What is tear osmolarity?

Tear hyperosmolarity (concentrated tears) occurs in meibomian gland dysfunction with an evaporative dry eye where there is an increased rate of tear evaporation.
Tear osmolarity is measured with an instrument made by TearLab and is done as an outpatient test we use here at Clinica London’s Tear Clinic, as part of the “work-up” of a patient with possible dry eyes. The tear osmolarity is measured in mOsm/L, so we get a measurable figure to record and monitor. We look at the overall figure and the difference between the two eyes.
Tear osmolarity can vary during the day from morning to evening, depending on the environment that the patient has been in and can vary depending on the health of the oily Meibomian glands and the degree of meibomian gland dysfunction (MGD). The tear film is dynamic, and fluctuations and changes in tear osmolarity can be helpful indicators of tear film instability.
When there are abnormal meibum (oil) quality and quantity with a decreased tear film lipid layer and tear hyperosmolarity, the eye gets irritated through simply mechanical irritation through increased friction, and this precipitates inflammation at the front of the eye which can lead to microscopic ocular surface damage.
Patients with meibomian gland dysfunction with obstruction of the meibomian orifices, the absence of gland structure or both, get more fluorescein due staining on examination of their conjunctiva and cornea than those without meibomian gland dysfunction, which is indicative of microdamage.
Increased tear evaporation, i.e. evaporative dry eye disease, causes ocular surface microdamage, which in turn causes a compensatory reflex increase in aqueous tear secretion. The patient experiences this hypersecretion as increased tear production, known as hyperlacrimation or reflex lacrimation.
Meibomian gland dysfunction (MGD) usually has a slow onset gradually getting worse with time, age and environment. Many extrinsic and intrinsic factors influence MGD. Therefore the symptoms of meibomian gland dysfunction are variable, and there can be patients who have clinically evident meibomian gland dysfunction on examination but are unaware that they have it. I have often wondered why this may be the case and I think it is because of the gradual onset of their MGD and dry eye, but to them it is normal.
Some patients with meibomian gland dysfunction report significant dry eye symptoms and ocular pain including foreign body sensation, photophobia, redness and blurring causing problems with reading, driving or watching TV. It may be that the level of inflammation associated with their meibomian gland dysfunction is what drives the symptoms, as well as whether the patient is accustomed to having meibomian gland dysfunction.
The hyperosmolarity and mechanical friction between the eyelids and the globe in meibomian gland dysfunction all contribute to an inflammatory environment for activation of various enzymes and pathways leading to the generation of the inflammatory cytokines and influx of inflammatory cells which cause symptoms.
One of the most important symptoms that has been grossly underrated in meibomian gland dysfunction is that of the detrimental effect on the visual acuity. They get intermittent blurring of vision. Because a healthy tear film lipid layer is required to provide a smooth optical surface between the tear film and cornea for clarity of vision, it is not surprising that alternations in the tear film lipid layer have significant implications on visual quality.
We are all becoming much more aware of the condition of meibomian gland dysfunction and its close association with dry eyes and the formation of chalazia. By understanding the pathology of the condition and recognising the clinical features, we can offer more appropriate targeted treatment and research in this area is also opening up areas of treatment which were previously unimagined.