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1 From the Medical Retinal Service and 2 Electrophysiology Department, Moorfields Eye Hospital; and the 3 Department of Visual Science, Institute of Ophthalmology, London; and the 4 Vitreo-Retinal Service, Ophthalmology Department, The Royal Liverpool University Hospital, Liverpool, United Kingdom.
| Abstract |
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METHODS. Thirty-one siblings from 15 families with SMDFFM were examined. Age of onset, visual acuity, and clinical features on fundus examination and fundus autofluorescence images, including presence or absence of central and peripheral atrophy and distribution of flecks, were recorded. In addition, electrophysiological studies were undertaken.
RESULTS. Large differences between siblings in age of onset (median, 12 years; range, 523 years) were observed in six of the 15 families studied, whereas in 9 families differences in age of onset between siblings were small (median, 1 year; range, 03 years). Visual acuity varied two or more lines among siblings in nine families. In 10 families (67%) siblings were found to have different clinical appearance on fundus examination and fundus autofluorescence images, whereas in 5 families (33%), affected siblings had similar clinical features. Electrodiagnostic tests were performed on affected members of 12 families and disclosed similar qualitative findings among siblings. In nine families there was loss of central function only; in two, global loss of cone function; and in one, global loss of cone and rod function.
CONCLUSIONS. In this series, although differences in age of onset, visual acuity, and fundus appearance were observed between siblings, electrophysiological studies demonstrated intrafamilial homogeneity in retinal function. The findings are difficult to reconcile with expression studies showing ABCR transcripts in rod photoreceptors but not in cones.
| Introduction |
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In SMDFFM there is wide variation in age of onset, clinical appearance, and severity of the disease.4 5 6 7 8 9 12 13 14 Furthermore, whereas in some patients the disease appears to be confined to the central retina, in others peripheral involvement ensues giving rise to very poor vision.4 9 This variation could be explained on the basis of different mutations in ABCR or the influence of other genes on the phenotype. Information regarding intrafamilial phenotypic homogeneity or heterogeneity would resolve this issue, but few such studies have been undertaken.4 5 6 14 Some reports have favored the existence of a similar pattern of involvement and severity of the disease within families.5 6 Others have described the coexistence of different phenotypes in members of the same family.4 9
The present study was designed to evaluate the phenotypic intrafamilial similarity or variation in SMDFFM.
| Methods |
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Patient demographics (age, sex), age of onset, duration of the disease, best corrected visual acuity, and fundus appearance (presence or absence of macular and peripheral atrophy, and distribution of flecks) were recorded. In addition, color fundus photographs and fundus autofluorescence images were obtained, and electrophysiological studies were undertaken.
The age of onset was defined as the age at which decreased visual acuity was first detected. The duration of the disease was calculated as the difference between the age at the examination and the age of onset.
Best corrected visual acuity was measured with Snellen visual acuity charts. Visual acuity was considered to be different between siblings when there was a difference of two or more lines with the better seeing eye.
Areas of atrophy and flecks were recorded by fundus autofluorescence imaging using published techniques.15 Autofluorescence images were compared with findings on fundus examination and color fundus photographs. Based on the presence or absence and extent of atrophy and presence and distribution of flecks within the retina, siblings were classified as having "similar" or "different" fundus appearance. The disease was considered to be confined to the posterior pole whenever atrophy or flecks were present only within the vascular arcades, with or without a few flecks nasal to the optic disc. The involvement was designated as peripheral whenever atrophy or flecks extended beyond the vascular arcades.
Twenty-two patients from 12 families underwent electrophysiological investigation. Protocols recommended by the International Society for Clinical Electrophysiology of Vision were used.16 17 Color contrast sensitivity, pattern and focal electroretinogram (ERG), full-field ERG (including rod-specific response, bright-white-flash mixed response, 30-Hz flicker response, and the photopic single-flash ERG), and electro-oculogram were performed. Qualitative functional differences between siblings were evaluated.
This study was approved by the ethics committee of Moorfields Eye Hospital and was conducted according to the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients.
| Results |
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Visual acuity was different between siblings in nine families. In 11
families (1, 2, 4, 7, 9, 10, 11, 12, 13, 14, and 15) the sibling with
longer duration of the disease and who usually also had an earlier age
of onset had the worse visual acuity (Table 1)
. Ten patients with a
median duration of the disease of 2 years (range, 015 years) had
visual acuity of 6/12 or better in at least one eye. Four of these
patients had marked asymmetry in visual acuity between eyes (
4
lines).
Areas of atrophy at the macula detected by fundus examination appeared as well-defined areas or multiple confluent foci of decreased signal compared with background on autofluorescence images. Those flecks that on biomicroscopy appeared to consist of an accumulation of white-yellow material at the level of the RPE were seen on confocal scanning laser ophthalmoscopic (cSLO) images as areas of increased signal compared with background. By contrast, when flecks appeared biomicroscopically as areas of depigmentation at the level of the RPE, fundus autofluorescence images disclosed focal areas of low-intensity signal compared with background. In some patients, areas of atrophy or flecks not detected biomicroscopically, usually located in the midperipheral retina, were easily recognized on cSLO images. In one patient (girl 8 years old, family 14) with decreased visual acuity but no obvious abnormality on fundus examination, cSLO disclosed focal areas of increased autofluorescence at the fovea.
In 10 families (67%) siblings had different fundus appearance (families 2, 3, 4, 8, 9, 10, 11, 12, 13, and 14; Table 1 ). In three families, the age at the time of the examination, the age of onset, and the duration of the disease were very similar between siblings (families 8, 9, and 13; Table 1 ). Figures 1 2 3 4 5 show fundus autofluorescence images of five of these sibling pairs (families 3, 4, 8, 12, and 13).
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In all patients tested (n = 21), pattern ERG (PERG) and
focal ERG showed very reduced (
0.5 µV) or abolished responses. Six
of these (nine eyes) had visual acuity of 6/12 or better (Table 1)
.
PERG and focal ERG abnormalities did not appear to be related to the
distribution of flecks. In addition, there was no close relationship
between the PERG amplitudes and the extent of atrophy. Some patients
with no atrophy had extinguished PERGs, and others with central atrophy
had residual (abnormal) activity.
Full-field ERG was performed in members of 12 families (25 patients). Eighteen patients of nine families (1, 2, 3, 4, 7, 8, 9, 12, and 13) had normal full-field ERGs implying that photoreceptor dysfunction was limited to the macula. Five patients of two families ( 6 and 10) had abnormal cone-derived responses with normal rod activity. Two siblings (family 5) had abnormal cone and rod ERGs. Electro-oculogram, performed in 23 patients from 11 families, was abnormal in 5 patients.
In all families siblings had similar qualitative electrophysiological abnormalities, although quantitative differences existed. Figure 7 shows scotopic rod-specific response, bright-white-flash mixed response, 30-Hz flicker, photopic single-flash ERG, and PERG of one sibling from families 4, 5, and 10.
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| Discussion |
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In the present study differences in age of onset, visual acuity, and fundus features were detected between siblings. A large difference in age of onset between siblings was found in 6 of the 15 families studied. Although the age of onset is subjective, being dependent on the patients ability to recognize visual symptoms, it appeared that the difference in age of onset observed in some families was real. Furthermore, in three families the diagnosis was established in the younger sibling first when the older affected sibling was still asymptomatic.
Given that the fundus appearance in patients with SMDFFM may change throughout the years,4 9 differences between siblings in the presence and distribution of fundus lesions could represent different stages in the progression of the disease. However, in this series differences in fundus appearance between siblings were detected even when the age, age of onset, and duration of the disease were very similar.
In 12 families in which electrodiagnostic studies were performed, the same qualitative functional abnormalities among siblings were detected whatever the disparity of age, age of onset, duration of the disease, or fundus appearance. It appeared that functional loss was either limited to the macula or generalized with loss of peripheral cone, or cone and rod function and that this was a characteristic of the disease in a single family. It appears that a similar conclusion can be drawn from the data published by Aaberg.4 If this is confirmed, it suggests that electrophysiological tests have a prognostic valuethat is, patients with early peripheral cone and rod involvement have a higher risk of development of peripheral visual loss and thus more severe disease.
Macular function was evaluated, in the present study, by using PERG and focal ERG. PERGs and focal ERGs were found to be severely reduced or abolished in all patients tested, even when visual acuity was still good. In our experience, this finding is uncommon in other inherited maculopathies. Although further studies in a large group of patients are needed to confirm this finding, it appears possible that abnormalities in PERG and focal ERG could be used to establish an early diagnosis and to differentiate SMDFFM from other macular dystrophies, especially in early stages of the disease when fundus flecks may not be evident.
Focal increased fundus autofluorescence an index of increased lipofuscin content in the RPE appeared to correspond well with those flecks consisting of white-yellow material at the level of the RPE. Similarly, well-defined areas of increased fundus autofluorescence compared with background were detected, at the macula, in some patients. This corresponds with light and electron microscopic studies, which show abnormal accumulation of intracytoplasmic material in the RPE, identified as lipofuscin,18 19 20 21 and with a previous study by Delori et al.,22 in which abnormally high levels of fundus autofluorescence were observed in five patients with SMDFFM. Although the distribution of white flecks and atrophy differed between siblings, this was not considered good evidence of qualitative differences in disease within the sibship. Fundus autofluorescence studies provided additional help in establishing the diagnosis of SMDFFM, especially in patients with the end stage of the disease in whom resorbed fundus flecks were very difficult to detect or were not visualized on slit lamp biomicroscopy.
Mutations in the ABCR gene, located in the short arm of chromosome 1, have been identified recently in patients with SMDFFM.10 Furthermore, it has been hypothesized that different mutations within the ABCR gene would probably account for the wide phenotypic heterogeneity observed in patients with SMDFFM.23 However, in an autosomal recessive disorder, siblings could be expected to have the same alleles, such that the variation between siblings could not be caused by different mutations or other sequence changes in the ABCR genes. Differences between siblings may indicate the influence of other "modifying genes." The differences in phenotype between siblings appear to be quantitative rather than qualitative with respect to the distribution and nature of photoreceptor dysfunction, which may reflect the influence of different mutations in ABCR. The modification of phenotype by other genes relates to the age of onset, distribution of fundus lesions, and speed of progression. Overall, the findings indicate that, when considering the putative influence of a mutation in the ABCR gene on cellular function, the qualitative attributes of functional loss should be taken into account rather than the age of onset, loss of visual acuity, and fundus features. It would be surprising if the distinction regarding distribution of photoreceptor dysfunction is absolute, because in most cases the disease occurs in the compound heterozygous state.
Finally, expression studies in mouse, rat, bovine, and macaque ocular tissues have demonstrated the presence of ABCR transcripts in rod photoreceptors but not in cones,10 24 which is difficult to reconcile with the gene causing macular dystrophy. It is even more difficult to explain the widespread cone involvement with normal rod function in members of two families presented here (6 and 10). It appears unlikely that the degree of cone dysfunction observed in some of these patients could be explained on the basis of a primary lesion within the rod photoreceptors, as has been previously proposed.10 Further studies are needed to resolve this dilemma.
| Footnotes |
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Submitted for publication October 28, 1998; revised March 30, 1999; accepted May 5, 1999.
Commercial relationships policy: N.
Corresponding author: Alan C. Bird, Medical Retina Service, Moorfields Eye Hospital, City Road, London EC1V 2PD, UK. E-mail: alan.bird{at}ucl.ac.uk
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