RESPIRATORY EFFECTS UPON THE VISUAL THRESHOLD

The Effect of Systemic Hyperoxia and Hypoxia on Scotopic Thresholds in People with Early and Intermediate Age-related Macular Degeneration

Purpose: Morphological retinal changes combined with functional evidence implicate hypoxia in the pathogenesis of age-related macular degeneration (AMD). However, the role of hypoxia in the scotopic threshold deficit reported in AMD has not been investigated. This study compared scotopic thresholds in participants with early and intermediate AMD recorded under conditions of systemic hypoxia, hyperoxia and normoxia. Materials and Methods: Over two sessions scotopic thresholds were measured with participants breathing 21% and 60% oxygen (n = 12 early AMD, n = 11 age-similar controls) or 21% and 14% oxygen (n = 16 early AMD, n = 20 age-similar controls). Thresholds were measured using a 'white', annular 12 degrees stimulus, using a QUEST procedure. Results: There was no statistically significant change in scotopic thresholds within the AMD or control group when breathing the hyperoxic gas mixture (60% oxygen) or the hypoxic gas mixture (14% oxygen) when compared to the normoxic condition (21% oxygen). There was also no statistically significant difference in scotopic thresholds between groups under the hyperoxic or hypoxic gas conditions. The difference between groups under the normoxic condition was not statistically significant for the hyperoxia study (p = .70), but did reach significance in the hypoxia study (p = .05). Conclusion: This study provided no evidence that breathing that breathing 14% or 60% oxygen altered scotopic thresholds in those with early AMD when compared to controls. However, the lack of elevated scotopic thresholds in the AMD group of the hyperoxia study is of note, as it is unlikely that hyperoxia would reduce thresholds which were not significantly raised at baseline, regardless of whether hypoxia was a factor in the disease pathogenesis. The findings of this study do not rule out a role for hypoxia in early AMD, but this needs to be assessed in future experiments using measures that differ significantly between people with AMD and controls.

Aviation-related respiratory gas disturbances affect dark adaptation: a reappraisal

This study examined the time course of early scotopic threshold sensitivity during dark adaptation under mild to moderate hypoxia, moderate hypocapnia and hyperoxia, measuring detection time displacement relative to normoxia. Cone rod inflection and early rod adaptation were highlighted using progressively dimmer green flash stimuli. Early scotopic sensitivity was significantly delayed by hypoxia and hastened by hypocapnia and hyperoxia. Effects of respiratory disturbance on dark adaptation include temporal shifts of early scotopic sensitivity while human rod photoreceptors appear functionally hypoxic when breathing air at one atmosphere. At night, supplementary oxygen may benefit aircrew visual sensitivity, even at ground level.

The effect of a moderate level of hypoxia on human color vision

This study reports the effect of a moderate level of hypoxia on human color discimination. We found a generalized loss of color vision affecting both red-green and blue-yellow discrimination at an altitude of 12,000 feet. Although the residual color discrimination at this altitude was within age-matched, sea-level norms, a statistically significant increase over sea level error scores was measured on the Farnsworth-Munsell 100-Hue test and the Pickford-Nicolson anomaloscope. An analysis of psychophysical and electrophysiological studies indicates that hypoxia acts by depressing retinal ganglion cell activity and that it can affect photopic visual processes as well as scotopic vision. We conclude that studies evaluating man's visual performance at altitude must consider post-receptoral processes.

Physiologic correlates of the ERG hyperventilatory response in dogs

Previous studies in man have shown that hyperventilation consistently enhances the amplitude of the electroretinogram (ERG) b-wave, but has little effect on the a-wave. The present study examined this phenomenon in the dog; central (centrifugal) and peripheral (sympathetic) neural effects and changes in arterial pO2 and blood pH were excluded as physiologic bases for the phenomenon. It was concluded, therefore, that the underlying mechanism must be decreased pCO2 (hypocapnia), although the specific site of action and the method by which the ERG b-wave is enhanced remain topics for future research.

The effect of breathing 100 percent oxygen on foveal thresholds

This study examined the effects of breathing 100% oxygen at sea level on the luminance thresholds necessary for perception of red (642 nm), green (521 nm), blue (468 nm), and yellow (584 nm), and the chromatic and absolute thresholds associated with these dominant wavelengths. Using two highly trained observers, thresholds were measured for one target size and exposure duration by the method of constant stimuli. For the foveal thresholds investigated, no differences were found between the condition of breathing 100% oxygen and breathing normal room air at sea level. There is at least some physiological basis for thinking that vision may improve in a high oxygen environment. At the other extreme, breathing high concentrations of oxygen under pressure is known to degrade vision. Evidence relating to the visual effects of high oxygen concentrations is evaluated and conclusions are drawn with reference to the findings of this and other studies.

Night vision and psychiatric disorders: a review of experimental studies

In recent years a number of studies have been undertaken in which the “night vision” or dark-adaptation of psychiatric patients has been compared with that of normal subjects. These studies have their origin in a wartime observation that the incidence of “night-blindness” among neurotics was higher than among normal Service personnel. Evidence of functional disorders of vision is of interest from several points of view, psychiatric and ophthalmological as well as psychological and physiological. The aim of this article is to make a critical evaluation of the results so far obtained, determine what generalizations are possible and consider implications for further research.