What Is Dry Eye Disease?
Dry eye disease (DED), also known as keratoconjunctivitis sicca, is a multifactorial condition of the ocular surface characterized by a loss of homeostasis of the tear film. In simpler terms, it occurs when your eyes either do not produce enough tears or when the tears that are produced evaporate too quickly or are of poor quality. This leads to inflammation, damage to the ocular surface, and a range of uncomfortable symptoms that can significantly impact daily life.
The Tear Film and Ocular Surface Society's Dry Eye Workshop II (TFOS DEWS II) defines dry eye as "a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles." This modern definition reflects our growing understanding that dry eye is not simply a matter of "not enough tears."
The Tear Film: Three Essential Layers
A healthy tear film is a complex, precisely balanced structure that protects, nourishes, and lubricates the ocular surface. It is approximately 3 micrometers thick and composed of three interdependent layers, each produced by different glands and structures around the eye.
The Lipid (Oil) Layer
The outermost layer of the tear film is the lipid layer, produced by the meibomian glands embedded in the upper and lower eyelids. This ultra-thin oily layer serves a critical function: it prevents the underlying aqueous layer from evaporating too quickly. There are approximately 30-40 meibomian glands in the upper lid and 20-30 in the lower lid. When these glands become blocked or dysfunctional -- a condition known as meibomian gland dysfunction (MGD) -- the lipid layer thins, and tear evaporation accelerates dramatically. MGD is the leading cause of evaporative dry eye and is present in the majority of dry eye cases.
The Aqueous (Water) Layer
The middle and thickest layer is the aqueous layer, produced primarily by the lacrimal glands located above each eye. This layer makes up approximately 90% of the tear film's thickness and contains water, electrolytes, proteins, antibodies, and growth factors. It serves multiple functions: it washes away debris and foreign particles, delivers oxygen and nutrients to the avascular cornea, and provides antimicrobial defense. A deficiency in this layer leads to aqueous-deficient dry eye, which is commonly associated with autoimmune conditions like Sjogren's syndrome.
The Mucin Layer
The innermost layer is the mucin layer, produced primarily by goblet cells in the conjunctiva. This layer acts as a bridge between the hydrophobic corneal surface and the aqueous tears above it, allowing the tear film to spread evenly across the eye with each blink. Without adequate mucin, the tear film cannot adhere to the ocular surface and breaks up rapidly, creating dry spots even when tear volume may be sufficient. Conditions that damage the conjunctival goblet cells, such as chemical burns, chronic inflammation, or certain medications, impair mucin production and contribute to tear film instability.
Types of Dry Eye Disease
Aqueous-Deficient Dry Eye
This type occurs when the lacrimal glands fail to produce sufficient aqueous tears. It accounts for approximately 10-15% of dry eye cases. It can be further classified as Sjogren's syndrome-related (caused by autoimmune destruction of the lacrimal glands) or non-Sjogren's (caused by age-related lacrimal gland decline, medications, or other factors). Patients with aqueous-deficient dry eye often have very low Schirmer's test scores and may experience severe symptoms including pain and corneal complications.
Evaporative Dry Eye
Evaporative dry eye is by far the more common type, accounting for approximately 85% of all dry eye cases. It occurs when the tear film evaporates too rapidly, most often due to meibomian gland dysfunction. Other causes include incomplete blinking, reduced blink rate (as seen with extended screen time), contact lens wear, and environmental factors. In evaporative dry eye, the lacrimal glands may produce adequate aqueous tears, but without a healthy lipid layer to seal in the moisture, those tears evaporate before they can adequately protect the ocular surface.
Mixed dry eye: In clinical practice, most patients present with a combination of both aqueous-deficient and evaporative mechanisms. This "mixed" presentation is so common that modern treatment approaches aim to address both components simultaneously for the best outcomes.
Prevalence and Impact
Dry eye disease is remarkably common. Global prevalence estimates range from 5% to 50% of the adult population, depending on the diagnostic criteria used and the population studied. In the United States alone, an estimated 16 million adults have been diagnosed with dry eye, and millions more are believed to have undiagnosed disease. The condition is more prevalent in women, older adults, and Asian populations.
The impact on quality of life should not be underestimated. Research has shown that the burden of moderate-to-severe dry eye on daily functioning is comparable to that of moderate angina (chest pain). Patients report difficulty with reading, driving (especially at night), working on computers, and enjoying outdoor activities. The economic burden is also substantial, with direct and indirect costs estimated at billions of dollars annually in the United States alone, including lost productivity, frequent eye care visits, and ongoing treatment expenses.
Despite its prevalence, dry eye disease remains widely underdiagnosed. Many patients attribute their symptoms to allergies, fatigue, or normal aging and delay seeking care for years. Early diagnosis and treatment are essential to prevent the cycle of inflammation and damage that characterizes progressive dry eye disease.