Mild systemic hypoxia and photopic visual field sensitivity

Influence of hypobaric hypoxic conditions on ocular structure and biological function at high attitudes: a narrative review

Background

With the development of science and technology, high-altitude environments, involving aviation, aerospace, and mountainous regions, have become the main areas for human exploration, while such complex environments can lead to rapid decreases in air and oxygen pressure. Although modern aircrafts have pressurized cabins and support equipment that allow passengers and crew to breathe normally, flight crew still face repeated exposure to hypobaric and hypoxic conditions. The eye is a sensory organ of the visual system that responds to light and oxygen plays a key role in the maintenance of normal visual function. Acute hypoxia changes ocular structure and function, such as the blood flow rate, and can cause retinal ischemia.

Methods

We reviewed researches, and summarized them briefly in a review.

Results

The acute hypobaric hypoxia affects corneal, anterior chamber angle and depth, pupils, crystal lens, vitreous body, and retina in structure; moreover, the acute hypoxia does obvious effect on visual function; for example, vision, intraocular pressure, oculometric features and dynamic visual performance, visual field, contrast sensitivity, and color perception.

Conclusion

We summarized the changes in the physiological structure and function of the eye in hypoxic conditions and to provide a biological basis for the response of the human eye at high-altitude.

Distance visual acuity in air force pilots and student pilots when exposed to +Gz acceleration in human centrifuge

Background/Aim. High speeds that modern aircraft develop during take-off, flight and landing place an additional strain on the organ of vision. Owing to its considerable practical implementation in air combat, the effect of +Gz acceleration on the organ of vision is considered increasingly important for research. Substantial changes in visual functions may occur during high acceleration onset rates. However, it is important for a pilot to maintain visual acuity in order to be able to monitor new functional displays for rapid orientation, scan the configuration of terrain, display of weapons systems and enemy aircraft and deal with additional issues of the complexity of spatial orientation. The aim of the investigation was to establish whether distance visual acuity in air force pilots and student pilots is affected when exposed to +Gz acceleration. ??thods. The study was performed on a defined population consisting of 95 respondents from 21 to 45 years of age divided into two groups. The first group included 65 air force pilots and the second group comprised of 30 student pilots, all of whom were exposed to an acceleration of +5.5 Gz. The testing was per-formed in a human centrifuge, which mimics conditions of real Gz acceleration, in the Department of Biodynamics in Aero Medical Institute (Zemun, Serbia). We examined the obtained differences in distance visual acuity before and after exposure to acceleration. Results. After the testing, all respondents in the group of air force pilots had distance visual acuity of 1.0, while in the group of student pilots a statistically significant difference in distance visual acuity was observed after being exposed to +Gz acceleration. Conclusion. Transient changes in distant visual acuity were more pronounced in the group of student pilots in comparison with the changes in visual acuity in the air force pilots when exposed to the same acceleration values (+5Gz acceleration). Since change in distance visual acuity is the most sensitive physiological indicator when exposed to high acceleration, individual physiological pilot training in the human centrifuge increases tolerance to accelerations, which is important for flight safety in both peacetime and combat conditions.

Electrophysiological mechanisms underlying hypoxia-induced deficits in visual spatial and non-spatial discrimination

Impaired visual cognition in residents of hypoxic environment has been widely reported; however, the underlying electrophysiological mechanisms remain unclear. In this study, 23 college students underwent three sessions of a Clock task test before a 30-day high-altitude exposure (Test 1) and 1 week (Test 2) and 3 months (Test 3) after they returned to lowlands. The Clock task consists of a visual spatial angle and a visual non-spatial color discrimination subtask. Simultaneously, electroencephalography (EEG) was recorded during the Clock task. The behavioral results showed that, compared with Test 1, accuracy in Test 2 was significantly decreased in both the Angle and Color tasks, and reaction time (RT) was significantly increased in the Angle task. The event-related potentials results showed that, during both tasks amplitudes of the occipital N1 and P3 components during both tasks were significantly decreased in Test 2, compared with Test 1. Moreover, N1 amplitude was negatively correlated with RT and positively correlated with accuracy. Further time-frequency EEG analysis showed that theta power at occipital sites was significantly decreased in both tasks in Test 2, compared with Test 1, and was negatively correlated with RT in the Angle task. In Test 3, both the behavioral performance and EEG activity recovered to the baseline level in Test 1. These findings suggested that hypoxia impairs both visual spatial and visual non-spatial discriminations, and these impairments can recover after subjects return to lowlands. Inhibition of brain electrophysiological activity in the visual cortex may explain the deficits in visual cognition.

Age-related visual signal changes induced by hypoxemic hypoxia: a study on aircraft pilots of different ages

PURPOSE

Exposure to high altitude leads to a series of alterations of higher nervous functions because of hypobaric hypoxia. Sensory systems, mainly the visual one, seem to be particularly involved. This study aimed to assess the effects of hypoxemic hypoxia on the transmission of the visual stimulus simulating a condition of breathing at an altitude of 18,000 feet (5,486 m) through the administration of an air mixture with 10% O2.

METHODS

The subjects involved in the study were 98 pilots of military aircraft (male, acclimated, healthy, 20/20 Uncorrected Visual Acuity (UCVA)/Best Corrected Visual Acuity (BCVA), and aged between 26 and 49 years) divided into 2 groups according to age (A: 26-36 years; B: 37-49 years). The visual evoked potentials were initially recorded at sea level (760 mm Hg) and subsequently at a simulated altitude of 18,000 feet (5,486 m) through the administration of an air mixture with 10% O2 that induced a blood saturation of 80% O2 after 15 minutes. The analysis was carried out using two different kinds of stimulus (15' and 60' of arc). The latency and the amplitude of N-75 (N1) and P-100 (P1) waves have been evaluated. Results obtained from visual evoked potentials were analyzed with Student t-test.

RESULTS

In the first group (pilots aged 26-36 years), an increase in both latency and amplitude of P-100 wave was observed and in the second group (pilots aged 37-49 years), an increase was found in latency and a significant reduction in amplitude.

CONCLUSIONS

The study suggests the existence of a mechanism or a particular anatomic and physiologic condition (probably the neurovascular coupling) that connects the local neuronal activity and the resulting changes in cerebral perfusion. This complex series of events binds together different structures and cell types, and it seems that younger people have a better resistance against the hypoxic insult to the central nervous system because of more efficient compensatory mechanisms.

The effect of hypercapnia on the sensitivity to flicker defined stimuli

BACKGROUND/AIMS

To investigate the effect of increased CO2 levels on flicker defined stimuli.

METHODS

The sensitivity of two flicker defined tasks was measured in nine healthy, trained observers using the Flicker Defined Form (FDF) stimulus of the Heidelberg Edge Perimeter (HEP; Heidelberg Engineering) and Frequency Doubling Technology (FDT) stimulus of the Matrix perimeter (Carl Zeiss Meditec) during normoxia and 15% hypercapnia (end-tidal CO2 increased by 15% relative to baseline). HEP-FDF and Matrix-FDT sensitivities were analysed for the global field, superior and inferior hemifields and at specific matched eccentricities, using repeated measures analysis of variance. The main effect of hypercapnia on flicker sensitivity was analysed using regression models.

RESULTS

Higher flicker sensitivity outcomes with increasing CO2 values were found for HEP-FDF and Matrix-FDT with a statistically significant main effect for HEP-FDF global, superior and inferior hemifields (p<0.01 for all) as well as 6°, 18°, 12° and 24° eccentricities (p=0.03, 0.04, 0.01, 0.05, respectively). When comparing mean sensitivity values between normocapnia and hypercapnia conditions, no statistically significantly different results were found for HEP-FDF and Matrix-FDT (p>0.05).

CONCLUSIONS

As CO2 levels were increased in healthy young individuals, there was an associated increase in visual sensitivity that was only significant for HEP-FDF stimuli, highlighting the different mechanisms involved in processing each of HEP-FDF and Matrix-FDT stimuli. Mean visual sensitivity outcomes were found to be similar for normocapnia and hypercapnia suggesting that a capability to compensate for a mild and stable increase in systemic CO2 levels may exist.

Evaluation of visual field parameters in patients with chronic obstructive pulmonary disease

PURPOSE

To evaluate the effects of chronic obstructive pulmonary disease (COPD) on retina and optic nerve.

METHODS

Thirty-eight patients with COPD and 29 healthy controls, totally 67 subjects, were included in the study. Visual evoked potentials (VEP) and visual field assessment (both standard achromatic perimetry (SAP) and short-wavelength automated perimetry (SWAP)) were performed on each subject after ophthalmological, neurological and pulmonary examinations.

RESULTS

Mean deviation (MD), pattern standard deviation (PSD) and corrected pattern standard deviation (CPSD) were significantly different between patient and control groups as for both SAP and SWAP measurements (p = 0.001, 0.019, 0.009 and p = 0.004,0.019, 0.031, respectively). Short-term fluctuation (SF) was not statistically different between the study and the control groups (p = 0.874 and 0.694, respectively). VEP P100 latencies were significantly different between patients with COPD and the controls (p = 0.019).

CONCLUSION

Chronic obstructive pulmonary disease is a systemic disease, and hypoxia in COPD seems to affect the retina and the optic nerve.