A thin film of fluid (the tear film) covers the exposed areas of the open eyes. The tear fluid has 3 layers; the outer lipid layer, the middle aqueous layer and the inner mucin layer (Bron et al. 2002).

The lipids in the outer layer, also known as meibomian lipids, are produced and secreted by the meibomian glands. These glands are in the upper and lower eyelids, and there are as many as 40 glands per lid. The aqueous layer is produced and secreted by a single lacrimal gland, situated above the top outer corner of the eyelid, and multiple accessory glands situated along the upper eyelid. The mucin layer is secreted from specialised goblet cells found in both the eyeball and eyelids (Bron et al. 2002).

The tear film provides a smooth moist surface for light to pass through. If tear fluid production is disrupted this may result in 'dry eye', which causes the eye to feel dry, gritty or sore. In some people, symptoms include red eyes, temporary blurred vision and the eyelids may stick together on waking up. Dry eye affects up to 1 in 3 people over the age of 65 years (NHS Choices 2014a; Lemp et al. 2012).

Dry eye can be classed as either evaporative dry eye or aqueous‑deficient dry eye. Evaporative dry eye is the most common type of dry eye and there are a number of types. One type of evaporative dry eye is caused by the tear film evaporating from the surface of the eye too quickly, because of a deficient lipid layer. Aqueous‑deficient dry eye is the result of an insufficient aqueous layer (Lemp et al. 2012).

Evaporative dry eye can also be caused by an obstruction or the abnormal functioning of the meibomian glands. This condition is called meibomian gland dysfunction (MGD; Lemp et al. 2012), and is the indication for the technology in this briefing. MGD can be caused by cell debris and meibomian lipids hardening in the terminal ducts of the glands and obstructing the secretion of meibomian lipids. MGD can also refer to the quality of meibomian lipids being secreted, which in turn will affect the quality of the lipid layer in the tear film. The prevalence of MGD is uncertain. In people over the age of 40 years, it has been reported to range from 68% in people of Asian origin to 19.9% in people of white origin. It is important to note that the clinical symptoms of MGD overlap with those of other types of evaporative dry eye, as well as those of aqueous‑deficient dry eye. This may account for the high and variable figures reported (Nichols et al. 2011).

Conventional treatment of MGD involves warming and massaging the eyelids to reliquefy the meibomian lipids and force the obstruction out of the glands. There is little consensus within the literature as to the specific frequency and method of application for the treatment of MGD (Finis et al. 2014a). Therefore, a standardised treatment regime for MGD, which relieves symptoms of dry eye, may improve patient outcomes.