Photoreceptor function in heterozygotes with insertion or deletion mutations in the RDS gene

PURPOSE

To understand the pathophysiology of human retinal degenerations caused by mutations in the peripherin/RDS gene.

METHODS

Three families with autosomal dominant retinal degeneration were found to have mutations in the peripherin/RDS gene. There were two frameshift mutations: a 1-base pair (bp) insertion at codon 32 and a 2-bp deletion at codon 193. For these mutations, the predicted proteins would be truncated by 303 and 131 amino acids, respectively. The third mutation would result in an 8-bp substitution for five nucleotides involving codons 67-69 and would be predicted to disrupt the second transmembrane domain of the protein. Heterozygotes were examined clinically and with rod and cone perimetry, dark adaptometry, and rod- and cone-isolated electroretinograms (ERGs).

RESULTS

Rod and cone sensitivity losses were present with perimetric testing in most patients; patients with advanced disease in all three families showed more pericentral than peripheral field dysfunction. The kinetics of dark adaptation were abnormal in all patients. Rod and cone ERG a-waves were normal in maximum amplitude in three younger patients but were reduced in all others; phototransduction was normal in most patients. There was equal loss of rod and cone a-wave amplitudes and equal elevation of rod and cone thresholds.

CONCLUSIONS

Heterozygotes with these different peripherin/RDS gene mutations showed variation in clinical presentation but a similar pattern of receptor abnormalities. Results of visual function tests were consistent with a normal amount of rod and cone outer segment membrane in early disease, progressing to reduced outer segments at later stages. There was an equal effect on rod and cone photoreceptor function at all stages of disease. This functional phenotype may represent the human analogue of the rds/+ mouse.

Chloroquine-induced lipidosis in the rat retina: functional and morphological changes after withdrawal of the drug

BACKGROUND

The antimalarial and antirheumatic drug chloroquine is one of the most infamous amphiphilic cationic drugs in clinical ophthalmology. It is known to cause lipidosis and photoreceptor degeneration in the human and the rat retina.

METHODS

We treated female albino Wistar rats (mean weight 200 g) orally with chloroquine (95 mg/kg body weight) for 12 weeks, followed by a period of 4 months with normal feed. After initial electroretinography in all rats, measurements were made after 4 and 12 weeks of treatment and 16 weeks after withdrawal. The rats were prepared for histological examination.

RESULTS

Treatment of rats with chloroquine caused severe lipidosis in the neuroretina; photoreceptor cell degeneration was slight. After 12 weeks of treatment, the b-wave amplitude was reduced to 30% of the initial value; the a-wave amplitude was reduced, but remained within the range of normal values. After withdrawal of chloroquine the lipidosis remitted, but the degeneration of the photoreceptor cell layer continued to progress. Despite remission of lipidosis, electroretinography demonstrated functional disturbances, marked by reduction of the a- and b-wave amplitudes to 25% and 16% of initial values, respectively.

CONCLUSIONS

Seen from the point of view of function, it is doubtful whether lipidosis is the primary cause of changes in the electroretinogram or of receptor cell degeneration.

Clinicopathologic effects of the Q64ter rhodopsin mutation in retinitis pigmentosa

PURPOSE

To correlate retinal histopathology with functional changes caused by the rhodopsin Q64ter mutation.

METHODS

A 50-year-old female heterozygote was evaluated clinically and with psychophysical and electroretinographic measurements of rod and cone function. The retinas obtained after death were examined microscopically, including immunolabeling with antibodies against the C- and N-termini of rhodopsin.

RESULTS

On clinical examination 4 months before death, patient's acuity was 20/60, and she had midperipheral scotomas with retained function centrally and in the far periphery. The rod electroretinogram (ERG) was undetectable, and the cone ERG was reduced in amplitude with abnormal receptoral and postreceptoral responses. A previous study of the phenotype of mildly affected family members of the donor suggested that the rod outer segments (ROS) were shortened and that only wild-type rhodopsin was functional. The retinas contained only scattered cones in the midperiphery; the maculas and far peripheral regions contained reduced numbers of rods and cones with short to absent outer segments. The ROS appeared to contain wild-type, but not mutant, rhodopsin, and many peripheral rods had sprouted long rhodopsin-positive neurites that projected into the inner retina. Many cone synapses were abnormal, and the axons of some peripheral cones reached the inner plexiform layer.

CONCLUSIONS

Microscopic changes in the donor retinas correlated well with the abnormalities in visual function in the patient donor and other family members. Postreceptoral ERG defects may relate to the abnormal photoreceptor processes found in the inner retina.

Light-induced acceleration of photoreceptor degeneration in transgenic mice expressing mutant rhodopsin

PURPOSE

Mutations at various loci on the rhodopsin gene have been shown to cause autosomal dominant retinitis pigmentosa (ADRP). One of the most common is a point mutation (P23H) near the N-terminus of the protein. The authors have studied the effects of light deprivation on the rate of degeneration in pigmented transgenic mice expressing the P23H mutation as well as two additional mutations near the N-terminus of opsin (V20G, P27L).

METHODS

Transgenic and normal littermates were reared in darkness or in cyclic light (approximately 7 foot-candle) for periods of 2, 4, or 6 months. Retinal structure and function were evaluated by electroretinography, retinal densitometry, light microscopy, and TUNEL labeling.

RESULTS

Retinas of normal animals, whether reared in darkness or in cyclic light, had no structural or functional abnormalities. The rate of photoreceptor degeneration in dark-reared transgenic mice was significantly slower than in transgenic mice raised under cyclic light conditions. Differences between the two groups of animals were evident in the retinal histology, the electroretinographically determined sensitivity to photic stimulation, and the rhodopsin levels in the retina. TUNEL labeling of retinal wholemounts showed that cyclic light-reared animals had a threefold higher incidence of photoreceptor cell death than their dark-reared counterparts; the density of apoptotic cells was greatest in the inferior retina, the region most severely affected in patients with the P23H mutation. In comparison, photoreceptor cell death was more uniformly distributed across the retina in dark-reared transgenic mice.

CONCLUSIONS

These findings suggest that light activation of rhodopsin contributes to the severity of the degenerative disease resulting from the P23H opsin mutation, and they raise the possibility that minimizing exposure to light may help to prolong useful vision of patients with this form of retinitis pigmentosa.

Photoreceptor function in infants and children with a history of mild retinopathy of prematurity

Five infants and children with a history of mild retinopathy of prematurity (ROP) were tested for postulated alterations in rod photoreceptor function. The photoreceptor responses were derived from the electroretinographic alpha waves. Postreceptoral components, the beta wave and the oscillatory potentials, were also examined. The saturated amplitude and sensitivity of the rod photoreceptor responses were low, except for the sensitivity in one patient. The beta-wave sensitivity was low, but saturated amplitudes were within the 95% prediction interval for normal. The amplitudes of the oscillatory-potential responses were also attenuated. The results indicate that retinal dysfunction may be present in patients with a history of mild ROP long after the ROP has completely resolved. Additionally, the data suggest that the photoreceptors are the primary site of retinal dysfunction in mild ROP.

Acute zonal occult outer retinopathy

Retinal photoreceptor dysfunction is an uncommon and often unrecognized cause of acute visual loss. Acute zonal occult outer retinopathy (AZOOR) has been reported to cause cone and rod dysfunction. Patients with AZOOR may present with normal visual acuity, normal fluorescein angiography, and a normal fundus examination despite severe loss of visual field. A healthy young white female presented with acute, unilateral loss of visual field and an afferent pupillary defect, but normal visual acuity, color vision, fundus examination, and fluorescein angiogram. A pattern visual evoked potential was normal, but an electroretinogram showed a unilateral peripheral photoreceptor dysfunction consistent with the diagnosis of AZOOR. Ophthalmologists should be aware of the diagnosis of AZOOR and should consider an ERG in the evaluation of any patient with unexplained visual field loss even in the presence of normal visual acuity, color vision, fluorescein angiography, or retinal examination.

Electroretinographic alterations in the Laurence-Moon-Bardet-Biedl phenotype

Maximal 0.5-Hz and cone 30-Hz ERG responses were recorded from 19 patients showing a Laurence-Moon-Bardet-Biedl (LMDD) phenotype. Off-line averaging of 80 to 100 iterations was routinely performed. When needed, our previously described low-noise techniques and off-line fast Fourier transform procedures were used. The maximal ERG response was non-detectable in 52.6% of cases. About half of the recordable signals were below 5% of the lower normal amplitudes. Cone 30-Hz ERGs were measurable in 64.7% of cases. Of these, 63% of tracings were below 5% of the lower normal range. In most cases no dystrophic pattern was definable, due to severe reduction of both signals. Statistical analyses showed no correlation between ERG amplitudes and residual visual field areas. Clinical and electroretinographic observations suggest that retinopathy in most LMBB patients is a widespread form of degeneration, initially affecting rods but rapidly involving cones as well. However, there are also cases with a clear-cut cone-rod pattern, with fairly well preserved maximal ERG responses. The lack of correlation between maximal ERG responses and visual field residual areas, different from non-syndromic retinitis pigmentosa (RP) patients, could be related either to a low reliability of visual field testing in LMBB patients or to mechanisms accounting for the ongoing retinal degeneration in LMBB syndrome that are different from those of pure RP. Variable findings are in line with the documented genetic heterogeneity of the syndrome.

Effects of mitomycin C on the rat retina

To evaluate the retinal toxicity of mitomycin C injection in the rat eye, we conducted an electroretinographic study and a histopathologic study. Three different concentrations of mitomycin C (0.2, 0.3 and 0.4 mg/ml) were injected into either the vitreous cavity or the anterior chamber of the experimental eyes. A full-field electroretinogram was recorded before injection and 2, 4 and 7 days after injection. The retinas of all eyes were examined by light and electron microscopy. We found no evidence of electroretinographic and histologic changes 2 and 7 days after injection of 0.4 mg/ml of mitomycin C into the anterior chamber. However, profound electroretinographic changes did follow injection of the drug into the vitreous. These were absent with the 0.2-mg/ml solution at 7 days, mild with the 0.3-mg/ml solution at 7 days and profound with the 0.4-mg/ml solution as early as 2 days. Intravitreal injection of 0.4 mg/ml, however, showed selective degeneration of Müller cell process at day 2, retinal pigment epithelium changes at day 4 and irregular arrangement of the outer nuclear layer and photoreceptors at day 7. Intravitreal injection of mitomycin C in a concentration comparable to the one used clinically could cause retinal disorders, both functionally and histologically.

Electrophysiological findings in patients with Oguchi's disease

Seven patients with Oguchi's disease were examined by conventional full-field rod and cone electroretinograms (ERGs), spectral characteristic studies with photopically matched short- and long-wavelength stimuli (3 of the 7 patients) and electrooculogram (EOG) (6 patients). ON and OFF photopic ERG responses, recorded with rectangular stimuli, in 3 patients were compared with those of 13 normal subjects and 13 patients with complete congenital stationary night blindness (CSNB). Conventional full-field ERGs recorded after 30 minutes of dark adaptation revealed absent rod, but essentially normal cone and flicker ERGs in all patients. With the dark-adapted standard flash ERG, the a-wave amplitude was significantly lower than normal in all patients. The spectral characteristic study indicated that the ERG recorded after 30 minutes of dark adaptation represented cone activity. The EOG was abnormal in all patients examined. The ON and OFF responses of the photopic ERG were normal in Oguchi's disease patients; however the ON responses were reduced in patients with complete CSNB. The absence of a rod a-wave, with the abnormal EOG, strongly suggests that the rod itself is abnormal in Oguchi's disease. This finding differs from the report by Carr and Gouras (1965) and Carr and Ripps (1967). The normal ON and OFF responses in the photopic ERGs of Oguchi's disease patients contrast with those of the complete CSNB patients in whom only the ON response is reduced.

[The pathogenesis of diabetic dyschromatopsia]

Essential data about the physiology of colour vision are reminded, with a stress layed on the possible contribution of the rodes in perception of blue and green colours. The main features of the diabetic dyschromatopsia are reviewed, together with their pathomorphological support and the two existing theories regarding its pathogenesis: vascular origin versus neuronal origin of the defect. The contribution of the photocoagulation to these alterations is also discussed.

Cone-rod dysfunction in patients with unexplained reduction in visual acuity

Electrophysiologic tests were performed in 233 patients who complained of reduced visual acuity with no satisfactory clinical explanation. The functional integrity of the retina was assessed from the light- and dark-adapted electroretinogram. Macular function and conduction in the optic nerves were estimated from the flash visual evoked potentials. Of the 233 patients 78 were grouped together on the basis of the electrophysiologic and clinical findings. They were characterized by subnormal electroretinogram responses with the cone system more affected than the rod system. The flash visual evoked potential responses were of abnormal waveform and prolonged implicit times. Most of these patients exhibited normal fundi. The reduction in visual acuity, the degree of electroretinogram deficits and the pattern of the visual evoked potential responses were similar in both eyes of each patient, indicating a symmetric disorder. Slight deterioration of visual acuity and electrophysiologic variables were observed in 37 of the patients who were followed up over a period of up to 8 years. The electrophysiologic findings indicate that about 20% of patients complaining of unexplained reduction in visual acuity were suffering from a diffuse retinal disorder affecting the peripheral retina as well as the macular region. On the basis of electrophysiologic findings and clinical symptoms, we suggest grouping these patients under a new entity: cone-rod dysfunction.

On- and off-responses in the photopic electroretinogram in complete-type congenital stationary night blindness

We examined the on- and off-responses of the photopic electroretinogram in patients with complete congenital stationary night blindness. Standard flash electroretinograms as well as those produced in a ganzfeld modified for long-duration light stimuli (500 msec) permitted the separation of on- and off-responses in four patients and four normal subjects. The amplitude and latency of the elctroretinogram on-response (a- and b-waves) and off-response (d-wave) in addition to the oscillatory potentials of the off-response in normal subjects and patients were compared. The abnormal on-response was demonstrated in all the patients, and the off-response with its oscillatory potentials were preserved. We showed that the second portion of the off-response (of inner retinal origin) is normal. If congenital stationary night blindness is a defect of depolarizing bipolar cells, these results preclude input of the depolarizing bipolar cells and support the hyperpolarizing bipolar cells as the cellular origin of the off-response electroretinogram.

Autosomal dominant cone-rod dystrophy associated with a Val200Glu mutation of the peripherin/RDS gene

OBJECTIVE

Mutations of the peripherin/RDS gene have been reported in several kinds of retinal dystrophy, and they show a variety of manifestations. The authors identified a novel Val200Glu mutation of the peripherin/RDS gene in a Japanese family with autosomal dominant cone-rod dystrophy (CRD). This report describes a genotype-phenotype correlation of the Val200Glu mutation.

PATIENTS AND METHODS

Fifteen members of one Japanese family with autosomal dominant CRD were screened for mutations in the peripherin/RDS and ROM 1 genes. Clinical features were identified by visual acuity, visual field testing, fundus examination, and electroretinography.

RESULTS

A Val200Glu mutation was found in all of the affected family members examined and was segregated with the disease. No patient had a mutation in the ROM 1 gene. Phenotypic characteristics of each affected member in this family showed intrafamilial similarity. Characteristic features included cone function more severely impaired than rod function and degenerative change in the macular region associated with peripheral retinal degeneration.

CONCLUSION

The mutation at codon 200 of the peripherin/RDS gene causes both cone and rod degeneration. The Val200Glu mutation results in a type of autosomal dominant CRD.

Preservation of inner retinal responses in the aged Royal College of Surgeons rat. Evidence against glutamate excitotoxicity in photoreceptor degeneration

PURPOSE

The aged Royal College of Surgeons (RCS) rat with advanced retinal degeneration loses the b-wave and shows a negative-going corneal electroretinogram (ERG) that has been attributed to loss of inner retinal function because of glutamate toxicity. The authors investigated the origin of this negative ERG and evaluated inner retinal function in late-stage RCS degeneration.

METHODS

The ERG a-wave, b-wave, and scotopic threshold response (STR) were used to follow degeneration in RCS dystrophic animals between 18 and 120 days of age. Glutamate analogs were given by intravitreal injection to suppress transmission from photoreceptors to second- and third-order neurons to identify the origin of the negative ERG observed in older RCS dystrophic rats.

RESULTS

In RCS dystrophic animals, the ERG developed normally up to day 27, but thereafter a- and b-wave sensitivity deteriorated more rapidly than the STR. By day 60, the STR threshold was elevated only 1 log unit, whereas a- and b-wave thresholds were > 2 log units higher than in controls. The STR range in dystrophic rats extended to brighter intensities previously dominated by the b-wave. Glutamate analogs eliminated the STR as well as the entire negative-going ERG in older dystrophic rats.

CONCLUSIONS

The negative ERG in older RCS dystrophic rats originates in the inner retina and not from photoreceptors. Inner retinal signaling remains sensitive despite major photoreceptors loss in RCS rats, consistent with previous psychophysical findings. The b-wave may not be as useful as the STR in detecting loss of quantal catch in degenerating retinas. The ERG provides no evidence of glutamate excitotoxic damage to neurons postsynaptic to degenerating RCS photoreceptors.

[Suppressive rod-cone interaction in complete and incomplete congenital stationary night blindness]

Dark-adapted rods have a suppressive effect on cone response (suppressive rod-cone interaction: SRCI). This phenomenon has been applied to analyze the pathological mechanism of congenital stationary night blindness (CSNB), but the results have been discrepant among the investigators. Alexander et al reported that 2 individuals with CSNB exhibited a normal SRCI, but Arden and Hogg reported an absence of SRCI in 4 cases of CSNB. We evaluated SRCI in complete and incomplete CSNB. SRCI was tested by measuring luminance threshold for flicker detection (685nm, 1.7 degrees in diameter, 20Hz) across the horizontal meridian of the visual field without and with full-field background illumination. In twelve normal subjects, a reduction of the flicker threshold was observed during presentation of the background. Five cases with complete CSNB showed normal SRCI, but in one case we failed to detect SRCI except at one tested locus. Similarly 5 cases of incomplete CSNB showed normal SRCI, but in one case we failed to detect SRCI except at one tested locus. These results may indicate that SRCI is normal in both complete and incomplete CSNB, but in some cases, SRCI is absent because of localized retinal dysfunction.

Functional consequences of oncogene-induced photoreceptor degeneration in transgenic mice

This study evaluated retinal function in mice following the expression of oncogenes under the control of photoreceptor-specific promoters in transgenic mice. Electroretinograms (ERGs) were recorded under stimulus conditions chosen to elicit rod- or cone-mediated components. In one transgenic line (MOT1), the simian virus 40 large tumor antigen was expressed under the control of the mouse opsin promoter. MOT1 mice exhibited an age-related decline in the amplitude of the rod-mediated ERG a-wave. In comparison, cone-mediated responses recorded from MOT1 mice remained normal up to four months of age, the oldest age tested. In the second transgenic line (CMYC), the rat c-myc gene was expressed under control of the human interphotoreceptor-retinoid binding protein promoter. CMYC mice exhibited a rapid reduction of cone-mediated responses and a gradual loss of the rod ERG a-wave. Analysis of rod ERG a-waves obtained from young MOT1 and CMYC mice indicated that the rod ERG abnormalities reflect a reduction in the number of rods contributing to the response with the retention of normal response properties in rods that remain. These results support the possibility that aberrant expression of oncogenes may underlie some forms of human rod and cone-rod dystrophy.

[Transplantation of retinal components]

BACKGROUND

The objective of recent animal experimental studies is to substitute destroyed or degenerated retina and to improve visual function by transplanting retinal components. Mature or immature retinal components were transplanted into the subretinal space of different animal species.

METHODS

Embryonic and non-embryonic retina components of different animals or of humans are used as grafts. They are transplanted to adult host animals. There is the possibility of utilizing a posterior transscleral approach or a vitreoretinal approach. The latter offers a better visual control. Transplantation of retinal pigment epithelium could rescue photoreceptors in adult rats with inherited or age-related macular degeneration.

RESULTS

After the transplantation it was possible to measure significantly higher visual evoked potentials in the transplanted eye than in the untreated fellow eye. Additionally, an almost normal pupillary reflex was identified. As it is not possible to adequately determine visual acuity in rodents, it is necessary to examine, as to whether visual improvement is also evident in primates.

CONCLUSION

The eye is an immunologically privileged location for transplants because of retinal autoantigenic specific immunosuppression that protects the transplant from immunologic rejection. Many questions regarding operation technique, gaining of transplantation tissue and immunoreaction are to be answered before the use is possible on humans. The counseling of the patients and the informational policies towards the media should be correspondingly retentive.

Chromosome 19q cone-rod retinal dystrophy. Ocular phenotype

OBJECTIVE

To describe the phenotype in a family with dominantly inherited cone-rod dystrophy with chromosome assignment to a 19q locus, and to correlate this with current classifications of this retinal dystrophy.

DESIGN

A detailed clinical examination including Goldmann perimetry was undertaken in all family members. Six members under the age of 30 years underwent dark-adapted electroretinography, color contrast-sensitivity measurement, dark-adapted static perimetry, and dark adaptometry.

PATIENTS

The study included 34 affected and 22 unaffected patients in four generations of a pedigree that manifested autosomal dominant cone-rod retinal dystrophy linked to a chromosome 19q locus by genetic linkage analysis.

RESULTS

Loss of visual acuity occurred in the first decade of life, onset of night blindness occurred after 20 years of age, and little visual function remained after the age of 50 years. Central and, later, peripheral retinal fundus changes were associated with central scotoma, pseudoaltitudinal field defects, and finally global loss of function. Psychophysical and electrophysiologic testing before the age of 26 years showed more marked loss of cone than rod function.

CONCLUSIONS

The phenotype associated with this mutation does not fit well into previous subtypes of cone-rod dystrophy. Further studies will be needed to correlate specific genetic mutations in this group of conditions with the various clinical phenotypes.

[Oscillatory potentials in retinitis pigmentosa with TCM treatment]

Computer-averaged oscillatory potentials (OPs) of electroretinogram (ERG) were recorded in 54 patients affected by different hereditary forms of retinitis pigmentosa (RP) before and after three-months of traditional Chinese medicine (TCM). This study was conducted in order to investigate the alterations of physiological function in inner part of the retina affected by RP. The results showed that OP waveforms were recordable in 22 and 27 patients before and after TCM treatment, respectively. The amplitudes of summed OPs (sigma OPs) were increased significantly in patients with different hereditary modes of RP, and also in those by clinical TCM classification after the treatment. The component-specific amplitudes of OPs were observed. In particular, OP2 showed an elevation in both dominant and nondominant patient groups, while OP3 only showed an elevation in the dominant group. The results suggest that OP2, which represents mainly cone OP activity, could be improved in the patients extensively; and OP3, which represents mainly rod OP activity, could be improved only in the patients with less advanced retinal degeneration. These data provide evidence that visual function in the inner retina of patient with RP could be improved to a certain extent with TCM treatment.

Abnormal electroretinogram associated with developmental brain anomalies

PURPOSE

We have encountered abnormal ERGs associated with optic nerve hypoplasia, macular, optic nerve and chorioretinal colobomata and developmental brain anomalies. Brain anomalies include cortical dysgenesis, lissencephaly, porencephaly, cerebellar and corpus callosum hypoplasia. We describe six exemplar cases.

METHODS

Scotopic and photopic ERGs adherent to international standards were performed as well as photopic ERGs to long-duration stimuli. CT or MRI studies were also done. The ERGs were compared to age-matched normal control subjects.

RESULTS

ERG changes include reduced amplitude b-waves to blue and red stimuli under scotopic testing conditions. Implicit times were often delayed. The photopic responses also showed reduced amplitude a- and b-waves with implicit time delays. The long-duration photopic ERG done in one case shows attenuation of both ON- and OFF-responses.

CONCLUSIONS

Common underlying developmental genetic or environmental unifying casualties are speculated to be at fault in causing these cases of associated retinal and brain abnormalities. No single etiology is expected. Multiple potential causes acting early in embryogenesis effecting neuronal induction, migration and differentiation are theorized. These occur at a time when brain and retinal cells are sufficiently undifferentiated to be similarly effected. We call these cases examples of Brain Retina Neuroembryodysgenesis (BRNED). Homeobox and PAX genes with global neuronal developmental influences are gene candidates to unify the observed disruption of brain and retinal cell development. The ERG can provide a valuable clinical addition in understanding and ultimately classifying these disorders.

Ocular findings associated with a 3 base pair deletion in the peripherin-RDS gene in autosomal dominant retinitis pigmentosa

Affected members of a family with autosomal dominant retinitis pigmentosa were found to have a 3 base pair deletion at codon 118 or 119 of the retinal degeneration slow gene. This mutation causes the loss of a highly conserved cysteine residue in the predicted third transmembrane domain of peripherin-rds, a photo-receptor specific structural glycoprotein localised to both rod and cone outer segment disc membranes. Four of these individuals underwent detailed clinical, psychophysical, and electroretinographic testing in order to characterise their photoreceptor dysfunction. Nyctalopia was reported early in the second decade by all patients. Global rod and cone dysfunction was recorded by the third decade with severe reduction of both photopic and scotopic function by age 30 years. This retinal degeneration slow gene mutation may lead to the primary loss of both rod and cone photo-receptor function.

Eye and head movements in patients with achromatopsia

The diagnosis of patients with rod monochromatism (RM) and blue-cone monochromatism (BCM) may be difficult. The relative direction and symmetry of nystagmus of the two eyes, as well as the existence or nature of rhythmic head movements, are not known. We analyzed simultaneous eye and head movement recordings of 16 patients with RM and three patients with BCM. Longitudinal examinations were performed in seven patients. Younger patients had pendular, intermittent or continuous oblique nystagmus with both eyes oscillating in phase or out of phase with equal amplitudes. Older patients had continuous symmetrical oblique jerk nystagmus with decreasing velocities in the slow phase. In two children, we demonstrated evolution from pendular to predominantly jerk nystagmus. Rhythmic head movements were detected in all children. Patients with RM and BCM exhibit a distinct entity of nystagmus and can be differentiated from patients with congenital or latent nystagmus. However, eye and head movements can mimic spasmus nutans.

Early selective neuroretinal disorder in prepubertal type 1 (insulin-dependent) diabetic children without microvascular abnormalities

The duration of diabetes before puberty is not considered relevant to the future development of complications. To evaluate the effects of diabetes on the neural retina, we analysed macular function by steady-state focal electroretinography in 20 prepubescent diabetic children without vascular retinopathy and in 39 sex- and age-matched normal children. The mean (+/- SD) response related to retinal cellular elements between the photoreceptors and ganglion cells was significantly lower in diabetic children than in the control group (0.38 +/- 0.12 vs. 0.51 +/- 0.13 microV; unpaired t-test = 3; P = 0.005). Similarly, ganglion cell function showed a significant impairment in diabetic children with respect to the control group (0.4 +/- 0.13 vs. 0.53 +/- 0.09 microV; unpaired t-test = 5.4; P = 0.0001), whereas the photoreceptors appeared unaffected. Metabolic control and disease duration were not correlated with functional deficits. Our results suggest that before puberty, early diabetes may have a selective effect on the neural retina prior to the appearance of microvascular changes. A focal electroretinogram could identify diabetic children with neurosensory disorders who may have a higher risk of developing microvascular retinopathy.

Electrophysiologic and electroretinographic evidence for photoreceptor dysfunction as a toxic effect of digoxin

PURPOSE

To investigate photoreceptor dysfunction caused by digoxin toxicity.

METHODS

First, a patient who experienced toxic side effects from digoxin was studied acutely by serial electroretinography and later during convalescence. Second, the light responses of isolated photoreceptors exposed to varying amounts of digoxin were studied in vitro.

RESULTS

Electroretinographic amplitudes were reduced and implicit times were delayed when digoxin levels were elevated and recovered slowly after return to normal digoxin levels. Isolated photoreceptors exhibited concentration-dependent reductions in the magnitude of the light response during digoxin exposure, suggesting reduction in the dark current due to blockade of the sodium-potassium-adenosine triphosphatase pump. Cones were about 50-fold more sensitive than rods.

CONCLUSIONS

Reversible rod and cone dysfunction occur during exposure to toxic levels of digoxin. Photoreceptor dysfunction is probably due to the diminution of the dark current in response to the sodium-potassium-adenosine triphosphatase blockade.

Understanding changes in the b-wave of the ERG caused by heterogeneous receptor damage

PURPOSE

To understand better the relationship between heterogeneous receptor damage and the changes in the b-wave of the rod ERG.

METHODS

A computational model of the b-wave is used to simulate b-waves from retinas with two regions (a healthier and a more affected region) differing in area and sensitivity. The peak-to-peak amplitudes of the simulated b-waves are fitted with a Naka-Rushton equation, and the parameters log K and Vmax are estimated. Insights gained from these simulations are tested against rod ERGs and rod visual fields from patients with autosomal dominant retinitis pigmentosa (n = 11) and cone-rod dystrophy (n = 17).

RESULTS

In the simulated retinas, Vmax decreases when the more affected region of receptors is less sensitive than the healthier region by more than 0.5 log unit. However, the relative change in Vmax does not match the relative area of the more affected region unless this region is depressed by 2.0 log units or more. As the more affected region loses sensitivity, log K at first increases but then decreases and approaches the log K of the healthier region for losses greater than 2.0 log units or so relative to the healthier region. For the patients' data, the simulations predict the general relationships observed between the summary statistics of the visual fields (e.g., area of field and mean of log sensitivity changes in the field) and the changes in log K and Vmax.

CONCLUSION

A decrease in Vmax indicates that regions of the patient's retina have lost 0.5 log unit or more of sensitivity. An increase in log K indicates that either the healthiest part of the patient's retina is abnormal by at most delta log K, a large part of the retina has lost considerable sensitivity (i.e., 0.5 to 2.0 log units) but is still contributing to the response, or both.