Purpose
To capture the key features patterning the transition from unaffected mutation carriers to clinically affected Leber hereditary optic neuropathy (LHON), as investigated by optical coherence tomography.
Design
Observational case series.
Methods
Four unaffected eyes of 4 patients with LHON with the first eye affected were followed across conversion to affected, from 60 days before to 170 days after conversion. The primary outcome measures were multiple timepoints measurements of peripapillary retinal nerve fiber layer (RNFL) thickness for temporal emi-side of the optic nerve (6 sectors from 6-11, clockwise for the right eye and counterclockwise for the left eye) in all patients and nasal emi-macular RNFL and ganglion cell layer (GCL) thickness in 2 patients.
Results
While the presymptomatic stage was characterized by a dynamic thickening of sector 8, the beginning of the conversion coincided with an increase in the thickness of the sectors bordering the papillo-acular bundle (6 and 7 for the inferior sectors, 10 and 11 for the superior sectors) synchronous with the thinning of sectors 8 and then 9. Conversely, the GCL did not undergo significant changes until the onset of visual loss when a significant reduction of thickness became evident.
Conclusion
In this study we demonstrated that the thinning of sector 8 can be considered the structural hallmark of the conversion from the presymptomatic to the affected state in LHON. It is preceded by its own progressive thickening extending from the optic nerve head toward the macula and occurs regardless of the amount of swelling of the rest of the peripapillary fibers.
L eber hereditary optic neuropathy (LHON) is the most common inherited mitochondrial disorder and usually affects young males. It is characterized by selective neurodegeneration of retinal ganglion cells (RGCs) and their axons, which typically leads to bilateral centrocecal scotoma due to preferential loss of papillomacular bundle nerve fibers. The primary mechanism of neurodegeneration is centered on the mitochondrial dysfunction induced by 1 of 3 frequent maternally inherited mitochondrial DNA point mutations affecting complex I subunit genes.
As LHON is characterized by incomplete penetrance, the disease clinically presents an asymptomatic stage, which may last through a lifetime in more than half of the mutation carriers. These asymptomatic individuals may present, however, with typical funduscopic changes including swelling of the peripapillary retinal nerve fiber layer (pRNFL) and microangiopathy. , Both may be related and part of a compensatory effect that may involve mitochondrial biogenesis.
The conversion from the asymptomatic to the symptomatic stage begins typically as a unilateral subacute optic neuropathy with, in most cases, subsequent asynchronous involvement of the second eye. The nature of the disease natural history makes it challenging and uncommon to detect the early structural and functional changes related to the disease conversion, which remains substantially uncharacterized in its granular details.
Only a few longitudinal spectral-domain optical coherence tomography (SD-OCT) studies have described the natural history of pRNFL and ganglion cell layer (GCL) changes during conversion of LHON, highlighting 2 important observations: 1) thickening and subsequent thinning of the pRNFL, which starts in the temporal and inferior quadrants and propagates to the other quadrants, and 2) early nasal GCL thinning with the progressive involvement of other sectors fitting the distribution of papillomacular fibers.
Other cross-sectional studies analyzed the progressive thinning of RNFL and GCL in acute and chronic stages and confirmed the typical pattern of RNFL loss caused by the disease and the preferential involvement of papillomacular bundle fibers. , , However, there is a dearth of studies addressing the relationship between RGC degeneration and fiber swelling, which would inform on their reciprocal role during the early phases of developing loss of vision. The new swept-source OCT device has greater temporal and spatial interpolation of data and allows us to obtain a better structural and topographical analysis of retinal layers.
In this study, we analyzed 4 eyes from 4 patients in the presymptomatic stage after involvement of the first eye. Our aim was to describe the transition from presymptomatic to subacute changes of RNFL and GCL, highlighting their temporal and topographic relationship in the attempt to gain further pathophysiologic insights about the key events occurring during disease conversion.
METHODS
All patients included in the present study (3 males and 1 female) had a molecularly confirmed diagnosis of LHON (3 patients had the m.11778G>A/ MT-ND4 mutation and 1 patient had m.3460G>A/ MT-ND1 ) and belonged to 4 unrelated pedigrees.
We recruited the patients when the first eye was already involved and the second still asymptomatic, and we included in the analysis the fellow asymptomatic eye of each patient across presymptomatic to subacute stages with long follow-up times. We defined the onset of the visual symptoms as the subjectively reported visual impairment before the first objectively documented decrease of visual acuity (above logarithm of the minimum angle of resolution 0) accompanied by a corresponding visual field defect in central vision. The patients underwent an extensive ophthalmologic examination, including best corrected visual acuity (VA) measurement, slit-lamp examination, intraocular pressure measurement, indirect ophthalmoscopy, and optic nerve head photography, automated visual field test (VF) (Humphrey Field Analyzer; Zeiss, San Leandro, CA). Eyes with the presence of any retinal pathology and/or optic nerve disease other than LHON, spherical and/or cylindrical refractive errors >3 and 2 diopters, respectively, and systemic conditions that may affect the vascular system were excluded. Patients gave their informed consent according to the Declaration of Helsinki, and the local Ethical Committee at the IRCCS Istituto delle Scienze Neurologiche di Bologna approved the study (121-2019 OSS AUSL BO).
INSTRUMENTATION AND PROCEDURES
OCT images were taken using 2 different devices: SS-OCT (DRI-OCT; Triton, Tokyo, Japan) and SD-OCT (Cirrus HD-OCT; Carl Zeiss Meditec, Inc, Dublin, CA).
The SS-OCT uses a wavelength of 1050 nm with a scan speed of 100,000 A-scans per second. The images were obtained using a 3-dimensional wide scan protocol with a size of 12 mm × 9 mm consisting of 256 B-scans, each comprising 512 A-scans. This allowed obtaining images of the macular and optic nerve head region in a single scan. The total acquisition time was 1.3 s per 12 mm × 9 mm scan. Following proper seating and alignment of each patient, the iris was brought into view using the mouse-driven alignment system, and the ophthalmoscopic image was focused. The analysis algorithm processed data from 3-dimensional volume scans with measured thicknesses of the macular and peripapillary regions. OCT protocols included the evaluation of the pRNFL, macular RNFL, and macular GCL. pRNFL thickness was measured with a 360° 3.46-mm–diameter circle scan centered automatically around the optic disc and across 8 sectors according to Early Treatment Diabetic Retinopathy Study (ETDRS) centered on the fovea in macular region. The segmentation analysis of the macula measured across 6 sectors (superotemporal, superior, superonasal, inferonasal, inferior, and inferotemporal) of the 6-mm–diameter circular annulus centered on the fovea included the GCL between the macular RNFL and the inner nuclear layer boundaries. The built-in SS-OCT eye-tracking system was used to provide reproducible measurements.
SD-OCT uses a wavelength of 840 nm with a scan speed of 27,000 A-scans per second. All scans were acquired using the Optic Disc Cube 200 × 200 protocol. To acquire the Optic Disc Cube, the ONH was centered on the live image and centering and enhancement were optimized. After the scanning process was launched, the instrument’s laser beam generated a cube of data measuring 6 mm × 6 mm after scanning a series of 200 B-scans with 200 A-scans per B-scan (40,000 points) in 1.5 seconds (27,000 A-scans/s). Cirrus SD-OCT algorithms were used to find the optic disc and automatically place a calculation circle of 3.46-mm diameter symmetrically around it. The system extracted from the data cube 256 A-scan samples along the path of the calculation circle.
All images were acquired by an experienced operator (M.C.) with both devices. The quality of each scan and accuracy of the segmentation algorithm were reviewed by experienced and blinded examiners (P.B., V.P., C.L.M., N.B.). Images with segmentation failures, significant motion artefacts, or signal strength <6 were excluded.
The pRNFL scan circle was segmented into 12 sectors clockwise for the right eye and counterclockwise for the left eye, to preserve the same topographic correspondence in each eye examined. We included, for the analysis, multiple time point measurements of pRNFL thickness for temporal emi-side of the optic nerve (6 sectors from 6-11) in all patients and nasal emi-macular pRNFL (nasal and inferior) and GCL (superonasal, inferonasal, and inferior) thickness for 2 patients.
STATISTICAL ANALYSIS
Due to the nature of the study, patients are described individually, without calculating inferential statistics.
The trend over time of the thickness with respect to the baseline (first examination) is shown for each patient ( Figures 1 A, through Figure 4 A).
Furthermore, we evaluated the percentage variations of the thicknesses with respect to the baseline for each patient, and then these were plotted ( Figure 1 B, through Figure 4 B and Figures 5 and 6 ).
RESULTS
DEMOGRAPHIC DATA
Table 1 shows the demographic data and clinical features of the 4 patients with LHON. We studied these 4 patients across the conversion of the unaffected eye (second eye). After the disease conversion (onset), the examinations were uniformly performed at 10 days, 1 month, about 2 months, and at least a final visit at about 6 months. We had available also different time points preceding the onset, which provided information on the preclinical stage immediately before conversion (Supplemental Figure S1). In patients undergoing SS-OCT (patients 1 and 2), both RNFL (peripapillary and macular) and GCL were examined while in patients undergoing HD-OCT only pRNFL was examined.
| Patient 1 | Patient 2 | Patient 3 | Patient 4 | |
|---|---|---|---|---|
| Age at onset (y) | 15 | 22 | 19 | 16 |
| Gender | Male | Male | Female | Male |
| Mutation | 11778/ND4 | 11778/ND4 | 3460/ND1 | 11778/ND4 |
| Interval between eyes’ onset (days) | 140 days | 151 days | 229 days | 10 days |
| Therapy before second eye’ onset (days) (dose) | 42 (idebenone 900 mg/day) | 80 (idebenone 900 mg/day) | 60 (vatiquinone 900 mg/day) | 10 (idebenone 900 mg/day) |
| Visual acuity at final visit (logMAR) | 1.8 | 1.32 | 1.3 | 1.3 |
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