Tear film is a thin film of fluid that covers the exposed areas of the eyes and extends under the eyelids. It has 3 layers: an outer lipid layer, a middle aqueous layer and an inner mucin layer (Bron et al. 2004).
The tear film is essential for maintaining the transparency and health of the cornea and conjunctiva, providing a smooth, moist surface for light to pass through. It also supplies nutrients and flushes away waste products, protects against shear forces produced by blinking and eye movements, and helps to protect the eye against environmental challenges. Dry eye disease (DED; also known as dry eye syndrome or keratoconjunctivitis sicca) was described in the Report of the International Dry Eye Workshop (DEWS 2007) as 'a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface'. DED often affects both eyes. The NICE clinical knowledge summary on dry eye syndrome states that symptoms of DED are most commonly caused by:
Decreased tear production, usually because of dermatosis of the eyelids, an adverse effect of some medicines, or allergic conjunctivitis. (This is referred to as aqueous DED [DEWS 2007].)
Increased evaporation of tears, usually because of low humidity, low blink rate (for example, from prolonged use of a computer or microscope), high wind velocity, an adverse effect of some medicines (such as antihistamines), or allergic conjunctivitis. (This is referred to as evaporative DED [DEWS 2007].)
Symptoms of DED may include dry, gritty, sore or red eyes, temporary blurred vision, and eyelids sticking together during sleep. DED symptoms vary in severity and are commonly classified using the 4‑grade DEWS severity grading scheme (DEWS 2007). Grade 1 refers to very mild or episodic DED, which tends to have limited visual symptoms, whereas grade 4 refers to severe and constant DED with potentially disabling symptoms. Severe symptoms can impair vision, limiting vision‑related daily activities such as reading and driving. The complications of DED include conjunctivitis, keratitis (infection of the cornea) and corneal scarring.
The condition is estimated to affect 15–33% of people aged over 65 years, and is about 50% more common in women than men (NICE clinical knowledge summary on dry eye syndrome; Lemp et al. 2012). DED is also more common in people who wear contact lenses, about 50–75% of whom report symptoms of ocular irritation (DEWS 2007).