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Dry eye is a disorder of the tear film due to tear deficiency or excessive evaporation that damages the ocular surface.
The tear film is the first refractive layer of the eye. It lubricates the ocular surface, possesses a flushing action, and has an antibacterial effect. It plays an important role in corneal health and ocular surface homeostasis. It also supports contact lenses.
The composition of tear film is not fully understood. It is a biochemical mixture consisting of water and electrolytes, mucins, immunoglobulins, lysosomes, growth factors, and polar and non-polar hydrophobic lipids. The tear film consists of three separate layers: an inner mucin or glycocalyx layer; a middle aqueous, hydrated gel layer; and an outer lipid layer. All three layers play a role in tear production.
Classification of Dry Eye
Dry eye can be classified as aqueous tear or mucin deficiencies, as lipid or lid surfacing abnormalities, and as epitheliopathies. The two main causes of dry eye are decreased tear production and increased evaporative loss. Both Sjögren's and non-Sjörgen's syndromes are caused by decreased tear production. Sjögren's syndrome is caused by lacrimal gland disease, while non-Sjörgen's syndrome is caused by loss of reflex tearing. Increased evaporative loss is seen in Meibomian gland disease such as meibomitis, in cases of exposure such as lagophthalmos, in persons with a blink abnormality, and in contact lens wearers.
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Clinical Features
Clinical symptoms of dry eye include irritation, foreign body sensation, grittiness, redness, photophobia, and decreased clarity of vision. The symptoms worsen as the day progresses and are exacerbated under dry conditions.
The clinical signs of dry eye may consist of reduced tear lake, mild congestion, increased debris in the tear lake, superficial punctate staining of the cornea, lusterless conjunctiva, Bitot spots, symblepheron, heperimic lid margin, abnormal meibum, corneal opacities with superficial punctate keratis, and filamentary keratitis.
Diagnosis
Diagnosis techniques include Schirmer's test to assess tear production, Rose Bengal staining, and measurement of the tear breakup time.
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