Dry eye is a condition that affects millions of people. The medical term for dry eyes is Keratoconjunctivitis sicca. Dry eye can range from a mild condition to a sight-threatening disability. It may be associated with illnesses such as rheumatoid arthritis and Sjogren’s syndrome. Fortunately, the understanding and treatment modalities for dry eyes have improved dramatically over the last decade.
Dry eyes are one of the most common reasons for a visit to the doctors at EDOW. Our doctors specialize in this condition and are involved with research to assist in its diagnosis and management.
Did You Know…
Dry eyes are a common reason for a visit to the doctors at EDOW. Our doctors have been associated with research to assist in the diagnosis and management of this condition.
Causes of Dry Eyes
Dry eye affects up to 15% of the adult population in the United States. It is more common in women and increases with age. The hormonal changes that occur during menopause are felt to affect tear secretion.
While one thinks of tears as “water,” in reality tears have a complex structure with three major components. Think of the tear film as a thick sandwich with a thin stabilizing layer of lipid on one side and mucin on the other side. In between these two layers is a salty mixture of water.
The lipid layer is on the outer surface of tears. The lipid layer is composed of oils. The Lipid layer plays a vital role in stabilizing tears. The lipids allow the tears to spread uniformly across the surface of the eye as well as reduce the rate at which tears evaporate. If the lipid layer is removed, tears will evaporate at 19 times the normal rate, and the eye will become parched.
The aqueous layer is the “middle layer” of tears. The aqueous layer is the main component of tears and is composed of water mixed with electrolytes, such as salt. The concentration of the electrolytes needs to be in balance (“isotonic”) with the underlying cells on the corneal and conjunctival surface.
The mucin layer is the inner portion of the tear film. The mucin layer allows the tears to adhere to the underlying cells on the surface of the eye (conjunctiva and cornea). The structure of the tear film is demonstrated on the slide below:
The aqueous (water) component of tears is produced by two different sets of lacrimal glands. Under normal conditions, the accessory lacrimal glands located in the upper and lower lids (see diagram below) produce the tears necessary to keep the eye moist. This is called “basal tear secretion” or your constant tears. When the eye is irritated, the main lacrimal gland located in the upper/outer portion of the orbit produces a flow of tears. This is called “reflex tear production.” When tears flow from the main lacrimal gland during reflex tear production, an excessive amount of tears is produced creating watery eyes.
When someone has dry eyes, there is almost always insufficient basal tear flow. The main lacrimal gland will then start to produce tears. Since the flow from the main lacrimal gland is usually too great, a common complaint from patients with mild dry eyes is actually “watery eyes.” It always seems paradoxical to patients when we explain that their watery eyes are caused by insufficient tear production. This is why it is important to understand the relative function of the two lacrimal glands – main (reflex tears) and accessory (basal tears).
Because we all live under varying environmental conditions, our level of tear production must vary to accommodate the circumstances. The two main methods to stimulate tear production are:
- Nerve innervation
Androgen hormones assist with the stimulation of tear production. As people mature, especially women, the changes in hormonal balance have an effect on tear production. Unfortunately, the use of traditional oral hormone replacement therapy has not demonstrated substantial efficacy in the treatment of dry eyes in women. It may also be associated with other illnesses such as a higher incidence of breast cancer.
There is a neural loop (see diagram below) between the surface of the eye and the lacrimal glands. When the ocular surface is irritated, a nerve impulse is sent from the surface of the eye to the brain. The signal is then sent on to the lacrimal gland. Once this message reaches the lacrimal gland, more tears are stimulated.
A substantial amount of ocular surface irritation can inhibit the eye’s ability to send a message to the brain to create more tears. This creates a vicious cycle in which the eye becomes irritated and, therefore, less nervous stimulus is sent to the brain creating less of a signal to create tears.
There are many causes of dry eyes; however, the most common causes are age and environmental conditions. With the aging process, the hormonal changes play a major role in the prevalence of this condition in mature women. There is also a mild reduction in nerve stimulation with the aging process. Conditions that may exacerbate dry eyes include the following:
Environmental Stresses: Such as low humidity, cigarette or cigar smoke, and air pollution can irritate the eyes. In the winter and summer, heaters and air conditioners tend to reduce moisture in the environment.
Allergies and Air Pollution: may increase ocular surface irritation that affects the neuronal loop that stimulates basal tear production.
Computer Terminals: when we stare at a computer terminal for a long period of time, the rate at which we blink our eyes tends to be reduced, causing ocular surface irritation.
Medications: certain medications especially those used for depression and allergies may cause a reduction in tear production. You should check with your physician if you use drugs for any of the following conditions:
- Breast Cancer
- Parkinson’s Disease
- High Blood Pressure
Soft Contact Lenses: almost all soft contact lenses have water as a major constituent. The lens will act as a sponge on the ocular surface to maintain the appropriate water content.
Certain medical conditions are associated with dry eyes. These include collagen vascular disorders such as Rheumatoid Arthritis, Lupus, and Sjogren’s syndrome. These conditions affect the immune system and may cause an immune-mediated inflammation in the eye.
Non-immune-related diseases may also affect dry eyes. Parkinson’s disease causes a poor or incomplete blink response. This may dry the ocular surface. Herpes Zoster (“Shingles”) and Bells Palsy affect the neural loop and may also cause dry eyes.