To examine retinal structure and changes in photoreceptor intensity after dark adaptation in patients with complete congenital stationary night blindness and Oguchi disease.
Prospective, observational case series.
We recruited 3 patients with complete congenital stationary night blindness caused by mutations in GRM6 , 2 brothers with Oguchi disease caused by mutations in GRK1 , and 1 normal control. Retinal thickness was measured from optical coherence tomography images. Integrity of the rod and cone mosaic was assessed using adaptive optics scanning light ophthalmoscopy. We imaged 5 of the patients after a period of dark adaptation and examined layer reflectivity on optical coherence tomography in a patient with Oguchi disease under light- and dark-adapted conditions.
Retinal thickness was reduced in the parafoveal region in patients with GRM6 mutations as a result of decreased thickness of the inner retinal layers. All patients had normal photoreceptor density at all locations analyzed. On removal from dark adaptation, the intensity of the rods (but not cones) in the patients with Oguchi disease gradually and significantly increased. In 1 Oguchi disease patient, the outer segment layer contrast on optical coherence tomography was 4-fold higher under dark-adapted versus light-adapted conditions.
The selective thinning of the inner retinal layers in patients with GRM6 mutations suggests either reduced bipolar or ganglion cell numbers or altered synaptic structure in the inner retina. Our finding that rods, but not cones, change intensity after dark adaptation suggests that fundus changes in Oguchi disease are the result of changes within the rods as opposed to changes at a different retinal locus.
Congenital stationary night blindness represents a heterogeneous group of inherited retinal disorders characterized by impaired night vision and can be inherited in an autosomal dominant, autosomal recessive, or X-linked fashion. Patients with congenital stationary night blindness have an abnormal rod electroretinogram (ERG) and an abnormal dark-adaptation curve. Additional ocular manifestations are variable, but can include reduced visual acuity, refractive error (commonly myopia but occasionally hyperopia), nystagmus, strabismus, and altered fundus appearance. Within congenital stationary night blindness with a normal-appearing fundus, X-linked and autosomal recessive congenital stationary night blindness may be subdivided further into complete and incomplete forms. Both forms are associated with an electronegative or negative ERG in response to a bright white flash in the dark-adapted eye, such that there is a reduced b-wave-to-a-wave ratio (ie, Schubert-Bornschein ERG). The discrimination between the complete and incomplete form is made according to whether there is a rod-specific ERG response to a dim light under dark adaptation.
In patients with complete congenital stationary night blindness, there is no detectable b-wave in the rod-specific ERG. Long-flash photopic ERG demonstrates attenuated b-wave amplitude with normal d-wave amplitude, consistent with involvement of the cone ON pathway as well. In patients with incomplete congenital stationary night blindness, there is a detectable rod-specific ERG, although the b-wave is reduced from normal. In addition, the long-flash photopic ERG demonstrates attenuated d-wave amplitude, consistent with impaired function of both the ON and OFF pathways. ERG evidence of inner retinal rod system dysfunction also may occur in autosomal dominant congenital stationary night blindness, but in association with normal cone ERGs. In other cases of autosomal dominant congenital stationary night blindness, ERG rod responses are attenuated with normal cone responses, but the standard bright flash response does not have a negative waveform ((ie, Riggs ERG).
Mutations in the NYX , GRM6 , TRPM1 , and GPR179 genes are associated with complete congenital stationary night blindness, being expressed in ON bipolar cells and encoding proteins that are part of the mGluR6 signaling cascade. Incomplete congenital stationary night blindness is associated with mutations in genes involved in glutamate release from the photoreceptors ( CACNA1F , CABP4 , and CACNA2D4 ). Mutations in genes encoding 3 components of the rod-specific phototransduction cascade all have been reported in association with autosomal dominant congenital stationary night blindness ( RHO , GNAT1 , and PDE6B ). Recent estimates suggest that approximately 20% of congenital stationary night blindness patients have an unknown genetic basis for the disease.
Oguchi disease is a distinctive form of autosomal recessive congenital stationary night blindness with an abnormal-appearing fundus first described in the Japanese population. Patients with Oguchi disease have a distinct dark adaptation curve compared with the other forms of congenital stationary night blindness; following a bleach, rod sensitivity recovers to normal after a prolonged period (>2 hours) of dark adaptation. Patients with Oguchi disease have no rod a-wave or b-wave, except after prolonged dark adaptation, when a normal single-flash response can be obtained. The fundus has a diffuse or patchy radiant appearance, described as a golden-yellow tapetal-like metallic sheen, with normal fundus coloration restored after prolonged dark adaptation (Mizuo-Nakamura phenomenon). Two genes have been implicated in Oguchi disease, both of which are involved in rod phototransduction: SAG (encoding arrestin) and GRK1 (encoding rhodopsin kinase).
Significant progress in our understanding of how the various mutations implicated in the subtypes of congenital stationary night blindness affect retinal structure has come from examination of mouse models. For example, Pardue and associates showed that the nob mouse had normal cytoarchitecture in the presence of disrupted retinal transmission. Rhodopsin kinase knockout mice have been shown to undergo light-induced rod degeneration. Across mouse models of congenital stationary night blindness, differences in retinal morphologic features have been noted between mutations in genes expressed presynaptically (incomplete congenital stationary night blindness) and mutations in genes expressed postsynaptically (complete congenital stationary night blindness). Despite this progress using mouse models of the disease, structural data in humans are sparse. A histopathologic report of a patient with congenital stationary night blindness of unknown genetic origin demonstrated normal rod and cone structure, and a second study of a patient with suspected congenital stationary night blindness showed normal rod outer segment structure. A recent investigation using spectral-domain optical coherence tomography (SD OCT) demonstrated thinned retinas in 5 patients with incomplete congenital stationary night blindness, 3 of whom had a mutation in the CACNA1F gene. Despite being more rare than congenital stationary night blindness without fundus abnormalities, anatomic studies in patients with Oguchi disease are rather numerous. Usui and associates reported diffuse, fine white particles on helium-neon laser imaging in the light-adapted retina that disappeared on dark adaptation and reappeared gradually on exposure to light, suggesting that accumulation of an abnormal product in the outer retina causes the golden metallic fundus appearance. A histopathologic report by Kuwabara and associates suggested the existence of an abnormal layer between the retinal pigment epithelium (RPE) and the outer segments of the photoreceptors along with displaced cone nuclei. More recently, SD OCT studies in patients with Oguchi disease have reported a variety of outer retinal findings, including shortened rod outer segments, hyperreflective outer segment regions that disappeared after extended dark adaptation, thinning of the parafoveal outer nuclear layer (ONL), and presumed shortening of photoreceptor outer segments (evidenced by the disappearance of the extrafoveal inner segment/outer segment [IS/OS] line in the partly dark-adapted state). Although congenital stationary night blindness (including the Oguchi variant) traditionally is believed to be nonprogressive, accumulating evidence suggests that not all cases are truly stationary. Thus, careful re-examination of retinal morphologic features with advanced retinal imaging tools may disclose previously unrecognized anatomic manifestations of congenital stationary night blindness.
We sought to apply noninvasive, high-resolution imaging tools and quantitative analyses to examine retinal structure in patients with complete congenital stationary night blindness resulting from mutations in GRM6 or from Oguchi disease caused by mutations in GRK1. We also examined the intensity of individual rod and cone photoreceptors under light-adapted conditions as well as after a prolonged period of dark adaptation. Our data indicate that defects in the phototransduction process can alter photoreceptor reflectivity and demonstrate altered inner retinal morphologic features in patients with GRM6 mutations. This quantitative imaging approach should prove useful in examining retinal structure in other diseases.
This prospective, observational case series included 3 patients diagnosed with complete congenital stationary night blindness caused by mutations in GRM6 , 2 brothers with Oguchi disease caused by mutations in GRK1 and 1 visually normal, healthy adult. A brief summary of the patient characteristics is provided in the Table . Color vision was assessed using the 2002 edition AO-HRR pseudoisochromatic plates (Richmond Products, Inc, Albuquerque, New Mexico, USA). Axial length measurements were obtained using an IOL Master (Carl Zeiss Meditec Inc, Dublin, California, USA) for calibrating the lateral scale of all retinal images. For all imaging sessions, each patient’s eye was dilated and accommodation was suspended using 1 drop each of phenylephrine (2.5%) and tropicamide (1%).
|Patient||Age (y)||Sex||BCVA (Right Eye; Left Eye)||Refractive Error, diopters (Right Eye; Left Eye)||Phenotype||Affected Gene||Allele 1||Allele 2||Source|
|JC_0677||23||F||20/12; 20/12||−1.50 sphere; −1.50 sphere||Normal||ND||ND||ND||Current study|
|DH_0491||41||M||20/16; 20/16||No correction||Oguchi disease||GRK1||c. 1129G → C, p.Ala377Pro||c.1139T → A, p.Val380Asp||Current study|
|DH_0819||47||M||20/32; 20/25||+0.25 sphere; no correction||Oguchi disease||GRK1||c. 1129G → C, p.Ala377Pro||c.1139T → A, p.Val380Asp||Current study|
|JC_0550||30||F||20/16; 20/16||−0.75/0.75 × 30; −0.50 sphere||Complete congenital stationary night blindness||GRM6||c.172G → A, p.Gly58Arg||IVS2-1G → T (c.722-1G → T)||Current study|
|JC_0682||13||F||20/32; 20/20||−1.00/−1.25 × 105; −0.50/−2.50 × 50||Complete congenital stationary night blindness||GRM6||c.2267G → A, p.Gly756Asp||c.2267G → A, p.Gly756Asp||Ref.|
|JC_0684||32||F||20/30; 20/30||−0.50/−1.00 × 80; −0.50/−1.50 × 105||Complete congenital stationary night blindness||GRM6||c.2029C → T, p.Arg677Cys||c.2029C → T, p.Arg677Cys||Ref.|
The methods of molecular genetic analysis and genotypes for two of the patients with complete congenital stationary night blindness (JC_0682 and JC_0684) were reported previously. For the third patient with complete congenital stationary night blindness (JC_0550), genomic DNA was isolated from whole blood using the ArchivePure DNA extraction kit (5 Prime, Gaithersburg, Maryland, USA). Polymerase chain reaction analysis was used to amplify the GRM6 gene using the primers previously described by Dryja and associates. For exon 1, exons 2 and 3, exons 5 and 6, and exon 8 (both the 5′ and 3′ regions) TaKara LA enzyme with the 2XGC buffer (TaKaRa Bio Inc, Otsu, Japan) was used according to the manufacturer’s instructions. All reactions were subjected to an initial denaturing step of 94 C for 1 minute and a final extension step of 72 C for 10 to 15 minutes. Between the initial and final incubations, PCRs for exon 1, exons 2 and 3, and both PCRs for exon 8 were subjected to 30 cycles of denaturing at 94 C for 30 to 45 seconds, annealing at 58 C for 1 minute, and extension at 72 C for 2 minutes. The PCR for exons 5 and 6 used the same denaturing, extension, and annealing temperatures, but the times were 45 seconds for denaturing, 2 minutes and 30 seconds for annealing, and 3 minutes and 45 seconds for extension. For exons 4, 7, 9, and 10, a hot-start PCR using the AmpliTaq Gold PCR kit (Applied Biosystems, Foster City, California, USA) was carried out according to the manufacturer’s instructions. All reactions were subjected to an initial denaturing step at 95 C for 5 minutes and a final extension step at 72 C for 10 to 15 minutes. Between the initial and final steps, reactions were subject to 30 cycles of denaturing at 94 C for 30 seconds; annealing for 30 seconds at 56 C for exon 9, 58.5 C for exons 4 and 7, and 60 C for exon 10; and extension at 72 C for 45 seconds. PCR products were sequenced directly using the same primers that were used for amplification and additional primers described by Dryja and associates for sequencing exons 5 and 6. Sequencing was performed using BigDye Terminator version 3.1 (Applied Biosystems) and AmpliTaq FS (Applied Biosystems), and sequencing reactions were analyzed on an ABI 7500 capillary sequencer. All sequencing was bidirectional.
DNA was extracted from 2 patients with Oguchi disease and their mother by following the manufacturer’s specifications for whole blood DNA extraction using Autopure LS instrument (Gentra Systems, Minneapolis, Minnesota, USA). The entire coding region of the GRK1 gene was sequenced in the proband using fluorescent dideoxynucleotides on an ABI 3730 automated sequencer (Applied Biosystems). Mutations were identified by the approximately equal peak intensity of 2 fluorescent dyes at the mutant base. Mutant nucleotides identified in the proband (DH_0491) then were evaluated in his sibling (DH_0819) and mother. All sequencing was bidirectional.
Spectral-Domain Optical Coherence Tomography
Volumetric images of the macula and optic disc were acquired using the Cirrus HD-OCT (Carl Zeiss Meditec, Inc). Individual volumes were nominally 6 × 6 mm and consisted of 128 B-scans (512 A-scans/B-scan). Retinal thickness was calculated using the built-in macular analysis software (software version 5.2), which is determined automatically by measuring the distance between the inner limiting membrane (ILM) and RPE boundaries. Thickness of the ganglion cell layer (GCL) plus inner plexiform layer (IPL) was calculated using the built-in analysis software (software version 6.0), which was determined by measuring the distance between the retinal nerve fiber layer−GCL interface and the IPL−inner nuclear layer interface.
We also acquired additional high-density SD OCT line scans through the fovea, with a sampling of 1000 A-scans/B-scan and 100 repeated B-scans (Bioptigen Inc, Durham, North Carolina, USA). Up to 40 B-scans from these 100-scan sequences were registered and averaged to reduce speckle noise in the image as described previously. A total of 6 layers were segmented manually in the averaged images (ILM, outer plexiform layer [OPL], external limiting membrane [ELM], inner segment ellipsoid [ISe], RPE1, and RPE2). Layer naming is based on the recent work of Spaide and Curcio. The innermost peak of the outer photoreceptor complex invariantly is attributed to the ELM. The second layer often has been referred to as the IS/OS layer, although herein we adopt the recent nomenclature of Spaide and Curcio, who through a detailed analysis attribute this band to the ellipsoid portion of the inner segment (ISe). The third layer is attributed to the RPE contact cylinder, whereas the fourth band is attributed to the apical portion of the RPE. Because the RPE contributes to both of these bands, we simply refer to the third and fourth layers as RPE1 and RPE2, respectively. Importantly, the distance between RPE1 and RPE2 is not the thickness of the RPE cell, but rather provides a convenient way to communicate the presence of multiple bands associated with the RPE.
Total retinal thickness (ILM to RPE2 distance), inner retinal thickness (ILM to OPL distance), and ONL thickness (OPL to ELM distance) was calculated using a custom Matlab program (Mathworks, Natick, Massachusettes, USA) and was compared with previously reported values for 93 controls (42 male, 51 female), with an average age of 25.7 ± 8.2 years (range, 11 to 40 years). For one of the patients (DH_0491), we acquired SD OCT images under light- and dark-adapted states. Longitudinal reflectivity profiles were generated as described previously to assess the relative intensity of the different retinal layers under these conditions.
Adaptive Optics Scanning Light Ophthalmoscope
Images of the photoreceptor mosaic were obtained using a previously described adaptive optics scanning light ophthalmoscope. The wavelength of the super luminescent diodes used for retinal imaging were 680 and 775 nm, subtending a field of view of 0.96 × 0.96 degrees. Separate image sequences of 100 to 200 frames each were acquired at various parafoveal and perifoveal locations. Parafoveal images usually were acquired by instructing the patient to fixate on one of the corners of the raster scan square, while the perifoveal images were acquired using an external fixation target. To increase the percentage of recorded frames with useable data, the image acquisition software had an active blink removal algorithm that discarded frames with a mean intensity below a specified threshold value.
To correct for intraframe distortions within the frames of the raw image sequence resulting from the sinusoidal motion of the resonant optical scanner, we estimated the distortion from images of a stationary Ronchi ruling and then resampled each frame of the raw image sequence over a grid of equally spaced pixels. After desinusoiding, a reference frame was selected manually from within each image sequence for subsequent registration using custom software. Registration of frames within a given image sequence was performed by dividing the frame of interest into strips, aligning each strip to the location in the reference frame that maximizes the normalized cross-correlation between them. The discrete set of locations for the image strips was used to generate a continuous transformation that would register the recorded frame to match the reference frame. After all the frames were registered, the 50 frames with the highest normalized cross-correlation to the reference frame were averaged to generate a final registered image with an increased signal-to-noise ratio for subsequent analysis.
Cell density was calculated over a 55 × 55-μm sampling window using a previously described semiautomated algorithm implemented in Matlab, with an additional modification that during the user review of the automated cell identifications, a cell removal tool also was available. Intrasession repeatability of the algorithm on parafoveal cone images was reported to be 2.7%. Because the rods greatly outnumber the cones in the perifoveal images, we used the algorithm to identify the rods by removing any cones during the user review step. Estimates of cone density for these images were obtained using manual identification of the large, coarsely spaced cones.
Analyzing Photoreceptor Reflectivity
Five of the 6 patients (JC_0677, DH_0491, DH_0819, JC_0682, and JC_0684) underwent a second imaging session using the adaptive optics scanning light ophthalmoscope. First, the eye of interest was aligned and optimal focus was established for a single parafoveal and a single perifoveal location (approximately 10 degrees of eccentricity). The eye then was patched and the patient was placed in a dark room for 2 hours. A second drop each of phenylephrine (2.5%) and tropicamide (1%) was administered approximately 15 minutes before the end of dark adaptation; this was done with care not to compromise the dark-adapted state of the retina.
Immediately on removal from dark adaptation, we began imaging with the 775-nm light source. The only additional illumination in the room was that from 2 computer monitors. Numerous image sequences were acquired at the predetermined parafoveal and perifoveal locations throughout a period of at least 45 minutes. Midway through this period, the room lights were turned on so as to return the eye fully to a light-adapted state. Although the light levels and adaption of the eye were not controlled precisely during the 45-minute imaging session, it was the same for the 5 patients, allowing us to make relative comparisons between the patients.
The reflectivity of individual cells was analyzed as previously described, and the method is summarized herein. The raw video sequences were processed as described above, and for a given imaging location, the average images from each time point were registered to each other using an affine transformation (i2kRetina; Dual Align LLC, Clifton Park, New York, USA). This aligned image stack then was cropped to a common area, a reference frame was selected, and the image stack then went through strip registration, as described above. Finally, the image series was normalized to the temporal mean of the nonzero portions of the stack. This aligned image stack then was averaged so as to generate a single reference image for each location. These images then were used to determine preliminary cone and rod coordinate locations. The position of perifoveal rods was determined by manual selection, whereas the position of parafoveal cones was identified using a modified version of a previously described semiautomated algorithm, which also allowed manual addition and subtraction of cones missed or selected in error. From these preliminary coordinates, the final coordinates were determined using custom Matlab software that identified the local maximum within a 3 × 3-pixel (approximately 1.25 × 1.25 μm) region around the initial cone (or rod) coordinate. Perifoveal cone photoreceptors show a multimodal intensity pattern that varies in both intensity and structure over time, a phenomenon not yet fully understood. In addition, the small number of cones (< 50) present in the perifoveal images would make any global conclusion about their reflectance behavior over time difficult. As such, we did not analyze the reflectivity of the perifoveal cones.
The final coordinates were adjusted for each frame within the aligned image stack to compensate for small errors in image registration. This was done by first projecting a mask for each cell through the aligned image stack. A square 3 × 3-pixel and a circular 5-pixel diameter mask were used for rods and cones, respectively. For each frame, each cells’ mask was repositioned to a local maximum, which never occurred more than 1 pixel away from the original final coordinate. Because the time stamp for each image was recorded, we could generate plots of intensity as a function of time for the parafoveal cones and perifoveal rods in each of the 5 patients.
Clinical Findings and Genetic Analysis Results
A summary of the clinical characteristics and molecular diagnosis for each patient is provided in the Table . Two of the complete congenital stationary night blindness patients (JC_0682 and JC_0684) were described previously, with their initial diagnosis based on nyctalopia and electrophysiologic findings. Both had homozygous mutations in GRM6 , p.Gly756Asp (JC_0682) and p.Arg677Cys (JC_0684). The third complete congenital stationary night blindness patient (JC_0550) was misdiagnosed with retinitis pigmentosa at age 7 years and sought treatment from us with a history of poor night vision as far as she could recall and no difficulty with her peripheral vision. A full-field ERG showed selective b-wave reduction under scotopic conditions using a bright flash stimulus. The single-flash photopic response showed evidence of an a-wave and absence of a b-wave, consistent with the loss of the ON-bipolar cell response. An isolated rod response was not detectable. She was found to be heterozygous for a mutation in GRM6 (p.Gly58Arg), which previously was reported to be pathogenic. She also was found to be heterozygous for a mutation at the intron 2−exon 3 junction within GRM6 that would be expected to result in a splicing defect (G → T at the 3′ end of intron 2).
The two brothers with Oguchi disease were found to harbor novel compound heterozygous mutations in GRK1 , p.Ala377Pro and p.Va1380Asp. Sequencing of their mother’s DNA revealed her to carry only the p.Ala377Pro variation, indicating that the Oguchi patients’ variations lie on different alleles. Neither of these variations has been observed in white (n = 81), Hispanic (n = 89), or black (n = 78) controls (unpublished observations). Also, neither of these alleles was observed among the more than 3000 exomes from white persons and more than 1800 exomes from black persons that are visible on the Exome Variant Server ( evs.gs.washington.edu/EVS/ ) of the National Heart, Lung and Blood Institute Exome Sequencing Project. DH_0491 originally had nyctalopia since early childhood, although he reported his night vision improved after 45 to 60 minutes of sustained dark adaptation. Maculae and peripheral fundi were normal with the exception of a diffuse hyperreflective sheen from the retinal surface on indirect ophthalmoscopic illumination. After 105 minutes of dark adaptation, the hyperreflective sheen was noted to be absent ophthalmoscopically, but reappeared after only 5 to 10 seconds of illumination (consistent with the Mizuo-Nakamura phenomenon). Fundus examination of his brother (DH_0819) revealed fundus discoloration typical of Oguchi disease.
Reduced Retinal Thickness in Complete Congenital Stationary Night Blindness and Normal Retinal Thickness in Oguchi Disease
Topographic retinal thickness maps obtained from the Cirrus HD-OCT showed reduced retinal thickness (at least 5 of 9 Early Treatment Diabetic Retinopathy Study segments with retinal thickness of less than 1% of normal distribution percentiles) in the extrafoveal region in the 3 complete congenital stationary night blindness patients with GRM6 mutations ( Figure 1 ). The 2 Oguchi disease patients with GRK1 mutations had predominantly normal retinal thickness, with each patient having a single Early Treatment Diabetic Retinopathy Study segment (superior retina, outer ring) with a thickness that was less than 1% of the normal distribution percentiles. Visual inspection of the entire volume scans disclosed no anomalies of any of the retinal layers, and detailed examination of high-resolution horizontal line scans demonstrated the presence of all retinal layers ( Figure 2 ).
To investigate the reduction in retinal thickness in more detail, we examined the thickness of the inner retina and the ONL from the high-resolution horizontal line scans in Figure 2 and compared it with previously reported normative data from 93 individuals. As shown in Figure 3 , all 5 patients had normal foveal thickness, although the complete congenital stationary night blindness patients with GRM6 mutations had reduced total retinal thickness (less than 2 standard deviations from the mean) outside the fovea, consistent with the results from the Cirrus volumetric data. The 3 patients with GRM6 mutations had normal ONL thickness, but reduced inner retinal layer thickness ( Figure 3 ). The GCL plus IPL analysis on the Cirrus macular volumes revealed significant thinning in the 3 patients with GRM6 mutations (the average GCL plus IPL thickness was 64 μm for JC_0550, 65 μm for JC_0682, and 59 μm for JC_0684). These data indicate that the thinning of the GRM6 retina is the result of inner retinal defects, as opposed to photoreceptor loss, and involves the GCL. Both patients with Oguchi disease caused by GRK1 mutations had normal ONL and inner retinal layer thickness ( Figure 3 ), and the average GCL plus IPL thickness was 70 μm and 76 μm for DH_0491 and DH_0819, respectively.
Integrity of the Photoreceptor Mosaic in Complete Congenital Stationary Night Blindness and Oguchi Disease Assessed With Adaptive Optics Retinal Imaging
All patients had a complete contiguous cone mosaic across the foveal region, with representative parafoveal images shown in Figure 4 . Parafoveal (approximately 0.6 degrees from fixation) cone density values were 73 058 cones/mm 2 (DH_0491), 83 966 cones/mm 2 (DH_819), 85 289 cones/mm 2 (JC_0682), 88 264 cones/mm 2 (JC_0550), 80 331 cones/mm 2 (JC_0684), and 83 967 cones/mm 2 (JC_0677). These cone density measurements were within previously measured normative values from our laboratory (mean ± standard deviation, 72 528 ± 8539 cones/mm 2 ). Perifoveal photoreceptor mosaic images showed normal rod and cone photoreceptor mosaics for all patients ( Figure 5 ). Rod density values measured at approximately 10 degrees from fixation were 95 868 rods/mm 2 (DH_0491), 79 669 rods/mm 2 (DH_0819), 98 512 rods/mm 2 (JC_0682), 82 644 rods/mm 2 (JC_0550), 89 917 rods/mm 2 (JC_0684), and 100 826 rods/mm 2 (JC_0677). Cone density values measured at approximately 10 degrees from fixation were 6612 cones/mm 2 (DH_0491), 7933 cones/mm 2 (DH_819), 8595 cones/mm 2 (JC_0682), 11 900 cones/mm 2 (JC_0550), 6281 cones/mm 2 (JC_0684), and 7934 cones/mm 2 (JC_0677). These values also were consistent with normal values reported previously from histologic and imaging studies.