Tear hyperosmolarity in renal dialysis patients asymptomatic for dry eye

Renal dialysis patients have hyperosmolar bodily fluids due to high urea levels, and accordingly their tears should be hyperosmolar. Tear hyperosmolarity is a sensitive and specific test for dry eye. The purpose of this study was to determine whether renal dialysis patients have hyperosmolar tears and dry eye. Ten renal dialysis patients completed the McMonnies and Ho dry eye questionnaire and underwent tear osmolarity testing before dialysis. Serum osmolarity was calculated from serum sodium, potassium, and blood urea nitrogen levels. Half of the subjects also participated in tear osmolarity testing after dialysis and underwent an eye examination that included rose bengal and fluorescein staining, Schirmer's testing, and tear break-up time testing. All of the subjects had both hyperosmolar serum and tears, and tear osmolarities were positive for dry eye in all subjects. However, none of the subjects tested positively for dry eye with the questionnaire. The subjects had some physical findings consistent with dry eye, and Schirmer's testing showed decreased tear production. Renal dialysis patients are a population that have hyperosmolar tears yet remain asymptomatic for dry eye. Urea has been used for years as a dermatologic treatment for dry skin. The high levels of urea in the tears of dialysis patients may protect them from developing dry eye. Tear hyperosmolarity due specifically to ionized solutes such as sodium and potassium is probably more pathogenic in causing dry eye than is hyperosmolarity in general. Nevertheless, tear osmolarity testing should not be used as a dry eye monitor in renal failure patients.