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"Don't it always seem to go, that you don't know what you've got 'till it's gone......."
- Joni Mitchell from Big Yellow Taxi.
I never gave my tears a second thought, until Sjogren's came along. Until I had to deal with Sjogren's-related dry eyes, I didn't realize most things about tears: that there are three different types of glands which produce three different substances which, combined, make ideal tears; and that there's several different types of tears.
Who knew? Read on for Tears 101. So settle down, class. There will be a quiz. (Julia wheels out the overhead projector and dims the lights.)
Image found here.
Tears appear on the eye surface in three layers. The innermost layer is mucous, secreted by cells in the conjunctiva. This mucous coats the cornea and promotes even distribution of tears over the eye surface.
The middle layer, or aqueous layer, is comprised of water and specialized proteins which provide moisture, help protect against infections, and maintain a normal osmotic balance. This layer is secreted by the lacrimal gland.
The outermost layer is the lipid layer, made of oils secreted by the meibomian glands. The lipid layer helps to protect the water component of the tears from evaporation, and to keep our tears from spilling out over our cheeks. Think of oil spreading out on the surface of water.
Our eyes produce several different types of tears:
Basal tears moisurize, nourish, and lubricate the eye surface. These are the tears that continually work to maintain a normal healthy eye.
Your body makes reflex tears in response to irritants such as foreign material, wind, or onion vapors. Reflex tears attempt to wash those irritants from the eye.
Psychic tears, or crying/weeping tears, are created in response to strong emotions such as sadness, joy, or laughter. These tears have additional components:1) Leucine-enkephalin — a mood-elevating and pain reducing endorphine 2) ACTH — a hormone considered to be the most reliable indicator of stress and 3) Prolactin — the hormone that regulates milk production in mammals. These components were analyzed and described by Dr. William Frey, a tear researcher at St. Paul Ramsey Medical Center. You can read more about Dr. Frey and his study of emotional tears here.
You can read more about the composition of human tears here.
So why do Sjogren's syndrome patients have decreased tears? Remember that Sjogren's syndrome and other autoimmune diseases create specialized lymphocytes, a type of white blood cell, that mistakenly identify healthy tissue as foreign tissue. These misguided lymphocytes attack the glands that secrete tears, saliva, and mucous in Sjogren's syndrome. Hence, decreased tears and saliva. Of course, Sjogren's is more complicated than that simple explanation and affects many other organs besides tear and saliva glands. You can read more about Sjogren's here.
(Julia flips lights back on and everyone blinks). That's enough for today, everyone. Next time we'll talk about treatment for Sjogren's dry eye.