What are the three layers of the tear film?

A healthy tear film has three layers: the outer lipid (oil) layer produced by the meibomian glands, which prevents evaporation; the aqueous (water) layer produced by the lacrimal glands, which makes up about 90% of tear thickness; and the inner mucin layer produced by conjunctival goblet cells, which anchors the tear film to the eye's surface.

What are the different types of dry eye disease?

There are two main types: aqueous-deficient dry eye, where the lacrimal glands fail to produce enough watery tears (roughly 10-15% of cases), and evaporative dry eye, where tears evaporate too quickly, most often due to meibomian gland dysfunction (roughly 85% of cases). In clinical practice, most patients present with a mix of both.

How common is dry eye disease?

Dry eye disease affects over 16 million diagnosed adults in the United States alone, with global adult prevalence estimated between 5% and 50%. It is about twice as common in women as in men, and remains widely underdiagnosed because symptoms are often mistaken for allergies, fatigue, or normal aging.

Understanding Dry Eye Disease

What Is Dry Eye Disease?

Dry eye disease (DED), also known as keratoconjunctivitis sicca, is a multifactorial condition of the ocular surface. In practical terms, it occurs when your eyes either don't produce enough tears, or when the tears that are produced evaporate too quickly or are of poor quality — leading to inflammation, surface damage, and a range of uncomfortable symptoms.

TFOS DEWS II Definition

"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."

Tear Film & Ocular Surface Society Dry Eye Workshop II (TFOS DEWS II)

This modern definition reflects a key insight: dry eye is not simply a matter of "not enough tears." It is a complex, self-perpetuating cycle of inflammation and surface damage that requires a structured, evidence-based approach to diagnose and treat effectively.

The Tear Film: Three Essential Layers

A healthy tear film is a precisely balanced structure — approximately 3 micrometers thick — that protects, nourishes, and lubricates the ocular surface. It is composed of three interdependent layers, each produced by different glands and structures around the eye.

Lipid (Oil) Layer — The Outermost Seal

Produced by the meibomian glands in the eyelids (30–40 in the upper lid, 20–30 in the lower). Prevents the aqueous layer from evaporating. When these glands become blocked — meibomian gland dysfunction (MGD) — tear evaporation accelerates dramatically. MGD is the leading cause of dry eye and is present in the majority of cases.

Aqueous (Water) Layer — The Middle Core

Makes up ~90% of the tear film's thickness. Produced by the lacrimal glands. Contains water, electrolytes, proteins, antibodies, and growth factors. Washes away debris, delivers oxygen to the cornea, and provides antimicrobial defence. Deficiency leads to aqueous-deficient dry eye, commonly associated with autoimmune conditions like Sjögren's syndrome.

Mucin Layer — The Inner Anchor

Produced by goblet cells in the conjunctiva. Acts as a bridge between the hydrophobic corneal surface and the aqueous tears above it, allowing the tear film to spread evenly with each blink. Without adequate mucin, the tear film breaks up rapidly — creating dry spots even when tear volume is sufficient.

Clinical assessment showing dry eye evaluation with diagnostic imaging

Advanced diagnostic imaging allows clinicians to assess the health of the meibomian glands and tear film stability objectively.

Types of Dry Eye Disease

10–15%

Aqueous-Deficient Dry Eye

The lacrimal glands fail to produce sufficient aqueous tears. Can be Sjögren's syndrome-related (autoimmune) or non-Sjögren's (age-related decline, medications). Often presents with very low Schirmer's test scores and severe symptoms.

~85%

Evaporative Dry Eye

By far the more common type. The tear film evaporates too rapidly — most often due to MGD. Other causes include incomplete blinking, reduced blink rate from screen use, contact lens wear, and environmental factors.

Most Common in Practice

Mixed Dry Eye

In clinical practice, most patients present with a combination of both aqueous-deficient and evaporative mechanisms. Modern treatment approaches aim to address both components simultaneously for the best outcomes.

In clinical practice, most patients present with a combination of both mechanisms — which is why a structured, comprehensive evaluation is essential before starting treatment.

Prevalence & Impact

16M+

diagnosed adults in the US alone

5–50%

global adult prevalence range

2×

more common in women than men

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. Patients report difficulty with reading, driving at night, working on computers, and enjoying outdoor activities.

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 self-perpetuating cycle of inflammation and damage that characterises progressive dry eye disease.

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