The human electroretinogram in the light and during dark adaptation

Human retinal dark adaptation tracked in vivo with the electroretinogram: insights into processes underlying recovery of cone- and rod-mediated vision

The substantial time taken for regaining visual sensitivity (dark adaptation) following bleaching exposures has been investigated for over a century. Psychophysical studies yielded the classic biphasic curve representing recovery of cone-driven and rod-driven vision. The electroretinogram (ERG) permits direct assessment of recovery at the level of the retina (photoreceptors, bipolar cells), with the first report over 70 years ago. Over the last two decades, ERG studies of dark adaptation have generated insights into underlying physiological processes. After large bleaches, rod photoreceptor circulating current, estimated from the rod-isolated bright-flash ERG a-wave, takes 30 min to recover, indicating that products of bleaching, thought to be free opsin (unbound to 11-cis-retinal), continue to activate phototransduction, shutting off rod circulating current. In contrast, cone current, assessed with cone-driven bright-flash ERG a-waves, recovers within 100 ms following similar exposures, suggesting that free opsin is less able to shut off cone current. The cone-driven dim-flash a-wave can be used to track recovery of cone photopigment, showing regeneration is 'rate-limited' rather than first order. Recoveries of the dim-flash ERG b-wave are consistent also with rate-limited rod photopigment regeneration (where free opsin, desensitising the visual system as an 'equivalent background', is removed by rate-limited delivery of 11-cis-retinal). These findings agree with psychophysical and retinal densitometry studies, although there are unexplained points of divergence. Post-bleach ERG recovery has been explored in age-related macular degeneration and in trials of visual cycle inhibitors for retinal diseases. ERG tracking of dark adaptation may prove useful in future clinical contexts.

Depletion of trace elements and acute ocular toxicity induced by desferrioxamine in patients with thalassaemia

High doses of intravenous desferrioxamine infused over a short period of time induce a large faecal and urinary iron excretion but also produce retinal abnormalities that are characterised by decreased amplitude on electroretinography and defective dark adaptation. This regimen also results in high faecal iron, zinc, and copper excretion, and reduced granulocyte zinc concentrations and alkaline phosphatase activity. The retinal abnormalities may be related to the zinc and copper deficiency and/or iron depletion 'per se' which interferes negatively with critical iron dependent enzymes.

The early receptor potential (ERP)

The ERP contains information on the function of the outer segments of the retinal photoreceptors. The authors have established normative values of the ERP and the factors which might be of influence hereupon. ERP findings in patients suffering from various eye diseases are given. Value and limitation of ERP recording are discussed in relation to the other clinically available electrodiagnostic tests.

Objective determination of dark adaptation: accuracy of the evoked potential method

The psychophysical threshold and the appearance of the visually evoked potential (VEP) were compared in the dark adapted state. Nineteen normal subjects and 11 persons with defective dark adaptation were studied. The VEP threshold was an average of 0.2 log units (standard deviation 0.2) above the subjective threshold in the normal group, and 0.4 log units above it in the night-blind subjects. In 7 normal subjects, the alpha-activity of the electroencephalogram interfered with the VEP recording and rendered the evaluation more difficult. The results indicate that the VEP can be used as an accurate method of objectively measuring dark adaptation.

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.

Sector retinitis pigmentosa. Electrophysiological and psychophysical study of the visual system

Combined clinical, psychophysical and electrophysiological examinations of the visual function were performed in two patients affected by sector retinitis pigmentosa. Psychophysical dark adaptation measurements, the electroretinogram (ERG) and the electro-oculogram (EOG) revealed that a larger area of the neural retina and pigment epithelium were pathologically involved than revealed by ophthalmoscopy and visual field. Five years of observation showed a stationary retinal defect in one patient, while in the other a slight clinical but a marked electroretinographical deterioration was found over a period of twelve years. However, visual acuity and the visually evoked potential (VEP) remained normal. In spite of subnormal ERG amplitudes, the photopic and scotopic peak latencies were normal in both cases. In the ophthalmoscopically normal relatives of one patient slight dark adaptation impairments, as well as EOG and ERG disturbances, were detected.