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For comfortable and successful contact lens wear experience,
it is important that the tear film in the eye is sufficient and
of good quality. If there is any disruption in the quantity and
quality of the tear film then it will lead to contact lens wear
intolerance and damage to the ocular surface.
What is Dry Eye?
Dry eye is a condition in which there are insufficient tears
in the eye to keep it optimally comfortable. A more comprehensive
explanation would be that this is a problem of inadequate quantity
of tear film or a disturbance in the quality or stability of tear
film causing damage to the interpalpebral ocular surface. The
tear film function is controlled by neurohormonal interactions.
The disturbance of tear film is associated with inflammation and
structural changes of the eye surface in turn leading to discomfort
in using contact lenses.
Factors Responsible for Dry Eye Disease
Dry eyes can occur as a result of mucin deficiency in which the
moistness of the eye surface is decreased. It can also be brought
about by insufficient lipids or abnormal lipid layers by which
the stability of the tear film is affected. The third cause can
be attributed to aqueous deficiency whereby there is less than
required volume of liquid to the tear film. Also abnormalities
in the eyelids and inappropriate blink function could lead to
the inability of the distribution of the tear film. Finally, if
there is eye surface alteration and abnormality, tear film stability
is hampered.
Whichever may be the reason, the conclusion drawn points towards
the important role of the tear film as a surface active layer,
which when disturbed either by physical, physiochemical or structural
causes results in problem of dry eyes.
Classification of Dry Eyes
Dry eyes can be further classified into two types- tear deficient
dry eye and evaporative dry eye. Tear deficient dry eye can be
further subdivided into Sjogren’s syndrome related and non-Sjorgen’s
syndrome tear deficiency. The former is more severe in nature
and is associated with systemic abnormalities (keratoconjunctivitis
sicca).
Tear Deficient Dry Eye
The structure of the tear film is not only constituted of water
but is an integrated layer of lipids, aqua and mucin components
whose function is not only as a cleansing and protective liquid
but a supportive film for the eye surface as well. Aqueous deficient
dry eye is often related to decrease in the production of tears.
Proteins present in tears such as lysozome and lactoferrin were
found to be much lower in tears of patients with dry eye. Topical
applications of different types of liquids and polysaccarides
and polymers are commercially available and can be used to replace
the volume of tear film. Also a variety of vitamins, electrolytes
and minerals have been included to improve lachrymal function
but these only soothe but do not solve the underlying problem
of dry eye. Despite all attempts, the perfect tear substitute
is yet to be created.
The presence of post inflammatory cytokines in the tears and
conjunctiva of dry eye patients, point to the fact that inflammation
leads to the development of aqueous deficient dry eye. Studies
have shown that androgen permits the inflammation when reduced
leading to dry eye. This explains why the incidence of dry eye
disease most commonly occurs in postmenopausal women and elderly
people.
Evaporative Dry Eye
This is caused by lipid insufficiency, blepharitis, eyelid pathogenic
conditions, contact lens wear and ocular surface abnormalities.
Evaporative dry eye is more often a result of meibomian gland
disease.
The tear film is composed of thixotropic layer with the interaction
of lipids covering the surface of tears. If there is increased
evaporation of tear film, which results in tear film instability,
it leads to meibomian gland disease. The presence of estrogen
and androgen receptors in the meibomian glands are similar to
the lachrymal gland surface and ocular surface and these glands
are controlled by hormonal interactions and may be part of the
disturbance of androgen metabolism present in aqueous deficient
dry eye. More evaporation often aggravates an aqueous deficient
dry eye because these two types are concurrent and frequently
happen together.
Contact lens wear and dry eye
Tear film is responsible for lubrication and moistening the contact
lenses. Movement of tears and hyration behind lenses assures flow
of oxygen to the cornea and proteins protect eye surface and decrease
contamination of the lenses by bacteria. Contact lenses can aggravate
evaporation of tear film and lead to dry eye reducing corneal
sensitivity and disrupts balance between tear secretion and ocular
surface. So, the least the contact lenses change the corneal sensation
and tear physiology, the better the tolerance level of lenses.
Thus, contact lenses are ideal for those having normal lid function
and tear film stability.
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