The ocular surface is composed of:
- The free margin of the eyelids
- The conjunctiva: transparent membrane which lines the inside of the eyelids as well as the sclera.
- The cornea
- The lacrimal glands:
- The principal lacrimal gland (in the upper lid)
- The Meibomian glands (in the upper and lower lids)
- The tear film
Eyelids have a function of protection to prevent injury to eyes and a function of tear film maintenance to secrete, distribute and drain.
Blinking performs several essential functions:
- Ensures tear film is evenly distributed over cornea
- Blink reflex protects against foreign bodies, removes irritants
- Normal blinking frequency: 1 blink every 3 to 6 seconds
The lacrimal tear is composed of 3 layers that have an essential role for the cornea:
- Mucin layer: it is composed of mucins which are glycoproteins forming a gel. Mucins are secreted mainly by the mucus cells (goblets) in the conjunctiva. Mucins are hydrophilic and are able to fix the tear film firmly to the cornea and conjunctiva. The mucin layer allows uniform spreading of the aqueous layer over the surface of the cornea.
- Aqueous layer: the thickest layer of the tear film (90%), it is composed of 98% of water, plus nutrients (glucose) and proteins. It is produced by the principal lacrimal gland and accessory lacrimal glands. It has several functions: protect the ocular surface, hydrate the cornea (water), oxygenate and nourish the cornea.
- Lipid layer: it is the layer of oil that is composed of triglycerides, cholesterol, waxes. It is secreted by the meibomian glands.It has several functions: to prevent the evaporation of the aqueous layer, to keep the tears in place and to facilitate the blinking.
Tears are secreted by glands located inside eyelids and the conjunctiva (1 to 2 ml/min). Tears are spread by the blinking action of eyelids thus forming the tear film (1 blink every 3 to 6 seconds). The tear film is eliminated throw evaporation and lacrimal pathways
Dry eye is a multifactorial pathology that results from tear film damage due to:
- Either an insufficient production of tears (aqueous deficience form),
- Either excess evaporation of the tears (evaporative form).
Dry Eye is multifactorial and the causes are numerous:
- Age: lacrimal glands atrophy resulting in fewer tears. Aggravated by hormonal changes (ex: menopause).
- Environment: pollution, dry air or air conditioning, cigarette smoke, computer use, smartphone, wind, high altitude
- Taking medications: antidepressants, analgesics, sleeping pills, antihistamines, anti-acne, anti-diarrhea, hormonal treatments
- Taking toxic substances: tobacco and cannabis
- Lasik or cataract surgery or glaucoma
- Diseases with inflammation of the eyelids (blepharitis): rosacea, seborrheic dermatitis, psoriasis, chronic eye allergies.
- General diseases: autoimmune diseases: syndrome of Gougerot-Sjögren, rheumatoid arthritis, HIV, hepatitis ...
- Some Ethnies are more affected: Hispanics and Asians
Dry eye can come from different disorders:
- Tear film disorders (lipid layer, aqueous layer, mucin layer)
- Corneal surface disorders
- Blinking mechanism disorders (incomplete blinking or insufficient blinking). Blinking mechanism disorders cause a bad emptying of the Meibomian glands (obstruction and atrophy of these glands).
- Eyelid disorders
Eye dryness affects more than 300 million people worldwide (25% in the USA, 24% in France) (1)
- The studies show that the prevalence is higher for some people:
- Elderly people
- Some Ethnies are more affected: the prevalence of severe symptoms is higher among Hispanics and Asians compared to Caucasian
- Prevalence is increasing due to:
- our environment and modern life style (pollution, air-conditioning, work on computer …)
- a better diagnosis due to a better comprehension of the physiopathology
1. SFO 2015 report (French Ophthalmic Society)