Introduction: The objective of this study was to test macular sensitivity, fixation stability and fixation location using microperimetry in patients with autosomal dominant optic atrophy (ADOA) and mutation-free relatives.
Material and methods: This was a cross-sectional study of 43 patients with exon 28 (2826 delT) mutation in OPA1 (age 11.7-71.5 years, best-corrected visual acuity (BCVA) 20/24-20/13). The patients and 49 mutation-free first-degree relatives (BCVA 20/25-20/10) underwent ophthalmic examination including macular microperimetry out to 12° eccentricity with registration of fixation stability and fixation location.
Results: The average (± standard deviation) sensitivity was significantly reduced in ADOA patients compared with controls, 14.9 (± 4.4) dB versus 19.7 (± 0.4) dB (p < 0.0001). In a retinotopic projection, the largest relative sensitivity deficits in ADOA were seen in the nasal macula (13.6 (± 5.7) dB versus 19.7 (± 0.7) dB) and in the central macula (14.2 (± 5.1) dB versus 19.9 (± 0.3) dB). The average sensitivity decreased with decreasing BCVA in ADOA (p < 0.0001). Stable fixation was found in 58% of ADOA patients versus 86% of controls, and relatively unstable fixation was observed in 35% of ADOA patients versus 14% of controls. Unstable fixation was found only in ADOA, where its prevalence was 7%.
Conclusion: ADOA was associated with unstable fixation and subnormal microperimetric sensitivity, especially in the central and nasal macula where the ganglion cell deficit is most pronounced.
Funding: The study was supported by Øjenfonden, Øjenforeningen, and Synoptikfonden.
Trial registration: NCT01522638.
Autosomal dominant optic atrophy (ADOA), also known as Kjer disease, (OMIM165500) is the most common inherited optic neuropathy [1, 2]. Both retinal nerve fiber layer and ganglion cell/inner plexiform layer thickness are lower than in healthy controls [3-6]. Patients are generally diagnosed with bilateral symmetric subnormal visual acuity during the first two decades of life and often before the age of ten years [7, 8]. Phenotype variation is considerable, both within and between affected families, where best-corrected visual acuity (BCVA) may range from normal to light perception [2, 9]. Patients typically have a central or centrocecal scotoma [2, 9, 10]. Nevertheless, many patients perform well in daily life into their older years , which suggests that there are lessons to learn from a close characterisation of visual function and its development with time in ADOA and other degenerative diseases that affect vision.
Microperimetry, which enables retinotopic mapping of fixation and localised fundus sensitivity, can be used to find small visual field defects that escape detection with conventional perimetry [12-15]. Studies of fixation patterns have been made in optic neuropathies other than ADOA. In the somewhat comparable Leber hereditary optic neuropathy, good visual acuity is related to stable fixation  and, likewise, severe glaucoma can be accompanied by poor, predominantly eccentric fixation .
The present study is part of a cross-sectional study of ADOA that aims to characterise retinal anatomy and visual fields in a homogenous group of patients with c.2826_2836delinsGGATGCTCCA mutation in OPA1 . The purpose of the present report is to describe microperimetric light sensitivity and fixation patterns in c.2826_2836delinsGGATGCTCCA ADOA and a control group of mutation-free relatives.
MATERIAL AND METHODS
This study was designed as a cross-sectional observational study and included 43 patients with c.2826_28 36delinsGGATGCTCCA mutation in OPA1 and a control group of 49 mutation-free first-degree relatives. The study population and the inclusion and exclusion criteria have been described elsewhere .
Patients and mutation-free first-degree relatives were recruited from a national register and examined in the order they volunteered. Written informed consent was obtained from all patients and healthy subjects. For participants under the age of 18 years, informed consent was obtained by their respective guardian. The study was approved by the Medical Ethics Committee of Copenhagen County and followed the tenets of the Declaration of Helsinki.
All subjects underwent an ophthalmologic examination, including refraction, determination of BCVA in Early Treatment Diabetic Retinopathy Study (ETDRS) letters and microperimetry visual field testing. Pupils were dilated using tropicamide 1% and phenylephrine hydrochloride 10%. In subjects younger than 15 years old, only tropicamide 0.5% was used.
Microperimetry (MP-1, Nidek Technologies, Padova, Italy) was performed with one eye covered at the time. Before each examination, a short test sequence was used to reduce learning effects. A Goldmann III stimulus of 200 ms duration was projected on a background intensity of 1.27 cd/m2 and tested using threshold strategy 4-2. Stimulus intensity ranged 0-20 dB and was set to start at 10 dB and titrated to 0.1 log scale accuracy, albeit with a sensitivity ceiling of 20 dB, which is reached by many healthy subjects in the central visual field. Automatic eye-tracking allowed each stimulus to be assigned to a specific location on a photograph of the fundus. The fixation target was a 1° red cross. The background illumination in the examination room was dim light. Before examination start, the instrument focus was adjusted to compensate for spherical refraction error. The blind spot was manually marked on the optic nerve head and automatically tested with a supra-threshold stimulus. The examination pattern was a macula 12° programme with 45 test points. For analysis, the test points were subdivided into five fields; the central, the superior, the nasal, the inferior and the temporal fields. Only fovea-centred fundus monitoring images of good quality were accepted.
Fixation stability was described automatically by a count of the percentage of fixation points located within the macular central 2° and 4°, respectively. The points were then divided into three groups; stable fixation (75% of fixation points within the central two degrees), relatively unstable fixation (less than 75% of fixation points within the central two degrees but 75% or more within the central four degrees) and unstable fixation (less than 75% of fixation points within the central four degrees). Fixation location was divided into three groups; predominantly central (more than 50% of fixation points within the central two degrees diameter circle), poor central (more than 25% of fixation points but less than 50% within the central two degrees diameter circle) and predominantly eccentric (less than 25% of the fixation points within the two degrees diameter circle).
Data from the microperimetry examination were analysed in relation to macular ganglion cell-inner plexiform layer (GC-IPL) thickness and peripapillary retinal nerve fiber layer (RNFL) thickness, which were determined as previously described . The position of the locus of fixation was determined in relation to the bottom of the foveal depression by aligning Cirrus optical coherence tomography (OCT) scans  by superimposing these on corresponding microperimetry images using Gimp 2.6.12 software. Retinal vessels were used as markers to ensure a complete match between the images. It was noted whether or not there were fixation points in the six different sectors and in the central field.
Data are presented as means ± standard deviations (SD) and full ranges. There was no statistical difference between the right and left eyes and therefore only right eyes are presented. Data were analysed using Mann-Whitney’s U test and the Spearman correlation coefficient (SAS 9.1 Software package, SAS Institute, Inc., Cary, North Carolina, USA).
Trial registration: NCT01522638.
The 43 patients with ADOA (22 males and 21 females) had a mean age of 39.3 (range: 11.7-71.5) years, BCVA ranged 7-94 (mean 56.9 ± standard deviation 21.5) ETDRS letters and mean axial length 24.3 ± 1.3 mm. A control group of 49 mutation-free first-degree relatives (25 males and 24 females) had a mean age of 32.8 (range: 8.9-68.7) years (p = 0.01), a mean BCVA of 88.8 ± 4.4 ETDRS letters (p < 0.0001) and a mean axial length of 23.5 ± 1.1 mm (p = 0.02).
In ADOA patients, the average sensitivity of the central 0-12° eccentricity visual field was subnormal compared with controls (p < 0.0001, Table 1), as were each of the five subfields (p < 0.0001, Table 1). Seen from the centre of the fovea, the fields with the lowest relative sensitivity in ADOA patients were the nasal subfield (13.6 ± 5.7 dB versus 19.7 ± 0.7 dB) followed by the central subfield (14.2 ± 5.1 dB versus 19.9 ± 0.3 dB) and the inferior subfield (14.8 ± 4.6 dB versus 19.8 ± 0.4 dB). The relative deficits amounted to 16-31%.
In ADOA, BCVA decreased with decreasing average sensitivity (r = 0.74, p < 0.0001, Figure 1). In three of the macular fields, sensitivity decreased with increasing age: superior (r = –0.49, p = 0.0009), inferior (r = –0.31, p = 0.04) and temporal (r = –0.35, p = 0.02). A borderline significant correlation was seen between average sensitivity and age (r = –0.30, p = 0.05). In the control group, average sensitivity decreased with age (r = –0.30, p = 0.04), but there was no correlation with BCVA (r = –0.19, p = 0.20). There was no effect of sex in any of the two groups.
In ADOA, average sensitivity increased with increasing GC-IPL thickness (r = 0.38, p = 0.012). There was no relationship between average sensitivity and the average RNFL thickness (r = 0.29, p = 0.06).
Stable fixation was found in 58% (25/43) of ADOA patients and 86% (42/49) of controls, relatively unstable fixation in 35% (15/43) of ADOA patients and 14% (7/49) of controls, whereas unstable fixation was found only in ADOA, where the prevalence was 7% (3/43) (Figure 2).
Predominantly macular central fixation location was found in 49% (21/43) of ADOA patients and 84% (41/49) of controls, poor central fixation location in 14% (6/43) of ADOA patients and 10% (5/49) of controls, whereas predominantly eccentric fixation location was found in 37% (16/43) of ADOA and 6% (3/49) of controls. The majority of patients, 53% (23/43) had a fixation pattern that covered the four superotemporal sectors and the central part. The same was true for 27% (13/49) of the healthy subjects. Fixation points that only covered the central field were seen in 7% (3/43) of ADOA patients and in 39% (19/49) of healthy controls. Only 2% (1/43) of the patients and 6% (3/49) of the controls had fixation points that covered the two inferonasal sectors and the central field. The remaining patients and controls had a more diffuse fixation pattern. No correlation was seen between fixation location and age, BCVA and average sensitivity in ADOA (Figure 2).
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