Anatomy of the Tear Film

• Having dry eyes for a while can lead to tiny abrasions on the surface of the eyes. In advanced cases, the epithelium undergoes pathologic changes, namely squamous metaplasia and loss of goblet cells.  Some severe cases result in thickening of the corneal surface,  corneal erosion,  punctate keratopathy,  epithelial defects,  corneal ulceration (sterile and infected),  corneal neovascularization,  corneal scarring,   corneal thinning,  and even corneal perforation.

Causes: Any abnormality of any one of the three layers of tears produces an unstable tear film, resulting in symptoms of keratitis sicca. 

• Deficient tear production

• Keratoconjunctivitis sicca is usually due to inadequate tear production.   The aqueous tear layer is affected, resulting in aqueous tear deficiency (ATD) or lacrimal hyposecretion.  The lacrimal gland does not produce sufficient tears to keep the entire conjunctiva and cornea covered by a complete layer.  This usually occurs in people who are otherwise healthy. Increased age is associated with decreased tearing.  This is the most common type found in postmenopausal women.
• Causes include idiopathic, congenital alacrima, xerophthalmia, lacrimal gland ablation, and sensory denervation.  In rare cases, it may be a symptom of collagen vascular diseases, including rheumatoid arthritis , Wegener's granulomatosis, and systemic lupus erythematosus.  Sjögren's syndrome  and autoimmune diseases associated with Sjögren's syndrome are also conditions associated with aqueous tear deficiency.  Drugs such as isotretinoin,  sedatives,  diuretics,  tricyclic antidepressants, antihypertensives,  oral contraceptives,   antihistamines,  nasal decongestants,beta-blockers,  phenothiazines,  atropine, , and pain relieving opiates such as morphine can cause or worsen this condition. Infiltration of the lacrimal glands by sarcoidosis or tumors, or postradiation fibrosis of the lacrimal glands can also cause this condition. 

• Abnormal tear composition
  • Keratoconjunctivitis sicca can also be caused by abnormal tear composition resulting in rapid evaporation  or premature destruction of the tears.  When caused by rapid evaporation, it is termed evaporative dry eyes.  In this, although the tear gland produces a sufficient amount of tears, the rate of evaporation of the tears is too rapid.  There is a loss of water from the tears that results in tears that are too "salty" or hypertonic. As a result, the entire conjunctiva and cornea cannot be kept covered with a complete layer of tears during certain activities or in certain environments. 
• Additional causes
  • Aging is one of the most common causes of dry eyes.  This is because tear production decreases with age. 
  • An eye injury or other problem with the eyes or eyelids, such as bulging eyes or a drooping eyelid can cause keratoconjunctivitis sicca.  Disorders of the eyelid can impair the complex blinking motion required to spread tears.
  • About half of all people who wear contact lenses complain of dry eyes. This is because soft contact lenses, which float on the tear film that covers the cornea, absorb the tears in the eyes.  Dry eyes also occurs or gets worse after LASIK and other refractive surgeries, in which the corneal nerves are cut during the creation of a corneal flap.  The corneal nerves stimulate tear secretion.  Dry eyes caused by these procedures usually resolves after several months.  Persons who are thinking about refractive surgery should consider this. 
  • Abnormalities of the lipid tear layer caused by blepharitis and rosacea, and abnormalities of the mucin tear layer caused by vitamin A deficiency, trachoma, diphtheric keratoconjunctivitis, mucocutaneous disorders and certain topical medications are causes of keratoconjunctivitis sicca. 
  • Persons with keratoconjunctivitis sicca have elevated levels of tear nerve growth factor (NGF).  It is possible that this ocular surface NGF plays an important role in ocular surface inflammation associated with dry eyes.

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Dry Eye Tests