DeeDee Doke/LONDON
The US military is on the verge of welcoming a new generation of aviators to its ranks with, it hopes, the sharpness of eye to match the precision of their weapons. Aspiring aviators could eventually benefit from a change in Department of Defense (DoD) policy to allow frontline combat forces to receive ocular surgery to improve their eyesight.
Ocular surgeries known as radial keratotomy (RK), photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK) have been off-limits to aviators because of concerns that the short- and long-term effects could hamper their wartime effectiveness (see box). The US Federal Aviation Administration allows civil pilots to undergo eyesight-improving ocular surgery - if they meet certain requirements - but the US military has not been alone in opposing it: the UK's Royal Air Force, for instance, also forbids its aviators to have such surgery.
The DoD, however, is buying least five new lasers to perform PRK surgery in US air force, army and navy medical centres in an initiative that will, according to one DoD official, "optimise our ability to recruit and retain the best people". Privately, some acknowledge that in a time of pilot shortages, such surgery could broaden the field of otherwise qualified candidates whose less than perfect eyesight prevents them from "making that one single gate".
The US Navy has already performed 3,000 PRKs on frontline combat forces, including non-pilot aviators, and will launch astudy next month to determine whether USN pilots whose eyesight has slipped below flight standards can be returned to flying duty after receiving PRK. A study of PRK's effects on the performance of new pilots entering training is planned.
The purpose of a 100-volunteer USAF study at Wilford Hall Medical Center at Lackland AFB, Texas, is to gauge the effect of PRK in airborne combat situations - although participation was restricted to non-aviators - such as "high g turns, high altitudes, night operations and low contrast situations - all those things a military aviator would perform", says Col Steve Waller, the centre's head of opthalmology.
In the USAF study, 20 participants are controls and do not receive PRK. Twenty others will simply receive PRK, 20 will go into altitude testing, 20 will undergo g-force training and 20 will be involved in cockpit visual simulation studies. "This is an attempt to look at visual performance in a cockpit environment following PRK with some subjects," says Col Douglas Ivan, who heads the ophthalmology department in the USAF's School of Aerospace Medicine at Brooks AFB, Texas. Ivan's department is teamed on the project with Wilford Hall and the Air Force Research Laboratory at Brooks. "What we're looking at is out-of-cockpit, canopy visual tasks with and without glare."
The USAF has had no problem in finding volunteers, but keeping them in the study can be a challenge as they encounter the intense physical demands of aircrew training.
Gravity or g-force training, for example, places non-aviators in a centrifuge "to get their g-training ginned up", says Ivan. "They have to learn how to sit, how to do g manoeuvres, how to take g levels up to nine. We get people hanging in there fairly well until about four. You might be motivated at four, but five starts hurting. Nine is a real bear. You have a lot of people, as we build up their tolerance, who drop out. You have to be g-tolerant or you never get to 9g. That's a logistical nightmare."
Areas of special interest to military researchers looking into optical surgeries are:
• Disability glare and contrast sensitivity, with and without night vision goggles (NVGs). "What PRK does is create a layer of new tissue that develops following surgery and creates a corneal haze," says Ivan. "It scatters light, the optical clarity of the cornea is no longer normal compared to what it was before. Haze can be very significant in some individuals. Between 4% and 8% of individuals will have significant residual haze that may not even be noticeable in the civilian world, but may have significant impact on night performance."
Contrast sensitivity is reduced temporarily for most PRK patients, adds Ivan. "When that comes back to acceptable limits for the USAF has yet to be defined."
USN Cdr Steve Schallhorn, director of cornea and refractive surgery at the Naval Medical Center, San Diego, says, however, that results of a USN study on NVG using non-pilot aircrew who had had PRK were favourable. That study found that the subjects' contrast sensitivity was "slightly reduced" for two weeks after the surgery, but was restored one month after surgery "and stayed there".
• Altitude. AUSN study placed participants who had had PRKin an altitude chamber, simulating 17,000ft for 4h and found no problems. Of the USAF, Ivan says: "We have missions that, operationally, require a very high, unpressurised altitude spectrum and supplemental oxygen. We need to know what happens in those circumstances to these corneas."
• g forces. "What's a cornea going to do, now that it's not as thick as it used to be, when you start applying g levels that are more than 1g, and current research is going as high as 12g environments?" says Ivan. "What does the g do to a cornea that's not the same cornea?"
The military' emphasises the possibilities of PRK only. RK, with its multiple corneal incisions, is considered outmoded. LASIK, while less painful to the recipient and quicker to heal than PRK, is problematic because of unknown factors surrounding the flap that is cut into the cornea. A USN study of LASIK determined that it produces "essentially identical" eyesight improvements as PRK, but that LASIK can lead to more complications, Schallhorn says. The USN and USAF have the same key concern: the flap. "We're worried that flap will not stay down," says Waller. "It could be lifted or ripped off and the person might need a corneal transplant. Relatively minor traumas, like being hit in the eye in racquetball become much more worrying." Suturing it would "destroy the vision you're trying to improve", says Schallhorn.
Schallhorn is optimistic about PRK's potential, and he notes that a non-pilot aircrew member who received PRK suffered no ill effects when he was forced later to eject from an aircraft. But Schallhorn warns: "It may take years before we know enough about PRK to say, 'It's OK for pilots'." And a DoD official says: "The door is open for all kinds of possibilities."
Source: Flight International