Rhodopsin density and visual threshold in retinitis pigmentosa

Novel snapshot imaging of photoreceptor bleaching in macaque and human retinas

PURPOSE

Various methods have been used to obtain a topographic map of bleached photopigments in human retinas in the past. The purpose of this study was to determine whether the bleaching topography of the photoreceptors could be obtained by snapshot imaging reflectometry.

METHODS

Four to five fundus photographs of one rhesus monkey and three healthy human subjects were taken by white flashes at intervals of 4 s, with a commercial fundus camera with minimal modifications. The flash-induced reflectance increases (bleaching) were calculated by dividing the reflectance of the first image into the subsequent images, pixel by pixel.

RESULTS

The topography of the bleached macula corresponded well with the anatomical distribution of the cones. The ratio of reflectance changes in the center to that in the surrounding tissue was high for red and low for green and blue images. These results indicate that the reflectivity changes were not artifacts but were derived from changes in the photopigment density in the cones and rods.

CONCLUSIONS

The topography of bleached photoreceptors obtained with a commercial fundus camera from one monkey and three healthy human subjects showed that this technique has potential as a new clinical method for examining photoreceptor function in both normal and diseased retinas.

Clinical applications of fundus reflection densitometry

Fundus reflection densitometry or retinal densitometry is a non-invasive technique to examine the visual photopigment kinetics in living eyes. The technique is based on the comparison of the reflected light from the fundus in a fully light adapted eye (when all visual photopigment has been bleached) with the reflected light following complete dark adaptation (when the retina contains its maximum amount of visual photopigment). The technique provides a measure of the density of visual photopigment, its time constant of regeneration, its distribution and spectral characteristics if measured at a series of wavelengths. Fundus reflection densitometry in the human eye was introduced 40 years ago. Presently, it is the only available technique from which direct and objective insight can be obtained into visual photopigment. This knowledge is particularly relevant in eyes where abnormalities of photoreceptor function are suspected. This paper summarizes the current knowledge of fundus reflection densitometry in the diseased and in the aging human retina, gathered over the last 30 years. Considerable improvements of the instrument for clinical purposes have been obtained, and are also discussed.

Visual pigments in the human macula assessed by imaging fundus reflectometry

Multispectral imaging fundus reflectometry and multiple linear regression fitting routines were used to simultaneously assess the spatial distributions of cone visual pigment and rhodopsin in the human macula. As expected from anatomic studies, the cone visual pigment distribution showed a peak in the central fovea and was elliptical, with the broader axis along the horizontal meridian. The rhodopsin distribution showed a minimum in the fovea and the rhodopsin density increased with eccentricity. Both visual pigment distributions showed striking variability among individuals. These data provide visual pigment distributions in the relatively unexplored parafoveal region.

Rod increment thresholds in cone-rod dystrophy

Rod system increment threshold functions (ITFs) were studied in patients with cone-rod dystrophy (CRD). Rod thresholds (I(t)) for a 104-min, 500-nm test stimulus (TS), superimposed on an 11 degrees long wavelength background (luminance I(b)), were measured in eleven CRD patients and fourteen normal subjects. Thresholds in normals were also measured using a 7-min TS. A modified version of Weber's law [logI(t) = logK + log(I(n)(b) + c)] was fitted to the data from each subject and test condition to yield a description of the ITF in terms of the free parameters K, c, and n. Four of the CRD patients exhibited an abnormally high absolute threshold; of these, two showed abnormalities in K and c consistent with a reduced efficiency of quantum capture by the rods. Abnormalities in the ITFs of CRD patients did not resemble the effect of reducing the diameter of the TS from 104 to 7 min in normals. This suggests that threshold abnormalities in the CRD patients did not result from altered spatial summation. The results illustrate use of a parametric representation of the ITF to evaluate the loss of sensitivity in visual disorders.

Probing visual function with psychophysics and photochemistry

New methods using computer based measurements and image analysis techniques can improve and expand our ability to investigate non-invasively the function of the retina in patients. These can provide insight into the underlying mechanism of an abnormality and further our understanding of disease processes.

Clinical investigation of retinitis pigmentosa

The diagnostic features and future research directions of retinitis pigmentosa were documented in this update and review of the subject. An extensive and current bibliography is provided.

Rod densitometry in night blindness: a review and two puzzling cases. Rod densitometry in night blindness

Since the non-invasive technque of retinal densitometry became available in 1955, rhodopsin kinetics could be studied in vivo. It was obvious that with this new tool investigators focussed attention on the aetiology of night blindness in various diseases. A brief review about the clinical developments in the past two decades is given. Also three case-reports are presented, which suggest that in some cases of congenital stationary night blindness (CSNB) the night blindness might arise from the absence of rhodopsin. This is contrary to the standing opinion and present problems regarding the integrity of the retina.

Visual function and rhodopsin levels in humans with vitamin A deficiency

Details of rod and cone dysfunction in vitamin A deficiency have been studied in two subjects with primary biliary cirrhosis and one with Crohn's disease, all of whom presented with symptoms of night blindness. Visual function in the mid-peripheral retina was monitored with two-color adaptometry and rhodopsin levels were measured by fundus reflectometry. Initially all three subjects had no measurable rod function and delayed cone adaptation. In one case the dark-adapted cone threshold was also elevated. Oral supplementation with vitamin A restored visual function to normal within 8 days in all subjects. During supplementation, cone function was restored more rapidly than that of rods, though the pattern of recovery was similar for each receptor type. Final thresholds improved first, though the rates at which they were reached were abnormally slow. As recovery continued, adaptation kinetics returned to normal. When rod adaptation was delayed, the regeneration of rhodopsin was also abnormally slow. When rod final threshold was 2 log units higher than normal, rhodopsin regeneration was incomplete, reaching about 70% of the normal level. The initial stages of visual dysfunction during onset of vitamin A deficiency were studied in one subject, and were found to mirror the pattern seen during recovery: rod adaptation was initially slower than normal, but reached completion. Cone adaptation remained normal until rod function was almost absent.

Clinical psychophysics

New developments in clinical psychophysics allow a non-invasive assessment of visual function which may otherwise not be possible. Measurements of spatial and temporal contrast sensitivity functions, perimetric rod and cone sensitivity, colour vision testing, and newer tests such as hyperacuity thresholds may provide information about the mechanism of an abnormality, allow earlier detection of damage, determination of retinal function in the presence of ocular media disturbances, and allow more sensitive detection of the effects of treatment on visual function. Some methods are more effective in screening or monitoring patients over time while others can be used as research tools to investigate the underlying causes of visual dysfunction. Emerging technologies such as those based on video displays and computer generated graphics and advances in methodology provide potential for new applications. The selection of which aspect of visual function to test depends on the condition (e.g., retinal degeneration or glaucoma), the goals of the investigation, and the facilities available. These non-invasive methods can provide accurate information about retinal function and further improve our ability to quantify and document this most important aspect of the eye--its role in visual function.

Dependence of cone b-wave implicit time on rod amplitude in retinitis pigmentosa

Computer-averaged full-field rod and cone electroretinograms (ERGs) were evaluated in 63 patients with retinitis pigmentosa and 116 normal subjects. Cone b-wave implicit times to 30 Hz white flicker and to 1.0 Hz white flashes on a white background were both inversely proportional to the log amplitude of the dark-adapted rod b-wave, even when controlling for the log amplitude of the dark-adapted cone b-wave. Cone ERGs were also obtained from a subset of 12 patients with retinitis pigmentosa in the presence of photopically-matched red and blue backgrounds that allowed direct assessment of the effect of rod function on cone b-wave implicit time in the patients. The effectiveness of the blue background in reducing cone b-wave implicit time relative to that obtained with the red background varied with the amplitude of the rod ERG. The results of both experiments suggest that delays in cone b-wave implicit time obtained with 30 Hz white flicker or with background adaptation in patients with retinitis pigmentosa are due, at least in part, to rod disease.

Prolonged rod dark adaptation in retinitis pigmentosa

The time course of rod dark adaptation was measured in 12 patients and carriers of retinitis pigmentosa (RP). In contrast to previous studies the rod absolute threshold was determined prior to any exposure to the bleaching light. For seven of the patients and carriers the recovery of rod sensitivity to the prebleach level was prolonged, with the delay ranging from mild to severe. The prolongation appeared to be limited to the late phase of bleaching recovery; the early portion of rod dark adaptation, measured following a weak bleach, was normal. These results suggest a selective impairment in the processes underlying rod dark adaptation in some patients with RP.

Human rhodopsin measurement using a T.V.-based imaging fundus reflectometer

An imaging fundus reflectometer for in vivo mapping of rhodopsin levels is described. The instrument is based on a high-sensitivity television camera attached to a Zeiss fundus camera, which enables areas of retina of angular subtense 25 degrees to be examined at a resolution of about 1 degree. Digital processing techniques are used to average the video signal spatially and temporally and to analyse the spatial information. Measurements with an artificial eye indicate that performance is comparable to that of photomultiplier-based systems. Rhodopsin levels and regeneration data for a normal human subject are presented; these are consistent with published values. The map of visual pigment levels derived from these normal data is contrasted with that for a subject with a patchy retinal dysfunction (autosomal dominant retinitis pigmentosa).

Rod and cone activity in patients with dominantly inherited retinitis pigmentosa: comparisons between psychophysical and electroretinographic measurements

Extended electroretinographic (ERG) testing has been carried out in a series of patients with retinitis pigmentosa, dominantly inherited. In 36 of 57 cases only cone b waves were evoked. In 20 of these, psychophysical tests showed only cones mediated vision (Massof class I), while in 16 statis scotopic perimetry demonstrated residual rod function (class II). In the remaining cases, where rod ERGs were seen, the light intensities required to evoke responses were not greatly elevated. A computer model was constructed to relate psychophysical threshold measurements to ERG data. This analysis of the results suggests that in one subgroup of patients the scotopic ERG is smaller than expected from the losses of visual field and that in another the psychophysical elevation of rod visual threshold is greater than expected from ERG measurements.

Determination of human cone pigment density difference spectra in spatially resolved regions of the fovea

A new method was developed to measure spectrally and spatially resolved cone pigment optical density difference in normal human subjects. Using digitized television images of human retinas scanned before and after bleaching the cone pigments with a bright light, unique high-resolution images of cone pigment density difference were produced. The spectral peak density difference within the fovea was found to be at 560 nm. These measurements demonstrate a decrease in cone pigment optical density difference with increased distance from the subject's central fixation point in the fovea. The cone pigment density difference was asymmetrically distributed in the fovea with higher amounts on the nasal side.

Investigation of retinitis pigmentosa

Retinitis pigmentosa is a solitary manifestation of separate genetically determined disorders in which there is progressive loss of vision and the appearance of characteristic fundus abnormalities. It is likely that each disease contained within this family of disorders has a different aetiology, a consideration which is important to the clinician, the researcher and the therapist. To the clinician it is essential to be able to identify the inheritance of the disorder in order to give educated genetic advice. It is the responsibility of the clinician to sub-divide retinitis pigmentosa into purer samples of disease since without such a sub-division research is unlikely to be fruitful. It is unreasonable to expect a biochemist to define systemic biochemical abnormalities if blood is analysed from a series of patients, each of which has a different disorder. If the cause of retinal degeneration in an animal homologue of human retinitis pigmentosa is identified the question is then raised as to whether the abnormality is relevant to human disease and, if so, to which one. Moreover, if a metabolic abnormality is identified in one disease it will not necessarily to be found in others and similarly, if therapy is effective in one form of the disease it may not be effective for all patients. A sub-division of retinitis pigmentosa may be made on the basis of inheritance of the disorder, on the basis of morphologic changes in the fundus, and on the qualitative functional changes identified. Such observations may also give some clues as to the pathogenesis of the different forms of RP or at least indicate in which cell system the primary disorders lie and will also show in what way the defect interferes with cell function. In this paper it is hoped to show that studies undertaken by the various disciplines within ophthalmology have now made limited achievements towards the goals of subdividing retinitis pigmentosa (RP) into purer samples of disease and typifying the functional and morphological attributes of the constituent disorders.

Carriers of X-linked recessive retinitis pigmentosa: investigation by vitreous fluorophotometry

The blood-retinal barrier was assessed by vitreous fluorophotometry in nine carriers of X-linked recessive retinitis pigmentosa. A breakdown of the blood-retinal barrier was apparent in all patients. In one carrier, the barrier was compromised even when abnormalities were not apparent in the retina ophthalmoscopically or by electroretinogram testing. The degree of abnormality of the blood-retinal barrier tended to parallel the extent of abnormalities noted on ophthalmoscopy, visual field examination, and electroretinogram testing.

Retinitis pigmentosa and allied diseases: applications of electroretinographic testing

Electroretinograms (ERGs) have provided criteria for establishing the diagnosis of retinitis pigmentosa in early life even at a time when fundus abnormalities visible with the ophthalmoscope are minimal or absent. Patients with widespread progressive forms of retinitis pigmentosa have shown not only reduced amplitudes but also delays in cone or rod b-wave implicit times, or both, while patients with self-limited sector retinitis pigmentosa or stationary forms of night blindness have had reduced amplitudes with normal b-wave implicit times. In families with retinitis pigmentosa ERGs can be used not only to identify which patients are affected but also to establish which patients are normal as those patients, age 6 and over, with normal cone and rod amplitudes and normal cone and rod b-wave implicit times have not been observed to develop primary retinitis pigmentosa at a later time. ERGs from patients with retinitis pigmentosa and allied night blinding disorders are presented to show their usefulness in genetic typing, documenting natural; histories, and defining possible pathogenetic mechanisms. The potential application of the ERG in evaluating the efficacy of therapeutic trials is also considered.

Retinitis pigmentosa

The authors review the symptomatic and genetic aspects of the various entities of isolated retinitis pigmentosa (R.P), both in its typical form and in the forms associated with the affection of other ocular tissues. Syndromes in which R. P. is associated with the affection of other organs and systemic disorders are also cconsidered. Origin, diagnosis and the course of the disease are discussed with regard to electrophysiology, histopathology, fluorescein angiography and biochemistry. Animal research has provided new realizations about the ultrastructure and physiological mechanisms of retinal photoreceptors, and better understanding of abnormal changes. The possible pathogenesis of the human disease, based on research findings, is onsidered. Although R.P. is generally thought to be to be an "untreatable" disease, therapy may be effective in several pathological entities. Methods and results of therapy with vitamins, light deprivation and vision aids are discussed.

Clinical assessment of rhodopsin in the eye. Using a standard fundus camera and a photographic technique

A technique based on the method of differential fundus reflectometry is used to assess the availability of rhodopsin in the eye. A defined part of the dark adapted fundus is bleached by a short intense flash of light. The fundus is subsequently photographed in order to record the flux reflected from the bleached area, the optogram, and the surrounding unbleached region. This procedure requires only a few simple modifications to a Zeiss fundus camera before it can be used routinely in the clinic.

X-linked retinitis pigmentosa

Of 107 consecutive patients with genetically-determined retinitis pigmentosa, 23 were provisionally diagnosed as having inherited the disease in an X-linked fashion. 42 affected males and 61 females were examined, and from the data obtained the following conclusions were drawn: (1) X-linked retinitis pigmentosa exists and is distinct from choroideremia. (2) In contrast to the results of previous surveys, X-linked retinitis pigmentosa is a common form of this disease and over 20 per cent. of retinitis pigmentosa is probably transmitted in an X-linked manner. (3) (a) In contradistinction to the findings of previous investigators, most if not all adult heterozygous females have detectable degenerative changes in the ocular fundus. (b) The ocular changes in heterozygous females are most easily detected by fundus examination, visual field testing, dark adaptation measurements, and estimation of retinal rhodopsin concentration. The single most frequent abnormality is peripheral retinal pigment epithelial atrophy, which is found in all adult heterozygous females. (c) The pattern of retinal dysfunction in heterozygous females, and in particular preservation of the ocular electrical responses, suggests that the disease in women is qualitatively different from that in men and in other genetic forms of retinitis pigmentosa. There is some evidience that the disease in heterozygous women is patchy. (d) Degeneration in heterozygous females is usually symmetrical, but great variation was found in the severity of degeneration amongst heterozygotes of similar ages. No non-genetic influences were found to account for this. No evidence came to light by which the importance of X-chromosome inactivation could be assessed in determining the phenotype of heterozygous women. (4) No evidience is available to determine the number of X-linked genes transmitting the disease.