Vol. 101, No. 4 Correspondence 501

Reference

1. Hogan, M. J., and Zimmerman, L. E.: Ophthal­mic Pathology, 2nd ed. Philadelphia, W. B.Saunders. 1962, p. 408.

_______ Reply _

EDITOR:We are reluctant to generalize about the co­

incident occurrence of two common abnormal­ities when we have only observed a smallnumber of patients.

In one of our patients diabetes was con­trolled by diet alone, but the other five pa­tients demonstrated no clinical evidence of di­abetes mellitus. Hence, the syndrome is notconfined to eyes with diabetic iris abnormali­ties. Despite our awareness of abnormalitiesin the diabetic iris, we do not know of suffi­cient data to demonstrate whether or notcomplications of posterior iris chafing fromintraocular lens implants is more frequentamong patients with diabetes. We too haveobserved a remarkably high incidence of dia­betes in subsets of patients (in our case, ofposterior chamber lens-pupillary block), butwe are reluctant to generalize about a causa­tive relationship from examination of a smallnumber of patients. We are grateful to Dr.Cykiert for his interest in our patients andwill watch closely for the development of pig­ment dispersion in our diabetic patients withintraocular lens.

]OHNR. SAMPLES, M.D.E. MICHAEL VAN BUSKIRK, M.D.

Portland, Oregon

The Effects of Hypotonic andHypertonic Solutions on the Fluid

Content of Hydrophilic Contact Lenses

EDITOR:The recent interest in the in vivo dehydra­

tion of hydrogel contact lenses has led to thedevelopment of the term "percent dehydra­tion." Unfortunately, this term does notclearly specify under what equation dehydra­tion has been calculated, and so data from

studies of dehydration are easily misinterpret­ed. Firstly, consider the example of a 75%water content lens that weighs 50 mg. It iscomposed of 12.5 mg of polymer and 37.5 mgof water. A typical amount of dehydrationduring wear would be a loss of around 10 mgof water. There is, therefore, 27.5 mg of waterin the lens after dehydration, and a newwater content of 68.8% may be calculated.The percentage decrease in the total lensmass is 20%. The percentage decrease in thewater mass is 26.7%. The decrease in watercontent is 6.3%, which is a relative decreasein water content of 8.3%. Thus, the value ofthe "percentage dehydration" could be 20%,26.7%,6.3%, or 8.3% depending on the def­inition.

Secondly, it is of interest to note that alow-water-content lens may have a greaterrelative decrease in water content than ahigh-water-content lens, and yet have a con­siderably less percentage decrease in totallens mass. This occurs because proportionate­ly large changes in total lens mass accompanysmall changes in the water content for high­water-content lenses.

Thirdly, when considering the effects of de­hydration, it is important to ascribe a particu­lar clinical effect to the correct definition. Asan example, the change in water contentshould be used for calculations of the effect ofdehydration on oxygen transmissibility, butthe percentage decrease in total lens massmay be more appropriate for consideringchanges to the shape of a lens with dehydra­tion.

In their article, "The effects of hypotonicand hypertonic solutions on the fluid contentof hydrophilic contact lenses" (Am. J. Oph­thalmol. 99:521, May 1985), J. P. Aiello andM. S. Insler described the results by the term"percent of water loss." Aside from the con­fusion that this term creates, the failure tostate clearly the definition under which theresults were calculated renders comparison topreviously reported data of little value. Exam­ination of their methods leads to the beliefthat they calculated percentage loss of lensmass. However, they compared their percent­age figures directly with the results of Andra­sko;' who calculated the percentage decreasein water content, not lens mass. They inap­propriately reported Andrasko as findingequilibrium values at 80% to 93% of the fullysaturated water weight-this should read