We performed a study to evaluate the influence on visual function of intraocular straylight after photorefractive keratectomy (PRK). We present 4 eyes of 4 myopic individuals, who had contacted our clinic for keratorefractive surgical treatment. PRK's were performed with a Summit laser, using a 5 mm ablation zone. The straylight meter was used to measure the amount of intraocular scattered light, the physical cause of glare complaints, before and after PRK. This apparatus uses the direct compensation method to assess the amount of intraocular light scatter. The results showed a significant increase in straylight values, in the tested eyes, during the first two weeks after PRK. After the initial rise, straylight values returned to preoperative levels, except for two eyes that clearly developed a haze higher than grade two. Instead of returning to baseline levels, straylight values remained significantly higher in these eyes.
The human electroretinogram (ERG), evoked by white flashes of extremely short duration (10 microseconds), shows a typical dependence on flash intensity. Increasing stimulus intensity increases the amplitude of the a-wave until saturation is reached. The amplitude of the b-wave reaches a maximum value with flashes of middle intensity, then decreases at higher stimulus strengths. The values of a-wave amplitude saturation, defined as 100%, may serve as a basis for standardizing the various amplitude-intensity relationships of the a- and b-wave. The b-wave function, calculated in this way, shows different maximum values depending on whether it was determined after light adaptation or in dark adaptation and low interindividual variability. However, the difference between bmax in dark and light adaptation is markedly decreased in the case of ethambutol intoxication. ERG changes are only detected in severe cases (total central scotoma) and are below the discrimination level in cases with moderate symptoms (relative central scotoma, visual acuity greater than 0.4-0.5).
Eight patients with tobacco-alcohol amblyopia, but no significant changes in ERG configuration, were subjected to a more extended ERG analysis applying normalized a- and b-wave amplitude-intensity relationships. The functions established significantly fell out of the range of normal subjects, with respect to the maximum of the b-wave function in dark and light adaptation. In this way, the ERG discrimination level in detecting retinal disturbances in tobacco-alcohol amblyopia can be shifted towards higher sensitivity.
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