Visual Fatigue Factors Associated with Night-Vision Training Among Military Pilots

METHODS

In the course of night-vision training in the Air Force Medical Center of Chinese People’s Liberation Army, some pilots also experienced different degrees of visual fatigue after training. In order to find out the possible factors leading to visual fatigue during night-vision training, so as to take corresponding mitigation measures in the later training, this study was carried out.

This study comprised 20 pilots who had undergone night-vision-related training at the Air Force Medical Center of Chinese People’s Liberation Army between October 2021 and March 2022, and we collected data on their visual function and any visual changes before and after training. We analyzed the factors related to visual fatigue caused by night-vision training by observing the visual function examination and visual quality evaluation of the pilots before and after wearing the night-vision device during night-vision training, laying the groundwork for finding solutions to alleviate visual fatigue in a night-vision environment.

RESULTS

The basic ocular assessment of the 20 male pilots who participated in the program revealed that they had good distance vision, near vision, dark adaptation, stereo vision, color vision, eye position, and contrast sensitivity. The specific results are listed below.

The visual acuity test is performed using a C-chart, with standard visual acuity above 1.0. Among the 20 pilots (40 eyes), 16 pilots (32 eyes) had distance vision greater than 1.0; 3 pilots (6 eyes) had distance vision greater than 0.6 in one eye while the opposite eye had distance vision greater than 1.0; and only 1 pilot (2 eyes) had distance vision of 0.4 in both eyes. In the near vision examination, for which the aeromedical standard is J2 (Jaeger2) or better, all 20 pilots (40 eyes) were higher than J2. In the dark adaptation examination, among the 20 pilots, the shortest dark-adaptation time was 12 s and the longest duration was 38 s; both were within the normal range. The Fly Stereo Acuity Test is used to detect the pilot’s stereo vision. The normal value range of stereo acuity is usually between 40–60". In the field of aviation medicine, there is currently no specific reference standard for stereoscopic vision function. Usually, the standards of the normal population are used as a reference by which the smaller the value, the better the stereoscopic vision function. In our study, the Fly Stereo Acuity Test with Lea Symbols (2007) showed stereoacuity thresholds of 20 arcsec in 10 pilots (50%), 25” in 4 pilots (20%), 32” in 3 pilots (15%), 40” in 1 pilot (5%), ad 50” in 2 pilots (10%). In the eye position examination (alternate cover method), eight pilots (40%) had heterophoria; the remaining eyes were orthophoric. After contrast-sensitivity testing, 10 pilots (50%) had normal contrast sensitivity in both eyes and the other 10 pilots (50%) had varying degrees of decline, including 4 eyes (10%) in bright light (naked eye examination), 12 eyes (30%) in dark light (wearing sunglasses), and 14 eyes (35%) in glare.

All participating pilots met the refractive-error standards for flight certification. Among the 20 pilots (40 eyes) who participated in night-vision training, 2 eyes had emmetropia, 3 eyes had myopia, 1 eye had presbyopia, 4 eyes had simple myopic astigmatism, 20 eyes had compound myopic astigmatism, and 10 eyes had mixed astigmatism. After night-vision training, the number of eyes with emmetropia and presbyopia decreased to 0, while the number of eyes with mixed astigmatism increased to 13 and the number of eyes with other refractive states did not change. In the group aged under 35 yr, 7 pilots showed ametropia type changes after training, while in the group aged 35 yr and over, 5 pilots showed ametropia type changes after training. A Chi-squared test showed no statistically significant difference in the changes (Table I, χ = 0.135, P = 0.535).

Table I. Comparison of the Type of Ametropia before and after Training after Grouping by Age.

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There were varying degrees of changes in the diopter examination after night-vision training. Among the 20 pilots (40 eyes), the negative value of spherical degree (myopia degree) increased in 11 eyes (27.5%), decreased in 15 eyes (37.5%), and did not change in 14 eyes (35%), with a range of change between −1.50 D–1.75 D. The negative value of cylindrical degree (myopic astigmatism) increased in 11 eyes (27.5%), decreased in 9 eyes (5%), and did not change in 20 eyes (50%). In addition, only 1 eye (2.5%) in 40 eyes decreased by 0.75 D, and 1 eye (2.5%) increased by 1.25 D. The cylindrical degree changes in the remaining 38 eyes were less than 0.50 D, and 50% had changes below 0.25 D. It can be seen that the change of diopter before and after training is mainly due to the change in spherical degree. Among the 20 pilots (40 eyes), there was no significant difference in spherical-equivalent change before vs. after training between right and left eyes (Z _{right eye} = −0.667, P _{right eye} = 0.505; Z _{left eye} = −0.119, P _{left eye} = 0.906; Z _{both eyes} = −0.378, P _{both eyes} = 0.706; all P > 0.05).

The negative value of spherical-equivalent difference (that is, the difference between the equivalent spherical degree before and after training) increased (that is, the negative value of the equivalent spherical degree after training was greater, which was closer to myopia) in 15 eyes (37.5%), decreased in 19 eyes (47.5%), and did not change in 6 eyes (15%). Moreover, among the 15 eyes with increased myopia after training, there were 10 eyes in those aged under 35 yr and 5 eyes in those aged 35 yr and over. There were no significant differences in the difference value between different age groups (Z _{right eye} = −0.346, P _{right eye} = 0.730; Z _{left eye} = −1.455, P _{left eye} = 0.146; Z _{both eyes} = −1.289, P _{both eyes} = 0.197; all P > 0.05).

After training, 17 pilots out of the 20 (85%) had changes in interpupillary distance, of which 13 (65%) had a decrease in interpupillary distance by 0.5–4.0mm, 4 (20%) had an increase in interpupillary distance by 0.5–1.0mm, and 3 (15%) had no change in interpupillary distance. The difference in interpupillary distance from before to after training was statistically significant (P = 0.017). Meanwhile, in the group aged under 35 yr, there was no significant difference in the change of interpupillary distance before and after training (P = 0.437). In the group aged 35 yr and over, the interpupillary distance after training was significantly reduced compared with that before training (P = 0.018) (Table II).

Table II. Comparison of the Interpupillary Distance before and after Training after Grouping by Age.

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When we evaluated visual fatigue symptoms based on the responses to the “Questionnaire on Visual Changes before and after Night-Vision Training” (supplemental Appendix A, found in the online version of this article), we found that the 20 pilots who participated in the training had varying degrees of eye swelling, eye pain, eye redness, eye itching, dry eyes, fatigue, foreign body sensation, increased number of blinks, and other symptoms of visual fatigue. Before training, the most common symptoms of visual fatigue were fatigue, dryness, ocular itching, and increased number of blinks, while the symptoms of visual fatigue after training were mainly fatigue, blurred vision, eye swelling, dryness, and increased number of blinks (Fig. 1).

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The Chi-squared test comparing different symptoms before and after training showed that the symptoms of blurred vision were most significantly aggravated after training (χ = 4.286, P = 0.041) (Table III), while the remaining symptoms had varying degrees of aggravation, with no statistical difference observed. At the same time, 11 of the 20 pilots (55%) who participated in the training reported varying degrees of worsening visual fatigue symptoms after training, while 5 pilots (25%) had reduced visual fatigue symptoms and 4 people (20%) had no significant change in visual fatigue symptoms. Also, when different age groups were compared, there was no statistically significant difference in the number of pilots who reported worsened visual fatigue symptoms after training (Table IV, χ = 0.002, P = 0.658). In addition, we also found that after night-vision training, 7 pilots (35%) reported worsening symptoms of visual fatigue with decreased interpupillary distance, 2 people (10%) had reduced symptoms of visual fatigue with increased interpupillary distance, and 3 people (15%) had worsening symptoms of visual fatigue with no change in interpupillary distance, but the difference was not statistically significant.

Table III. Comparison of Blurred Vision before and after Training.

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Table IV. Comparison of Worsening Visual Fatigue Symptoms in Different Age Groups after Night-Vision Training.

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DISCUSSION

Wearing and using night-vision goggles (NVG) has become increasingly prevalent in modern combat, as the share of night operations has increased. In the 1990s, the Combat Training Research Division of the U.S. Air Force Research Laboratory began developing NVG system training, and they soon implemented it in flight units.³ NVG are a class of photoelectric imaging devices using low-light-level image enhancement technology, which can increase the brightness of the external view of an airplane cabin by at least 3 orders of magnitude at night.⁴

However, the use of modern NVG technology still has certain shortcomings, such as providing pilots with an observation field of view of only about 40° and requiring them to move their heads to obtain more observation information.⁵ Although binocular night-vision equipment can improve the field of view to around 100°, the problem of limited field of view persists.² Furthermore, because of the low level of NVG imaging, the stereoscopic vision of pilots is reduced when using night-vision equipment. This, coupled with the loss of vision in a dark environment and the difference between the imaging spectrum of night-vision equipment and the spectrum of human sensitization, makes judging altitude and distance during flight quite challenging.⁶

North Atlantic Treaty Organization (NATO) studies have demonstrated that pilots with a visual acuity of 20/15 who wear NVG cannot achieve the visual level of those with a visual acuity of 20/20 who wear NVG.⁷ As a result of this, pilots often experience varying degrees of visual fatigue after wearing night-vision devices.

Through eye examinations before and after training of pilots who underwent night-vision training in our hospital between October 2021 and March 2022, we found that night-vision training aggravated their symptoms of visual fatigue to varying degrees. In the visual fatigue questionnaire survey, more than 50% of the trainees of different age groups reported worsening symptoms after night-vision training, indicating that pilots of different ages may have visual fatigue after night-vision training, and this phenomenon did not significantly worsen with age.

Apart from disease-related factors, the factors that typically cause visual fatigue are divided into two categories: regulation-related factors and convergence-related factors. The regulation-related factors refer to ciliary muscle contraction which relieves suspensory ligament tension, resulting in a convex lens and increased refractive power. In this study, a single eye having presbyopia and 2 eyes having emmetropia changed to mixed astigmatism, simple myopic astigmatism, and compound myopic astigmatism after training, and 12 eyes (30%) had distinct myopia-related refractive state changes. The refraction examination further confirmed that the subjects had varying degrees of diopter changes of increased negative values in the spherical degree after training, while the cylindrical degree was almost the same before and after training.

Although the difference in the equivalent spherical before and after training was not statistically significant, when we calculated the equivalent spherical difference (that is, the difference between the equivalent spherical after training and the equivalent spherical before training), we found that 15 of 40 eyes (37.5%) had a negative value of the equivalent spherical difference. In other words, the equivalent spherical after training had a greater negative value, meaning that the myopic diopter was higher. This was especially so in the group aged under 35 yr where the proportion of subjects with a negative value of equivalent spherical difference was 45.5% (i.e., 10 of 22 eyes had negative values), compared with 27.8% in the group aged 35 yr and over (i.e., 5 of 18 eyes had negative values), indicating that a certain degree of myopia adjustment occurred in the eyes after night-vision training, particularly in pilots aged under 35 yr.

Apart from regulation-related factors, visual fatigue is also caused by convergence-related factors. In this study, 65% of subjects had significantly decreased interpupillary distance after training. Specifically, 7 of the 9 pilots aged 35 yr and over had decreased interpupillary distance, accounting for 77.8%, and the remaining 2 had no change in interpupillary distance. This indicates that despite having more flight experience, the regulation ability of the eye muscles in older pilots is relatively weak. The interpupillary distance reduces after training with night-vision equipment, and it is harder for the eye muscles to relax quickly after convergence actions. It was easier to obtain measurement data of reduced interpupillary distance, and the data were statistically distinct.

In contrast, among the 11 pilots aged under 35 yr, 6 showed reduced interpupillary distance after training, accounting for 54.5%, which was significantly lower when compared to those who were 35 yr and over. Moreover, another 4 showed increased interpupillary distance, accounting for 36.4%, and the remaining 1 had no change in interpupillary distance. This suggests that pilots in the group aged under 35 yr had better eye-muscle regulation ability, which allowed them to quickly adjust and adapt when putting on night-vision devices and immediately relax after removing them. As a result, the difference in change in interpupillary distance was not significant. This demonstrates that after wearing the night-vision device, sustained convergence also contributes to visual fatigue.

In conjunction with the responses of the trainees to the “Questionnaire on Visual Changes before and after Night-Vision Training", we found that the training aggravated their symptoms of visual fatigue to varying degrees. Among the 14 symptoms of visual fatigue investigated in the questionnaire, the trainees reported an increase in 10 symptoms after night-vision training (Fig. 1). Specifically, the incidence of blurred vision was significantly higher after training, which was related to the blurred vision caused by an increase in myopia diopter and reduced interpupillary distance after wearing night-vision gear.

In this study, 35% of the pilots had increased interpupillary distance and worsened visual fatigue symptoms, 15% had increased interpupillary distance and alleviated visual fatigue symptoms, and 15% had no change in interpupillary distance but aggravated visual fatigue symptoms. Despite the fact that we did not detect any statistically significant difference due to the small sample size, it is clear that decreasing the interpupillary distance was closely related to the appearance of visual fatigue symptoms. Moreover, the pilots reported an increase in symptoms of seeing double, eye swelling, and eye pain, though the differences were not statistically significant. It is nevertheless possible that night-vision training aggravated the symptoms of visual fatigue in the trainees to a certain extent. However, the causes of color perception changes and visual rotation in individual trainees were not known, and these may be related to factors such as color perception fatigue caused by continuous green visual field in a night-vision environment, visual rotation resulting from optical illusion training, and the like, which require further research.

In conclusion, the visual fatigue caused by wearing night-vision devices can be multifactorial, and proficiency in the use of night-vision equipment can reduce the workload of pilots. The North Atlantic Treaty Organization (NATO) issued the standardized protocol STANAG 7147 - Aviation Medical Issues of Night-Vision Equipment Training in 2007, which established standards for night-vision training for pilots.⁸ The protocol stipulates that even experienced pilots need to continuously improve their knowledge of night-vision equipment and undergo training for its proper use. If flight training cannot be carried out within 3 mo of ground training, they must undergo night-vision ground training again to ensure that the training is effective.⁸ The U.S. Army developed a 4.5-h training program in 1975 for the use of second-generation night-vision equipment.⁹ The identification training related to NVG of the U.S. military was increased to 10 h in 1983 and 20 h in 1999.⁹

In this study, we found that night-vision training aggravated the symptoms of visual fatigue in trainee pilots to varying degrees. When wearing night-vision devices, regulation and convergence actions often occur simultaneously, which were manifested as concurrent changes in diopters and reduced interpupillary distance. This, in turn, causes changes in refractive type, resulting in the presence or aggravation of visual fatigue in subjects at the end of training. Therefore, it is very necessary to formulate a reasonable training plan, including reasonable training duration and frequency, and further exploration is needed. And during the training, whether before or after night-vision training, pilots can be advised to actively look far away whenever visual fatigue symptoms occur, and if necessary, intervention with appropriate eye drops such as “Stulln Mono” (a commonly prescribed drug) may be given to relieve symptoms. This pharmacological intervention may also be used in pilots who exhibit signs of visual fatigue before training.

Through this study, we found that the change of diopter and the reduction of interpupillary distance of the trainee pilots were consistent with the aggravation of visual fatigue symptoms, especially blurred vision. Also, pilots aged under 35 yr were more likely to develop regulation-related visual fatigue, while pilots aged 35 yr and over were more likely to develop convergence-related visual fatigue. Therefore, paying attention to the change of relevant indicators before and after training, such as the measurement of pupil distance, the change of diopter, and the questionnaire related to visual fatigue, can help the pilot better adapt to night-vision training.