APPLIEDPHYSIOLOGY

The main lacrimal glands produce about 95% of the aqueous component of tears and the accessory
Lacrimal glands of Krause and the Wolforing produce the remainder. Secretion of tears has basic
(resting) and much greater reflex components. Reflex secretion occurs in response to corneal and conjunctival sensory stimulation, tear break-up and dry spot formation or ocular inflammation.
it is reduced by topical anaesthesia . Although in the past basic secretion was ascribed to the accessory lacrimal glands, it is now thought that the whole mass of lacrimal tissue responds as one unit. The precorneal tear film consists of three layers:
(1) lipid
(2)aqueous
(3) mucin
Each of this have separate functions.

OUTER LIPID LAYER:

This is secreted by the meibomian glands.
(1) Functions:
To retard evaporation of the aqueous layer of the tear film.
To lower surface tension of the tear film. This, in turn, draws water into the tear film and thickens the aqueous layers.
To lubricate the eyelids as they pass over the surface of the globe.
(2) Dysfunction of this layer may result in an evaporative dry eye.

MIDDLE AQUEOUS LAYER:

This is secreted by the lacrimal glands and consists of proteins, electrolytes and water.
(1)Functions:
To supply atmospheric oxygen to the avascular corneal epithelium.
Antibacterial function due to presence of IgA, lysozyme and lactoferrin.
To abolish any minute irregularities of the anterior corneal surface.
To wash away debris and noxious stimuli and allow the passage of luecocytes after injury.
(2) Deficiency of this layer results in a hyposecretive dry eye.

INNER MUCIN LAYER

This is secreted by the conjunctival goblet cells, the crypts of Henle and the glands of Manz.
(1) Functions:
Wetting of the cornea by converting the dorneal epithelium from a hydrophobic to a hydrophilic surface.
Lubrication
(2)Deficiency of this may be a feature of both hypo secretive and evaporative states.

The tear film is mechanically spread over the ocular surface through a neuronally controlled blinging mechanism and after a period of time is cleared through the nasolarcrimal drainage system .
The three factors required for effective resurfacing of the tear film are: (1) normal blink reflex,(2) congruity between the external ocular surface and eyelids and (3)normal corneal epithelium.

CAUSES:

The term ‘dry eye’ and ‘keratoconjunctivitis sicca’(KCS) are synonymous. The two main categories: (a) hypo secretive,which may be sjogren, and (b) evaporative, although the two are mutually exclusive.

SYMPTOMS:

The most common are irritation ,foreign body sensation,burning, a stringy mucus discharge and transient blurring of vision. The less frequent symptoms are itching, photophobia and a tired or heavy feeling .patients with filamentary keratitis may complain of severe pain brought on blinking.
Surprisingly, patience seldom complain that their eyes are dry. Although some may report a lack of emotional tears or a deficient response when peeling onions. The symptoms of KCS are frequently exacerbated on exposure to conditions associated with increased tear evaporation (e.g air conditioning ,wind,central heating ) or prolonged reading ,when blink frequency is reduced . Symptoms may be improved by lid closure .

TEAR FILM ABNORMALITIES:

1.Mucus starands and debris are an early sign . In the normal eye, as the tear film breaks down,the mucin layer becomes contaminated with lipid but is washed away . In the dry eye ,the lipid-contaminated mucin accumulates in the tear film and tends to move with each blink. Mucin also has the interesting property of drying very quickly and rehydrating very slowly.
2. The marginal tear meniscus is a crude measure of the volume of aqueous in the tear film. The normal meniscus varies in height between 0.1 and0.5 mm and forms a convex band with a regular upper edge . In KCS the tear meniscus becomes concave,irregular,thin or absent.
3. Froth in the tear film or along the eyelid margin occurs in meibomian gland dys function.

SPECIAL INVESTIGATIONS :

Tear film break -up time:
The tear film break-up time(BUT)is an index of precorneal tear film stability. It is measured as follows :
a. Fluorescien is instilled into the lower forix.
b. The patient is asked to blink several times and then stop.
c. The tear film is examined with a broad beam and a cobalt blue filter . After an interval of time, black spots or line indicating the formation of dry areas will appear .

The BUT is the interval between the last blink and the appearance of the first randomly distributed dry spot. The development of dry spots always in the same location should be ignored because this is caused by by a local corneal surface abnormality and not by intrinsic instability of the tear film. A BUT of less than 10 seconds is abnormal.

ROSE BENGAL:

This dye has an affinity for dead or devitalized epithelial cells and mucus. It stains the exposed bulbar conjunctiva, resulting in the typical staining pattern of two triangles with their bases at the limbus. Corneal filaments and plaques are also shown up more clearly by the dye.

TREATMENT:

The main for the treatment of KCS are to remove the discomfort ,provide a smooth optical surface and to prevent structural corneal damage . One or more of the following measure may be used simultaneously .

Preservation of existing tears :

1. Reduction of room temperature ,to minimize evaporation of tears.
2.Room humidifier
3.A small lateral tarsorrhaphy which decreases the surface area of the interpalpebral fissure may be helpful.

Tear substitutes

1. Drops
2. Gels
3.oinments

Mucolytic agents

Acetylcysteine 5% drops may be useful in patients with corneal filaments and mucous plaques. They are used q.i.d. and may cause irritation following instillation. It has a limited bottle life ,so that it can only be used for up to 2 weeks.

Reduction of tear drainage:

1. Temporary occlusion
2. Reversible long-term occlusion
3.permanent occlusion

OTHER OPTIONS

1.Topical cyclosporin
2. Oral cholinergic agents