optimal conditions, such as type of active electrodes and fixation stability. In the literature, mean inter-sessional variability of P1 amplitude was reported to be as much as 35% in terms of coefficient of variation [3]. Coefficient of variation is the ratio of standard deviation to mean. Statistically, two standard deviations from the mean approximately equals 95% confidence interval. This means that individual P1 amplitude may vary by up to 70% in terms of percentage in intersessional recordings. For this reason, 13 eyes may cause a false positive statistically significant difference between the recordings. Although Lai et al. [1] did not report the localizations of the epiretinal membranes (ERM), they were probably just on or around the macula. If the epiretinal membranes were out of the macula, use of mfERG for the retinal functional changes of the central 25° would be meaningless. On the other hand, we believe that use of mfERG for investigating the ICG toxicity after central epiretinal membranes is also meaningless, because the traction on the macula during the membrane peeling may well impair macular functions just after the surgery. It is also possible for impaired macular functions due to macular traction and/or microtrauma to heal in the chronic phase and further complicate mfERG evaluation. Moreover, changes in fixation stability in patients with macular abnormalities or in patients who have undergone epimacular membrane peeling may affect mfERG results, especially in ring 1 in mfERG concentric ring analysis. In clinical practice we use the RetiScan system (RolandConsult, Wiesbaden, Germany) with 61 hexagons. With increasing eccentricity, the number of hexagons increases from 1 (central hexagon), to 6 (ring 1), 12 (ring 2), 18 (ring 3) to 24 (ring 4) in this system. Noises such as fixation instability or blinking during the recording are spread Graefe’s Arch Clin Exp Ophthalmol (2007) 245:1237–1238 DOI 10.1007/s00417-007-0545-1