الفهرس 
Anatomy and physiology of the tear filmOnly 14 pages are availabe for public view
 |  | from | 52 |  |  |
| | | from | 52 | | |
Abstract
Dry eye disease, though common, is a frequently under-recognized clinical state whose aetiology and management are challenging. Many external and internal factors have been studied, which affects the stability of tear film. Dry eye is multifactorial diseases of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface.
Ocular surface inflammation is associated with excessive tear evaporation, which leads to tear film instability. The symptoms of dry eye include feeling of dryness, grittiness or foreign body sensation that gets worse throughout the day, burning sensation, red eyes, eyelids that stick together on waking up, and temporarily blurred vision which usually improves with blink. The causative mechanisms of dry eye include tear hyperosmolarity and tear film instability.
Dry eye syndrome (DES) is a common ocular disease that is characterized by tear instability, ocular surface inflammation, and irritable eye symptoms. DES is divided into two types: aqueous deficiency and evaporative type. The reduction of tear secretion from lacrimal glands leads to aqueous deficiency DES. Eyelid margin inflammation and meibomian gland dysfunction (MGD) have been suggested as major causes of evaporative type DES. DES has been shown to cause inflammation of the ocular surface that is evidenced by increased levels of inflammatory cytokines in the tear fluid and corneal and conjunctival epithelia, and infiltration of CD4+ T cells into the conjunctiva.
Treatment for dry eye includes the use of artificial tears, anti-inflammatory drugs, autologous serum, and punctal occlusion. Artificial tears contain carboxy methylcellulose or hyaluronate sodium and act as a lubricating agent at the ocular surface. Artificial tears provide palliative relief of eye irritation in patients with aqueous tear deficiency, but do not treat the underlying inflammation or reverse conjunctival squamous metaplasia in chronic dry eye. Conventional treatments using topical drugs and punctal occlusion are not effective in the treatment of chronic ocular pain in some patients with dry eye.
Recently, vitamin D deficiency has been suggested to be a contributory factor in DES. An association between DES and serum 25-hydroxyvitamin D (25(OH) D) concentration has been suggested. It has been reported that vitamin D plays an immuno-modulatory role in innate and adaptive immunity. Vitamin D and the vitamin D receptor (VDR) regulate several genes involved in inflammation, immunity, cellular proliferation, differentiation, and apoptosis. However, the effect of vitamin D supplementation on dry eye has not been reported.
Vitamin D is a multifunctional hormone that can be obtained from the diet and is also produced in the skin. Apart from its well‐known role in calcium metabolism and bone health, vitamin D is now recognized to have anti‐inflammatory and immunoregulatory properties. Researchers have identified vitamin D receptor expression on the majority of immune cells (i.e., antigen‐presenting cells, T cells and B cells).
Binding between active vitamin D and the VDR regulates several genes involved in inflammation, immunity, cellular proliferation, differentiation and apoptosis. Previous studies suggest an association of vitamin D deficiency with chronic illness, autoimmune disease and malignancy. Recently, vitamin D deficiency has been suggested to be a contributory factor in dry eye. It is presumed that patients with vitamin D deficiency are prone to dry eye. However, some studies have been unable to demonstrate an association between dry eye and a low vitamin.