Photoutlus

Dehydration of Hydrogel Contact Lenses

Joe B. Goldberg, OD, FAA0

Hydrogel contact lenses dehydrate when they are worn. The degree of dehydration is a variable. It is affected by the swelling pressure and water conductivity of the hydrogel, the lens thickness, and the characteristics of the blink.’

Why does a hydrogel contact lens dehydrate on the eye when it has bound water and is always bathed in tears?

One hypothesis states that the cornea1 metabolic de- mands are not satisfied when there is an acidotic shift in the tears. A lactic acidosis shift induces a pH change in the lens environment and causes the lens to tighten.’ This results in the trapping of mucus and debris under the lens and dem- onstrates its inability to exchange tears.

Fatt’ reported that some mechanisms cause a cyclic de- hydration and rehydration at the anterior surface of the lens (see Table 1). There is an osmotic withdrawal of water from the lens by a temporarily hyperosmotic tear film during the open-eye part of the blink cycle. A cyclic application of iso-osmotic and hyperosmotic tear fluid may lead to cyclic dehydration and rehydration of the lens surface. Further-

more, temperature cycling at the anterior lens surface is another possible mechanism.

An accepted hypothesis is that water loss is all or mostly due to evaporation. ’ When a hydrogel lens is placed on the eye, evaporation from the front surface begins when the tear film is broken. There will be no dehydration when the amount of water lost during the period when the tear film is broken is recovered in an equal amount on the next blink and the lens is immersed in tears.

It was reported that high water content lenses lose more water when worn than do low water contact lenses.3 They probably lose more water than they can recover from one blink cycle to another.

Another hypothesis is that a hydrogel lens will imbibe water at a lower temperature of the open eye. However, it will lose it as the lens is warmed by the descending lid.’

Table 1. Possible Mechanisms That Cause Dehydration’

1. Tear film breakage followed by evaporation 2. Greater dehydration in an environment of low relative

humidity 3. Evaporation from the tear film followed by water

withdrawal from the lens by an osmotic process

Address reprint requests to Dr. Joe B. Goldberg at 4217 ginia Beach Boulevard, Virginia Beach, VA 23452.

Accepted for publication May 1990.

0 1990 Butterworth-Heinemann

Figure 1. Scleral impression of a dehydrated soft lens.

ICLC, Vol. 17, July/August 1990 197

Figure 2. Cornea1 abrasion. Figure 4. Debris under a dehydrated soft lens.

Figure 3. Scattered cornea1 stippling.

When dehydration of a hydrogel lens is from moderate to severe, it will create several changes observed with biomi- croscopy (see Figures 1-S). Cornea1 punctate staining oc- curs when the cornea is in contact with a hydrogel lens whose hydration varies with time.4 There is dessication of the anterior epithelial cells during the low-hydration part of the hydration cycle. The thin lipid-mucin layer between the lens and the cornea breaks in spots and allows the lens to dehydrate the epithelial cells in the vicinity of the break.

It is doubtful that we can control dehydration of hydrogel lenses worn for either daily or extended wear. However, I have found that disposable lenses have reduced the inci- dence and degree of dehydration and cornea1 insult.

Figure 5. Pockets of trapped lacrimal fluid.

References

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Fatt I: A predictive model for dehydration of a hydrogel con- tact lens in the eye. J Br Contact Lens Assoc 1989;12(2):15- 31. Lembach RG, Keates RH: Permalens extended wear contact lenses, in Hartstein, J (Ed): Extended Wem Contact Lenses for $h$iu and Myopia. C.V. Mosby, St. Louis, MO 1982, pp

Efron N, Brennan NA, et al.: Dehydration of hydrogel lenses under normal wearing conditions. CLAO J 1987;13:152- 156. Orsbom GN, Zantos SG: Cornea1 dessication staining with thin high water content contact lenses. CIAO ] 1988; 14:81-85.

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