pigmentary glaucoma associated with posterior chamber intraocular lenses: reply
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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