Full-field electroretinogram recorded with skin electrodes in 6- to 12-year-old children

Because light stimulation can induce retinal electrical activity, electroretinograms (ERGs) are used extensively for clinical identification of retinal diseases and evaluation of their severity [1]. Recently, the International Society of Electrophysiology of Vision (ISCEV) standard specified six recording conditions for the ffERG and an extended protocol for the photopic negative response (PhNR) [2]. These conditions were established to reflect the function of the main physiological generators located in the different retinal layers [3‐5].

Currently, the negative a-wave and the positive b-wave are the main ERG components used in clinical practice. Under photopic conditions, the negative a-wave is thought to predominantly reflect activity from cone cells and OFF bipolar cells [6]. The b-wave is mainly generated by ON bipolar cells [7], with some contributions of other post-receptoral sources [8]. The PhNR is predominantly generated by retinal ganglion cells [9].

Abnormal photopic ERG components were previously reported in diseases such as retinitis pigmentosa [10], optic nerve hypoplasia [11], age-related macular degeneration [12], diabetic retinopathy [13], and aniridia [14]. A reduction in the PhNR amplitude was also described in pathological diseases such as glaucoma [15, 16], multiple sclerosis [17], childhood optic gliomas [18], and optic neuropathy [19]. Although standard ffERG values from the normal retina are required for assessment of pathological retina, these values have not been reported in a large cohort of healthy children. It also remains unclear whether the a-wave, b-wave, and PhNR amplitudes change with age especially in 6- to 12-year-old children. Furthermore, children have a preference for skin electrodes, although these typically produce lower amplitude responses than those obtained with a contact lens [20, 21]. Previous studies using skin electrodes also had relatively small sample sizes. For example, Soekamto et al. [22] recorded scotopic (rod) and photopic (cone) responses from only 20 healthy patients. Additionally, although there are some other reports of normal electrophysiological values [23‐26], they are typically recorded from older subjects or do not assess the PhNR.

Therefore, the aims of this study were to record the LA 3 ERG and PhNR with a skin electrode in a large population of healthy children and to determine the relationship between these electrophysiological parameters and age.

Changes in the ffERG have been reported in children with various retinal disorders, including Stargardt disease [27] and retinopathy of prematurity [28]. However, the normal ffERG values for children in those studies were derived from healthy control groups with a small sample size and a wide age range. ERG is also affected by the type of electrophysiological recording system, subject age, light stimulation parameters, and electrode type [18, 29‐33]. Therefore, in the present study, we determined the normal values of LA 3 ERG and PhNR in a large cohort of children and also examined whether the a-wave, b-wave, and PhNR changed with age especially in 6- to 12-year-old children. Furthermore, we used skin electrodes, which were reported to provide accurate ffERG recording in clinical practice [7, 34, 35] to improve cooperation in children.

Using the Metrovision electrophysiological system, we recorded LA 3 ERG from 214 healthy subjects (mean age, 8.81 years; range, 6–12 years). The mean peak time in the a-wave (16.20 ms) was similar to that reported by Schwitzer et al. (18.6 ms) [29], Esposito et al. (15.73 ms) [31], and Bhatti et al. (14.3 ms) [36]. Furthermore, the mean peak time in the b-wave (33.41 ms) was similar to that reported by Schwitzer et al. (35.80 ms) [29], Esposito et al. (32.16 ms) [31], Abed et al. (30.29 ms) [18], and Bhatti et al. (29.3 ms) [36]. By contrast, the wave amplitudes in our study were similar with previous studies using skin electrodes and were lower than those previously reported using other electrodes. For example, the mean a-wave amplitude (4.25 μV) was similar with Esposito et al. (6.09 μV) [31] and was lower than that reported by Schwitzer et al. (10.8 μV) [29], Lin et al. (91.4 μV) [30], and Bhatti et al. (22.1 μV) [36]. Similarly, the mean b-wave amplitude (14.78 μV) was similar with Esposito et al. (17.37 μV) [31] and Abed et al. (22.35 μV) [18] and was lower than that reported by Schwitzer et al. (48.0 μV) [29], and Bhatti et al. (95.0 μV) [36]. The amplitude in our study which is lower than that in previous studies reported by Lin et al. and Bhatti et al. significantly, because amplitudes are lower with skin electrodes [34]. We can see that the values of different literatures differ greatly, and their sample size is relatively small except for Bhatti et al. study, and age range is relatively large. The data of our large sample may be able to somehow correct bias caused by small samples.

The amplitude of the PhNR (13.90 μV) in the present study was higher than that reported by Mortlock et al. (11.43 μV) [32], Esposito et al. (9.40 μV) [31] and was lower than Abed et al. (19.17 μV) [18] using the same types of electrodes. These differences may relate to differences in the light stimulation parameters and subject age. The amplitudes in all of these studies, including ours, were lower than those reported by Banerjee et al. [33] and Bhatti et al. [36], which likely relates to the different electrodes used in that study.

In the present study, there was only a small difference in the peak time of the b-wave between the 6- to 8-year-old and the 9- to 12-year-old groups, which may be of limited clinical significance, while there were no differences in any other parameters. This indicates that the electrophysiology of retinal cells in school-age children does not change with age. It is also possible that because the amplitude obtained by the skin electrode is small, the correlation is not easily detected.

The significant correlation between the amplitudes of the a-wave and b-wave in LA 3 ERG was consistent with the findings of Esposito et al.[31], which confirms the reliability and repeatability of our study. Skin electrodes placed farther from the eyelid margin can reduce amplitude responses, though less effect on peak time [37]. In order to be more reflective of retinal neuronal processing times, we also analyzed correlation between the peak time of the a-wave and b-wave. The significant correlation between the a-wave and b-wave peak time also existed. These findings suggest the existence of a strict functional relationship between cone cell pathway components in the eye [31]. We also found a positive correlation between PhNR amplitude and the a-wave and b-wave amplitudes, as reported by Esposito et al. [31], suggesting that ganglion cell function is affected by more distal retinal elements in the eye. In addition, the farther the active electrode is from the eyelid margin, the lower the recorded signal amplitude [37, 38]. So, we made sure that our electrode positioning is consistent with Esposito et al. However, the extent to the amplitude of each wave reduction remains unclear. So, the influence of electrode position on correlation needs to be further explored.

By contrast, we found no correlation between the electrophysiological parameters and AL. Previous studies have reported a reduction in electrophysiological amplitudes in high myopia and pathological myopia patients [39, 40]. In adults, axial elongation of the myopic eye can stretch the retina across the interior of the globe, thereby reducing the sampling density of retinal neurons and altering retinal physiology [41]. Previous study has reported peak time showed minimal delay with increase in axial length in adults. However, this has not yet been reported in children. Additionally, the present study only included children with a refractive error <  ± 6.00 D mean sphere, which resulted in a small AL range (21.37–26.32 mm). Thus, we did not find the same relationship as observed in adults. Further studies are required in children with a larger AL range.

There are some limitations to our study. First, the electrophysiological examination was not repeated for every child, although only stable electrophysiological results from cooperative children were selected for analysis. Second, because we used skin electrodes, the ffERG values are not comparable to studies using corneal electrodes. However, skin ERG electrodes can facilitate better testing cooperation in children. Thirdly, we recorded only LA 3 ERG and PhNR. However, in order to record the ERG responses for the younger children, who are not able to cooperate with the longer examination, we could not record other 5 standard ffERG responses. Due to the difficulty of examining children, and to ensure the reliability of the data, most of the published papers did not report records including 6 standard full-field ERG responses. Although establishing laboratory-specific reference values is the most optimal process, now lacking large sample electrophysiological data in children as a basic reference, we recorded a large number of electrophysiological results in 6- to 12-year-old children, especially including PhNR, which can at least provide a reference value for public. Fourthly, our study filtered to a much narrower range of frequencies than is conventional, but the ffERG results obtained in the 1–35 Hz range were more stable and reproducible in our laboratory with reference to previous research [42]. In addition, we detected ffERG following ISCEV standard protocols as strictly as possible.

To our knowledge, this study provides the largest dataset of LA 3 ERG and PhNR parameters in a population of healthy children. These electrophysiological parameters may be useful reference values when evaluating children with potential retinal defects. However, these results should be interpreted with caution because of different skin electrodes and narrower range of frequencies.