Introduction
Leber hereditary optic neuropathy (LHON) is a rare inherited disease predominantly affecting males with a prevalence in Europe of approximately 1:45,000 considering the three primary (m.11778G>A, m.14484T>C and m.3460G>A) maternally transmitted mitochondrial DNA (mtDNA) mutations [
1]. These mtDNA mutations are associated with abnormal cellular respiratory chain function causing oxidative stress [
2] that severely affects retinal ganglion cells (RGCs) causing progressive optic atrophy [
3‐
6]. The changes in RGCs and in the retinal nerve fiber layer (RNFL) strongly impact the central retina leading to low vision or irreversible legal blindness with small number of patients that eventually recover vision partially [
7,
8]. Equivalent mitochondrial alterations can be also caused by biallelic mutations in genes of the nuclear DNA leading to an autosomal recessive form of LHON, as recently reported [
9,
10].
There are no specific signs of retinal alterations preceding the conversion to the acute phase in LHON patients [
11]. The discovery of retinal biomarkers preceding the conversion could be relevant for early interventions. In addition to the structural examination with optical coherence tomography (OCT), visual electrophysiology plays an important role in the assessment of retinal integrity in LHON. Typically, affected LHON patients show abnormal pattern-reversal visual evoked potential (VEP) and pattern electroretinogram (PERG) with abnormal N95 amplitude or N95/P50 amplitude ratio and shortening of P50 peak time, revealing the primary dysfunction of RGCs [
12‐
17]. The
a-wave and the
b-wave of the standard (ISCEV) full-field flash ERG may be classified as normal or slightly reduced in LHON patients [
16,
18]. On the other hand, the photopic negative response (PhNR) originating in the inner retina, dependent on RGCs’ integrity [
19], has been described as altered and associated to the disease progression in LHON patients [
17,
18,
20].
In addition to the PhNR, oscillatory potentials (OPs) have been long reported to originate in the inner part of the retina, reflecting inhibitory/excitatory interactions involving bipolar and amacrine cells in the inner plexiform layer of the primate retina [
21,
22]. Each individual OP wavelet may be originated by a different subset of cells which may provide the possibility of accessing specific retinal mechanisms by evaluating individual and consecutive OP peaks [
23‐
26]. Considering the proximity and physiological interdependence of RGCs and the OP generators, it could be speculated that LHON patients are at higher risk of presenting OP dysfunctions. Reduction in OP amplitudes were reported in patients with dominant optic atrophy with OPA1 gene mutations [
27] and glaucoma [
28,
29], conditions predominantly affecting the retinal ganglion cells. We are not aware of any prior study investigating OPs in LHON.
In the present study, OPs were extracted from DA and LA full-field ERG signals with the application of digital filters using different cutoff frequencies in genetically confirmed LHON patients to evaluate whether OP changes reflect retinal alterations caused by LHON.
Discussion
In addition to the well-known, and confirmed by this study, inner retina dysfunction reflected by pattern ERG (PERG) and photopic negative response (PhNR) changes, the present report shows that oscillatory potentials (OPs) are also affected in LHON patients. Retinal alterations caused by LHON were reflected in full-field dark-adapted (DA) and light-adapted (LA) ERG OPs. Interestingly, LA OP abnormalities were more evident when slow components were filtered out using a higher-band-pass digital filter. The present data also confirm preserved or relatively preserved photoreceptor to bipolar cell (outer retina) mechanisms as revealed by normal
a-wave and
b-wave values in the DA responses (Fig.
1A) and close to normal values in the LA responses (Fig.
1B) in our cohort, as previously reported [
17,
18,
20]. In addition, LHON patients also showed affected inner retinal components of the multifocal ERG [
32].
The OPs of the full-field flash ERG are low-voltage high-frequency electric oscillations consistently observed in the rising phase of the
b-wave [
26,
33‐
35]. DA OP analysis is recommended by ISCEV [
30] using band-pass (~ 75–300 Hz) filter of signals obtained with the standard (3.0 cd·s/m
2) DA full-field ERG. Interestingly, individual OPs are generated by distinct retinal mechanisms which are also differentially influenced by the flash intensity and the state of adaptation [
24‐
26,
35‐
37]. It has been proposed that the early OPs may have more distal retinal origins while intermediate and late OPs show inner retinal spiking generators [
38]. Since the late OPs are believed to originate from the inner retina (amacrine/ganglion cells) which is severely affected in LHON, we investigated the integrity of the five major DA and LA OPs. DA OPs have been consistently reported to be sensitive to retinal changes caused by diabetes [
39‐
41] and other conditions affecting inner retinal mechanisms [
24]. OP alterations may accompany other optic neuropathies as they have been reported in patients with autosomal dominant optic atrophy [
27]. However, OPs were not specifically studied in LHON patients. Abnormal OPs have also been reported in patients with glaucoma primarily affecting retinal ganglion cells. These findings suggested amacrine cells alterations in addition to ganglion cells alteration [
28,
29]. DA OPs in non-human primate with experimental glaucoma were not consistently different from control eyes [
42], while LA OPs were not specifically studied.
In the present report, DA OP differences between controls and LHON patients with completely intact DA
a-wave and
b-wave were slight for the sum of the DA OPs (Fig.
3B). The comparisons of individual OPs revealed that OP2 was mainly affected. The LA OPs seemed more affected in LHON patients. Importantly, significant differences between LHON patients and controls for OPs extracted with the 100 Hz low-cutoff frequency filter suggested that fast OPs (105–215 Hz) [
23] likely originated at inner retinal cells [
38] might be involved in the pathogenies.
There are no current standard recommendations to derive OPs from LA ERG signals although they are clinically relevant. For instance, reduced or absent LA OP2 has been observed in inherited retinal conditions affecting signal transmission between photoreceptors and bipolar cells [
43]. Interestingly, in patients with congenital stationary night blindness (CSNB), LA OP2 and OP3 were absent while OP4 was preserved [
44]. Likely, abnormal OP2 and OP3 was probably due to a defect in the on-bipolar cells, with normal OP4 as off-bipolar cells were properly functioning. The present data shows asymmetric LA OP alterations in LHON patients with OP4 more prominently reduced. Although the origins of the ERG OPs have been debated for several decades [
22,
25,
45,
46], early observations indicated that they may originate in the retinal interneurons with different OPs representing the electrical manifestation of a distinct retinal event [
35]. Possibly, the lower OP amplitudes found in LHON patients represent functional changes of the amacrine cells, similar to what has been reported in patients with autosomal dominant optic atrophy [
27]. However, a direct effect of RGCs dysfunction on the OP amplitudes [
45] may also be taken into consideration when analyzing OP changes in LHON patients.
Visual processing driven by the RGCs that are responsible for sharp vision and color discrimination are at high risk of suffering from mitochondrial dysfunction [
47] which may influence inner retinal mechanisms. DA OPs have been long reported to be specifically affected in diabetic patients with no detectable signs of diabetic retinopathy [
48], with lower amplitudes of the early OPs correlated with the vascular changes [
49]. Microvascular changes have also been considered a pathogenic mechanism and a potential biomarker in LHON patients [
50]. Special microvascular properties are present in the optic nerve head that is affected in LHON [
5] and may therefore influence DA OP2 amplitudes perhaps in presymptomatic stages. However, other mechanisms affecting the synaptic conduction of affected ganglion cells may play a role in the chronic stage as observed in the patients included in this study.
The limitations of the study were small sample size and the genetic / clinical heterogeneity of LHON patients included in the study. In order to confirm whether this group can be representative of the LHON disease, future studies may consider investigating genetically homogeneous groups as well as subjects with and without microvascular changes (OCT) in presymptomatic stages.
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