What Is Dry Eye?
Dry eye is a condition in which the tears do not keep the cornea sufficiently hydrated. The cornea is a transparent dome-shaped tissue that forms the front part of the eye. It functions as a window and allows light to enter the eye.
What Are Tears?
The tear film of the eye serves many purposes, including keeping the eye hydrated; providing nutrients to, and removing waste from, the ocular surface; providing a smooth interface for eye movement and lid movement over the eye; creating a smooth optical surface over the cornea to allow clear focusing of images for vision; and fighting infections via antibacterial enzymes and by flushing harmful substances and bacteria away from the eye.
The tear film is composed of an intricate layering of three different substances:
- The innermost layer is the mucin layer. This layer consists of a thick, slippery, mucus material. It acts as a protective lubricant and provides a smooth surface for even distribution of the tear film over the surface of the eye.
- The middle layer is the aqueous layer. This layer consists of water, proteins, oxygen, and salts (which is why tears taste salty). It provides nutrients to the surface of the eye, removes waste products, and prevents infection.
- The outermost layer is the lipid layer. This layer consists of oils that coat the aqueous layer, act as a sealant to prevent the tears from evaporating, and prevent the tears from flowing over the lower eyelid.
What Are Symptoms of Dry Eye?
Dry eye can cause irritation, a gritty sensation, a "foreign body" sensation in the eye, itching, sensitivity to light, and blurry vision.
Paradoxically, a dry eye often results in the production of excess tears. In patients with dry eyes, dryness of the cornea stimulates a reflex tear production from the lacrimal gland, which produces the aqueous layer of tear film. However, these reflex tears are of poor quality and do not relieve the dry eye.
What Causes Dry Eye?
In dry eye, there is either inadequate quantity or quality of tears, or there is increased evaporation of tears. Many factors can cause or contribute to dry eye.
Conditions related to inadequate quantity or quality of tears include:
- Age: There is an increased risk of dry eye as individuals age and secrete fewer and poorer quality tears.
- Medications: A variety of medications, including blood pressure medications, antihistamines, and psychiatric medications, can reduce the ability of the lacrimal glands to produce tears.
- Medical conditions: Certain medical conditions, such as Sjögren's syndrome and rheumatoid arthritis, can decrease the ability of the lacrimal gland to produce tears. Other medical conditions, such as Riley-Day syndrome, injury to the fifth cranial nerve, and diabetes, result in damage to the nerves that allow the eye to sense dryness and thus prompt a reflex response of producing tears.
- Eye conditions: Diseases of the conjunctiva; diseases of the cornea; Vitamin A deficiency; history of eye surgeries, such as refractive surgeries; contact lens wear; and use of certain eye drops (especially those with preservatives) can cause dry eye by reducing production of any of the three components of tears, which makes the tear film unstable, or damaging some of the nerves responsible for sensing and responding to dryness.
Other conditions cause dry eye by allowing a higher than normal rate of evaporation of tears from the surface of the eye:
- Abnormal eyelid position: Bell's palsy (in which there is neurological damage to the 7th cranial nerve, preventing the eye from blinking) and diseases that cause a mechanical abnormality in the eyelid position result in an inadequate covering or closure of the eye by the eyelids. This is called exposure and causes an increased evaporation of tears
- Other eyelid conditions: Blepharitis (inflammation of the eyelids) or blockage of the glands that secrete the lipid layer of tears (the meibomian glands), may result in increased evaporation of tears
- Environmental factors: Low humidity or high winds promote can cause increased evaporation of tears.
- Decreased blink rate: Aging and medical conditions (such as Parkinson's disease), allows for increased exposure of the eye surface to the air, which results in increased evaporation of tears. In addition, it has been well established that when individuals are focusing on a task (computer use, reading, or watching television), their blink rate decreases. As a result, patients with dry eye may notice symptoms worsen during such tasks.
How Is Dry Eye Treated?
Dry eye is a chronic condition that generally requires a multifaceted treatment approach, often in a step-by-step manner, with modifications over time:
- Treatment of the underlying condition: The key to dry eye treatment is identifying any underlying causes or contributing factors and treat them directly. Alternatives for any medications that may be contributing to the condition should be considered when possible. Any underlying medical or ocular condition should be treated. If a lid position abnormality is causing the dry eye, surgery should be considered to reposition the eyelid.
In addition, a variety of other steps may be taken to treat dry eye:
- Tear replacement: The use of artificial substances to replace the tears is a key first step in the treatment of dry eye. These are over-the-counter eye drops created to simulate the composition of normal tears.
People with dry eye should use preservative-free artificial tears. Many rewetting solutions, especially those advertised to "get the red out," contain preservatives that are actually toxic to the eye and can make dry eye worse in the long term. Preservative-free artificial tears should be used four or more times a day, although one can never use too many artificial tears.
- Conserving tears: Tears drain from the surface of the eye through tiny holes (one in each upper and lower lid) called puncta. These punctal openings allow tears to drain into the lacrimal (tear) drainage system and ultimately into the nose and throat.
One strategy for the treatment of dry eye is to decrease the drainage of tears through the lacrimal drainage system, so that they remain on the surface of the eye for longer than usual.
This can accomplished by the placement of punctal plugs, which are tiny silicone or collagen plugs. These plugs are inserted into the lower lid punctal opening of each eye, although the upper lids may also need to be treated in more severe cases. This procedure is usually performed in the doctor's office. The puncta can also be closed by laser, heat treatment, or surgery.
- Creating more of your own tears: A medication called Restasis (cyclosporine), available in drop form, has been shown to increase production of tears. This medication can require consistent use for at least two months before it is effective.
- Improving the quality of tears: Omega-3 fatty acids, in the form of fish, fish oil, or flaxseed oils, may be recommended to boost production of tears, especially the lipid layer. Warm compresses, eye drops, eye ointments, or even oral antibiotics may be recommended to treat any blepharitis (inflammation of the eyelids) or meibomian gland dysfunction (glands that secrete the lipid, or oily, layer of tears) that leads to a poor composition of the lipid/oily layer of the tear film.