Photoreceptor orientation in iris coloboma

PURPOSE

Investigate photoreceptor orientation and alignment in an observer with a simple coloboma of the iris and no retinal involvement.

METHOD

Using a psychophysical procedure, extensive measurements of the Stiles-Crawford functions of the first kind (SCE I) were obtained from this observer.

RESULTS

In this observer, the usual center-of-the-exit-pupil-of-the-eye alignment tendency of photoreceptors (inferred from the peaks of the SCE I functions) is absent. The inferred orientation of photoreceptors at numerous retinal loci tested lies outside the pupil, opposite the direction of the downwardly displaced pupil center.

CONCLUSIONS

These results add support to previous suggestions that nonphototropic forces such as retinal tractional forces should be considered as an integral part of the mechanism(s) subserving human photoreceptor alignment in both normal and anomalous observers.

Photoreceptor dysfunction in central areolar choroidal dystrophy

Central areolar choroidal dystrophy (CACD) is a rare, bilateral, autosomal dominant macular dystrophy. Early in the course of the dystrophy, there is fine, symmetric mottling of the retinal pigment epithelium in the macula, often in a bull's-eye pattern. This progresses to geographic atrophy of the macula without noticeable drusen or flecks. This article describes the funduscopic, angiographic, and electrophysiologic findings in three patients with three generations of a pedigree with CACD. Generalized photoreceptor dysfunction was present in the oldest patient and manifested by delayed electroretinographic implicit timing. Age-related generalized photoreceptor abnormality is hypothesized in CACD.

[Acute eye damage from ultraviolet radiation]

The eyes of rabbits were UV-irradiated in a range close to the natural spectrum at a dose of 23.2 kJ/m2 (biologically effective dose 1.3 kJ/m2). The condition of optic media was estimated, as well as the parameters of electroretinogram (ERG) induced by single light stimulus of varying intensity and paired light stimuli with different intervals between them. As a result a distinct photokeratoconjunctivitis developed, the amplitude of the ERG a-wave decreased by 30-50% and that of b-wave by 35%, while the latencies of the both waves and the critical frequency of flashes remained unchanged. The cornea transparency did not decrease markedly even during the period of maximum photokeratoconjunctivitis and the changes in the ERG parameters were mostly due to the retina damage by the indirect effect of UV irradiation.

Butterfly-shaped macular dystrophy. Longitudinal case study of two siblings

Two brothers with butterfly-shaped macular dystrophy (BSMD) are reported. The anatomical and functional data were analyzed in 11 family members. Progressive photoreceptor dysfunction, supported by ERG abnormalities, was documented in both patients. The progression of the disease was observed over a period of seven years. The present observations emphasize that BSMD, at least in some cases, can be a chronic progressive disorder with secondary involvement of the photoreceptors.

Genetic mapping of a cone and rod dysfunction (Aland Island eye disease) to the proximal short arm of the human X chromosome

A five generation family with an X linked ocular disorder has been investigated. The major clinical features were reduced visual acuity, nystagmus, and myopia. Although impaired night vision was not a symptom, using psychophysical and electrophysiological testing both rod and cone function were found to be abnormal in all affected males. No abnormality was detected in carrier females. Gene location studies showed X linked transmission of a gene that maps to proximal Xp11. The findings observed in this cohort are similar to those previously reported in both congenital stationary night blindness type 2 (CSNB2) and Aland Island eye disease (AIED). This study addresses whether CSNB2 and AIED are a single entity or whether the latter is a subset of the former.

[Peripheral retinal correspondence after retinal detachment]

BACKGROUND

Occasionally, patients who underwent surgery for complicated retinal detachment report spatial distortions. This may be due to a translocation of photoreceptors when they become reattached to the pigment epithelium. We wanted to find out if, and to what extent, similar translocations occur after a simple buckling operation.

METHODS

Binocular retinal correspondence was used as an indicator of photoreceptor translocation. Correspondence was measured in the peripheral visual field at eccentricities of 5, 10 and 20 degrees, using a mirror haploscope. We examined two normal observers, one patient having had surgery for complicated retinal detachment including retinotomy, and 23 patients who had simple radial buckling operations.

RESULTS

The patient who had received complicated detachment surgery including retinotomy revealed a tremendous shift in correspondence. Of the 23 patients who had simple radial buckle operations, only 10 were able to sustain central fixation while being tested in the periphery. Of these 10 patients, 2 showed distortions of peripheral retinal correspondence in the area of previous retinal detachment. However, only one of them remembered some spatial distortion after surgery.

CONCLUSION

As a rule, photoreceptors become correctly reattached after radial buckle operations for retinal detachment. Rarely, vertical or horizontal translocations of about 1 to 2 degrees occur, but this does not usually lead to metamorphopsia.

The migratory potential of vitally labelled microglial cells within the retina of rats with hereditary photoreceptor dystrophy

We employed a recently developed technique of labelling retinal microglial cells to identify these cells within the retinal parenchyma and to determine their role during degenerative diseases. In a first set of experiments we retrogradely labelled the ganglion cells from the superior colliculus of neonatal pups of the royal college of surgeons (RCS)-strain with the fluorescent dye 4Di-10ASP [N-4-4-(4-didecylaminostyryl-N-methylpropidium iodide)] at the day of birth. In a second group of young rats of the same strain retrograde staining of ganglion cells was performed by applying the fluorescent dye at the cross-sectional area of the transected optic nerve. In both experimental groups, prelabelled ganglion cells die. In the neonatal rats ganglion cells disappear because the initial neuronal cell population becomes naturally reduced by about 35% in the course of the programmed cell death, which takes place during the first 2 weeks of life. In the mature rat, ganglion cells die as a consequence of the axotomy. In both cases, the fluorescent dye, which is a lipid inserted into the cell membrane, labels the ganglion cell bodies after retrograde transport from the axons. In the course of their degradation the dye is then selectively taken up by intraretinal microglial cells in the ganglion cell and inner plexiform layers. In both groups of experiments, fluorescently labelled microglial cells were redistributed from the inner retina toward the photoreceptor cell layer, which is afflicted by chronic photoreceptor dystrophy. Similar migration does not occur when the same experiments were repeated with non-dystrophic normal rats. The experimental results suggest that intraretinal microglial cells possess a migratory potential and make use of it when the cells are exposed to degeneration-induced stimuli, the nature of which remains to be characterized.

[The assessment of blue mechanism by measuring the threshold versus intensity curve under flashed background]

The threshold versus intensity curve with brief blue stimuli with a flashing background of the same wavelength was measured in normal subjects and subjects with several kinds of optic nerve and retinal diseases. In young normal subjects the threshold curve rose sharply away from the line measured under the same conditions but without flashing blue background, as has been reported in measurements with light of long wavelengths. In the convalescent stage of optic neuritis, diabetic retinopathy, and central serous chorioretinopathy, the threshold curves presented two patterns; one pattern was shifted upward and the other shifted upward and to the right. We suggest that this method with short wavelength light is useful in differentiating retinal diseases from the change of optic media, and for layer-by-layer diagnosis of retinal diseases.

Cone dystrophies with negative photopic electroretinogram

A scotopic electroretinogram with an a-wave amplitude larger than the b-wave amplitude traditionally is termed 'negative'. Six male patients with negative photopic electroretinograms were examined; three of them suffered from progressive cone dystrophy, in which negative electroretinograms are unusual. Another patient without symptoms was the brother of a patient with cone dystrophy. These patients are compared with others who characteristically have negative electroretinograms-one patient with incomplete congenital stationary night blindness and another with X linked congenital retinoschisis. Differential diagnosis between these unusual cases of cone dystrophies and X linked retinoschisis or congenital stationary night blindness was possible with funduscopy, adaptometry, and evaluation of progression, but not with the electroretinogram. Inner retinal defects may occur in cone dystrophies as indicated by the negative electroretinogram. The waveform variations between our patients may be due to different inner retinal defects. The findings in two brothers indicate that cone dystrophy and inner retinal defects may be inherited separately.

Clinical subtypes of cone-rod dystrophy

OBJECTIVE

To determine possible distinct phenotypic subtypes of cone-rod dystrophy.

PATIENTS

Thirty-three patients with cone-rod dystrophy (from 25 families) were assessed prospectively on electroretinography, visual field testing, psychophysical threshold profiles, and fundus features. The clinical records of an additional 150 patients with cone-rod dystrophy were examined retrospectively in terms of the classification schema derived from the prospective study.

RESULTS

Based on electroretinographic recordings, two major types of cone-rod dystrophy were differentiated. In type 1, cone amplitudes were reduced to a greater degree than were rod amplitudes on electroretinography, while in type 2, cone and rod electroretinographic amplitudes were reduced in equal proportion. These two types were further subdivided on the basis of patterns of visual field loss and threshold elevation. In type 1a, there was a central or paracentral scotoma, and cone thresholds were more elevated centrally than peripherally. In type 1b, there was no central scotoma, and cone thresholds were more elevated peripherally than centrally. In type 2a, there was a central scotoma, cone thresholds were more elevated centrally than peripherally, and rod thresholds were more elevated peripherally than centrally. In type 2b, a partial or complete ring scotoma was present, cone thresholds were more elevated peripherally than centrally, and rod thresholds were more elevated in the midperipheral than in either the central or far peripheral region of the retina. Of the 150 additional patients with cone-rod dystrophy, data sufficient for classification were available for 95 patients, and all but two had findings that were consistent with classification into one of these four subtypes.

CONCLUSION

Our results identify four functionally distinct subtypes of cone-rod dystrophy that may be useful for patient counseling and future molecular genetic studies.

Survival of cone responses in postmortem human retina

A cone-mediated electroretinogram was recordable from human retinas 17-18 hours postmortem, after regeneration of visual pigments by application of 11-cis and 9-cis retinal. The cone was separated from the rod component by stimulation with flickering red light. In addition to these neuronal responses, a P2 and a slow P3 were present.

Pattern of dysfunction in progressive cone dystrophies--an extended classification

An extended classification for progressive cone dystrophies is proposed on the basis of the retrospective analysis of the clinical and electrophysiological findings obtained in a series of 91 patients with progressive cone dystrophies and of a review of the literature. This classification depends on the different patterns of electroretinographic responses. Four main categories and ten subgroups are distinguished. Generalized cone dystrophies are most frequent (76/91), affect all types of cones, and may be subdivided according to the degree of rod involvement. In selective cone dystrophies (8/91), the three cone types are affected differently as detected with the color electroretinogram. They are subdivided on the basis of the cone type predominantly involved. Additional inner retinal transmission defects may occur in cone dystrophies (3/91). They are identified by an alteration in the b/a-wave ratio on the electroretinogram and may affect the cone or rod pathway. Localized cone dystrophies (4/91) are limited to certain retinal areas.

Retinal cone dysfunction of supernormal rod ERG type. Five new cases

Five patients, not related to each other, showed clinical signs, including electroretinograms, of a retinal dysfunction which mainly affected the cone system, but also involved the rod responses in a peculiar way. ERG b-wave threshold under dark adapted conditions was elevated. In contrast, rod b-wave sensitivity was enhanced with medium to high intensity flash stimulation. Furthermore, all patients had a severe reduction of the oscillatory potentials. The findings are discussed with special emphasis on a hypothetical disturbance in the cyclic guanosine monophosphate metabolism, involving both photoreceptors and cells of the inner plexiform layer responsible for retinal feedback mechanisms.

Colour vision anomalies following experimental glaucoma in monkeys

Spectral sensitivity defects, associated with chronic elevated intraocular pressure (IOP) produced by Argon laser trabeculoplasty, were studied in monkeys. Increment-threshold spectral sensitivity (ITSS) and threshold versus intensity (TVI) functions were measured using a behavioural model. Elevated IOP resulted in short wavelength (SW) sensitivity losses characteristic of many ocular diseases. The amount of SW sensitivity loss for ITSS functions depended upon the intensity level and chromatic composition of the background field. The optimum condition identifying the greatest SW sensitivity reduction was a yellow background of moderate intensity (100-1000 Td). In the early stages of experimental glaucoma, the cone mechanisms and the rod mechanism typically showed decreased test and field sensitivities. The SW cone pathway has slightly greater threshold elevation (approximately 0.3 log unit) compared to the rod and cone pathways. On the other hand, in the advanced stages of experimental glaucoma, the largest sensitivity losses were in the longer-wavelength, red-green opponent mechanisms, with the rod and SW cone pathways showing smaller losses. The similarities of the colour vision anomalies in this animal model with those of patients with glaucoma, provides support for its use as an experimental model for human glaucoma.

Foveal cone involvement in retinitis pigmentosa progression assessed through psychophysical impulse response parameters

PURPOSE

To compare psychophysical impulse response parameters in retinitis pigmentosa (RP) patients and healthy controls using a temporal contrast sensitivity threshold paradigm, and to measure changes in these parameters with RP progression.

METHODS

Sixty-six RP patients and 10 healthy control subjects were tested, and the amplitude and timing parameters of the psychophysical impulse response function were computed through time-domain transformation under assumption of minimum-phase properties.

RESULTS/CONCLUSIONS

The initial rise time of the impulse response, ta, was significantly lengthened in RP patients compared to controls, as was the fall time from peak to trough, tp-->t. The log peak-to-trough amplitude, log R, was significantly reduced. With disease progression, all impulse response parameters continued to move away from the normal range. Only minor distinctions according to RP pathophysiologic subtype or mode of inheritance were found, supporting the hypothesis of a common course of the secondary retinal degeneration across different RP subcategories.

Foveal cone involvement in retinitis pigmentosa progression assessed through flash-on-flash parameters

PURPOSE

To compare psychophysical Naka-Rushton parameters in retinitis pigmentosa (RP) patients and healthy controls using a flash-on-flash increment threshold paradigm, and to measure changes of these parameters with RP progression.

METHODS

Sixty-six RP patients and 10 normal subjects were tested, and their maximum response (Rmax), half-saturation intensity (sigma), and slope (n) parameters were estimated.

RESULTS/CONCLUSIONS

Rmax in RP patients is decreased significantly with respect to the range in normal controls and continues to decrease (0.024 log units/yr) with disease progression. The distribution of sigma in RP patients differs from that in normal subjects, showing lower values in general, but no progression. Small differences in parameter distributions among genetic or pathophysiologic RP subcategorizations were found, but these do not fulfill stricter statistical criteria required for multiple comparisons. Measurement noise, inherent in the flash-on-flash paradigm, exert considerable influence on the quality of the data, as was demonstrated through repeated measures and a Monte Carlo simulation.

Effects of early diabetic retinopathy on rod system sensitivity

Previous studies have shown that S-cone pathway sensitivity is selectively decreased in the early stages of diabetic retinopathy. In the present study, rod system sensitivity was evaluated in a group of diabetic patients using psychophysical techniques. The course of dark adaptation was first determined, then absolute thresholds were measured in the horizontal and vertical meridians. For all patients, although the recovery of the initial portions of rod dark adaptation were normal, absolute thresholds were increased in both the horizontal and vertical meridians. The findings provide evidence that patients with early diabetic retinopathy show a generalized dysfunction of the rod system.

Electrophysiologic monitoring of diabetic retinopathy in pregnancy

The authors studied the correlations between electroretinographic, ophthalmologic, and clinical parameters in 30 pregnant women (20 with diabetes and 10 control subjects). Diabetic patients were divided into two groups: group 1 included 11 cases of insulin-dependent diabetes (IDDM); and group 2 included 6 cases of noninsulin-dependent diabetes (NIDDM) and 3 cases of gestational diabetes (GDM). Adapto-electroretinography (AERG) was used as the main monitoring parameter, and in particular, the relationship between the cone-mediated (b1) and rod-mediated (b2) components of the b wave (b2/b1 ratio) 7 minutes after photobleaching was studied. The results indicate that the b2/b1 ratio can detect functional modifications before the onset of ophthalmoscopically detectable retinopathy. Significant statistical correlations were demonstrated both between the type of diabetes and AERG responses, and between metabolic control (HbA1c values) and AERG alterations. A higher maternofetal complication rate in those patients with severe and frequent AERG alterations during pregnancy also was found.

X-linked retinitis pigmentosa: functional phenotype of an RP2 genotype

Rod- and cone-mediated function was studied with psychophysics and electroretinography in members of an X-linked retinitis pigmentosa pedigree with the RP2 genotype. An asymptomatic hemizygote with an early stage of the disease had cone dysfunction in the mid-periphery and an abnormal cone electroretinogram (ERG); rod function was normal. Hemizygotes with more advanced disease had cone and rod dysfunction in the mid-peripheral retina and cone dysfunction in the far periphery; cone and rod ERGs were abnormal. At very advanced stages, there was an absolute mid-peripheral scotoma and marked cone and rod dysfunction in the far peripheral and central retina. Cone and rod ERGs were severely abnormal or not detectable. Heterozygotes showed tapetal-like reflexes, patches of pigmentary retinopathy, and a range of functional findings from no detectable abnormalities to moderate levels of retinal dysfunction. There were regions of normal function adjacent to dysfunctional patches that had greater cone than rod sensitivity losses or comparable cone and rod losses. The results suggest that the phenotype of this RP2 genotype of X-linked retinitis pigmentosa, unlike other forms of retinitis pigmentosa, is first expressed as a cone photoreceptor system dysfunction, and as the disease progresses, both rod and cone systems are involved.

Foveal cone electroretinograms in patients with central visual loss of unexplained etiology

Foveal cone electroretinograms (ERGs) were recorded in five patients (aged 24 to 66 years) referred because of central visual loss of unexplained etiology. These patients had no family history of visual loss and no diagnostic fundus abnormalities seen on ophthalmoscopy or fluorescein angiography. Foveal cone ERGs were elicited with a 4 degrees white stimulus flickering at 42 Hz centered within a steady 10 degrees white surround presented through the dilated pupil to the fovea by a hand-held, dual-beam stimulator-ophthalmoscope. All five patients showed reduced foveal cone ERG amplitudes. Their abnormal responses were similar to those previously reported in cases of clinically apparent macular degeneration and contrasted with previously reported normal responses in patients with optic atrophy or strabismic amblyopia. The progressive loss of central vision as determined by history and these subnormal foveal cone ERG amplitudes suggest that these patients have a form of macular degeneration.

Visual function and the subsequent development of exudative age-related macular degeneration

The eyes of 47 subjects with exudative age-related macular degeneration in the fellow eye were tested with a battery of visual function tests at baseline and followed for at least 18 mo. Fundus photographs also were obtained at baseline. These photographs were used to verify the absence of exudative lesions in the 47 eyes tested. Functional and funduscopic baseline data each were compared against outcome data obtained typically at 18 mo. The baseline data were analyzed for their ability to distinguish eyes that had developed detectable exudative age-related macular degeneration from eyes that had not. Eyes with relatively slow foveal dark adaptation rates despite low foveal quantum absorption capabilities (as inferred from the effects of test area on the Rayleigh color match) were especially likely to develop subretinal neovascularization. The resulting sensitivity/specificity and odds ratios were comparable to those of the most effective funduscopic risk indicators. Low S (blue) cone-mediated sensitivity also was associated with an exudative outcome.

Still no evidence for a photoreceptor-level abnormality in dyslexia

In reply to Grosser and Spafford's (1992) defence of their theory that there is a photoreceptor-level abnormality in dyslexia we argue that (1) their original experiments do not tap rod functions, (2) the theory fails at the quantitative level, and (3) the proposition that rod inputs generate transient responses is false.

Foveal photopigment kinetics--abnormality: an early sign in myotonic dystrophy?

Twelve subjects with minimal expression of the myotonic dystrophy (MyD) gene were investigated by retinal densitometry, a technique which has been used to study the properties of photopigments in the living eye and to detect photoreceptor abnormalities. Other investigations included slit-lamp examination, funduscopy, raleigh matches with the anomaloscope, tonometry, and neurological examination, including electroretinography (ERG) and pattern visual evoked potentials recording. Foveal densitometry demonstrated reduced values of the macular photopigment density difference with normal photopigment kinetics in early phases of the disease, even in asymptomatic individuals. The densitometric values correlated with decreased amplitudes of the photopic ERG a-wave. These findings may be explained by loss or dysfunction of the outer segments of foveal receptors. It is yet unknown whether or not these changes are secondary to other observed neuroretinal abnormalities in MyD. The most likely explanation might be an abnormality of the Na, Ca:K exchanger at the level of the outer segments of the photoreceptors whether or not in combination with a dysfunction of voltage generation systems, involving both photoreceptors and retinal pigment epithelium.

Neuro-ophthalmology

The anatomy, neurophysiology, semiology, and pathology of the pupillary reflexes are reviewed. Recent advances in the demonstration of midbrain pathways projecting to and from the Edinger-Westphal (EW) nucleus are discussed. Observations of the pupillary diameter and reflexes in premature infants can be helpful in the diagnosis of neurological disorders. A relative afferent pupillary defect (RAPD) without visual disturbances can be suggestive of midbrain lesions. Automated pupil perimetry is proposed as an objective method for the evaluation of the visual field. Tonic pupil, the pupil in diabetics, and blue-cone monochromatism are also discussed.