Computer-Assisted Analysis of Clinical Electroretinographic Intensity-Response Functions

Maturation of rod function in preterm infants with and without retinopathy of prematurity

OBJECTIVES

To establish normal development of rod electroretinograms in preterm infants and to assess the effects of retinopathy of prematurity (ROP).

STUDY DESIGN

We measured 88 Naka-Rushton functions from 41 preterm infants at maturities from 30 to 72 weeks postmenstrual age (PMA). Outcomes (log sigma, retinal sensitivity and V(max), retinal responsivity) were compared between control (no ROP), untreated ROP, and treated ROP.

RESULTS

In control infants, sensitivity increased by 1.5 log units from 30 to 40 weeks PMA and by a further 0.5 log units by 50 weeks PMA but was 0.5 log units less than in similarly-mature, healthy, term-born infants. Average retinal responsivity increased from 23 microV to 90 microV between 30 and 40 weeks PMA and was 35 muV greater at 40 weeks PMA than in similarly-mature term-born infants. At around 36 weeks PMA, (when onset of ROP peaks), infants with untreated ROP had average retinal sensitivity 0.2 log units lower than control infants; sensitivity was reduced further in infants treated for ROP. Retinal responsiveness did not differ between control subjects and untreated infants with ROP but was greatly reduced in infants treated for ROP.

CONCLUSIONS

Maturation of rod sensitivity appears to be slowed by preterm birth whereas maturation of rod responsivity is accelerated. ROP reduces retinal sensitivity, and treated ROP reduces both sensitivity and responsivity.

Zinc and the Eye

Zinc, a trace element that influences cell metabolism through a variety of mechanisms, appears to play an integral role in maintaining normal ocular function. This element is present in high concentrations in ocular tissue, particularly in retina and choroid. Zinc deficiency has been shown in a number of species to result in a variety of gross, ultrastructural and electrophysiologic ocular manifestations. The physiological functions for zinc have been studied predominantly in retina and retinal pigment epithelium where zinc is believed to interact with taurine and vitamin A. modify photoreceptor plasma membranes, regulate the light-rhodopsin reaction, modulate synaptic transmission and serve as an antioxidant. Suboptimal zinc status in North America may influence the development and progression of several chronic eye diseases. Zinc supplementation trials and epidemiological studies have produced conflicting results concerning the role of zinc in age-related macular degeneration. Additional well-controlled supplementation trials are indicated to clarify the role of zinc in this disease. Future investigations must also expand our understanding of the mechanisms by which zinc regulates ocular morphology and function.

Electrophysiology in the investigation of acquired retinal disorders

Electrophysiological research on acquired retinal disorders, both common and rare, is reviewed. Age is a major factor influencing electroretinogram (ERG) and electro-oculogram (EOG) findings. Bipolar or Müller cell death in the aging retina could account for much of the amplitude decline that is observed with age. In diabetic retinopathy, the oscillatory potentials can monitor the progression of the disease and indicate neuronal alterations rather than diabetic angiopathy of the retina. Human ERG studies on glaucoma concentrated on ERG measures that are dominated by inner retinal contributions. It has been shown that the pattern ERG can serve as a predictor of ocular hypertension's progression to glaucoma. In retinal disorders caused by endogenous intoxication, such as hepatic retinopathy, or exogenous intoxication from chronic lead exposure, ERG changes give an objective measure of the damage and allow to study the pathophysiological mechanisms that are involved. Inflammations of the choroid and the retina affect the standard ERG when they are diffuse. In central serous chorioretinopathy, functional disturbances can be revealed not only in the photoreceptors but also in the middle and inner retinal layers with the use of focal stimuli. Choroidal melanoma leads to large reductions of the EOG light peak-to-dark trough ratio through its influence on the transepithelial potential of the retinal pigment epithelium (RPE). In cancer-associated retinopathy, both the rod and cone ERGs are reduced. However, selective cone dysfunction has been described. In melanoma-associated retinopathy, the long flash ERG may reveal a specific pathophysiological mechanism, namely the affection of the ON-pathway with preservation of the OFF-pathway. ERG measurements can reveal vitamin A deficiency and are altered in cases with a mutation in the gene for the retinol binding protein in which other organs are not affected. Photochemical damage to the retina from light emission by the operating microscope can be assessed by electrophysiological methods.

Reproducibility of electroretinograms recorded with DTL electrodes

The purpose of this study was to examine whether the use of the DTL fiber electrode yields stable and reproducible electroretinographic recordings. To do so, luminance response function, derived from dark-adapted electroretinograms, was obtained from both eyes of 10 normal subjects at two recording sessions spaced by 7-14 days. The data thus generated was used to calculate Naka-Rushton Vmax and k parameters and values obtained at the two recording sessions were compared. Our results showed that there was no significant difference in the values of Vmax and k calculated from the data generated at the two recording sessions. The above clearly demonstrate that the use of the DTL fiber electrode does not jeopardize, in any way, the stability and reproducibility of ERG responses.

Fundus pigmentation and the dark-adapted electroretinogram

Dark-adapted electroretinograms were obtained over a 3.6-log range of stimulus intensities from 17 black and 15 white normal subjects. Subjects were grouped on the basis of light or dark fundus pigmentation, determined from digitized fundus photographs. B-wave amplitudes for each group were fitted by the Naka-Rushton equation, and the measures Vmax, log K, and n were determined. The luminance-response functions revealed that subjects with light fundi had larger b-wave amplitudes at all luminance levels. There was a significant difference between groups for Vmax and n but not for log K. A comparison of b-wave implicit times showed no significant difference between subjects with dark and light fundi. Ancillary tests and multiple regression analysis suggested that the relationship between Vmax and fundus pigmentation could not be attributed to age, gender, refractive error, axial length or intraocular pressure. The results have implications for the collection of normative electroretinographic data and for the interpretation of electroretinogram results.

Dark-adapted luminance-response functions with skin and corneal electrodes

Normative dark-adapted electroretinograms were recorded simultaneously with a skin electrode and corneal electrode for varying stimulus intensities. The electroretinogram b-wave amplitudes for each electrode were fitted by the Naka-Rushton equation, and the parameters Vmax, K and n were evaluated. A comparison of parameters between the two electrodes showed a significant difference for Vmax and K but not for n. Vmax was approximately eight times smaller and K was 0.3 log unit smaller for the skin electrode than for the corneal electrode. B-wave amplitude and implicit time were also compared between the two electrodes. The b-wave amplitude ratio of the corneal electrode to that of the skin electrode increased with luminance and ranged from 1.83 to 7.68. Overall, b-wave implicit time for the skin electrode was approximately 10 ms shorter than that of the corneal electrode.

Electrophysiologic characteristics of human and rat retinas in vitro

To promote studies on the human retina, we investigated the survival of function in postmortem specimens. Visual pigment has been regenerated in normal human retinas, 5 to 58 hours postmortem, by exposure to retinal isomers in the dark. Levels from 0.1 to 0.41 nmol/mg protein were reached. Photoresponses were obtained in 9 of 13 retinas: P III maximum amplitudes ranged from 20-398 microV and thresholds, taking the criterion amplitude as 3 microV, ranged from 8.8-1340 quanta/micros2. In three cases, the b-wave was also seen. The P III amplitude vs. log intensity curves gave values of n between 0.6 and 1.0, and sigma (the stimulus intensity for a half maximal response) between 132-3700 quanta/microns2. Recovery of sensitivity did not always correspond to that of maximum response.

Autosomal dominantly inherited macular dystrophy with preferential short-wavelength sensitive cone involvement

We found an apparently inherited tritan-like color vision defect in five members of a family, spanning three generations. The defect was associated with mild macular pigmentary changes, poor foveolar reflexes, or slightly reduced visual acuity in four of the affected individuals. The inheritance pattern appeared to be autosomal dominant. Results of various color vision tests indicated preferential involvement of the short-wavelength sensitive cone system, with relative preservation of the middle- and long-wavelength sensitive cone systems. Both anomaloscope testing with larger (8-degree) fields and short-wavelength sensitive electroretinography indicated some short-wavelength sensitive cone system involvement beyond the central macula in the three affected individuals on whom testing was performed. The condition appeared to be a familial macular dystrophy with preferential short-wavelength sensitive cone involvement. The abnormal macular findings and mild reduction in visual acuity distinguish this condition from congenital tritanopia; the normal optic disks distinguish it from autosomal dominant optic atrophy.

Naka-Rushton equation parameters in electroretinogram analysis of daunomycin effects on retinal function

Intraocular daunomycin may inhibit intravitreal cellular proliferation in proliferative vitreoretinopathy after rhegmatogenous retinal detachments and retinal reattachment surgery. Doses of 15 nmol or more have shown histological retinal toxicity when injected into the intact vitreous of rabbit eyes. We injected 30, 20, 10 and 0 nmol doses of daunomycin into rabbit eyes with gas-compressed vitreous to better simulate the clinical conditions in which it would be used. We then evaluated effects on retinal function by examining scotopic b-wave amplitudes, measured for a four log unit set of intensities. We used the three independent parameters Rmax, log K, and n of the Naka-Rushton equation to measure changes in retinal function. All three doses of daunomycin failed to measurably depress retinal function (i.e., there were no dose-related losses of Rmax, log K or n) in this experimental model. We used this approach to monitor alterations in retinal function since it can show selective changes in each parameter. This selectivity is an advantage over monitoring retinal function with single intensities or comparisons limited solely to electroretinogram amplitudes.