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Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
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OCR for page 10
2
Adverse Effects of Contact Lenses
FACTORS ELICITING ADVERSE El ~ ECTS
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:
· mechanicalfactors 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 Articulates;
and physical forces such as rapid decompression or high G-forces
from acceleration;
physiologicalfactors, 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 plm alterations due to the combined action of the lens and
environmental factors such as low humidibr 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.
10
OCR for page 11
ADVERSE EFFECTS OF CONTACT LENSES
Hypoxia
11
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 pi 02/cm2/hr Parke 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 hypo~a-induced
edema without adverse affects. Low levels of corneal swelling under 5
percent—are experienced regularly dunog 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 oxy-
gen 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
OCR for page 12
12
CONTACT I F;NS USE UNDER ADVERSE CONDITIONS
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 rou-
tinely 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.
OCR for page 13
ADVERSE EFFECTS OF CONTACT I FNsES
AI)VERSE EFIECTS
13
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.
Infection
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 be-
neath 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 pro-
mote 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 conse-
quence of hydrogel wear (Schein, 1990~. When examined, a high per-
centage 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
OCR for page 14
14
CONTACT CONS USE UNI)ER ADVERSE CONDITIONS
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 ~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 grayish-white lines called striae begin
to appear in the posterior stroma. At 1~12 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 keratiti&
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 diagno-
sis, treatment can range from refitting with a more oxygen-permeable lens
to antibiotic therapy to discontinuance of lens use due to chronic allergic
response.
OCR for page 15
AD~:RSE EFFECTS OF CONTACT LENSES
Excess Mucus Production
15
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 below3.
Depending on its cause, treatment of excess mucus may require re-
moval 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 Micro~sts
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 discon-
tinued 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
Infiltrates appear as hay, 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 eve" 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. 1b prevent recurrence, patients may have to reduce
wear time, change lens type or fit, or alter their lens care regime.
-a ---r
OCR for page 16
16
CONTACT LENS USE UNDER ADVERSE CONDITIONS
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~. Lens-
induced changes of the endothelium thus may disrupt this fluid regulation,
resulting in corneal swelling (edema) or an inability to deswell after hypona-
induced 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. Iteatment may involve adopting a different lens design or reducing
wear time.
Giant Papillary Conjunctivitis
Giant papillary conjunctivitis (GPC) is a common complication oc-
currine largely among soft contact wearers especially those following an
O ---=--~ --------= -a ~ --r-~ -J ~~~~~~ ~~ - ~--~~-o
extended-wear regimen (citron and Golden, 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 conjunctive) thickens, becoming congested with blood and de-
veloping 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 for-
eign body sensation and mucus production becomes copious (Allansmith,
1990~.
. .
OCR for page 17
ADVERSE EFFECTS OF CONTACT LENSES
17
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 mi-
gration 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 trans-
missibility 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
OCR for page 18
18
CONTACT LENS USE UNDER ADVERSE CONDITIONS
can all trigger immune responses resulting in symptoms severe enough to
warrant discontinuance of lens wear.
Meibomitis
Oil exacted 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, 19803. 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 reha-
bilitation of the meibomian glands through a daily home treatment program
and repeated office visits for physical expression of the blockage. The pre-
cise 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 d~yness-related
symptoms momentarily. Longer-term management of dryness involves ad-
justment of lens thickness and water content, as described previously. In
OCR for page 19
ADVERSE EFFECTS OF CONTACT LENSES
19
general, however, strategies to cope with the symptoms of dryness, such as
increasing lens thickness, lead to decreased oxygen levels available to the
cornea.
Representative terms from entire chapter:
contact lenses
