Contact Lens Use Under Adverse Conditions Applications in Military Aviation (1990) / Chapter Skim
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2. Adverse Effects of Contact Lenses
2. Adverse Effects of Contact Lenses
Pages 10-19
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From page 10... ...
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.
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From page 11... ...
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.
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From page 12... ...
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.
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From page 13... ...
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)
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From page 14... ...
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)
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From page 15... ...
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.
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From page 16... ...
As the condition progresses, the inner surface of the upper eyelid (palpebral conjunctive) 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.
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From page 17... ...
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.
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From page 18... ...
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~.
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From 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.
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