The present study demonstrated no significant differences in the prevalence of ADs and/or BDs between the CRS and emmetropic pilots, and binocular vision dysfunction was a common finding in this population. To our knowledge, this was the first study to investigate ADs and/or BDs among civilian pilots. Thus, some aspects of these abnormalities need to be addressed.
The lack of significant differences in the prevalence of ADs and/or BDs between the CRS and emmetropic groups suggests that CRS in civilian pilots with low-moderate myopia might not generally impact binocular functions. These findings are consistent with the results of a recent study [23], which showed that diplopia and strabismus are rare complications after CRS in the U.S. military population. According to Kushner et al. [24], patients with less than 4 D of anisometropia, no prisms in their spectacles, and no history of diplopia or strabismus should be considered to have low risk of postoperative binocular function decompensation. In addition, García-Montero M et al. [4] found that most decompensation of binocular vision after CRS were in fact preoperative disorders. In this study, the mean preoperative SE of the CRS group was −1.51±1.15 D with no other significant preoperative medical histories. Thus, the pilots who underwent CRS met the low risk standards of postoperative ADs and/or BDs. Therefore, it might be reasonable that the differences in the prevalence of ADs and/or BDs between CRS and emmetropic groups were nonsignificant in this study.
In this study, the prevalence of overall ADs and/or BDs was estimated to be 15.7% and 15.2% in the CRS and emmetropia pilots, respectively. As shown in Table 5, this study obtained a lower prevalence of ADs and/or BDs than those reported in three studies [16, 25, 26] employing adult population. Lara et al. [25] examined a sample of 265 symptomatic participants who consecutively attended an optometric clinic and found that 59 (22.3%) presented some type of AD or BD. Martin et al. [26] examined the prevalence of ADs and/or BDs in a clinical population of 415 Chinese participants, finding that 178 patients (42.9%) in the total sample had general binocular disorders. The samples from both studies were taken from a clinical population seeking solutions to visual symptoms, which might have contributed to the higher prevalence of visual anomalies than the general population. In addition, Martin et al. [26] deliberately did not consider subjective symptoms when classifying participants with diagnostic criteria. Thus, the data obtained by Martin et al. [26] might have provided an overestimation of binocular vision dysfunctions.
Table 5 Summary of studies on accommodative and binocular dysfunctions
|
Study
|
Country
|
Age (year)
|
Number
|
Population
|
Prevalence (%)
|
|
|
|
|
|
AD
|
BD
|
total
|
García-Muñoz et al [20] (2016)
|
Spain
|
18~35
|
175
|
University (randomized)
|
2.29
|
8.00
|
13.15
|
Lara et al [21] (2001)
|
Spain
|
10~35
|
265
|
Clinic
|
9.4
|
12.9
|
22.3
|
Porcar et al [22] (1997)
|
Spain
|
19~25
|
65
|
University
|
17.0
|
15.3
|
32.3
|
Martin [23] (2019)
|
China
|
21~38
|
415
|
Clinic
|
Emmetropia: 10.5
Low myopia: 5.6
Moderate myopia: 5.6
High myopia: 4.7
|
Emmetropia: 38
Low myopia: 41
Moderate myopia: 29.9
High myopia: 36.7
|
Emmetropia: 48.5
Low myopia: 46.6
Moderate myopia: 35.5
High myopia: 41.4
|
Present study
|
China
|
18~35
|
207
|
Civilian pilots
|
refractive surgery: 4.63 Emmetropia: 3.03
|
refractive surgery: 11.1 Emmetropia: 12.1
|
refractive surgery: 15.7
Emmetropia: 15.2
|
AD=accommodative dysfunction; BD=binocular dysfunction; CRS=corneal refractive surgery
|
Esteban [17] selected 65 university students aged approximately 22 years old, 32.3% of whom showed ADs or BDs. This percentage is much higher than those obtained in the present study. Although the participants included in these studies were all university students and thus similar in age to the subjects in this study, drawing comparisons between them is still difficult, as each study included different populations, measurement methods and diagnostic criteria [9, 10]. For example, Esteban [17] applied “moderate to high exophoria at near >6△” to diagnose CI , while the present study adopted the standard criteria (“Near exophoria at least 4△ greater than distance exophoria”) suggested by Scheiman [6, 22] to diagnose CI. Therefore, the diagnostic criteria of ADs and/or BDs used in these two studies differ substantially, and this should be considered one of the main factors leading to the varying prevalence figures between studies.
The prevalence in this study was slightly higher than that obtained by Ángel [16], who found that 23 university students (13.15%) among the total sample presented some type of AD and/or BD. They used criteria for diagnosing ADs and/or BDs similar to those used in the present study. However, the prevalence of ADs and/or BDs was different because of the characteristics of the study participants. The participants of the present study were civilian pilots who are required to perform considerable amounts of near work, such as reading a panel during a long-duration flight; thus, they are more likely to develop symptoms and signs related to ADs and/or BDs. In addition, lack of sleep [27], fatigue [28], and cervical symptoms [29] are also known to aggravate the problem [30]. Therefore, studying the prevalence of ADs and/or BDs among this specific population is important for planning appropriate intervention.
Regarding visual symptoms, the higher COVD-QOL scores in this study suggest that most pilots with ADs and/or BDs indeed experience many visual problems in their daily lives. These findings are consistent with the results of previous studies [20, 31], which showed that individuals with binocular vision anomalies had more visual discomfort symptoms than those with normal binocular vision. These visual complaints may include asthenopia, headache, blurred vision, loss of concentration when reading or doing near work [6-8]. These visual symptoms may have a negative effect on flight performance and leisure activities. Headaches, for example, can diminish pilots’ quality of life by giving them constant pain. Asthenopia and blurred vision can seriously affect pilots’ daily activities. Hence, all pilots who complain of visual symptoms should be tested for ADs and/or BDs. Detecting and managing these dysfunctions as early as possible are important, as pilots are required to operate under both physiologically and psychologically stressful conditions, and they often face a high visual workload demand within a degraded visual environment [18].
This study separately analyzed subjects with an asymptomatic binocular vision anomaly, revealing that only subjects with CI had abnormal clinical measurements without symptoms. The results of this study are similar to the results reported by Hussaindeen et al. [11], who found that 58 school children (6.3%) in the total sample were asymptomatic but still failed the binocular vision tests.
Several factors may account for this mismatch between signs and symptoms. Firstly, some professionals have argued that CI is not a highly symptomatic condition. Some subjects with CI who were not symptomatic might have suppression, avoidance of near visual tasks, or monocular occlusion [32, 33], but this was not assessed directly in this study. Secondly, this study also revealed significantly lower accommodative amplitude and binocular accommodative facility in symptomatic subjects with CI compared with asymptomatic subjects with CI. According to the results of Marran et al. [32], children with accommodative insufficiency (AI) only and children with both AI and CI had more visual symptoms than children with CI only. The outcomes obtained in the current study further corroborate the conclusion of Marran et al. [32] that elevated symptoms in CI may be the result of comorbid AI. Therefore, determining the symptoms specific to CI due to the high comorbidity of CI and accommodative dysfunction would be beneficial for future studies.
Furthermore, the subjective responses of pilots may not be reliable. Most binocular tests used to diagnose ADs and/or BDs are based on subjective responses. However, this study cohort included two groups of motivated and highly competitive pilots who were required to faithfully complete all measurements, and they may have “overachieved” on the subjective response tests. Therefore, the prevalence results of ADs and/or BDs among the civilian pilots in this study represent the best-case scenario.
This study has several limitations. One limitation was subject selection. As mentioned above, only 110 pilots had CRS in Southwest China; therefore, this study did not have sufficient subjects to randomize the sample. Secondly, although only adult participants were included, cycloplegia was not applied to avoid disrupting the evaluation of accommodation. However, the plus lens (+2.00 D) test was conducted on all participants to exclude latent hyperopia. Lastly, these data were based on the currently outdated broad beam laser and cannot be directly compared with contemporary techniques, such as femtosecond laser technology. Further studies are needed in this area.
In conclusion, these findings suggest that CRS for civilian pilots with low-moderate myopia might not have a general impact on binocular function. ADs and/or BDs are commonly present in pilots with both CRS and emmetropia. Pilots with increased visual symptoms may benefit from binocular vision evaluation to assess for the presence of ADs and/or BDs, and future research investigating the potential risks associated with aviation accident experience of civilian pilots with ADs and/or BDs might provide more insight into improving their vision efficiency and daily lives.
Table 1 Diagnostic criteria for non-strabismic binocular and accommodative dysfunctions.
|
Convergence Insufficiency
|
Requires 1, 2, and 3
|
|
1. Near exophoria at least 4△ greater than distance exophoria
2. NPC break point ≥6 cm
3. Reduced near PFV (break point ≤15△ or failed Sheard’s criterion)
|
Convergence Excess
|
Requires 1 and at least 1 sign from 2 ~ 3
|
|
1. Near esophoria greater than distance esophoria by ≥4△
2. Reduced near NFV, ≤8/16/7 for blur, break and recovery (at least one of three)
3. Near VF ≤12 cpm
|
Divergence Insufficiency
|
Requires 1 or 2 + 3
|
|
1. Distance esophoria greater than near esophoria by ≥10△
2. Distance esophoria greater than near esophoria by ≥4△
3. Reduced distance NFV (break point ≤4△ or failed Sheard’s criterion)
|
Divergence Excess
|
Requires 1 or 2 + 3
|
|
1. Distance exophoria greater than near esophoria by ≥10△
2. Distance exophoria greater than near esophoria by ≥4△
3. Reduced distance PFV, ≤ 4/ 10/ 5 Δ (at least one of three)
|
Basic Exophoria
|
Requires 1, 2 and at least 1 sign from 3 ~ 5
|
|
1. Difference between near and distance exophoria ≤3△
2. Subjects needs to be exophoria at both distant and near
3. PFV at far ≤ 4 /10/ 5Δ and ≤ 11/14/3 Δ at near (at least one of three)
4. NPC break point ≥6 cm
5. Near VF ≤12 cpm
|
Fusional Vergence Dysfunction
|
Requires 1, 2 and at least 1 sign from 3 ~ 4
|
|
1. No significant phoria at distance and near (distance: exophoria ≤2△ to orthophoria; near: exophoria ≤5△ to orthophoria)
2. No other vergence dysfunction diagnosed
3. Reduced NFV or PFV (PFV break point ≤15△ or NFV break point≤7△ or failed Sheard’s criterion)
4. Near VF ≤12 cpm
|
Accommodative Insufficiency
|
Requires 1
|
|
1.Monocular AA at least 2 D below minimum age-based norms as defined by Hofstetter’s formula (15-age/4)
|
Accommodative Infacility
|
(Requires 1 or 2
|
|
1.MAF≤6 cpm with ±2.00 D lenses
2.BAF≤3 cpm with ±2.00 D lenses
|
Accommodative excess
|
1. Variable visual acuity findings
2. Variable static retinoscopy and subjective refraction
3. MAF≤6 cpm with +2.00 D lenses
4. BAF≤3 cpm with +2.00 D lenses
5. MEM< +0.25 D
6. NRA < +1.50 D
|
NPC Near point of convergence, PFV Positive fusional vergence, NFV Negative fusional vergence, cpm cycle per minute, AA Amplitude of accommodation, BAF Binocular accommodative facility, MAF Monocular accommodative facility, MEM Monocular estimated method, NRA Negative relative accommodation, VF Vergence facility.
|