Long-Term Follow-Up of Helicopter Pilots Using Monocular Helmet Displays Shows No Anisometropia

INTRODUCTION: Pilots of multiple rotary-wing platforms of the Royal Netherlands Air and Space Force operate with monocular helmet-mounted displays (HMDs) that project flight or weapon symbology over the right eye. In particular, AH-64 Apache pilots continuously use the Integrated Helmet and Display Sight System, whereas AS532 Cougar pilots use a monocular HMD mainly during degraded visual environments. Although these systems are calibrated to project imagery at optical infinity, incorrect calibration may induce accommodation. There is concern that prolonged monocular accommodation could lead to myopization and, ultimately, anisometropia. METHODS: Longitudinal aeromedical screening data from Dutch rotary-wing pilots were retrospectively analyzed over a 12-yr period (2012-2023). In total, 336 pilots with 3,169 ophthalmological screenings were included. Apache and Cougar pilots constituted the experimental groups and were compared with pilots from other rotary-wing platforms without monocular HMD use. Anisometropia was defined as a ≥1 diopter difference in spherical equivalent between both eyes. RESULTS: Among Apache (N = 126) and Cougar (N = 62) pilots, only 1 case of anisometropia was identified, occurring in an Apache pilot with prior refractive surgery and without progression over time. In the control group (N = 150), 2 cases of anisometropia were observed, 1 of which was present at baseline. DISCUSSION: These longitudinal data show no evidence that prolonged use of monocular HMD is associated with the development of permanent anisometropia in rotary-wing pilots. Cornelissen SJWM, de Jong M, Jalink MB. Long-term follow-up of helicopter pilots using monocular helmet displays shows no anisometropia. Aerosp Med Hum Perform. 2026; 97(5):372-374.