What is Dry Eye Syndrome?
Dry Eye Syndrome & The Ocular Surface
The ocular surface is a very complex environment. The tear film which coats the eye is mainly composed of 3 substances; water produced by the lacrimal gland, oil which is produced by glands that line the upper and lower lids, and mucous produced by goblet cells which are located on the conjunctiva. There are many other substances in the tear film such as proteins, electrolytes, antibodies and growth factors. This elaborate mixture pools (known as the tear lake) where the lower lid touches the globe. When you blink, the upper lid touches the tear lake and upon opening, the upper lid drags the tear film over the cornea.
Without the proper amounts of all 3 main tear film elements, the tear film will become unstable and fail to coat the eye properly. Dry patches on the eye will form in between blinks and symptoms such as stinging, burning, or scratchy sensation will begin. There may also be eye fatigue or a feeling of “heaviness” after short periods of reading. Many people experience increased eye irritation or burning from wind, air-conditioning or ceiling fans. Blurred vision, light sensitivity, or even tearing can also be caused by an unstable tear film.
Water, the majority of the tear film, is produced by the lacrimal gland which is located above and on the outside of each globe within the bony orbit. The water component lubricates the eye and washes away irritants, allergens, bacteria, fungus, and viruses. Although aging is the most common cause of decreased lacrimal gland output, (common over the age of 40) the condition afflicts women more than men, especially after menopause. Diabetes, rheumatoid arthritis, lupus, scleroderma, Sjogren’s syndrome, and vitamin A deficiency can all diminish the lacrimal gland’s output.
Essential for tear film stability, fatty oils called lipids are produced by a line of meibomian glands on the upper and lower lid edge. The oil stabilizes the tear film and retards evaporation. Blocked and inflamed meibomian glands (meibomitis/ meibomian gland dysfunction) are another common cause of dry eye syndrome symptoms. Instead of producing a thin flowing oil, patients with meibomitis have thickened (sometimes described as toothpaste like) oils which often completely block the glands. Blocked glands result in further inflammation and sometimes cause styes and chalazia to form. Meibomitis are commonly seen in people with rosacea or blepharitis.
Mucin, which is the last of the three main tear components, is produced by goblet cells that line the conjunctiva. Mucin allows the tear film to spread evenly over the ocular surface. Conditions like Stevens Johnson Syndrome and Ocular Cicatricial Pemphigoid can damage goblet cells and result in decreased mucous production.
The successful treatment of dry eye syndrome symptoms can be difficult as most people have deficiencies in 2 or more of the tear film components. In addition, they often have confounding problems like blepharitis and eyelid malposition. All these factors must be carefully evaluated and treated in order to cure you of your dry eye syndrome symptoms.