Macular Degeneration Spectrum in ABCA4-RD
-RD can be associated with the full spectrum of macular health from the nearly normal retina to complete chorioretinal atrophy.1–12
illustrates some of the features of macular degeneration in ABCA4
-RD using NIR-RAFI imaging to define RPE melanin abnormalities and OCT imaging to define retinal abnormalities in five patients. P6 and P24 () exemplify some of the milder stages of macular disease involving the fovea (~5°-diameter circle centered on the foveola) or parafovea (~2°-wide annulus around the fovea) or both. There can be loss of RPE melanin associated with abnormal inner and outer segments; foveal regions can have partially retained photoreceptor nuclei (B) or complete atrophy (C). More advanced disease stages include greater involvement of photoreceptors and the RPE of the perifovea (~5°-wide annulus around the parafovea) as exemplified by P5 (D) or near complete loss as exemplified by P20 and P25 (E, F). The extramacular retina can appear to be normal (B, C), mildly involved (D), or severely affected (E, F). In severe stages of ABCA4
-RD, an annular parapapillary region is often retained immediately outside the macula. Previous studies have shown the relative preservation of this region structurally and functionally.9,10,44,45
P5, P20, and P25 illustrate the relative retention of the RPE melanin and photoreceptor structure in the parapapillary region (D–F).
Figure 1. Spectrum of macular disease severity caused by mutations in the ABCA4 gene. NIR-RAFI (upper) and OCT (lower) imaging results shown in a healthy subject (A) and in five patients (B–F). Red horizontal lines on NIR-RAFI images demarcate the location (more ...)
Macular Function Testing Appropriate for the ABCA4 Disease Spectrum
What is the appropriate sampling grid to use with retina-tracking perimetry to evaluate macular function in ABCA4
-RD? Considering the spectrum of macular disease severity (), the parafoveal region should be densely sampled to detect any abnormal function at early disease stages, when most of the retina is normal. At intermediate stages of the disease, the perifoveal region should be densely sampled to reliably measure the expected centrifugal expansion of degeneration over time. At later stages of the disease, the parapapillary region should be sampled to register any detectable visual function. Although the parapapillary region is not strictly within the traditional definition of macula, it often becomes the preferred retinal locus for ABCA4
-RD patients and thus represents an important “central” component of their vision.9
Densely sampling all macular regions in two dimensions would be technically possible but practically onerous because of the many hours of testing time that would be required. Thus standard patterns sample the macula at 2° intervals.14,15,17,20,21,23,24,27,29
An alternative approach involves a higher sampling density along one or more carefully selected meridians. We have previously proposed9,10
using an FPP along the horizontal meridian extending from the fovea nasally and crossing the ONH as one such alternative for ABCA4
Sensitivity Values along the FPP in Patients with ABCA4-RD and RP
FPP sensitivities in a representative ABCA4
-RD patient are shown in a pseudocolor scale registered to the NIR-RAFI results (A). The patient is not able to perceive the brightest available stimuli within the central atrophic region as well as within her pBS. Between 6° and 10° eccentricity from the fovea, she has abnormally reduced but detectable vision (A). We have previously reported the existence of a dysfunctional penumbra in structurally normal-appearing regions surrounding scotomas in other patients with ABCA4
Of note, at and near the parapapillary boundary, she has normal sensitivity (A).
Figure 2. Sampling of the macular vision along the FPP with retina-tracking microperimetry. (A) Sensitivity values along the FPP coded to a pseudocolor scale and overlaid onto the NIR-RAFI image of a representative ABCA4-RD patient, P23. Gray bars represent loci (more ...)
To better understand the range of macular dysfunction measurable, FPPs were performed in three groups of patients. The first group was the subset of ABCA4-RD patients (n = 14, ages 12–58; ) with retained foveal fixation (B). At the parafovea, all sensitivities were abnormally reduced or undetectable, whereas at the perifovea the sensitivities could be normal or near normal. There was a tendency for sensitivities to increase with eccentricity along the FPP toward the parapapillary region (B). The second group was a more commonly encountered subset of ABCA4-RD patients (n = 17, ages 11–60; ) who fixated using an extrafoveal retinal region (C). Parafoveal vision tended to be more severely affected in this group of patients, but sensitivities also increased with eccentricity as the parapapillary region was approached. Across the group of all ABCA4-RD, the mean eccentricity of first measurable threshold with MP1 was 6.2° (±3.2°), and the eccentricity of the central edge of the physiological blind spot was 13.7° (±1.5°) providing a 7° (±3.4°) wide region of measurable thresholds. The third group was composed of patients with RP (n = 7, ages 17–67; ) and with foveal and stable fixation. As expected, many of the RP patients showed normal or near-normal sensitivities at the parafoveal regions with increasing abnormalities with eccentricity (D), thus forming a control group to evaluate statistics of parafoveal vision often lacking in ABCA4-RD patients.
Source of the Visual Function Measured by Retina-Tracking Microperimetry
We determined the relationship of visual function measured with retina-tracking microperimetry to absolute sensitivities measured in a subset of patients; this subset had foveal fixation and thus allowed comparison with conventional projection perimetry at the same retinal locations along the FPP. Structural and functional data from P12, a representative of the group of ABCA4-RD patients with foveal fixation, are shown in A. OCT shows abnormally thinned foveal and parafoveal regions with no detectable inner or outer segments but with still detectable ONL remaining at the fovea. MP1 sensitivity loss was shown as 20 dB in the parafoveal region where the patient was not able to see the brightest stimulus. Absolute thresholds show a 25-dB sensitivity loss consistent with MP1 results. At 4° of eccentricity, there are detectable but abnormal inner and outer segments and a thin ONL. MP1 sensitivity loss is 4 to 7 dB, and this is matched by the absolute sensitivity loss of 5 dB. From 6° eccentricity nasally, a nearly normal retinal structure is matched by normal or nearly normal sensitivities with both methods. Comparison of the absolute sensitivities to 500- and 650-nm stimuli showed rod (R) mediation at all loci except at 2°, which was indeterminate (A). Rod mediation of absolute thresholds at 650 nm suggests that MP1 thresholds could also be mediated by rods.
Figure 3. Comparison of sensitivity losses estimated with retina-tracking microperimetry and free viewing perimetry at corresponding retinal locations in the subset of patients with foveal fixation. (A, B) Results from representative patients with ABCA4-RD (A) (more ...)
A representative of the group of RP patients with foveal fixation is P5 (B). There is relative structural preservation in the foveal and parafoveal regions surrounded by increasing photoreceptor degeneration with greater eccentricity toward the ONH, contrasting the pattern typically seen in the maculopathy of ABCA4-RD. Up to 6° eccentric MP1 thresholds and absolute thresholds to 650 nm were within the normal range. From 7° eccentricity onward, there were increasing losses of sensitivity, but estimates from MP1 thresholds and absolute thresholds were similar (B). Comparison of the absolute sensitivities showed mixed (M) mediation (650-nm stimulus detection by cones and 500-nm stimulus detection by rods), implying that MP1 thresholds to red stimuli would be expected to be mediated by cones.
Data from all available retinal loci from all patients with foveal fixation (45 loci from 9 patients) were used to summarize the relationship between the loss of absolute sensitivity at 650 nm as measured with dark-adapted projection perimetry and the loss of increment sensitivity to the red stimulus as measured by microperimetry (C). Because of the differences in the sizes of the stimuli, three neighboring MP1 thresholds corresponded to each location of the absolute thresholds. For the comparison, the smallest loss of microperimetric sensitivity among the three neighbors was used assuming, in locally heterogeneous regions, the healthiest patch of retina illuminated would dominate the sensitivity to the larger stimulus. There were no obvious clusters formed by the diagnostic category (ABCA4-RD vs. RP) or by the type of photoreceptors mediating the absolute thresholds. Overall there was a strong (r2 = 0.74) linear correlation. The associated regression line had an intercept near zero (1.9 dB), and the slope was 0.83, suggesting a small underestimate by the microperimetric increment sensitivity loss compared with the absolute sensitivity loss (C). Ideally the relationship may be expected to have a unity slope. Small deviations from unity slope may be related to the limited sample size. Additionally or alternatively, the smaller stimulus size and the dim mesopic background used in the microperimetry system could have contributed to the small deviation from unity slope.
Test-Retest Repeatability of Retina-Tracking Microperimetry Thresholds
To evaluate variability of microperimetric testing with the FPP pattern, we obtained two sets of thresholds either on the same day (n = 21) or within a short interval (6.9 ± 2.8 months; n = 8). P7 is a representative of the subset (n = 7) of ABCA4-RD patients with foveal fixation (A). The NIR-RAFI image shows the central area of demelanization surrounding a small island of foveal preservation (A, upper; foveal island is not visible because of overlaid fixation locations). The mean fixation location (A, white dot) is at the fovea, and a circle of 0.9° radius encompasses 95% of fixation variation in this patient (A, white circle). The two sets of FPPs obtained show reasonable concordance (A, lower) with test-retest differences ranging up to 4.7 dB at individual locations. A representative of the subset (n = 15) of ABCA4-RD patients with extrafoveal fixation is P31 (B). The NIR-RAFI image shows a central region of RPE demelanization that has unmasked the choroidal melanin autofluorescence evidenced by visibility of choroidal blood vessels (B, upper). The mean fixation location is 6° superior to the anatomic fovea, and a larger circle of 3.1° radius is required to encompass the fixation instability (B, white dot and white circle). Two sets of FPPs obtained on the same day show high concordance (B, lower) with test-retest differences ranging up to 2 dB at individual locations.
Figure 4. Test-retest variability along the FPP using retina-tracking microperimetry. (A, B) NIR-RAFI images of P7 and P31 representing ABCA4-RD patients with foveal and extrafoveal fixation, respectively. Overlaid are the fixation loci (black crosses) and the (more ...)
Next, we considered all available test locations with a measurable sensitivity (n = 360) among ABCA4-RD patients (n = 22). The TRTSD was 1.52 dB, yielding 95% test-retest limits of ±4.21 dB for a single test location measured twice. The estimate of TRTSD was almost identical (P = 0.92) in ABCA4-RD patients with foveal (TRTSD, 1.53 dB; 145 points, 7 patients) and extrafoveal (TRTSD, 1.52 dB; 215 points, 15 patients) fixation. The test-retest variance component from foveally fixating RP patients (102 points, 7 patients) was smaller (TRTSD, 1.15 dB) but not significantly so (P = 0.495). The signed test-retest difference was not statistically different from zero (P = 0.084), ruling out a major learning effect.
Next we queried whether the overall repeatability coefficient estimated from all test locations in all ABCA4
-RD patients was a good descriptor of expected variability when patient-specific and locus-specific parameters were considered. In terms of retinal location, most of the ABCA4
-RD data clustered in the perifoveal eccentricities, but there was no obvious change in variability as a function of eccentricity from the fovea (C; r2
relating TRTSD to eccentricity was 2%; P
= 0.529). In terms of mean thresholds, Bland-Altman plots of test-retest difference against mean sensitivity showed no significant relationship (D; the r2
relating TRTSD to mean sensitivity was <1%; P
= 0.68). Next we considered the relationship between variability and local slope (Supplementary Fig. S1A, http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.11-8415/-/DCSupplemental
) to evaluate whether variability increases at transition zones between regions of normal or near-normal sensitivity and scotomas. A mixed linear model analysis found no effect (P
= 0.914) of local slope on TRTSD. Among the person-specific parameters was age at the time of testing. With the exception of a fairly large variability in one young patient, no relationship with age was evident on visual inspection (Supplementary Fig. S1B, http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.11-8415/-/DCSupplemental
), and no correlation was found (including that young patient's observations) between TRTSD and age (r2
= 0.02; P
= 0.593). Similarly, the r2
values relating TRTSD to fixation instability (Supplementary Fig. S1C, http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.11-8415/-/DCSupplemental
) and eccentricity of the fixation location (Supplementary Fig. S1D, http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.11-8415/-/DCSupplemental
) were 0.01 (P
= 0.72) and 0.02 (P
= 0.53), respectively, and indicated no relationship. Results in RP patients (C, D, Supplementary Fig. S1A, http://www.iovs.org/lookup/suppl/doi:10.1167/iovs.11-8415/-/DCSupplemental
) were generally similar to those from ABCA4
Variability at the Border of Deep Scotomas
There are at least two important reasons for evaluating variability of microperimetric thresholds near deep scotomas. First, in a clinical trial involving ABCA4
-RD patients, the expected rate of (centrifugal) expansion of the scotoma boundary may contribute strongly to the design of outcome measures of visual function. Second, previous studies46,47
have suggested that the variability of psychophysical thresholds increases near deep scotomas. It is important to note that our analyses (estimating variability as a function of local slope) did not include retinal loci at the immediate boundaries of deep scotomas because, by definition, a value for the slope at these locations is indeterminate. Our FPP test, designed to take advantage of the known boundary at the pBS and the relative parapapillary preservation of vision in most ABCA4
-RD, afforded the opportunity to evaluate variability as a function of distance from a deep scotoma. First, the eccentricity of the temporal papillary boundary intersecting the FPP was estimated by examining the NIR fundus image obtained during MP1 testing and determining the eccentricity of the first nondetectable threshold location at or near this boundary. The papillary boundary was 13.7° ± 1.2° and 13.7° ± 1.7° eccentric to the fovea for groups of ABCA4
-RD patients with and without foveal fixation, respectively. Next, MP1 thresholds for each subject were reassigned to an alternative coordinate system with an origin starting at the temporal papillary boundary with increasingly negative eccentricities toward the fovea (). Normal results were analyzed equivalently (papillary boundary at 13.3° ± 0.8°), and limits of normal sensitivity were determined along the same alternative coordinate system.
Figure 5. Test-retest differences in the immediate vicinity of the pBS. (A, B) Microperimetric sensitivities replotted as a function of the distance from the temporal papillary boundary defining the start of the pBS in ABCA4-RD patients with (A) or without (B) (more ...)
Individual sensitivity profiles in the group of ABCA4-RD patients with foveal fixation (A) and with extrafoveal fixation (B) are shown. By definition, there is a steep increase in sensitivity within 1° of the papillary boundary reaching normal limits in the majority of the patients. Test-retest differences plotted along the alternative coordinate system allow examination of variability in the vicinity of the deep scotoma formed by the pBS (C). Qualitatively, the overall 95% test-retest limits (C, solid lines) appeared to encompass the data well and corresponded to the ±2 SD limits calculated at each eccentricity (C, dashed lines). In a mixed linear model analysis, eccentricity from the pBS had no effect on TRTSDs, either when included as a fixed effect (P = 0.63) or as a linear covariate (P = 0.97). In the model with eccentricity from the pBS included as a linear covariate, the slope relating TRTSD to eccentricity from the pBS was 0.017 (SE, 0.272), which is very close to zero.