What causes dry eyes?

The terms "dry eye" and "dry keratoconjunctivitis" are synonyms. There are 2 main forms:

1. hyposecretory dry eye Sjogren, in particular Sjogren's or non-Sjogren's syndrome,
2. disruption of tear evaporation.

But these two conditions are not mutually exclusive.

Clinical Physiology

The main lacrimal glands produce approximately 95% of the aqueous component of tears, and the accessory lacrimal glands of Krause and Wolfring produce 5%. Tear secretion can be either primary (constant) or a much more pronounced reflex production. Reflex tear production is produced in response to sensory stimulation of the cornea and conjunctiva, tear film rupture, and the formation of a dry spot or inflammatory process. Reflex tear production is reduced by local anesthetics. Previously, primary tear production was attributed to the accessory lacrimal glands, and reflex tear production to the main lacrimal glands. Now it is believed that the entire mass of lacrimal tissue works as a single whole. The precorneal tear film has 3 layers: lipid, aqueous, and mucin.

Outer lipid layer

The outer lipid layer is secreted by the meibomian glands.

Functions of the lipid layer

• Delays evaporation of the aqueous layer of the tear film.
• Reduces the surface tension of the tear film, which in turn attracts the aqueous component into the tear film and thickens the aqueous layer.
• Lubricates the eyelids, which follow the contours of the eye surface.

Dysfunction of the lipid layer can lead to dry eye syndrome due to increased tear evaporation.

Middle water layer

The middle aqueous layer is secreted by the lacrimal glands and consists of proteins, electrolytes and water.

Functions of the water layer

• Delivery of atmospheric oxygen to avascularized corneal epithelium.
• Antimicrobial protection due to the presence of IgA, lysozyme and lactoferrin proteins in tears.
• Removal of foreign bodies from the surface of the cornea.
• Cleansing the wound from inflammation products.

Deficiency of the aqueous layer leads to secretory "dry" eye.

Inner mucin layer

The inner mucin layer is secreted by the goblet cells of the conjunctiva, the crypts of Menle, and the glands of Manz.

Functions of the inner mucin layer

• Moisturizing the cornea by transforming the hydrophobic surface of the corneal epithelium into a hydrophilic one.
• Lubrication.

Insufficiency of the inner mucin layer can be the cause of both hyposecretion and a condition with increased tear evaporation.

The tear film is spread over the surface of the eye mechanically by reflexive blinking movements and after a certain period of time is removed through the lacrimal ducts. Three factors are necessary for normal distribution of the tear film: a normal blinking reflex, full correspondence between the anterior surface of the eye and the eyelids, and normal corneal epithelium.

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Hyposecretory dry eye (keratoconjunctivitis sicca) Sjogren

Sjogren's syndrome is an inflammatory process caused by a cytokine reaction that affects the lacrimal glands and ducts, causing disruption of the tear film and can often lead to damage to the ocular surface.

1. Primary Sjogren's syndrome is characterized by dry mouth (xerostomia) and the presence of antibodies that are characteristic of an autoimmune process.
2. Secondary Sjogren's syndrome is characterized by the presence of systemic autoimmune connective tissue disorders and manifests itself in the form of the following diseases: rheumatoid arthritis, lupus erythematosus, systemic sclerosis, dermatomyositis and polymyositis, mixed connective tissue pathology, relapsing polychondritis or primary liver cirrhosis. All these conditions complement the primary Sjogren's syndrome.

Hyposecretory dry eye (keratoconjunctivitis sicca) non-Sjogren

1. Primary, age-related - the most common.
2. Destruction of lacrimal gland tissue caused by a tumor or inflammation (eg, pseudotumor, endocrine ophthalmopathy, or sarcoidosis).
3. Absence of the lacrimal gland due to surgical interventions, rarely congenital.
4. Obstruction of the lacrimal gland ducts due to cicatricial changes in the conjunctiva (for example, cicatricial pemphigoid and trachoma).
5. Neurological disorders such as familial vegetative-vascular dystonia (Rilay-Day syndrome).

Dry eye, which is associated with impaired tear evaporation

1. Lipid deficiency most often occurs due to dysfunction of the meibomian glands.
2. Violation of the integrity of the covering of the surface of the eye with a tear film due to misalignment of the edges of the eyelids or a disruption of the blinking process.

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