Abstract
Systemic dehydration due to inadequate water intake or excessive water loss, is common in the elderly and results in a high morbidity and significant mortality. Diagnosis is often overlooked and there is a need for a simple, bedside diagnostic test in at-risk populations. Body hydration is highly regulated with plasma osmolality (pOsm) being tightly controlled over a wide range of physiological conditions. By contrast, normal tear osmolarity (tOsm) is more variable since the tear film is exposed to evaporation from the open eye. While plasma hyperosmolality is a diagnostic feature of systemic dehydration, tear hyperosmolality, with other clinical features, is diagnostic of dry eye. Studies in young adults subjected to exercise and water-deprivation, have shown that tOsm may provide an index of pOsm, with the inference that it may provide a simple measure to diagnose systemic dehydration. However, since the prevalence of both dry eye and systemic dehydration increases with age, the finding of a raised tOsm in the elderly could imply the presence of either condition. This diagnostic difficulty can be overcome by measuring tear osmolality after a period of evaporative suppression (e.g., a 45 min period of lid closure) which drives tOsm osmolality down to a basal level, close to that of the pOsm. The arguments supporting the use of this basal tear osmolarity (BTO) in the diagnosis of systemic dehydration are reviewed here. Further studies are needed to confirm that the BTO can act as a surrogate for pOsm in both normally hydrated subjects and in patients with systemic dehydration and to determine the minimum period of lid closure required for a simple, “point-of-care” test.
Highlights
Water-loss dehydration, due to a net loss of hypotonic body fluids [1,2,3,4], is a common condition in the elderly, responsible for functional disability, poor health outcomes, and death [3,5] on a global scale
This was the case in a group of 10 uraemic patients studied by Charlton et al [155] where both tear osmolarity (tOsm) and plasma osmolality (pOsm) measured by DFP were raised, but despite tOsm levels of 347.2 ± 17.4 mOsm/L there were no relevant signs of Dry eye disease (DED)
It appears that if high humidity is to be used to drive down tOsm to the basal tear osmolarity (BTO), it will be necessary to expose the eyes to an relative humidity (RH) of 80–90% and the efficacy of this approach will need to be validated by parallel pOsm measurements
Summary
Water-loss dehydration, due to a net loss of hypotonic body fluids [1,2,3,4], is a common condition in the elderly, responsible for functional disability, poor health outcomes, and death [3,5] on a global scale. It has been shown that when tear evaporation is prevented by eye closure, the tOsm is driven down to a basal level that is close to that reported for the plasma [11,12,13]. This is assumed to be the lowest tOsm level that can be achieved in resting eye conditions and we have termed this value the Basal Tear Osmolarity (BTO). We have proposed it as a metric in the diagnosis of systemic dehydration. Fuller details of the personal research cited here are published elsewhere [11,12,13]
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