Refractive Surgery Safety at Altitude

Introduction

Since the introduction of radial keratotomy (RK) over 30 years ago, there has been a gradual evolution and refinement in corneal refractive surgery. Through technical advances in instrumentation and a better understanding of corneal biomechanics, these procedures have become less invasive and far more predictable in terms of visual outcome and comfort. Photorefractive keratectomy (PRK) is now approved for aviators in the United States Army, Navy, and Air Force (Stanley et al., 2008). Recently, an astronaut has flown in space following PRK with no visual disturbances related to the procedure (personal communication, NASA). Despite this consistent improvement in surgical technique, all such procedures are still based upon a precise, controlled surgical disruption of the corneal stroma. The desired endpoint of this disruption is largely a change in the anterior corneal curvature that corresponds to the desired new refractive error. However, this same structural alteration, designed to improve the visual acuity, may set the stage for unexpected visual disturbances with exposure to changes in environmental oxygen partial pressure. Many people who climb or travel to altitude are interested in refractive surgery, but are unsure of what might happen following one of these procedures. The purpose of this review is to provide guidelines to clinicians so that they may effectively advise patients who have had or are considering having these procedures. We will examine the basic mechanism of several common corneal refractive surgery procedures and explain how hypoxia may alter the postoperative dynamics, leading to visual changes with altitude exposure. Although such changes are rarely blinding, a patient may benefit from some knowledge of what to expect and how to be prepared should changes occur. We will also discuss surgical cataract removal with the insertion of an intraocular lens. This is another very common form of refractive surgery that does not involve surgical alteration of the cornea.

Normal Corneal Anatomy and Physiology

The normal cornea, untouched by surgical procedures, has a very predictable and consistent response to extended hypoxia. Simply stated, the cornea thickens in a uniform manner but remains clear and the vision does not change even at extreme altitude (Bosch et al., 2010; Morris et al., 2007). Corneal metabolism is largely dependent on ambient environmental oxygen as opposed to vascular enrichment. The normal cornea is extremely sensitive to changes in ambient oxygen levels. Even the decrease in ambient oxygen caused by lid closure during sleep causes a slight increase in corneal thickness upon awakening (Santos et al., 1998). Corneal thickening from hypoxia in the normal cornea is the result of swelling of the corneal stroma that makes up more than 90% of the corneal mass. The stroma is composed of a cross-hatched meshwork of collagen fibrils which are oriented in all directions in a horizontal plane. This stromal swelling is likely due to the effects of hypoxia on the corneal endothelium. The endothelium is a uniform sheet of cells on the undersurface of the cornea that actively pumps water from the corneal stroma into the anterior chamber. The endothelium's function is to produce a relatively dehydrated cornea. This dehydration is largely responsible for corneal clarity. Hypoxia is thought to decrease the activity of the O₂-dependent endothelial pump mechanism, which results in corneal thickening. Corneal collagen in the normal cornea is arranged in such a fashion that corneal expansion occurs with increasing hypoxia, but it happens uniformly such that the anterior corneal surface contour remains unchanged. Since the anterior corneal surface largely determines the refractive power of the eye, it follows that visual acuity is not noticeably changed in normal corneas, even in the face of prominent corneal thickening. Although poorly documented, some climbers without prior eye surgery have reported decreased visual acuity at high altitude, usually described as “foggy vision.” This is hypothesized to occur as a result of pre-existing endothelial anomalies such as Fuchs dystrophy, which may predispose these individuals to corneal edema under hypoxic conditions.

Radial Keratotomy

In the cornea following refractive surgery, the mechanical stress of altitude-related corneal swelling coupled with surgically induced structural changes to the corneal stroma set the stage for changes in anterior corneal shape and resultant visual changes. RK has been studied extensively at high altitude and perhaps best illustrates some of the basic concepts of corneal response to altitude following surgical manipulation. The RK procedure usually consists of 4 to 8 radial incisions in the mid-peripheral cornea. These extend very deep into the cornea with a desired depth of about 90%. The central 3 mm or so comprising the visual axis is spared. The radial incisions, made with a microblade, largely sever fibrils running generally perpendicular to the length of the incision and leave most radially oriented fibers intact. The basic idea of the procedure is to weaken the peripheral cornea such that it bows forward slightly in the circumferential periphery. This leads to a slight flattening of the corneal surface, a lessening of corneal refractive power, and a decrease in myopia. When the RK cornea is exposed to hypoxia, there is an overall thickening of the entire corneal stroma. However, because the mid peripheral cornea has been weakened by incisions, this area of the cornea swells preferentially. This further elevates the peripheral cornea, augments the effect of the RK, and causes a hyperopic (farsighted) refractive shift (Mader et al., 1996; McMann et al., 2002; Winkle et al., 1998). The amount of hyperopic shift corresponds to the degree of hypoxia but regardless of the extent of hyperopic shift, the cornea remains clear (Mader and White, 1996; Mader and White, 2010). It is important to keep in mind that these corneal changes are metabolic in nature and the direct mechanical effect of atmospheric pressure has no effect on the cornea (Ng et al., 1996). This means that extended exposure to hypoxia of at least 12 hours or so is needed before these changes become manifest. This explains why many climbers with RK may climb to a new base camp with no visual changes but wake up in the morning with obvious visual anomalies. This fact also accounts for the lack of visual complaints in those RK patients exposed to brief periods of hypoxia such as an airplane flight.

RK patients may experience a wide range of visual changes with increasing altitude exposure. Beck Weathers' tragic and well-publicized visual loss on Mt. Everest represents the extreme end of the spectrum of visual change following RK (Krakauer, 1997). Although visual changes in RK patients had been described previously (White and Mader, 1993; Mader and White, 1995; Mader and White, 1996), this incident brought the RK at altitude issue into public focus in a rather dramatic fashion. Most RK patients, on exposure to altitude, do not suffer such incapacitating changes. However, it is still important to anticipate what changes can occur. The most common scenario involves the RK patient who resides at sea level and travels to moderate altitudes for a ski or hiking trip. Some ski resorts such as those in Colorado or Utah are at elevations in excess of 9000 feet, and many popular hiking areas are well in excess of 10,000 feet. Nontechnical treks to the Andes or Himalayas may easily exceed 15,000 feet. Following an overnight stay at such altitudes, RK patients commonly awake with visual changes. As noted above, all such changes occur because of a flattening of the cornea with altitude exposure and a resultant hyperopic shift.

The extent of the change is largely dependent on three factors. One is the residual refractive error following surgery. If a patient's residual refractive error following RK is myopic, then a hyperopic shift may actually improve distant vision but lessen near vision. If a patient is left with residual hyperopia (farsightedness) following RK surgery, then he or she will become even more hyperopic with increasing altitude. This translates into a loss of near vision and far vision. The second factor is the age of the patient because it largely determines the accommodative reserve of the individual. Accommodation describes the ability of the eye to increase its refractive power. Accommodation occurs when the ciliary body contracts, the lens zonules relax, and the lens assumes a more spherical shape, thus increasing the refractive power. As one ages, the ability to accommodate decreases. Therefore a 20-year-old RK patient may have a large hyperopic shift with altitude exposure, but due to his abundant accommodative reserve he may not even notice any visual change. A 60-year-old, on the other hand, with the same amount of hyperopic shift, may become visually incapacitated. The third factor affecting degree of change with hypoxic exposure is the extent of RK surgery. Patients with more and longer RK cuts have a larger corneal area subject to change. Thus, a patient with a 16 cut RK for 8D of myopia would likely have a larger refractive change than a patient with a 4 cut RK performed for 2 D of myopia.

The main point for RK patients to remember is to anticipate visual change during altitude exposure. Due to the variables of altitude achieved, age, degree of surgery, and refractive error, it is difficult to predict the precise refractive shift an individual RK patient will encounter with altitude exposure (Mader and Tabin, 2003; Mader and White, 1995). Following a night's sleep at 17,000 feet, one 46-year-old RK climber awakened to a very annoying situation in which he could not read his watch, assemble his cookstove, or even quickly put on his crampons (Mader and White, 1995). Nothing was in its usual perfect focus at any distance. In contrast, another 47-year-old RK climber successfully reached the summit of Mt. Everest without corrective lenses (Mader et al., 2002). Although he noted a prominent loss of near vision, he only experienced mild blurring at distance. Previous studies of RK eyes have documented a spherical equivalent hyperopic shift of 1.00 and 2.00 diopters at altitudes of 12,000 and 17,000 feet, respectively (Mader and White, 1995).

Laser in Situ Keratomileusis (LASIK)

LASIK is currently the most commonly performed corneal refractive procedure worldwide. In this technique, a thin flap ranging from 100 to 180 micrometers thick is created in the cornea using a metal blade or femtosecond laser microtome. The flap is lifted in order to expose the underlying corneal tissue. An excimer laser is then used to selectively ablate portions of the anterior corneal stroma. The flap is then replaced. The flap now rests on reshaped tissue that permanently alters the anterior corneal curvature of the flap, thus changing the refractive power of the eye. The advantage of this procedure over RK and PRK is that it provides faster visual rehabilitation and less discomfort. Several studies have documented that LASIK corneas are relatively stable with exposure to hypoxia. Using tight fitting goggles, LASIK and normal control eyes were exposed to an anoxic environment for 2 hours (Nelson et al., 2001). A slight corneal steepening and myopic shift were documented in LASIK corneas. Similar small changes in vision have been documented at high altitude (Boes et al., 2001; White and Mader, 2000). In one study of 12 LASIK eyes, the vision of 6 climbers was carefully documented during an ascent of Mt. Everest (Dimmig and Tabin, 2003). Three of the climbers ascended Mt. Everest with no visual difficulties. Three other climbers did report blurry vision in excess of 26,000 feet, which improved with descent. Although these corneas could not be examined at the altitude at which the changes occurred, this information suggests that minor visual changes may be observed with extreme altitude exposure.

A slight myopic shift would not be unexpected given the structural changes that take place during the LASIK procedure. The central portion of the LASIK cornea is structurally weakened while the peripheral cornea is largely preserved. When the corneal stroma expands in response to hypoxia the central cornea preferentially elevates slightly, thus steepening the cornea. This central steepening increases the refractive power of the eye producing a myopic shift.

Photorefractive Keratectomy

With PRK, the corneal epithelium is removed and discarded prior to the procedure. Thereafter, an excimer laser is used to permanently reshape the anterior corneal stroma just under the corneal epithelium. During this procedure a computer system tracks the patient's eye position in order to ensure precise laser placement. Following the laser application, the corneal epithelium slowly grows back over the defect over a period of days. PRK does not involve a microtome or any corneal incisions.

PRK appears to be stable with exposure to hypoxia. No visual or corneal curvature changes were documented in six PRK patients (12 eyes) exposed to 14,000 feet for 72 hours (Mader et al., 1996). As noted above, there are no corneal incisions involved in the procedure and the laser ablates the cornea in a uniform fashion. When the PRK cornea is exposed to hypoxia, the cornea swells as expected. However, since there are no focal structural defects, the swelling is uniform and there is no change in the curvature of the anterior corneal surface. Thus, even with extreme hypoxia, no visual changes would be expected.

Laser Epithelial Keratomileusis (LASEK) and EPI-LASIK

LASEK is a newer laser refractive surgical technique, similar to PRK. In this procedure, a trephine is used to cut a superficial circular opening in the peripheral corneal epithelium. Following the application of a dilute alcohol solution, a very thin epithelial flap is gently lifted away from the corneal surface and folded away from the opening. At this point, an excimer laser is used to sculpt the underlying cornea. Afterward, this thin epithelial flap is replaced onto the corneal surface. EPI-LASIK is a similar procedure in which a plastic epithelial separator is used to detach a thin corneal epithelial layer, followed by excimer laser ablation. LASEK and EPI-LASIK may be performed on any cornea, but these procedures are ideally suited for corneas that may be too thin for LASIK.

Intraocular Lenses (IOL)

Lens removal with IOL insertion has been performed for decades and is one of the most successful and commonly performed surgical procedures in the world. In this procedure a tiny incision, less than 2 mm across, is made in the peripheral cornea. Through this incision, a circular opening is then created in the anterior lens capsule and a phacoemulsification device is used to remove the contents of the lens. The remaining lens capsule is preserved. Thereafter, through this same small corneal opening, a folded IOL of the proper power is inserted into the remaining capsular bag. The IOL then slowly unfolds within the capsular bag and remains permanently centered in this stable anatomic position. This procedure is most commonly performed on patients with visually significant cataracts. However, refractive lens exchange may be performed on patients with clear lenses who desire a more optimal distant vision. In this procedure, the lens is removed and replaced with an IOL of the proper power to provide excellent uncorrected distant visual acuity. Since the first report of their successful use by pilots (Mader et al., 1987) these lenses have been used by flight personnel for decades in all three military services and have even proven stable in an astronaut during space flight (Mader et al., 1999).

Of historical note, IOL's actually had their origins in a high altitude environment. During the Battle of Britain, the canopies of British Spitfires and Hurricanes were composed of polymethylmethacrylate (PMMA). During aerial combat, these canopies were shattered by cannon fire from German fighter planes and small fragments became embedded within the eyes of Royal Air Force pilots. These canopy fragments were noted to be inert and well tolerated within the eye. This observation eventually led to Dr. Harold Ridley's first use of plastic intraocular lenses in humans in Great Britain in 1949 (Ridley, 1952).

Summary of Key Concerns and Possible Solutions