A 12-month Prospective Study of Tear Osmolarity in Contact Lens Wearers Refitted With Daily Disposable Soft Contact Lenses

Tear osmolarity refers to the amount of osmotically active particles in tears and is defined as the number of osmoles per litre of solution. It is mostly determined by the electrolytes of the aqueous component of tears and less significantly by proteins and sugars. It has been described as a single clinical measure that gives insight into the balance between tear production, evaporation, drainage and absorption of tears. Studies support the use of tear osmolarity as the best single diagnostic test for dry eye disease (DED) and the objective numerical measure for diagnosing, grading severity, and managing DED. Clinical application was facilitated after introducing a chip-based osmometer (TearLab® Osmolarity System, TearLab Corp, San Diego, California, USA)7, which allows collection of a relatively small sample of 50 nL from the inferior tear meniscus and automatic determination of tear osmolarity, based on the sample's conductivity.

The osmolarity of 308 mOsm/L is considered to be the most sensitive threshold to distinguish normal from mild/moderate forms of DED and 315 mOsm/L the most specific cut off. Also the inter-eye difference in osmolarity increases with DED severity and beyond the threshold of 8 mOsm/L it is considered an indication of the loss of tear film homeostasis. In normal subjects this variation between eyes is around 6.9 ± 5.9 mOsm/L. Tear film with a contact lens undergoes biophysical and biochemical changes, which have the potential to influence tear function and impact comfort and contact lens tolerance. Many factors have been associated with lowering of comfort and changes in tear dynamics during lens wear, however the literature is divided over whether or not contact lens wear increases, or has no effect on tear osmolarity.

The differences in reported values are likely a function of the wearer's ability to maintain tear film homeostasis, overcoming the drying effect of contact lenses.

Tear osmolarity was reported as the least variable of all the common signs across a clinically relevant timeframe, and was the only sign to reduce its variation upon application of effective therapy.

Tear hyperosmolarity is a clinical starting point in the pathogenesis of DED, responsible for ocular surface damage, both directly and by initiating a chain of inflammatory responses leading to ocular surface damage. Contact lens wear increases the risk of developing DED from between 2.01 and 2.96 times. Wearers are much more likely to report discomfort, mostly described as dryness and end of day dryness, than non-wearers and this factor remains the single largest cause of contact lens wear discontinuation. However, a multifactorial nature of DED, disguised as a lack of association between clinical signs and reported symptoms, leads to underlying cause of contact lens discomfort and remains poorly understood. Contact lens related DED may be explained by increased tear film thinning times resulting in increased tear film osmolality or by the loss of corneal sensation particularly associated with long-standing wear of hard and extended-wear contact lenses. Lowered corneal sensitivity leads to decreased tear secretion in response to reflex stimuli presumably causing an increase in tear osmolarity. Tear hyperosmolarity has been reported in daily and extended wear of both soft and hard contact lens wearers, especially in symptomatic subjects. Extended-wear soft contact lenses can decrease corneal sensitivity more than daily-wear soft contact lenses. A significant increase in tear osmolarity could not be confirmed in subjects wearing daily disposable soft contact lenses.