Dry eye occurs when the eyes do not generate adequate tears or if the tears generated evaporate quickly. This definition has changed over the years. Currently, it is held that dry eye is a multifactorial disorder that can arise because of the inflammation of the ocular surface and the lacrimal gland, neurotrophic deficiency, and dysfunction of the meibomian gland. This has led to the development of new and effective treatment and medications.
The causes of dry eye can be due to different factors, which can be grouped under three categories:
- Aging and how gender difference has an impact on dry eye
- Dry eye induced by diseases, eyelid disorder, ocular surgeries, and medications
- Environmental causes
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Predominantly, older people are affected by dry eye. The meibomian gland, which generates the oily part of tears, becomes dysfunctional with age; in addition, many other significant factors also contribute to the tear film evaporation rate.
Involutional eyelid malposition, particularly horizontal lid laxity, is most common in elderly people causing poor distribution of the tear film resulting in abnormal flow of tears. These conditions develop into eye diseases such as blepharitis and dry eye. Antiaging reagents such as retinoid may increase tear generation by the meibomian glands.
The Influence of Gender, Race and Ethnicity
Women are more prone to dry eye symptoms, particularly those treated for oestrogen substitute therapy. During menopause, women undergo changes in the endocrine glands combined with the aging effects in the pathogenesis of dry eye.
Increase in the immune reaction in the lacrimal duct cells, destruction of acinal cells by prolactin, and epidermal growth phenomenon during pregnancy can affect the tear film and cause dry eye. Dryness can also be induced because of the dehydration resulting from nausea and vomiting. This can be further aggravated by taking anti-nausea medications.
There are few studies that exist on the causes of dry eye while using oral contraceptives. In regards to race and ethnicity, there is greater evidence of dry eye in the Chinese, Asian, Hispanic, and Pacific Island populations.
Dry Eye Induced by Diseases
Evidence of dry eye is found in patients with a history of the following diseases/deficiencies:
- High level in cases of collagen vascular disease, occult thyroid eye disease, vitamin A deficiency, hepatitis C and androgen insufficiency
- Medium level in cases of diabetes mellitus, HIV/HTLV1 infection, systemic chemotherapy, keratoplasty, sarcoidosis and ovarian dysfunction
Several systemic diseases are linked to dry eye syndrome and some of them include rheumatoid arthritis, scleroderma, polymyositis, lymphoma, amyloidosis, hemochromatosis, sarcoidosis, and systemic lupus erythematosus.
A Triad of Causes - Eyelid Disorders, Surgery, Medications
Eyelid disorders (such as infection, swelling or malignancies) may lead to dry eye. A healthy eyelid is a remarkable but frequently ignored structure. Exposure keratitis (a condition where eyelids are half-closed during sleep) may cause dry eye. Skin infection or inflammation in the eyelid, and when the eyelids do not blink properly and regularly, can also result in dry eye. If there is a severe inflammation in the conjunctiva, i.e. the lining membrane of the eyelid, dry eye can occur.
Ophthalmologic surgeries may induce dry eye. Some of them are cataract surgery with an incision, phacoemulsification surgery, photorefractive keratectomy, as well as laser in situ keratomileusis. If the patient has a history of dry eye, premedical treatment is required prior to the surgery, or the treatment of ocular surface in the case of post refractive surgery. During the surgery, long exposure periods to microscopic light may also lead to dry eye.
Moreover, studies have proved that using antihistamines involves a high risk factor for dry eye. Medications such as beta-blockers, antidrepressants, and serotonin inhibitors may have a medium risk of causing dry eye. Low level of evidence is found with regard to anticholinergic drugs like antipsychotics or anxiolytics.
Environmental Causes of Dry Eye
Dry eye syndrome is induced by the workplace and living environment, and increases with the use of electronic devices and certain outdoor environmental factors. Variations in airflow, moisture level in the air, low relative humidity, exposure to toxic elements, inadequate ventilation, and waft from air-conditioning or heater - all these can lead to dry eye development.
Extreme hot or cold weathers, exposure to UV radiations, polluted environment, chemical fumes, tobacco smokes, and atmospheric dusts aggravate ocular dryness. Cigarette smoking and alcohol consumption may associate with the risk of dry eye syndrome.
There are several additional factors that influence dry eye development. Spending more time in front of computers and televisions has increased in these days. Working at computer terminals for more than 8 hours daily is one of the significant causes for dry eye. In these instances, the blink rate is reduced or it leads to incomplete blinks.
Studies have revealed that long-term rigid gas-permeable (RGP) or soft contact lens (SCL) wearers may be diagnosed with dry eye. It is important for such wearers to diagnose and treat dry eye at the early stage. The main factor for contact lens-induced dry eye is because of the tear film instability, which leads to reduced lipid film of tears.