When the edges of the eyelids are inflamed, problems with the meibomian glands may occur. These small glands are found at the edges of eyelids. They are responsible for the production of oil in tears. The oil is important in tears, because it stops the water behind it from evaporating too rapidly. The oil is the last layer within tears. Dry eyes may occur when there is not enough oil produced. This usually happens if the meibomian glands are obstructed or clogged. [source]
Antihistamines can decrease tear production, as noted on the EyeCare Source website. According to a 2007 article in the "Review of Optometry" journal, oral antihistamines are a main culprit. Examples of antihistamines include diphenhydramine, also known as Benadryl, and cetirizine, or Zyrtec.
- Oil. The outer layer of the tear film, produced by small glands on the edge of your eyelids (meibomian glands), contains fatty oils called lipids. These smooth the tear surface and slow evaporation of the middle watery layer. If your oil glands don't produce enough oil, the watery layer evaporates too quickly, causing dry eyes. Dry eyes are common in people whose meibomian glands are clogged. Meibomian dysfunction is more common in people with inflammation along the edge of their eyelids (blepharitis), rosacea and other skin disorders.
- Water. The middle layer is mostly water with a little bit of salt. This layer, produced by the tear glands (lacrimal glands), cleanses your eyes and washes away foreign particles or irritants. If your eye produces inadequate amounts of water, the oil and mucus layers can touch and cause a stringy discharge.
- Mucus. The inner layer of mucus helps spread tears evenly over the surface of your eyes. If you don't have enough mucus to cover your eyes, dry spots can form on the front surface of the eye (cornea).
CAUSE 1: EVAPORATIVE DRYNESS
Evaporative dry eye is caused by a deficiency in the lipid layer of the eye’s natural tear film, resulting from blockages in the lipid-producing meibomian glands located in the eyelids, called meibomian gland dysfunction. A lipid deficiency can lead to evaporation of tears at a faster rate than normal, which can leave eyes feeling dry, irritated, tired and red. Of the more than 100 million dry eye sufferers worldwide, approximately 65 percent have evaporative dry eye. [Source]
By staring at the computer/TV and not blinking enough, a large surface area is presented to the eye, which creates significant loss of tears. The cause here is NOT the loss of the tear production capacity, but the habit which creates significant loss of tears. Working on a computer, reading and driving at night can contribute to hyper-stressed tear film states due to insufficient blinking frequency.
The human tear film is typically described as having mucin, aqueous and lipid components. At a more complex level, the tear film is composed of electrolytes (e.g., sodium, chloride), antioxidants (e.g., vitamin C, glutathione), anti-infectives (e.g., lactoferrin, lysozyme), immune and inflammatory proteins (e.g., sIgA, IgG, IL-2), proteins involved in tear-film maintenance (e.g., lipocalins, mucins), proteins involved in corneal wound healing and maintenance (e.g., fibronectin, TGF-alpha, insulin), neuropeptides (e.g., calcitonin gene-related peptide, subP) and lipid components (e.g., wax esters, cholesterol esters).2-4 These tear-film components originate from a variety of sources, including the lacrimal glands, ocular surface cells, nerve endings and meibomian glands.
The concentrations of these components vary drastically from one tear film to another, and deficiencies in an individual’s quality or quantity of the components can lead to dry eye.
- Are able to form a continuous tear layer over the ocular surface including the damaged epithelium.
- May have high oncotic pressure capable of dehydrating injured water-logged epithelial tissue to normal level.
- May contain polar lipids at low level to strengthen the lipid layer of the tear film.
CAUSE 2: INFLAMMATION
Other notes on causes
The technical name of what I have is asthenopia. http://en.wikipedia.org/wiki/Asthenopia
COMPUTER VISION SYNDROME
This is also known as the computer vision syndrome: http://en.wikipedia.org/wiki/Computer_vision_syndrome (see definition here)
Apparently eyestrain will not permanently damage the eyes or cause a loss of vision. However, it can be very uncomfortable and lead to a loss of productivity. Anyone who uses a computer can take measures to reduce eye discomfort. The good news is some strains and pains can be temporary and can be resolved with rest and simple techniques and possibly medication.