Hydrogel Contact Lens Water Content is Dependent on Tearfilm pH.

Introduction: Based on clinical inferences, investigators theorized in situ soft lens hydration was linked to the precorneal tearfilm pH. Methods: Forty-one myopic subjects at Fort Rucker, AL, USA were fitted with one of two types of extended-wear soft contact lenses, and were followed quarterly for a period of 33 mo. The anterior soft contact lens surface pH was measured in situ, while the in vitro lens water content was measured immediately after lens removal, using a hand-held refractometer on one lens, and a gravimetric means of hydration measurement on the other lens. Results: The in situ pH increased logarithmically across extended wearing time, reaching an asymptote at approximately 5 d' wearing time at a pH of 7.45 ± 0.03. Lens water content was shown to similarly decrease at an inversely logarithmic rate, leveling off at 4-5 d' extended wearing time. Both means of hydration assessment correlated well with each other across days' extended wearing time (R = 0.98; p < 0.0001). Discussion: A log-log dual conversion yielded significantly different linear slopes (p < 0.001), based on a multifactorial analysis of both lens types, by the pH, and by their water content. The differing ionic status of each material accentuated their varied polar attraction characteristics. Two soft lenses, identical in all patient-based parameters, could provide differing oxygen availability, as well as differing physical fits in patients of identical physical characteristics, as a result of their unique tearfilm pH differences. Conclusion: The in situ hydrogel lens water content is directly dependent on the precorneal tearfilm pH.

Human Contrast Acuity Variability.

BACKGROUND: Army vision standards have varied little from Aviation's nominal birth. On the basis of classic Snellen acuity, we simply cannot predict threshold skill levels of any one individual(s). A growing number of Army Flight Surgeons, clinicians, and vision scientists have argued for the inclusion of contrast acuity metrics within flight physical standards. METHODS: Previous monitoring of operational contact lens utility in 223 Apache pilots, visual acuity data were gathered under two conditions: high illuminance; low illuminance combined with low contrast. Spectacle, contact lens, and aging influences were evaluated. RESULTS: The high-contrast Snellen acuities clustered at 20/15 and 20/20. Low-contrast acuities stretched from 20/25 to 20/125. LogMAR analysis highlighted statistical significance between the two acuity sets (p < 0.001) to an unanticipated data spread. The known underlying mechanisms possibly related to this effect are poorly documented; all such variables collectively explain <30% of the known variation in low-illuminance vision. DISCUSSION: Some pilots possessed the capacity to resolve 20/25 lettering under obfuscating conditions; others were adversely influenced by those same conditions. Snellen acuity involves target recognition; contrast acuity detects threshold differences; both aspects can be important. CONCLUSION: Prescreening under both vision assessment conditions will help identify and select superior vision performers. The validity and predictability of documenting this effect is targeted within planned future research efforts.

Brief report: a hypothetical construct based on limited data visual system recovery after refractive surgery.

Laser refractive surgery, involving the computer-controlled application of a 193-nm beam of excimer laser "light," is utilized to resculpt the central cornea, thus reducing its apical thickness. On casual inspection, this simple matter of removing or excising a specific amount of central corneal avascular tissue is a smooth, seamless alteration with few apparent secondary issues or sequelae. Normal postoperative recovery is typically gauged by the recovery of high-contrast visual acuity to the same (or better) degree as was previously obtained with a spectacle correction. However, although this is an acceptable means of determining operative success, it is not indicative of the complex challenges imposed upon the neurosensory system. The secondarily imposed strain upon the visual system, regarding the return to its pre-existing visual line-of-sight organization occurs only by bringing multiple adaptations into subtle and seamless play. This process is initiated and completed in a relatively short time period, such that most patients (but not all) are not even marginally aware of the challenges imposed to the visual system. This article is meant to probe those system challenges, serving to highlight this postoperative plasticity, seeking to gain a broader understanding and appreciation of the perceptual range of the visual recovery process.

Military aircrew eyewear survey: operational issues.

BACKGROUND: Army aircrew are required to operate under stressful conditions within a degraded visual environment. To gain an understanding of current eyewear mission needs, we surveyed aircrew regarding operational issues associated with the current U.S. Army-issue aircrew eyewear. METHODS: An anonymous online survey was completed by 976 U.S. Army aircrew personnel recruited through an invitational e-mail. RESULTS: Of the respondents, 34% reported wearing corrective lenses. Nearly 50% of them reported dissatisfaction with the HGU-4/P Aviator spectacles. The leading problems reported were discomfort (15%), "hot spots" around the ears (13%), and break of the earcup seal (11%). Nearly 41% of the respondents wore multifocal lenses and, of those, 16% reported that the HGU-4/P's restricted field of view (FOV) adversely affected duty performance. Of those who wore Modified HGU-4/P Apache spectacles, 79% had an unacceptable FOV with the helmet display unit. A similar number of aircrew modified the Apache eyewear to increase the FOV. In addition, wind and dust created eye discomfort or affected the vision of 46% of all respondents when performing flight duties. Over 82% expressed the need for an improved aircrew spectacles and 73% desired aviation-compatible protective eyewear. DISCUSSION: The current Army HGU-4/P Aviator and the Modified HGU-4/P Apache spectacles have significant operational deficiencies and compatibility issues. Such deficiencies have led to poor wear compliance and unauthorized modification of the Army-issue Aviator eyewear, posing readiness and safety issues. The results of this survey formally identify the need for military combat eye protection that is compatible with flight systems.