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National Research Council (US) Working Group on Contact Lens Use Under Adverse Conditions. Contact Lens Use Under Adverse Conditions: Applications in Military Aviation. Washington (DC): National Academies Press (US); 1990.

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Contact Lens Use Under Adverse Conditions: Applications in Military Aviation.

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2Adverse Effects of Contact Lenses


Adverse effects due to contact lens wear can be acute or chronic in nature and can span the range from a mere annoyance to a disabling condition that results in permanent ocular damage or loss of the eye. In general, contact lens complications are the result of one or more of the following factors:

  • mechanical factors causing irritation or abrasion of the eye or lid due to: lens materials, inappropriate designs, or improper fitting; lens interactions with foreign bodies such as dust or other particulates; and physical forces such as rapid decompression or high G-forces from acceleration;
  • physiological factors, such as the eye's response to reduced ambient oxygen levels at altitude; infection; or chemical exposure, including the preservatives in many lens care solutions;
  • immunological factors, such as allergies, that can result in general lens intolerance;
  • tear film alterations due to the combined action of the lens and environmental factors such as low humidity or high air flow; altering the tear film can disrupt its normal functions of removing waste products and clearing foreign matter from the eye, lubricating it, and preventing its desiccation.

Among the many factors covered by these broad categories, it is useful to note three of special relevance to military aviation: reduced oxygen levels (hypoxia), low humidity, and the mode of lens wear—whether daily or extended wear.


The cornea does not have blood vessels (except near its edges) and must obtain the oxygen it requires for metabolism from the surrounding air. By modifying access to this ambient air, most contact lenses reduce the oxygen available to the cornea. A pilot flying at altitude also experiences a reduced level of oxygen in the ambient air relative to sea level, and this reduction, compounded by the action of contact lenses, results in a hypoxic condition whose effects vary with the degree of oxygen deprivation and the length of exposure time.

Normally, the cornea consumes from 3 to 10 µl O2/cm2/hr (Larke et al., 1981; Lowther, 1990). When sufficient oxygen is not available, the cornea's metabolism is stressed and a buildup of lactic acid occurs as the cornea begins to respire anaerobically. This buildup of lactic acid creates an osmotic load, drawing water into the cornea faster than it can be removed and resulting in corneal swelling or edema (Mertz, 1990) (see discussion below). In addition, hypoxia depletes the cornea's energy reserves in the form of glycogen—another indication that the cornea is under stress.

The cornea can apparently tolerate moderate levels of hypoxia-induced edema without adverse affects. Low levels of corneal swelling—under 5 percent—are experienced regularly during sleep when the eye is closed. The closed-eye environment offers perhaps only a third of the oxygen available to the open eye (Mertz, 1990) and has been used as a guide for lens designers in determining tolerable levels of corneal edema. In the absence of contact lenses, sleep-induced edema normally requires only a few hours to disappear once the eyes are opened.

The amount of oxygen available under a lens varies with lens material and thickness. With rigid lenses, some oxygen is supplied in the tears pumped under the lens with each blink. In RGP lenses, additional oxygen diffuses through the lens itself, the amount depending on the exact chemistry and thickness of the lens.

Hydrogel lenses do not allow substantial tear pumping under the lens. Rather, oxygen reaches the cornea by diffusing through the water in the lens itself. Thus, oxygen transmissibility is directly related to water content, with higher water content translating to higher levels of oxygen available to the eye. This has implications for cockpit use of hydrogels, since the low humidity found there will contribute to water loss in the lens, a phenomenon known to decrease its ability to transmit oxygen (O'Neal, 1990).

Low Humidity

Humidity is of critical importance in contact lens performance since it directly contributes to a drying of the ocular environment, which can affect the quality of vision, the amount of oxygen reaching the cornea, and the wearer's ability to tolerate the lens. The movement of dry air past the eye, as provided by forced air heating or air conditioning systems, can greatly exacerbate these low-humidity effects.

Low humidity (15-20 percent or less relative humidity), such as routinely encountered in aircraft, can cause the tear film to dry on the lens, depositing solid lipid and protein residues that prevent proper wetting of the lens (Refojo, 1990). A nonwetting lens lacks the normal lubrication provided by the tear film and can create mechanical irritation of the lid, cornea, and surrounding tissues, leading to pain or discomfort.

In addition, markedly dry conditions can lead to rapid dehydration of a hydrogel lens from evaporation through the lens, since most of the water in a hydrogel lens is not bound in the polymer matrix (Refojo, 1990). As a hydrogel lens dries, its optical properties are affected as its refractive index changes and its radii of curvature steepen. The shape of the lens will also change upon dehydration as its shrinks and thins, with consequent effects on fit. Very rapid drying may induce the edges of the lens to curl, which may, in turn, lead to expulsion of the lens from the eye (Refojo, 1990).

Humidity-induced drying of hydrogel lenses can be affected by blink rate, tear composition, lens thickness, lens water content, and, to a limited extent, lens material (Henriquez and Korb, 1981). In general, higher water content lenses dehydrate more rapidly than lower water content lenses, and thinner lenses dehydrate more quickly than thicker ones. Thus, the use of thicker lenses with a lower water content may help ameliorate the effects of low humidity on soft contact lenses.

Lens Wear Time

It should be noted that the length of time contact lenses are worn, as measured by the time elapsed between insertion and removal, is a significant factor in determining the risk of complications due to contact lens wear. In general, longer wear times translate to a higher risk of complications. Thus, extended-wear lenses present a greater risk to their wearers than standard (daily) wear lenses, all other factors being equal. The Food and Drug Administration has estimated that those wearing extended-wear hydrogel lenses are 10 times more likely to develop complications than those with daily-wear hydrogel lenses (Green, 1990). More recently, a controlled case study has shown that the relative risk of ulcerative keratitis is 10 to 15 times as great for users of extended-wear lenses as for users of daily wear lenses (Schein et al., 1989). Cumulative wear time—the number of months or years of overall lens wear—is also recognized as an important factor in eliciting complications, but this effect has not been reliably quantified.


Complications resulting from the factors listed above are many and varied. Indeed, nearly all the frontal (anterior) structures of the eye can be affected by wearing contact lenses (Efron and Holden, 1986a). The most likely complications to pertain in the context of military aviation are described below.


Microbial infection of the cornea is a recognized danger of contact lens wear—especially the wearing of hydrogel lenses. Incidence of infections is lowest among PMMA and RGP lens wearers and highest among wearers of extended-wear hydrogel lenses. Symptoms cover a wide range, including burning, itching, redness, irritation, the sensation of a foreign object beneath the lens, and acute pain. Infections can lead to corneal ulcers with both acute and chronic repercussions on visual performance. Serious cases may require surgical intervention or may result in loss of the eye.

Contact lens wear gives rise to two factors that work in concert to promote infection: (a) compromise of the corneal surface (epithelium) either through mechanical abrasion or oxygen deprivation and (b) contamination of the cornea with bacteria (Schein, 1990).

Bacterial contamination appears to be the nearly inevitable consequence of hydrogel wear (Schein, 1990). When examined, a high percentage of hydrogel lenses from patients without symptoms of infection show such contamination (Schein, 1990) and bacteria can be found in a significant percentage of commercial lens solutions (Donzis et al., 1987). Even compliance with strict lens cleaning procedures cannot guarantee an absence of infections, especially among extended-wear soft contact lens users (Mondino, 1986).

Bacteria adhere readily to the surface deposits on soft contact lenses and can multiply rapidly there, creating a bacterial biofilm (Schein, 1990) able to supply infectious agents to an abraded or stressed cornea. The contact lens environment also seems to favor some of the more harmful bacteria, such as pseudomonas, a particularly destructive organism in the cornea (Green, 1990; Cohen et al., 1987).

Recovery from infection can be quick or quite prolonged, depending on the rapidity of treatment, and usually requires discontinuance of contact lens use for the duration of the recovery period.

Corneal Edema

As discussed earlier, corneal edema consists of the swelling of the midsection of the cornea (stroma) as it is stressed, either from an inadequate oxygen supply or from other causes. Because it progresses in recognizable stages and can be readily measured, stromal edema is the most frequently used indicator of the physiological integrity of the cornea. Even without contact lenses, edema of less than 5 percent occurs regularly during sleep without apparent symptoms and eventually reverses itself upon awakening. An edema level of 5 percent is considered acceptable during daily wear of contact lenses, and overnight edema levels as high as 8-10 percent are considered acceptable in extended wear if the edema subsides within 2-3 hours upon awakening (Efron and Holden, 1986a).

Beyond 5 percent edema, vertical greyish-white lines called striae begin to appear in the posterior stroma. At 10-12 percent swelling, folds also appear and contact lens wear is usually modified to increase oxygen levels. Edema beyond 15 percent is considered pathological; at about 20 percent, the cornea begins to cloud and visual acuity drops. Within a few days other complications such as infiltrates and epithelial microcysts occur. The induction of blood vessel growth within the cornea may also occur if this level of swelling persists.

Upon removal of the cause, acute corneal edema usually resolves itself within a matter of hours, but chronic edema such as experienced by some extended wear patients after months of wear may take up to 7 days to disappear (Efron and Holden, 1986a).

Superficial Keratitis

Irritation of the outermost layer of the cornea—superficial keratitis—can result from several causes, including mechanical irritation, infection, allergies, lens care solutions, or a combination of these. Symptoms vary, but often involve scratchiness, pain, foreign body sensation, and other complaints. Treatment also varies according to the cause, and may mean refitting, revised wear schedule, or treatment with antibiotics. Clearly, this complication may herald other more serious complications and is thus worthy of note.

Red Eye

“Red eye” describes a common but potentially serious symptom that may derive from a host of different causes or a synergy of these. Possible causes include infection, allergy, low humidity, hypoxia, dust or foreign particles, lens deposits that lead to irritation or immune response, warped lenses, or poorly fitting lenses (Michaels, 1985). Depending on the diagnosis, treatment can range from refitting with a more oxygen-permeable lens to antibiotic therapy to discontinuance of lens use due to chronic allergic response.

Excess Mucus Production

As with red eye, excess mucus production can accompany a number of other complications and is perhaps best viewed more as a symptom than a separate complication. Excess mucus is sometimes an early sign of corneal ulceration or infections of other types. It is also associated with alterations of the tear film as may occur with dry eye syndrome or allergies. A thick, ropy mucus coating on contact lenses usually indicates the presence of giant papillary conjunctivitis (see below).

Depending on its cause, treatment of excess mucus may require removal of lenses for a period, modification of lens design or wear schedule, or treatment with antibiotics. Incidence of the condition is higher among extended-wear than among daily-wear users.

Epithelial Microcysts

Microcysts are small (15 to 50 microns in diameter) cystlike areas located throughout the layers of the cornea. They are most often observed in patients using extended-wear lenses and are thought by many to indicate chronic metabolic distress in the cornea, perhaps due to prolonged hypoxia; mechanical irritation may also be a contributing factor. Thus, while not dangerous in themselves, microcysts are worrisome for what they may portend.

If microcysts are present in large numbers, lens use is usually discontinued until they clear up and resumed only with an alternate wear strategy (reduced wear time; refitting with a lens with higher oxygen transmissibility, etc.).


Infiltrates appear as hazy, grey areas in the midregion of the cornea (the stroma) and are most likely aggregations of inflammatory cells. While infiltrates themselves may be asymptomatic, they are often found associated with other complications, such as “red eye” reaction, whose symptoms include scratchiness, pain, photophobia, and lacrimation.

The exact cause of infiltrates is unknown, but prolonged hypoxia, immune responses, physical irritation, and local infection have all been suggested as factors in its development (Efron and Holden, 1986a). The presence of infiltrates usually indicates that a serious tissue reaction in the eye is imminent; medical care must be sought and lens use must be discontinued immediately until the infiltrates have disappeared, which can take up to two months. To prevent recurrence, patients may have to reduce wear time, change lens type or fit, or alter their lens care regime.

Endothelial Polymegethism

Endothelial polymegethism refers to an abnormal variation in the size of the cells making up the endothelium. The corneal endothelium is a single layer of cells at the base of the cornea that is thought to be primarily responsible for maintaining the cornea's proper fluid balance. The cornea is dehydrated compared with its surroundings—about 78 percent water versus nearly 100 percent for the tears and aqueous tissues (Mertz, 1990). Lensinduced changes of the endothelium thus may disrupt this fluid regulation, resulting in corneal swelling (edema) or an inability to deswell after hypoxiainduced edema.

Polymegethism appears to be the indirect result of corneal hypoxia and thus relates to the oxygen transmissibility of the contact lens, the lens design, the hours of wear, and the wear mode (whether extended or daily wear) (Schoessler, 1990). Generally, polymegethism begins shortly after contact lens wear commences and progresses slowly as wear continues. Recovery time after lens removal is as yet unknown, if indeed it occurs at all (Efron and Holden, 1986b).

Corneal Molding

Molding refers to changes in the shape of the cornea due to contact lens wear. It is most likely with intrinsically misshapen (toric) corneas subject to chronic oxygen deprivation (Michaels, 1985). It may also occur due to the action of bubbles formed under contact lenses at reduced pressures, as experienced during flight at high altitudes. Molding may result in decreased visual acuity when spectacles are used immediately after contact lenses are worn. Treatment may involve adopting a different lens design or reducing wear time.

Giant Papillary Conjunctivitis

Giant papillary conjunctivitis (GPC) is a common complication occurring largely among soft contact wearers, especially those following an extended-wear regimen (Efron and Holden, 1986b). Its earliest symptoms are easily ignored and often unreported: increased mucus upon arising and itching after lenses are removed.

As the condition progresses, the inner surface of the upper eyelid (palpebral conjunctiva) thickens, becoming congested with blood and developing abnormally large papillae; symptoms increase to obvious mucus production, mild blurring of vision, and considerable movement of the lens on blinking. In its advanced stages, lenses wear causes an intolerable foreign body sensation and mucus production becomes copious (Allansmith, 1990).

Chronic hypoxia beneath the upper lid, mechanical irritation of the lid from lens movement, reaction to lens solution preservatives, and an immunological reaction to deposits on the surface of the lens seem to be the four key factors in development of GPC (Efron and Holden, 1986b).

Lens use is usually interrupted if symptoms are severe, but minor cases may be treated while maintaining lens wear. Change in lens design or type, a rigorous lens cleaning regimen, and cessation of the use of solutions containing preservatives are all important aspects of treating GPC. Papillae can remain for weeks, months, or years. Many patients with mild symptoms can persist in wearing lenses indefinitely, but they do require frequent follow-up (Efron and Holden, 1986b).

Corneal Vascularization

The cornea is normally avascular—without blood vessels—except near its edge. Invasion of blood vessels more than 2 mm into the clear corneal region is considered abnormal and is most often observed in soft contact lens wearers, especially those with extended-wear lenses (Schein, 1990). Vascularization usually causes no symptoms until the new blood vessels encroach far enough to cover the pupil, thereby reducing visual acuity.

A combination of causes is thought to bring about vascularization. Chronic lens-induced trauma of the corneal epithelium may promote migration of inflammatory cells to the site of injury. These cells may then release vaso-stimulating agents that result in blood vessel growth into the corneal stroma, which has already been softened by chronic hypoxia-induced edema (Efron and Holden, 1986a).

If vascularization is mild, changing to a lens with higher oxygen transmissibility may be enough to solve the problem. However, if vessels have encroached the pupil, lens use must be permanently ended.

Lens Intolerance

The contact lens is, in essence, a foreign body placed in contact with the sensitive tissues of the eye on a regular basis. It is not surprising, then, that the body should marshal its normal defenses to foreign bodies (Allansmith, 1990) in reaction to the intrusion of the lens. In some patients, this will result in an inability to tolerate the lens due to pain or the symptoms accompanying allergic reactions, such as itching, redness, irritation, or mucus production.

Although light-complected people with sensitive skin or those with a history of allergies may be more prone to such lens intolerance, there is no general rule to define those who will not tolerate lenses. Chronic mechanical irritation, lens deposits, and preservatives used in lens care can all trigger immune responses resulting in symptoms severe enough to warrant discontinuance of lens wear.


Oil excreted by the many meibomium glands found within the upper and lower eyelids comprises an important constituent of the tear film remaining on the cornea after each blink. This oil prevents the rapid evaporation of the tear film and thus helps the eye avoid desiccation. Sufficient lubrication of the eye with oil-containing tears is important to its ability to tolerate contact lenses.

Disruption of meibomium gland function (meibomitis) can occur when keratinized oil plugs the glands, leading to a reduction in lubrication and accompanying “dry eye” symptoms (Korb and Henriquez, 1980). These symptoms can result in lens discomfort and may ultimately cause the patient to discontinue lens use.

Preservatives present in lens care solutions may be a primary factor in the development of meibomial dysfunction. Meibomial plugging also occurs more frequently among those with seborrhea or other skin problems or those with light complexions. In addition, the tendency toward meibomial plugging increases with age.

Treatment of meibomian dysfunction and meibomitis involves the rehabilitation of the meibomian glands through a daily home treatment program and repeated office visits for physical expression of the blockage. The precise treatment is dependent upon the severity, may take months, and may require disruption of lens wear (Korb and Henriquez, 1980).

Dryness-Related Effects

“Dry-eye” problems—typically associated with low humidity—are some of the most prevalent and difficult to treat among contact lens wearers, and a wide spectrum of complications result from this condition. Among RGP users, dryness gives rise to a sensation of scratchiness or a stinging pain as irritation of the lid or the cornea and its surrounding tissues occurs (superficial limbal keratitis). These symptoms are usually relieved only by increasing the humidity level or switching to hydrogel lenses.

Among users of hydrogel lenses, symptoms of dryness are frequently more subtle but revolve around the same scratchy sensation indicative of lid irritation as the lens dries. In some soft lens wearers, low humidity environments also produce a significant epithelial disruption (Josephson, 1990). Unlike with hard lenses, blinking usually relieves dryness-related symptoms momentarily. Longer-term management of dryness involves adjustment of lens thickness and water content, as described previously. In general, however, strategies to cope with the symptoms of dryness, such as increasing lens thickness, lead to decreased oxygen levels available to the cornea.

Copyright © National Academy of Sciences.
Bookshelf ID: NBK234044


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