Psychophysical assessment of low visual function in patients with retinal degenerative diseases (RDDs) with the Diagnosys full-field stimulus threshold (D-FST)

To determine whether the Diagnosys full-field stimulus threshold (D-FST) is a valid, sensitive and repeatable psychophysical method of measuring and following visual function in low-vision subjects. Fifty-three affected eyes of 42 subjects with severe retinal degenerative diseases (RDDs) were tested with achromatic stimuli on the D-FST. Included were subjects who were either unable to perform a static perimetric field or had non-detectable or sub-microvolt electroretinograms (ERGs). A subset of 21 eyes of 17 subjects was tested on both the D-FST and the FST2, a previous established full-field threshold test. Seven eyes of 7 normal control subjects were tested on both the D-FST and the FST2. Results for the two methods were compared with the Bland-Altman test. On the D-FST, a threshold could successfully be determined for 13 of 14 eyes with light perception (LP) only (median 0.9 +/- 1.4 log cd/m2), and all eyes determined to be counting fingers (CF; median 0.3 +/- 1.8 log cd/m2). The median full-field threshold for the normal controls was -4.3 +/- 0.6 log cd/m2 on the D-FST and -4.8 +/- 0.9 log cd/m2 on the FST2. The D-FST offers a commercially available method with a robust psychophysical algorithm and is a useful tool for following visual function in low vision subjects.

Lipofuscin accumulation, abnormal electrophysiology, and photoreceptor degeneration in mutant ELOVL4 transgenic mice: a model for macular degeneration

Macular degeneration is a heterogeneous group of disorders characterized by photoreceptor degeneration and atrophy of the retinal pigment epithelium (RPE) in the central retina. An autosomal dominant form of Stargardt macular degeneration (STGD) is caused by mutations in ELOVL4, which is predicted to encode an enzyme involved in the elongation of long-chain fatty acids. We generated transgenic mice expressing a mutant form of human ELOVL4 that causes STGD. In these mice, we show that accumulation by the RPE of undigested phagosomes and lipofuscin, including the fluorophore, 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-1-(2-hyydroxyethyl)-4-[4-methyl-6-(2,6,6,-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]-pyridinium (A2E) is followed by RPE atrophy. Subsequently, photoreceptor degeneration occurs in the central retina in a pattern closely resembling that of human STGD and age-related macular degeneration. The ELOVL4 transgenic mice thus provide a good model for both STGD and dry age-related macular degeneration, and represent a valuable tool for studies on therapeutic intervention in these forms of blindness.

Interocular amplitude differences of the full field electroretinogram in normal subjects

AIMS

To determine the interocular amplitude response difference of the electroretinogram (ERG) in normal subjects.

METHODS

79 subjects, without retinal changes of clinical significance, underwent ERG testing. They included 63 men and 16 women, with a mean age of 44 (SD 12) years and range of 18-65 years. Isolated rod, scotopic maximal, dark adapted 30 Hz flicker, photopic single flash, and light adapted 30 Hz flicker responses were recorded in both eyes following the International Society for Clinical Electrophysiology of Vision (ISCEV) standard protocol. The interocular percentage differences of the ERG b-wave amplitudes were calculated and presented as percentiles (25th, 50th, 75th, 95th), means (SD), and medians.

RESULTS

The median interocular percentage differences in the b-wave amplitudes for the above ERG stimulus responses were 10%, 8%, 10%, 11%, and 10%, respectively. The mean interocular percentage differences were 11%, 11%, 12%, 13%, and 14%. The 95th percentiles for the interocular percentage differences were 28%, 27%, 36%, 33%, and 35%, respectively.

CONCLUSIONS

The interocular percentage differences in the ERG b-wave amplitudes for five different stimulus responses were similar in our cohort of individuals without clinically significant retinal changes and ranged from a median of 8-11% and a 95th percentile of 27-36%. Our findings should be useful for determining sample sizes in future therapeutic trials on retinal diseases with monocular therapeutic strategies and may also have application for the more accurate detection of asymmetric retinal disease.

Visual function in patients with cone-rod dystrophy (CRD) associated with mutations in the ABCA4(ABCR) gene

Mutations in the ABCA4(ABCR) gene cause autosomal recessive Stargardt disease (STGD). ABCR mutations were identified in patients with cone-rod dystrophy (CRD) and retinitis pigmentosa (RP) by direct sequencing of all 50 exons in 40 patients. Of 10 patients with RP, one contained two ABCR mutations suggesting a compound heterozygote. This patient had a characteristic fundus appearance with attenuated vessels, pale disks and bone-spicule pigmentation. Rod electroretinograms (ERGs) were non-detectable, cone ERGs were greatly reduced in amplitude and delayed in implicit time, and visual fields were constricted to 10 degrees diameter. Eleven of 30 (37%) patients with CRD had mutations in ABCR. In general, these patients showed reduced but detectable rod ERG responses, reduced and delayed cone responses, and poor visual acuity. Rod photoresponses to high intensity flashes were of reduced maximum amplitude but showed normal values for the gain of phototransduction. Most CRD patients with mutations in ABCR showed delayed recovery of sensitivity (dark adaptation) following exposure to bright light. Pupils were also significantly smaller in these patients compared to controls at 30 min following light exposure, consistent with a persistent 'equivalent light' background due to the accumulation of a tentatively identified 'noisy' photoproduct.

Impaired synthesis of DHA in patients with X-linked retinitis pigmentosa

Many patients with X-linked retinitis pigmentosa (XLRP) have lower than normal blood levels of the long-chain polyunsaturated omega3 fatty acid docosahexaenoic acid (DHA; 22:6omega3). This clinical trial was designed to test whether down-regulation of DHA biosynthesis might be responsible for these reduced DHA levels. DHA biosynthesis was assessed in five severely affected patients with XLRP and in five age-matched controls by quantifying conversion of [U-(13)C]alpha-linolenic acid (alpha-LNA) to [(13)C]DHA. Following oral administration of [U-(13)C]alpha-LNA, blood samples were collected at designated intervals for 21 days and isotopic enrichment of all omega3 fatty acids was determined by gas chromatography/mass spectroscopy. Activity of each metabolic step in the conversion of alpha-LNA to DHA was determined by comparison of the ratios of the integrated concentration of (13)C-product to (13)C-precursor in plasma total lipid fractions. The ratio of [(13)C]DHA to [(13)C]18:3omega3 (the entire pathway) and that of [(13)C]20:5omega3 to [(13)C]20:4omega3 (Delta(5)-desaturase) were significantly lower in patients versus controls (P = 0.03 and 0.05, respectively). The estimated biosynthetic rates of [(13)C]20:5omega3, [(13)C]22:5omega3, [(13)C]24:5omega3, [(13)C]24:6omega3, and [(13)C]22:6omega3 were significantly lower in XLRP patients (42%, 43%, 31%, 18%, and 32% of control values, respectively; P < 0.04), supporting down-regulation of Delta(5)-desaturase in XLRP. The disappearance of (13)C-labeled fatty acids from plasma was not greater in XLRP patients compared with controls, suggesting that XLRP was not associated with increased rates of fatty acid oxidation or other routes of catabolism.Thus, despite individual variation among both patients and controls, the data are consistent with a lower rate of Delta(5)-desaturation, suggesting that decreased biosynthesis of DHA may contribute to lower blood levels of DHA in patients with XLRP.

Essential fatty acids in visual and brain development

Essential fatty acids are structural components of all tissues and are indispensable for cell membrane synthesis; the brain, retina and other neural tissues are particularly rich in long-chain polyunsaturated fatty acids (LC-PUFA). These fatty acids serve as specific precursors for eicosanoids, which regulate numerous cell and organ functions. Recent human studies support the essential nature of n-3 fatty acids in addition to the well-established role of n-6 essential fatty acids in humans, particularly in early life. The main findings are that light sensitivity of retinal rod photoreceptors is significantly reduced in newborns with n-3 fatty acid deficiency, and that docosahexaenoic acid (DHA) significantly enhances visual acuity maturation and cognitive functions. DHA is a conditionally essential nutrient for adequate neurodevelopment in humans. Comprehensive clinical studies have shown that dietary supplementation with marine oil or single-cell oil sources of LC-PUFA results in increased blood levels of DHA and arachidonic acid, as well as an associated improvement in visual function in formula-fed infants matching that of human breast-fed infants. The effect is mediated not only by the known effects on membrane biophysical properties, neurotransmitter content, and the corresponding electrophysiological correlates but also by a modulating gene expression of the developing retina and brain. Intracellular fatty acids or their metabolites regulate transcriptional activation of gene expression during adipocyte differentiation and retinal and nervous system development. Regulation of gene expression by LC-PUFA occurs at the transcriptional level and may be mediated by nuclear transcription factors activated by fatty acids. These nuclear receptors are part of the family of steroid hormone receptors. DHA also has significant effects on photoreceptor membranes and neurotransmitters involved in the signal transduction process; rhodopsin activation, rod and cone development, neuronal dendritic connectivity, and functional maturation of the central nervous system.

Pupil size following dark adaptation in patients with retinitis pigmentosa

According to the equivalent light hypothesis, molecular defects in the photoreceptor lead to a continuous activation of the photoreceptor cascade in a manner equivalent to real light. The consequences in diseases such as retinitis pigmentosa (RP) are as disruptive to the cells as real light. Two forms of the equivalent light hypothesis can be distinguished: strong - mutations in rhodopsin or other cascade proteins in some forms of RP continuously excite the visual phototransduction cascade; weak - disruption of outer segments in all patients with RP eliminates circulating dark current and blocks neurotransmitter release in a manner similar to real light. Both forms of the equivalent light hypothesis predict that pupils of patients with RP will be constricted like those of normal subjects in the light. The purpose of this study was to test the equivalent light hypothesis by determining whether steady-state pupil diameter following full dark adaptation is abnormally small in any of a sample of patients with RP. Thirty-five patients with RP and 15 normal subjects were tested. Direct steady-state pupillometric measures were obtained from one eye in a full-field dome after 45 min of dark adaptation by videotaping the pupil with an infrared camera. Mean pupil diameter in the dark was comparable (t = -0.15, P = 0.88) between patients with RP (6.85 +/- 0.58 mm) and normal subjects (6.82 +/- 0.76 mm). The results of the present study are clearly counter to the prediction of the second (weaker) form of the equivalent light hypothesis.

Delayed dark-adaptation and lipofuscin accumulation in abcr+/- mice: implications for involvement of ABCR in age-related macular degeneration

PURPOSE

To examine the ocular phenotype in mice heterozygous for a null mutation in the abcr gene.

METHODS

Retinas and retinal pigment epithelia (RPE) were prepared from wild-type, abcr+/-, and abcr-/- mice. Fresh tissues were homogenized and analyzed by normal phase high-performance liquid chromatography (HPLC) for the presence of retinoids and phospholipids. In another study, fixed tissues were sectioned and analyzed by light and electron microscopy. Finally, anesthetized mice were studied by electroretinography (ERG) at different times after exposure to strong light.

RESULTS

A2E, the major fluorophore of lipofuscin, and its precursors, A2PE-H(2) and A2PE, were approximately fourfold more abundant in 8-month-old abcr+/- than in the wild-type retina and RPE. The levels of these substances in abcr+/- mice were approximately 40% those in abcr-/- mice. Lipofuscin pigment-granules were also visible in abcr+/- RPE cells by electron microscopy. Accumulation of A2PE-H(2) and A2E in abcr+/- retina and RPE, respectively, was strongly dependent on light exposure. Heterozygous mutants also exhibited delayed recovery of rod sensitivity by ERG. This delay was correlated with elevated levels of all-trans-retinaldehyde (all-trans-RAL) in retina after a photobleach and was not caused by a reduction in quantum-catch due to depletion of 11-cis-retinaldehyde (11-cis-RAL).

CONCLUSIONS

Partial loss of the ABCR or rim protein is sufficient to cause a phenotype in mice similar to recessive Stargardt's disease (STGD) and age-related macular degeneration (AMD) in humans. These data are consistent with the suggestion that the STGD carrier-state may predispose to the development of AMD.

Mutations in NYX, encoding the leucine-rich proteoglycan nyctalopin, cause X-linked complete congenital stationary night blindness

During development, visual photoreceptors, bipolar cells and other neurons establish connections within the retina enabling the eye to process visual images over approximately 7 log units of illumination. Within the retina, cells that respond to light increment and light decrement are separated into ON- and OFF-pathways. Hereditary diseases are known to disturb these retinal pathways, causing either progressive degeneration or stationary deficits. Congenital stationary night blindness (CSNB) is a group of stable retinal disorders that are characterized by abnormal night vision. Genetic subtypes of CSNB have been defined and different disease actions have been postulated. The molecular bases have been elucidated in several subtypes, providing a better understanding of the disease mechanisms and developmental retinal neurobiology. Here we have studied 22 families with 'complete' X-linked CSNB (CSNB1; MIM 310500; ref. 4) in which affected males have night blindness, some photopic vision loss and a defect of the ON-pathway. We have found 14 different mutations, including 1 founder mutation in 7 families from the United States, in a novel candidate gene, NYX. NYX, which encodes a glycosylphosphatidyl (GPI)-anchored protein called nyctalopin, is a new and unique member of the small leucine-rich proteoglycan (SLRP) family. The role of other SLRP proteins suggests that mutant nyctalopin disrupts developing retinal interconnections involving the ON-bipolar cells, leading to the visual losses seen in patients with complete CSNB.

Impact of early dietary intake and blood lipid composition of long-chain polyunsaturated fatty acids on later visual development

BACKGROUND

In contrast to human milk, current infant formulas in the United States do not contain omega3 and omega6 long-chain polyunsaturated fatty acids. This may lead to suboptimal blood lipid fatty acid profiles and to a measurable diminution of visual function in developing term infants. The need for docosahexaenoic acid and arachidonic acid supplementation in the infant diet was evaluated in a double-blind, randomized clinical trial.

METHODS

Healthy term infants were randomized to diets of (1) commercial formula, (2) docosahexaenoic acid-enriched formula (0.35% of total fatty acids), or (3) docosahexaenoic acid- (0.36%) and arachidonic acid- (0.72%) enriched formula. Eighty-seven infants completed the 17-week nutritional trial, and 58 were observed until 52 weeks of life. A reference group was exclusively breast fed for at least 17 weeks (n = 29). Outcome measures included electroretinographic responses, visual evoked potentials, and blood fatty acid analysis in infants at birth and at 6, 17, and 52 weeks of age.

RESULTS

Commercial formula-fed infants had 30% to 50% lower content of docosahexaenoic acid in total red blood cell lipids during the 17-week feeding trial compared with breastfed infants. Significant differences persisted at the 1-year follow-up. Arachidonic acid content was consistently reduced in the commercial formula group by 15% to 20%. Infants fed long-chain polyunsaturated fatty acid-enriched formulas had docosahexaenoic acid and arachidonic acid blood lipid profiles resembling those of human milk-fed infants. Infants receiving this enriched formula had more mature electroretinographic responses than commercial formula-fed infants at 6 weeks of age. Human milk-fed and docosahexaenoic acid-enriched formula-fed infants had better visual acuity than commercial formula-fed infants at both 17 and 52 weeks of age. Early (17-week) fatty acid profiles in blood lipids were correlated with later (52-week) visual function development in study infants.

CONCLUSIONS

Results from this clinical trial demonstrate that long-chain polyunsaturated fatty acid supplementation of formula in term infants produces blood lipid fatty acid profiles that are similar to those observed in breast-fed infants. This supplementation leads to better visual function later in life (i.e., 1 year of age) than that shown by infants fed commercial formula.

Effect of stimulus size on static visual fields in patients with retinitis pigmentosa

PURPOSE

To determine the effect of stimulus size on sensitivity of patients with retinitis pigmentosa (RP) as measured by automated static perimetry.

DESIGN

Comparative case series.

PARTICIPANTS

Thirty-nine patients with RP and a control group of 10 healthy volunteers.

METHODS

Automated static perimetry (full threshold programs 24-2 or 30-2) was performed twice on one eye of each participant using stimulus sizes III (0.43 degrees diameter) and V (1.72 degrees diameter). Data from the same 50 test locations were used from each field.

MAIN OUTCOME MEASURES

At each location, for each participant, the size effect was computed as the difference (in decibels) in sensitivities for sizes V and III, and the average sensitivity was computed as the mean of sensitivities for the two sizes.

RESULTS

For both patient and control groups, the size effect was negatively correlated with average sensitivity (r(2) > 0.124; P: < 0.001). The mean size effect was significantly greater for the patient group than for the control group: 8.6 (+/- 3.6) dB versus 5. 4 (+/- 2.2) dB (t = 18.0; P: < 0.001). The percentage of abnormal locations (more than 8 dB below mean normal) tended to be lower for size V than for size III, with a mean of 67% for size V versus 95% for size III. The percentage of absolute defects was also lower for size V than for size III, with a mean of 35% for size V versus 54% for size III.

CONCLUSIONS

In damaged regions of the visual fields of patients with RP, increase in stimulus size from III to V can produce abnormally large increases in perimetric sensitivity. Size III may be more useful than size V for detection of field abnormality, whereas size V may be more useful than size III for observing progression of advanced RP.

Comparison of the Goldmann-Weekers dark adaptometer and LKC Technologies Scotopic Sensitivity tester-1

The standard for dark adaptation has long been the Goldmann-Weekers Dark Adaptometer(Haag-Streit). More recently, portable, relatively inexpensive LED-based dark adaptometers have become commercially available. These devices have potential use in areas with limited resources to screen for night-blindness, commonly caused worldwide by vitamin A deficiency. In order to determine the sensitivity to detecting changes in night vision, this study compared one such device, LKC Technologies Scotopic Sensitivity Tester-1 (SST-1) to the Goldmann-Weekers in patients with hereditary retinal degeneration and loss of rod function. Dark-adapted final thresholds and rod full-field ERG responses were obtained from 87 patients and 24 normal subjects. Linear regression analysis, discrepancy analysis, and receiver operator characteristic curves for both devices show that the SST-1 quantifies psychophysical rod function nearly as well as the Goldmann-Weekers, within some limitations. We conclude, therefore, that the SST-1 is a viable alternative to the Goldmann-Weekers for the screening of night-blinding retinal disorders.

Visual phenotype in patients with Arg41Gln and ala196+1bp mutations in the CRX gene

Our aim was to describe the visual function characteristics of affected members from two unrelated families with different dominant mutations in the CRX gene. Standard full-field ERGs and high-intensity a-wave series were obtained. In addition, in most subjects, dark-adapted (DA) thresholds, color vision function (arrangement tests), and static perimetry were assessed. A point mutation in codon 41 of the CRX gene (Arg41Gln) was identified in family members from the RFS087 family who were tested on several occasions since 1983. Depending on age, affected members showed varying degrees of acuity loss, normal or slightly elevated DA thresholds, reduced cone a- and b-wave amplitudes, normal or minimally delayed cone b-wave implicit times, and normal rod and cone phototransduction gain parameters. An insertion mutation (Ala196+1bp) was found in two members of another family (RFS014). Affected members showed reduced visual acuity, normal or slightly elevated DA thresholds, relatively preserved rod ERG and substantially reduced or undetectable cone ERG, and normal rod phototransduction gain parameters. The Arg41Gln was associated with a late-onset, slowly progressing mild form of cone-rod dystrophy with cone loss but preserved rod and cone sensitivity until later in life. The Ala196+1bp mutation was associated with an early-onset, severe form of cone-rod dystrophy similar to that described in the original CORD2 family (Evans et al., Arch Ophthalmol 1995;113:195-201).

Mutations in a new photoreceptor-pineal gene on 17p cause leber congenital amaurosis. Nat gen 2000;24:79-83

Leber congenital amaurosis (LCA, MIM 204000) accounts for at least 5% of all inherited retinal disease1 and is the most severe inherited retinopathy with the earliest age of onset2. Individuals affected with LCA are diagnosed at birth or in the first few months of life with severely impaired vision or blindness, nystagmus and an abnormal or flat electroretinogram (ERG). Mutations in GUCY2D (ref. 3), RPE65 (ref. 4) and CRX (ref. 5) are known to cause LCA, but one study identified disease-causing GUCY2D mutations in only 8 of 15 families whose LCA locus maps to 17p13.1 (ref. 3), suggesting another LCA locus might be located on 17p13.1. Confirming this prediction, the LCA in one Pakistani family mapped to 17p13.1, between D17S849 and D17S960—a region that excludes GUCY2D. The LCA in this family has been designated LCA4 (ref. 6). We describe here a new photoreceptor/pineal-expressed gene, AIPL1 (encoding arylhydrocarbon interacting protein-like 1), that maps within the LCA4 candidate region and whose protein contains three tetratricopeptide (TPR) motifs, consistent with nuclear transport or chaperone activity. A homozygous nonsense mutation at codon 278 is present in all affected members of the original LCA4 family. AIPL1 mutations may cause approximately 20% of recessive LCA, as disease-causing mutations were identified in 3 of 14 LCA families not tested previously for linkage.

Prevalence of AIPL1 mutations in inherited retinal degenerative disease

Leber congenital amaurosis (LCA) is the most severe form of inherited retinal dystrophy and the most frequent cause of inherited blindness in children. LCA is usually inherited in an autosomal recessive fashion, although rare dominant cases have been reported. One form of LCA, LCA4, maps to chromosome 17p13 and is genetically distinct from other forms of LCA. We recently identified the gene associated with LCA4, AIPL1 (aryl-hydrocarbon interacting protein-like 1) and identified three mutations that were the cause of blindness in five families with LCA. In this study, AIPL1 was screened for mutations in 512 unrelated probands with a range of retinal degenerative diseases to determine if AIPL1 mutations cause other forms of inherited retinal degeneration and to determine the relative contribution of AIPL1 mutations to inherited retinal disorders in populations worldwide. We identified 11 LCA families whose retinal disorder is caused by homozygous or compound heterozygous AIPL1 mutations. We also identified affected individuals in two apparently dominant families, diagnosed with juvenile retinitis pigmentosa or dominant cone-rod dystrophy, respectively, who are heterozygous for a 12-bp AIPL1 deletion. Our results suggest that AIPL1 mutations cause approximately 7% of LCA worldwide and may cause dominant retinopathy.

Rom-1 is required for rod photoreceptor viability and the regulation of disk morphogenesis

The homologous membrane proteins Rom-1 and peripherin-2 are localized to the disk rims of photoreceptor outer segments (OSs), where they associate as tetramers and larger oligomers. Disk rims are thought to be critical for disk morphogenesis, OS renewal and the maintenance of OS structure, but the molecules which regulate these processes are unknown. Although peripherin-2 is known to be required for OS formation (because Prph2-/- mice do not form OSs; ref. 6), and mutations in RDS (the human homologue of Prph2) cause retinal degeneration, the relationship of Rom-1 to these processes is uncertain. Here we show that Rom1-/- mice form OSs in which peripherin-2 homotetramers are localized to the disk rims, indicating that peripherin-2 alone is sufficient for both disk and OS morphogenesis. The disks produced in Rom1-/- mice were large, rod OSs were highly disorganized (a phenotype which largely normalized with age) and rod photoreceptors died slowly by apoptosis. Furthermore, the maximal photoresponse of Rom1-/- rod photoreceptors was lower than that of controls. We conclude that Rom-1 is required for the regulation of disk morphogenesis and the viability of mammalian rod photoreceptors, and that mutations in human ROM1 may cause recessive photoreceptor degeneration.

A randomized controlled trial of early dietary supply of long-chain polyunsaturated fatty acids and mental development in term infants

The effects of dietary docosahexaenoic acid (DHA) supply during infancy on later cognitive development of healthy term infants were evaluated in a randomized clinical trial of infant formula milk supplemented with 0.35% DHA or with 0.36% DHA and 0.72% arachidonic acid (AA), or control formula which provided no DHA or AA. Fifty-six 18-month-old children (26 male, 30 female) who were enrolled in the trial within the first 5 days of life and fed the assigned diet to 17 weeks of age were tested using the Bayley Scales of Infant Development, 2nd edition (BSID-II) (Bayley 1993) at the Retina Foundation of the Southwest, Dallas, TX. These children had also been assessed at 4 months and 12 months of age for blood fatty-acid composition, sweep visual evoked potential (VEP) acuity, and forced-choice preferential looking (FPL) acuity (Birch et al. 1998). Supplementation of infant formula with DHA+AA was associated with a mean increase of 7 points on the Mental Development Index (MDI) of the BSID-II. Both the cognitive and motor subscales of the MDI showed a significant developmental age advantage for DHA- and DHA+AA-supplemented groups over the control group. While a similar trend was found for the language subscale, it did not reach statistical significance. Neither the Psychomotor Development Index nor the Behavior Rating Scale of the BSID-II showed significant differences among diet groups, consistent with a specific advantage of DHA supplementation on mental development. Significant correlations between plasma and RBC-DHA at 4 months of age but not at 12 months of age and MDI at 18 months of age suggest that early dietary supply of DHA was a major dietary determinant of improved performance on the MDI.

Mutations in a new photoreceptor-pineal gene on 17p cause Leber congenital amaurosis

Leber congenital amaurosis (LCA, MIM 204000) accounts for at least 5% of all inherited retinal disease and is the most severe inherited retinopathy with the earliest age of onset. Individuals affected with LCA are diagnosed at birth or in the first few months of life with severely impaired vision or blindness, nystagmus and an abnormal or flat electroretinogram (ERG). Mutations in GUCY2D (ref. 3), RPE65 (ref. 4) and CRX (ref. 5) are known to cause LCA, but one study identified disease-causing GUCY2D mutations in only 8 of 15 families whose LCA locus maps to 17p13.1 (ref. 3), suggesting another LCA locus might be located on 17p13.1. Confirming this prediction, the LCA in one Pakistani family mapped to 17p13.1, between D17S849 and D17S960-a region that excludes GUCY2D. The LCA in this family has been designated LCA4 (ref. 6). We describe here a new photoreceptor/pineal-expressed gene, AIPL1 (encoding aryl-hydrocarbon interacting protein-like 1), that maps within the LCA4 candidate region and whose protein contains three tetratricopeptide (TPR) motifs, consistent with nuclear transport or chaperone activity. A homozygous nonsense mutation at codon 278 is present in all affected members of the original LCA4 family. AIPL1 mutations may cause approximately 20% of recessive LCA, as disease-causing mutations were identified in 3 of 14 LCA families not tested previously for linkage.

Disruption of a retinal guanylyl cyclase gene leads to cone-specific dystrophy and paradoxical rod behavior

One of two orphan photoreceptor guanylyl cyclases that are highly conserved from fish to mammals, GC-E (or retGC1) was eliminated by gene disruption. Expression of the second retinal cyclase (GC-F) as well as the numbers and morphology of rods remained unchanged in GC-E null mice. However, rods isolated from such mice, despite having a normal dark current, recovered from a light flash markedly faster. Unexpectedly, the a- and b-waves of electroretinograms (ERG) from dark-adapted null mice were suppressed markedly. Cones, initially present in normal numbers in the retina, disappeared by 5 weeks, based on ERG and histology. Thus, the GC-E-deficient mouse defines a model for cone dystrophy, but it also demonstrates that morphologically normal rods display paradoxical behavior in their responses to light.

Insights into the function of Rim protein in photoreceptors and etiology of Stargardt's disease from the phenotype in abcr knockout mice

Rim protein (RmP) is an ABC transporter of unknown function in rod outer segment discs. The human gene for RmP (ABCR) is affected in several recessive retinal degenerations. Here, we characterize the ocular phenotype in abcr knockout mice. Mice lacking RmP show delayed dark adaptation, increased all-trans-retinaldehyde (all-trans-RAL) following light exposure, elevated phosphatidylethanolamine (PE) in outer segments, accumulation of the protonated Schiff base complex of all-trans-RAL and PE (N-retinylidene-PE), and striking deposition of a major lipofuscin fluorophore (A2-E) in retinal pigment epithelium (RPE). These data suggest that RmP functions as an outwardly directed flippase for N-retinylidene-PE. Delayed dark adaptation is likely due to accumulation in discs of the noncovalent complex between opsin and all-trans-RAL. Finally, ABCR-mediated retinal degeneration may result from "poisoning" of the RPE due to A2-E accumulation, with secondary photoreceptor degeneration due to loss of the RPE support role.

Current popularity of pneumatic retinopexy

PURPOSE

To compare the popularity of pneumatic retinopexy (PR) in 1997 with its popularity in 1990 among retinal specialists.

METHODS

In 1997, a survey was mailed to the 1994-1995 members of the Retina or Vitreous Societies who lived in the United States or Canada, asking how they would manage a hypothetical retinal detachment. The choices were limited to PR, segmental scleral buckling, scleral bucking with encircling, primary vitrectomy, and Lincoff balloon. The results of the survey were compared with those previously reported by a similar survey in 1990.

RESULTS

The majority (55%) of respondents selected PR, which is a twofold increase over those who preferred it in 1990 (odds ratio 2.08; 95% confidence interval 1.53, 2.85). The popularity of PR was inversely proportional to the length of time the respondents had been in practice. If the eye with the hypothetical detachment had pseudophakia, only 30% of respondents selected PR. If the eye had additional tears, vitreous hemorrhage, or lattice degeneration, only about one-sixth preferred PR.

CONCLUSION

Pneumatic retinopexy was much more popular in 1997 than it was in 1990. Its popularity continues to be influenced by the age of the surgeon and by the complexity of the detachment.

Fatty acid profile of buccal cheek cell phospholipids as an index for dietary intake of docosahexaenoic acid in preterm infants

Cheek cells (buccal epithelia) were utilized as a noninvasive index of fatty acid status in a study of the effects of n-3 long chain polyunsaturated fatty acid supplementation on visual function in preterm infants. The fatty acid profile of cheek cell phospholipids was directly correlated with the dietary docosahexenoic acid (DHA) intake of infants receiving: (i) primarily human milk; (ii) n-3 fatty acid-deficient, corn oil-based, commercial formula (CO); (iii) alpha-linolenic acid-enriched, soy oil-based, commercial formula; or (iv) experimental formula enriched with soy and marine oils providing a DHA level equivalent to that in human milk. In a subset of infants with complete cheek cell fatty acid profiles and visual function assessments, preterm infants at both 36 wk (n = 63) and 57 wk (n = 45) postconceptional age had significantly (P < 0.0005) reduced cheek cell phospholipid DHA levels in the n-3-deficient, CO-fed group compared to the other diet groups. The DHA content in cheek cell phospholipids was highly correlated (P < 0.0005) with that of both red blood cell lipids and plasma phospholipids at the 36- and 57-wk time points. The DHA content in cheek cell lipids of infants at 36 wk was significantly correlated with electroretinographic responses (r = -0.29; P < 0.03) and visual acuity (r = -0.31; P < 0.02) as measured by visual-evoked potentials (VEP). Cheek cell DHA was highly correlated (r= -0.57; P < 0.0005) with VEP acuity at the 57-wk time point. These results suggest that the fatty acid profile of cheek cells is a valid index of essential fatty acid status, can be monitored frequently, and is associated with functional parameters in infants.

Retinal degeneration in retinitis pigmentosa and neuronal ceroid lipofuscinosis: An overview

Retinal degeneration is an early consequence of the group of lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). This review details specialized techniques that have evolved for retinal assessment in patients with hereditary retinal degeneration. A standard ERG protocol is described for assessing rod- and cone-mediated function. Standardization will be crucial for planning and implementing multicenter trials as rational therapeutic intervention becomes available. In recent years, there has been a dramatic increase in knowledge of the molecular biological bases of retinitis pigmentosa and allied retinal degenerations. Rather than attempting a comprehensive summary, this review stresses the concepts of genetic, allelic, and clinical heterogeneity, which have obvious parallels in the NCLs. Many of the mutations that cause retinal degeneration are in genes that encode photoreceptor cascade proteins; others are in genes that encode photoreceptor structural proteins. Recent advances in linking the retinal degeneration slow (RDS) and ATP-binding cassette transporter retina (ABCR) genes to a variety of disease phenotypes will be summarized. Clinical heterogeneity even among family members with the same mutation raises the possibility that modifying factors, either genetic or environmental, could influence the severity of the disease. Here, we focus on vitamin A and docosahexaenoic acid, two potential nutritional modifiers that have received considerable attention in recent years.

Yearly rates of rod and cone functional loss in retinitis pigmentosa and cone-rod dystrophy

OBJECTIVE

To provide the first measures of the relative rates of rod and cone functional loss in patients with retinitis pigmentosa (RP) or cone-rod dystrophy (CRD).

DESIGN

Five-year, prospective natural history study.

PARTICIPANTS

Ninety-six patients (67 with RP and 29 with CRD) retaining measurable rod-mediated visual function and 5 normal subjects were tested at baseline and annually for 4 consecutive years.

MAIN OUTCOME MEASURES

Tests of visual function included visual acuity, dark-adaptation thresholds, dark-adapted static perimetry, and rod and cone computer-averaged electroretinograms (ERGs), which were obtained over a range of retinal illuminances. Intervisit variability for each measure was obtained in a subset of patients who were tested twice within a 2-month interval and was used to determine whether an individual patient had shown progression, regression, or no change over a particular study interval.

RESULTS

Over a 4-year interval, a significant number of patients with RP (60%) and CRD (62%) showed a decline in cone ERG amplitude. For rod ERG amplitude, the percentage of patients with RP or CRD showing progression was 64% and 45%, respectively. Although visual acuity, dark-adapted threshold, and rod visual field area also declined significantly over the 4-year period, the mean rate of change and the numbers of patients showing progression on these measures were lower than those for ERG measures. On specialized ERG testing, the yearly change in rod ERG threshold in RP was greater than the yearly change in cone ERG threshold, and the rate of progression varied significantly among inheritance types. For patients with CRD, the yearly change in rod threshold was comparable to the yearly change in cone ERG threshold.

CONCLUSIONS

This study helps to define the natural progression of rod-mediated and cone-mediated functional loss in patients with RP and CRD.

A novel locus (RP24) for X-linked retinitis pigmentosa maps to Xq26-27

Two genetic loci, RP2 and RP3, for X-linked retinitis pigmentosa (XLRP) have been localized to Xp11.3-11.23 and Xp21.1, respectively. RP3 appears to account for 70% of XLRP families; however, mutations in the RPGR gene (isolated from the RP3 region) are identified in only 20% of affected families. Close location of XLRP loci at Xp and a lack of unambiguous clinical criteria do not permit assignment of genetic subtype in a majority of XLRP families; nonetheless, in some pedigrees, both RP2 and RP3 could be excluded as the causative locus. We report the mapping of a novel locus, RP24, by haplotype and linkage analysis of a single XLRP pedigree. The RP24 locus was identified at Xq26-27 by genotyping 52 microsatellite markers spanning the entire X chromosome. A maximum LOD score of 4.21 was obtained with DXS8106. Haplotype analysis assigned RP24 within a 23-cM region between the DXS8094 (proximal) and DXS8043 (distal) markers. Other chromosomal regions and known XLRP loci were excluded by obligate recombination events between markers in those regions and the disease locus. Hemizygotes from the RP24 family have early onset of rod photoreceptor dysfunction; cone receptor function is normal at first, but there is progressive loss. Patients at advanced stages show little or no detectable rod or cone function and have clinical hallmarks of typical RP. Mapping of the RP24 locus expands our understanding of the genetic heterogeneity in XLRP and will assist in development of better tools for diagnosis.

Visual acuity and the essentiality of docosahexaenoic acid and arachidonic acid in the diet of term infants

The need for a dietary supply of docosahexaenoic acid (DHA) and arachidonic aid (AA) in term infants was evaluated in a double-masked randomized clinical trial of the effects of supplementation of term infant formula with DHA (0.35% of total fatty acids) or with DHA (0.36%) and AA (0.72%) on visual acuity development. One hundred and eight healthy term infants were enrolled in the study; 79 were exclusively formula-fed from birth (randomized group) and 29 were exclusively breast-fed (gold standard group). Infants were evaluated at four time points during the first 12 mo of life for blood fatty acid composition, growth, sweep visual evoked potential (VEP) acuity, and forced choice preferential looking acuity. Supplementation of term infant formula with DHA or with DHA and AA during the first 4 mo of life yields clear differences in total red blood cell (RBC) lipid composition. Supplementation of term infant formula with DHA or with DHA and AA also yields better sweep VEP acuity at 6, 17, and 52 wk of age but not at 26 wk of age, when acuity development reaches a plateau. The RBC lipid composition and sweep VEP acuity of supplemented infants was similar to that of human milk-fed infants, whereas the RBC lipid composition and sweep VEP acuity of unsupplemented infants was significantly different from human milk-fed infants. Differences in acuity among diet groups were too subtle to be detected by the forced choice preferential looking protocol. Infants in all diet groups had similar rates of growth and tolerated all diets well. Thus, early dietary intake of preformed DHA and AA appears necessary for optimal development of the brain and eye of the human infant.

Evidence for genetic heterogeneity in X-linked congenital stationary night blindness

X-linked congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder characterized by disturbed or absent night vision; its clinical features may also include myopia, nystagmus, and impaired visual acuity. X-linked CSNB is clinically heterogeneous, and it may also be genetically heterogeneous. We have studied 32 families with X-linked CSNB, including 11 families with the complete form of CSNB and 21 families with the incomplete form of CSNB, to identify genetic-recombination events that would refine the location of the disease genes. Critical recombination events in the set of families with complete CSNB have localized a disease gene to the region between DXS556 and DXS8083, in Xp11.4-p11.3. Critical recombination events in the set of families with incomplete CSNB have localized a disease gene to the region between DXS722 and DXS8023, in Xp11.23. Further analysis of the incomplete-CSNB families, by means of disease-associated-haplotype construction, identified 17 families, of apparent Mennonite ancestry, that share portions of an ancestral chromosome. Results of this analysis refined the location of the gene for incomplete CSNB to the region between DXS722 and DXS255, a distance of 1.2 Mb. Genetic and clinical analyses of this set of 32 families with X-linked CSNB, together with the family studies reported in the literature, strongly suggest that two loci, one for complete (CSNB1) and one for incomplete (CSNB2) X-linked CSNB, can account for all reported mapping information.

Rod photoresponses in 6-week and 4-month-old human infants

Rod-only electroretinograms (ERGs) were recorded from 6-week and 4-month-old normal human infants. The leading edge of the rod a-wave was fitted with a model of the activation phase of phototransduction to provide estimates of S (a sensitivity parameter) and RmP3 (the maximum saturated photoreceptor response) at each of the investigated ages. Both S and RmP3 increased over the first postnatal months but followed different developmental time courses with S approaching adult-like values sooner than RmP3. The changes in S and RmP3 can be interpreted within the context of a model incorporating the combined effects of increased levels of rhodopsin and the changing structure of the rod outer segment during development.

Submicrovolt full-field cone electroretinograms: artifacts and reproducibility

The clinical utility of submicrovolt full-field 30-Hz (cone) electroretinograms was assessed by quantifying their contamination by electrical and photoelectric artifacts from xenon-flash stimulators and their test-retest variation in patients with retinitis pigmentosa. Artifacts obtained in saline with four commonly used electrodes varied with electrode type and consisted of an early, brief electrical component and a superimposed, extended photoelectric component. Techniques for minimizing these artifacts are described. Electroretinogram recordings from patients with advanced retinitis pigmentosa or congenital rod monochromatism indicate that these artifacts can be virtually eliminated with bipolar lenses. To assess test-retest variation, narrow-band-filtered responses were obtained twice during 6 weeks from patients with amplitudes less than 1 microV; threshold criteria for significant (p < 0.05) change in amplitude with this technique were approximately 0.25 log unit for each of two different systems.

Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: applications and methods

The impact of a disease on phototransduction can be assessed by fitting the leading edge of the rod a-wave to high-energy flashes with a quantitative expression. Two parameters of rod receptor activity are obtained, S (sensitivity) and Rm (maximum response). In this study, the meaning of these parameters and examples of conditions that change them were examined. In addition, a new protocol was developed for obtaining these parameters. A set of three to five white flashes were first presented in the dark and then on an adapting field (30 cd/m2). Subtracting the light-adapted responses from the dark-adapted responses yielded isolated rod a-wave responses. A clinical protocol was developed based on a single white flash energy. It is possible to determine whether a disease is producing a change in S and/or Rm with this single flash energy without the use of any equations.

Spectrum of mutations in the RPGR gene that are identified in 20% of families with X-linked retinitis pigmentosa

The RPGR (retinitis pigmentosa GTPase regulator) gene for RP3, the most frequent genetic subtype of X-linked retinitis pigmentosa (XLRP), has been shown to be mutated in 10%-15% of European XLRP patients. We have examined the RPGR gene for mutations in a cohort of 80 affected males from apparently unrelated XLRP families, by direct sequencing of the PCR-amplified products from the genomic DNA. Fifteen different putative disease-causing mutations were identified in 17 of the 80 families; these include four nonsense mutations, one missense mutation, six microdeletions, and four intronic-sequence substitutions resulting in splice defects. Most of the mutations were detected in the conserved N-terminal region of the RPGR protein, containing tandem repeats homologous to those present in the RCC-1 protein (a guanine nucleotide-exchange factor for Ran-GTPase). Our results indicate that mutations either in as yet uncharacterized sequences of the RPGR gene or in another gene located in its vicinity may be a more frequent cause of XLRP. The reported studies will be beneficial in establishing genotype-phenotype correlations and should lead to further investigations seeking to understand the mechanism of disease pathogenesis.

Light reduction and the electroretinogram of preterm infants

AIMS

To examine the effects of light on retinal development and function in preterm infants as measured by the electroretinogram (ERG). Secondary outcomes included visual acuity testing, the incidence of retinopathy of prematurity, and general wellbeing, reflected in feeding tolerance, rate of weight gain, and length of hospital stay.

METHODS

Eligibility criteria for enrollment were birthweight < or = 1250 g and gestational age < or = 31 weeks. Sixty one infants were randomly allocated by 6 hours after birth to a control or treatment group which wore 97% light filtering goggles for a minimum of four weeks or until the infant reached 31 weeks postmenstrual age.

RESULTS

There were no significant differences between the two groups in the numbers of electroretinograms performed at 36 weeks of postmenstrual age. Although the sample size was not large enough to exclude clinically important differences in secondary outcomes, no significant differences were observed between the groups in visual acuity testing at 4-6 months corrected age, incidence of retinopathy of prematurity, weight gain, or length of stay.

CONCLUSION

These data support the safety and feasibility of this intervention. A much larger study will be needed to determine whether light reduction to the eyes of very low birthweight infants will reduce the incidence of retinopathy of prematurity or enhance general well-being.

In-utero and post-delivery supplementation of motor neuron degeneration mutant mice with polyunsaturated fatty acids does not alter the clinical or pathological course

We have studied the effects of polyunsaturated fatty acid (PUFA) supplementation in utero and throughout life in mnd mutant mice, a proposed model for juvenile neuronal ceroid lipofuscinosis (CLN-3). Unlike our earlier in-vitro studies in humans with CLN-3, and in-vitro studies in CLN-3 lymphoblasts, we saw no beneficial effects in electroretinographic, electron microscopic or clinical studies in the mnd mice. Electron microscopy of brain revealed a pattern which was not consistent with the characteristic ceroid patterns in CLN-3. Our data suggest that the mnd mouse is not responsive to PUFA supplementation and may not be an appropriate animal model for CLN-3.

Generation and analysis of transgenic mice expressing P216L-substituted rds/peripherin in rod photoreceptors

PURPOSE

In this study, the authors present the biochemical, morphologic, and physiological analyses of a transgenic mouse model for retinal degeneration slow (RDS)-mediated retinitis pigmentosa caused by a proline 216 to leucine (P216L) amino acid substitution in rds/peripherin.

METHODS

The authors assembled a mutant rds transgene that encodes rds/peripherin with a P216L substitution. Transgenic mice were generated on wild-type (+/+), heterozygous (rds-/+), and homozygous (rds-/rds-) null genetic backgrounds. These mice were analyzed biochemically, by light and electron microscopy, and by electroretinography.

RESULTS

In P216L-transgenic mice on a +/- background, the authors observed expression-level-dependent photoreceptor degeneration and outer-segment shortening. Expression of the P216L transgene on an rds-/+ background resulted in more severe photoreceptor degeneration and outer-segment dysplasia than seen in nontransgenic rds-/+ mutants. Severely dysplastic outer segments were detectable in P216L transgenics on an rds-/rds-null background. The reduction in b-wave amplitudes by electroretinography were well correlated with the degree of photoreceptor degeneration, but not outer-segment dysplasia in these different rds mutants.

CONCLUSIONS

The phenotype in P216L-transgenic mice on an rds-/+ genetic background probably is caused by a combination of two genetic mechanisms: a direct dominant effect of the P216L substituted protein, and a reduction in the level of normal rds/peripherin. The expression pattern of the normal and mutant genes in these animals is similar to that predicted for humans with RDS-mediated autosomal-dominant retinitis pigmentosa. These mice may thus be considered an animal model for this disease.

Photoresponses of human rods in vivo derived from paired-flash electroretinograms

In the human eye, domination of the electroretinogram (ERG) by the b-wave and other postreceptor components ordinarily obscures all but the first few milliseconds of the rod photoreceptor response to a stimulating flash. However, recovery of the rod response after a bright rest flash can be analyzed using a paired-flash paradigm in which the test flash, presented at time zero, is followed at time t by a bright probe flash that rapidly saturates the rods (Birch et al., 1995). In ERG experiments on normal subjects, the hypothesis that a similar method can be used to obtain the full time course of the rod response to test flashes of subsaturating intensity was tested. Rod-only responses to probe flashes presented at varying times t after the test flash were used to derive a family of amplitudes A(t) that represented the putative rod response to the test flash. These rod-only responses to the probe flash were obtained by computational subtraction of the cone-mediated component of each probe flash response. With relatively weak test flashes (11-15 scot-td-s), the time course of the rod response to the test flash derived in this manner was consistent with a four-stage impulse response function of time-to-peak approximately 170 ms. A(170), the amplitude of the derived response at 170 ms, increased with test flash intensity (Itest) to a maximum value Amv and exhibited a dependence on Itest given approximately by the relation, A(170)/Amo = 1 - exp(-kItest), where k = 0.092 (scot-td-s)-1. In steady background light, the falling (i.e. recovery) phase of the derived response began earlier, and the sensitivity parameter k was reduced several-fold from its dark-adapted value. As the sensitivity, sensitivity, kinetics, and light-adaptation properties of the derived response correspond closely with those of photocurrent flash responses previously obtained from isolated rods in vitro, it was concluded that the response derived here from the human ERG approximates the course of the massed in vivo rod response to a test flash.

Abnormalities of the retinal cone system in retinitis pigmentosa

Patients with retinitis pigmentosa (RP) show delayed inner retinal responses as measured by the cone ERG response to a 30 Hz stimulus. To determine the extent to which this delay results from abnormalities of cone phototransduction, cone ERGs to single flashes were obtained from 21 patients with RP and a model of cone phototransduction was fitted to the leading edge of the a-waves of these ERGs. Nearly all patients showed an abnormally low sensitivity of cone phototransduction consistent with a reduction in the amplification of transduction. This abnormality can account for part of the delayed 30 Hz response. Analysis of post-receptoral potentials indicated that RP also slows the responses of the inner retina. A combination of these two factors, a sensitivity change at the receptor and a delay in the response of the inner retina, produces the delayed response of the cone flicker ERG in patients with RP.

Recovery kinetics of human rod phototransduction inferred from the two-branched alpha-wave saturation function

Electroretinographic data obtained from human subjects show that bright test flashes of increasing intensity induce progressively longer periods of apparent saturation of the rod-mediated electroretinogram (ERG) alpha wave. A prominent feature of the saturation function [the function that relates the saturation period T with the natural logarithm of flash intensity (ln I(f)] is its two-branched character. At relatively low flash intensities (I(f) below approximately 4 x 10(4) scotopic troland second), T increases approximately in proportion to ln I(f) with a slope [delta T/delta (ln I(f)] of approximately 0.3 s. At higher flash intensities, a different linear relation prevails, in which [deltaT/delta(ln I(f) is approximately 2.3 s [Invest. Ophthalmol. Vis. Sci. 36, 1603 (1995)]. Based on a model for photocurrent recovery in isolated single rods [Vis. Neurosci. 8, 9 (1992)], it was suggested that the upper-branch slope of approximately 2.3 s represents tau R*, the lifetime of photoactivated rhodopsin (R*). Here we show that a modified version of this model provides an explanation for the lower branch of the alpha-wave saturation function. In this model, tau E* is the exponential lifetime of an activated species (E*) within the transducin or guanosine 3', 5'-cyclic monophosphate (cGMP) phosphodiesterase stages of rod phototransduction; the generation of E* by a single R* occurs within temporally defined, elemental domains of disk membrane; and Ex, the immediate product of E* deactivation, is converted only slowly (time constant tau Ex) to E, the form susceptible to reactivation by R*. The model predicts that the decay of flash-activated cGMP phosphodiesterase (PDE*) is largely independent of the deactivation kinetics of R* at early postflash times (i.e., at times preceding or comparable with the lifetime tau E*) and that the lower-branch slope (approximately 0.3s) of the a-wave saturation function represent tau E*. The predicted early-stage independence of PDE* decay and R* deactivation furthermore suggests a basis for the relative constancy of the single-photon response observed in studies of isolated rods. Numerical evaluation of the model yields a value of approximately 6.7s for the time constant tau Ex.

Beta wave of the scotopic (rod) electroretinogram as a measure of the activity of human on-bipolar cells

The beta wave of the human electroretinogram (ERG) is widely believed to reflect the activation of on-bipolar cells. However, the shape of the beta wave is also influenced by the activity of other cell types. To assess how the activity of on-bipolar cells is reflected in the human ERG, rod ERG's were recorded in the dark and on the steady fields. Derived P2 responses were obtained by computer subtraction of the receptor contribution to the ERG. The light-adapted derived P2 was shown to have properties similar to those predicted from previous studies of on-bipolar activity. This was also true of the dark-adapted derived P2 if a small (less than 10%) contribution from a negative potential was taken into consideration. The derived P2, and under certain conditions the beta wave, can be used to study rod on-bipolar activity.

Phototransduction in human cones measured using the alpha-wave of the ERG

To study human cone phototransduction, the alpha-wave of the ERG was recorded from color normals, dichromats, and patients with retinitis pigmentosa. A model of the activation phase of phototransduction, previously fitted to responses from single rods and the rod alpha-wave, was modified and fitted to the human cone alpha-wave. The modified model fits the cone alpha-wave well and allows questions about human cone phototransduction to be addressed. In particular, we conclude that: (1) the amplification of the activation phase of human cone transduction is comparable to that of the human rods. (2) Steady lights have relatively little effect on the amplification of cone transduction. (3) The normal alpha-wave elicited by red flashes is dominated by the L cones, consistent with a ratio of L:M cones of > 1. (4) Retinitis pigmentosa caused by mutations of the rhodopsin gene can affect cone phototransduction. Finally, a simpler computational expression is shown to approximate the modified model's responses.

Rod transduction parameters from the a wave of local receptor populations

The analysis of electroretinogram a waves from locally stimulated populations of rods is complicated by the presence of scattered light within the eye. Scattered-light and cone contributions can be assessed after brief flashes of light designed to saturate only rods in the locally stimulated area. Subtracting the scattered-light and the cone responses from the local electroretinogram gives a pure rod a wave that can be fitted with models of photoreceptor activity. We demonstrate the feasibility of this technique by recording local rod a waves from a group of five normal subjects and by fitting the a waves with the rod model to derive transduction parameters. The local rod a waves are compared with expected responses derived from simulations in which the response of the entire retina to heterogeneous illumination is mimicked.

Abnormal activation and inactivation mechanisms of rod transduction in patients with autosomal dominant retinitis pigmentosa and the pro-23-his mutation

PURPOSE

The leading edge of the rod a-wave in normal human subjects can be fit with a computational model of the activation phase of transduction to provide parameters analogous to those obtained from individual photoreceptors. The authors extend this work to the kinetics of recovery after saturating flashes.

METHODS

Electroretinograms were recorded from three patients with autosomal dominant retinitis pigmentosa and the pro-23-his rhodopsin mutation, two patients with rod monochromatism, and five normal subjects. Rod-only a-waves were obtained for a series of flashes ranging from 4.4 to 10.1 ln (1.9 to 4.4 log) scot td-sec. One set of parameters describing the activation process was derived from fits to the a-wave model. A double-flash paradigm was used to study inactivation mechanisms. The first flash was achromatic and varied in intensity (I(f)) from 6.1 to 13.9 ln (2.6 to 6.0 log) scot td-sec. The second flash was a short-wavelength probe held constant at 9.3 ln (4.0 log) scot td-sec. Cone components were elicited with a photopically matched long-wavelength stimulus and were computer subtracted. Recovery at each I(f) was followed by measuring the amplitude to the probe flash at various interstimulus intervals (ISI). The critical time (Tc) before the initiation of rod recovery was determined from the function relating relative rod amplitude to ISI.

RESULTS

Recovery from activation was similar in normal subjects and in patients with rod monochromatism. Over a large range of I(f) above rod saturation, Tc increased in proportion to ln I(f). The mean slope of the function relating Tc to I(f) was 2.3 s/ln I(f) when I(f) varied between 11 and 13.9 ln scot td-sec. Patients with retinitis pigmentosa and the pro-23-his rhodopsin mutation had a decrease in the gain of activation. They also had significantly slower than normal recovery after high test flash intensities, such that the slope of the function relating Tc to ln I(f) was 12.1 seconds.

CONCLUSION

Available data from other species imply that complete, transient activation of transducin (T saturation) occurs within or below the investigated range of flash intensities. Based on the slope of the delay function (delta Tc/ delta ln I(f)) above 11 ln scot td-sec, the authors hypothesize that the lifetime of activated rhodopsin (R) in normal human rods is approximately 2.3 seconds. In patients with the pro-23-his mutation, the gain of the activation mechanism is reduced and the reaction determining the delta Tc/ delta ln I(f) slope is markedly slowed. The activated species that exhibits this prolonged lifetime could be the mutant rhodopsin itself.

Docosahexaenoic acid in red blood cells of patients with X-linked retinitis pigmentosa

PURPOSE

Abnormalities in the distribution of long-chain polyunsaturated fatty acids (LCPUFA) have been documented in plasma of patients with X-linked retinitis pigmentosa (XLRP). In this study, fatty acid profiles of red blood cells (RBC) were used as an index for LCPUFA metabolism in patients with XLRP because RBC lipids reflect membrane-associated fatty acids. Correlations between LCPUFA content and electroretinographic (ERG) function were assessed.

METHODS

Mean ages for the XLRP group (n = 18) and control group (n = 28) were 22 +/- 18 years and 24 +/- 16 years, respectively. Electroretinographic assessment included the International Society for the Clinical Electrophysiology of Vision standard protocol. Methyl esters of RBC fatty acids were analyzed by capillary column gas chromatography.

RESULTS

The content of the omega 3 fatty acid, docosahexaenoic acid (DHA), was 40% lower in the group with XLRP (23.1 +/- 5.9 micrograms/ml RBC [mean +/- 1 SD]) than in normal subjects (38.6 +/- 9.4 micrograms/ml RBC, t = 6.24, P < 0.0001). Total omega 3 LCPUFA content in patients with XLRP was reduced by 30% from normal levels compared to a 10% reduction in omega 6 LCPUFA levels. Elongation reactions for omega 3, omega 6, saturated fatty acids, and monounsaturated fatty acids were markedly lower for patients with XLRP than for normal subjects. Multiple regression analysis revealed that RBC-DHA was a significant determinant for amplitude and implicit time of cone ERG responses.

CONCLUSIONS

The overwhelming majority of patients with XLRP have lower levels of DHA in RBCs compared to normally sighted control subjects. An analysis of fatty acid profiles suggests a metabolic defect in fatty acid chain elongation mechanisms. The significant association between DHA content and cone ERG response parameters is consistent with an effect of lipid abnormalities on membrane environment and physiology in retinal photoreceptors.

Rod phototransduction in retinitis pigmentosa. Distinguishing alternative mechanisms of degeneration

PURPOSE

To test alternative hypotheses concerning the mechanisms of rod degeneration in retinitis pigmentosa (RP).

METHODS

Full-field rod electroretinograms and rod visual fields were measured for 15 patients with RP and a normal control group. The rod a-wave was fitted with a computational model based on known transduction biochemistry. The values of td (the initial delay), S (a sensitivity parameter), and RmP3 (the maximum amplitude) were estimated. Rod b-wave amplitudes were fitted with the Michaelis-Menten equation, and the parameters Kbw (the semisaturation intensity--a sensitivity parameter) and Vmax (the maximum amplitude) were estimated.

RESULTS

The patients all had significantly reduced values of RmP3, indicating rod receptor damage, and a wide range of S values. Three patients had S values in the normal range. Four had abnormal S values but normal thresholds in some locations in their visual field. The remaining patients had abnormal values of S and entirely abnormal visual fields. Three of those had a history of large elevations in Kbw. For all patients, the changes in Kbw and Vmax followed closely the changes in S and RmP3, respectively.

CONCLUSION

Retinitis pigmentosa has a varying initial impact on the activation phase of rod transduction. Available evidence suggests that the activation of transduction is initially normal in most patients with RP. In some patients, RP appears to result in a reduced transduction amplification from birth. In all patients, subsequent degeneration of the rods effects progressive reductions in transduction amplification but no other major functional changes. Outer segment shortening and local dropout of rods appear to have little functional impact.

Metabolism of omega-3 fatty acids in patients with autosomal dominant retinitis pigmentosa

Abnormalities in lipid metabolism have been reported in numerous patients with retinitis pigmentosa. As an initial step in evaluating these anomalies, two trials of fatty acid intervention were conducted with autosomal dominant retinitis pigmentosa (adRP) patients and controls. The first trial addressed absorption and incorporation of omega 3 long-chain fatty acids from a fish-oil concentrate into red blood cell (RBC) lipids. The utilization of omega 3 long-chain fatty acids by adRP patients was found to be equivalent to that of controls. The second trial addressed the conversion of precursor, eicosapentaenoic acid (EPA, 20:5 omega 3), to end-product, docosahexaenoic acid (DHA, 22:6 omega 3), following oral supplementation of EPA ethyl ester. Although the levels of EPA and the intermediate, docosapentaenoic acid (22:5 omega 3), were both elevated by EPA supplementation in RBCs of adRP patients with rhodopsin gene mutations and controls, DHA production was elevated only in controls. Based on these results, we suggest the presence of a metabolic defect in the final stages of DHA biosynthesis.

Rod phototransduction in retinitis pigmentosa: estimation and interpretation of parameters derived from the rod a-wave

PURPOSE

Relationships between the Lamb and Pugh model of transduction in single rod photoreceptors and the parameters derived from human rod a-waves are examined to evaluate transduction abnormalities in retinitis pigmentosa (RP) and cone-rod dystrophy (CRD).

METHODS

ERGs were obtained from 15 patients, 11 with RP and 4 with CRD, and from 15 normal subjects. Rod-only responses were derived from responses to blue (W47B) flashes over a wide range of flash energies by computer-subtracting estimates of the cone contribution. A model of the rod's response was fitted to the a-waves of the rod-only responses as well as to the responses to the blue flashes by estimating three parameters-sensitivity (S), delay (td), and maximum response (RmP3). To assess the limits of the cone contributions to the parameters of the model, ERGs were obtained from two of the patients, two normal subjects, and three additional patients with nondetectable rod responses using white flashes.

RESULTS

All patients had significantly decreased values of RmP3, and eight had significantly decreased values of S compared to the control subjects. The values of the parameters estimated from the responses to the blue flashes were reasonably similar to those for the rod-only responses except in patients with nondetectable rod signals. For the white flashes, the estimate of S was considerably larger and the fit of the model noticeably poorer.

CONCLUSIONS

The authors reached two conclusions. First, some forms of RP and CRD alter the activation stages of transduction. Transduction appears normal in other patients, despite extensive receptor damage as indicated by markedly reduced RmP3 values. Second, the cone contribution to a blue flash only affects the estimate of the key parameters in patients in whom the rod component is minimal. The cone contribution to a white flash contaminates the fit and the estimate of the parameters in all patients and normal subjects.