Tears are essential to keep eyes healthy and to provide good vision. As there are no blood vessels in the cornea, basal tears are responsible to nourish and keep the cornea wet. The liquid film washes the bacteria in the eye and the antibiotic proteins kill the bacteria.
Whenever the eyes are opened, the tear film supplies the oxygen and the thin liquid layer spreads across the cornea, creates a good eye surface and performs this activity each time the eye blinks. The friction caused due to the eye movement is limited as the basal tears act as a lubricant. While tears perform such a complex role, when the production and drainage of tears is not balanced, it could lead to a medical condition known as the dry eye syndrome (keratoconjunctivitis sicca).
Dry Eye Animation
Structure of the Tear Film
The tear film is made up of three layers with each layer contributing to a purpose. The outer oily, lipid layer produced by meibomian glands is responsible for keeping the tear surface smooth and ensure that the surface is not drained quickly.
The middle watery layer, the aqueous layer, is composed of water, proteins, and electrolytes. Produced by the lacrimal glands, this layer is responsible to keep the eye clean. Making up most of the tears it washes away foreign particles from the eyes.
The inner layer, the mucus layer, consists of water, secreted mucins and electrolytes, and it is produced by the conjunctival goblet cells. This layer ensures the watery layer is spread over the surface of the eye so that the eye is wet. It is due to this layer that the tears stick to the eye.
Types of Dry Eye
Dry eye is classified into two categories based on the causes:
- Lacrimal tear deficiency or aqueous tear deficiency
- Evaporative tear deficiency
Lacrimal Tear Deficiency or Aqueous Tear Deficiency
The lacrimal gland plays a great role in protecting the eyes. The acinar cells of the lacrimal glands produce tears in aqueous contributing a major volume to the tear film. This additional fluid keeps the eye surface moist, while the meibomian gland supplies lipocalin and lipids making the tears to be highly thick and water dilutes the substances.
Proteins in the epidermal keratinocytes, such as retinol (a vitamin A derivative) are produced by lacrimal glands and contribute to protect the eyes from infections and help in healing wounds. When the lacrimal glands fail to secrete or secrete tears in poor quantities, it causes dryness in the eye resulting in lacrimal tear deficiency or aqueous tear deficiency.
Aging and medications (such as decongestants, antihistamines, antidepressants, medicines used for controlling blood pressure) can lessen the production of tears. Furthermore, the deficiency in Vitamin A can damage the acinar cells of the lacrimal glands and result in aqueous tear deficiency, which is a condition known as xerophthalmia.
Evaporative Tear Deficiency
In some cases, although the lacrimal gland secretion is normal, the tears evaporate quickly either due to eye diseases or when exposed to certain external circumstances. This condition is termed as evaporative dry eye.
Conditions such as poster blepharitis caused due to dysfunction of the meibomian gland, seborrheic dermatitis, and ingestion of polychlorinated biphenyls through food result in evaporative dry eye. Indulging in games where while focusing on the goal, the eyes usually gaze upward will lead to increased exposure of the ocular surface and make the surface dry.
Reduced blink rates will increase the water loss to the ocular surface until the next blinking occurs. Studies reveal that people who wear contact lenses are more prone to evaporative dry eye when compared with people who wear spectacles.
Tear Hyperosmolarity and Tear Film Instability
In both the types of dry eye discussed above, two factors are vital: tear hyperosmolarity and the instability of the tear film. Reduction in aqueous tear production or increase in evaporation of tears, or both, may constitute tear hyperosmolarity.
The surface of the epithelium can be damaged by tear hyperosmolarity and this may lead to tear film instability. The damaged epithelium is the cause of various dry eye symptoms such as burning sensation in the eye, excess tear secretion, pain and redness of the eye, blurred vision, feeling of heaviness in eyelids, decreased ability to read books, watch television, or work on computer, as well as eye fatigue.
Treatment and Prevention
Treatments include adding artificial tear solutions, conserving natural tears by blocking the tear ducts, and increasing tear production. However, the inflammation around the ocular surface can be reduced by applying eye drops, ointments, and warm compresses.
Self-help such as avoiding long episodes of staring at computers/TV screens and blinking regularly, increasing air humidity in work and home environment using humidifiers, protecting eyes by wearing sunglasses to avoid exposure to sun, intake of nutritious food rich in fatty acids, as well as drinking plenty of water and staying hydrated will lessen the symptoms of dry eye syndrome.
Further Reading