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Chapter 1. Dry Eye Disease
KEY LEARNING
Dry eye disease
• Group of disorders of the tear
film and ocular surface
• Due to reduced tear production
or excessive tear evaporation
• Associated with ocular
discomfort and visual symptoms
• Potentially damaging to the
ocular surface, especially the
cornea
• Associated with increased
osmolarity of the tear film
• Associated with inflammation
of the ocular surface
Chapter 1. Dry Eye Disease
DEFINITION OF DRY EYE
FORMS OF DRY EYE
As we previously learned, dry eye disease is a group of disorders of the tear film and
ocular surface that are due to reduced tear production or excessive tear evaporation. In
either case, dry eye disease, or keratoconjunctivitis sicca, is associated with ocular
discomfort, visual symptoms, and tear film instability with potential damage to the ocular
surface. It is accompanied by increased osmolarity of the tear film and inflammation of
the ocular surface.1 Dry eye is very common in the United States, affecting approximately 14% of people over the age of 40, or over 20 million Americans.2, 3
Classifying Dry Eye Disease
In 2007, the Tear Film and Ocular Surface Society published an extensive report of the
International Dry Eye WorkShop (DEWS).1 The DEWS classification of major causes of
dry eye is summarized in Figure 2.
KEY LEARNING
The 2 major classes of dry eye are insufficient tear production, or
aqueous-deficient dry eye, and excessive tear evaporation, or evaporative dry eye.
These classes are not mutually exclusive—they may coexist or interact with each other
in ways that can increase the severity of dry eye.1 To further emphasize that point,
recently the American Academy of Ophthalmology made modifications to the original
DEWS classification through their Preferred Practice Patterns publication, where they
specifically illustrate the “combination” classification of dry eye. It is worth noting that
Sjögren’s syndrome can be classified under either aqueous-deficient dry eye or
combination dry eye as it routinely impacts all layers of the tear film, not just the
aqueous layer.
• Lipid layer—composed of oil
secreted by the meibomian
glands of the eyelids
Tear Film and the Ocular Surface
Let’s take a moment to review the tear film and the ocular surface because they are
integral to the understanding of dry eye. Tears help maintain eye health and clear vision
by bathing the surface of the eye, keeping it moist, and washing away dust and debris.
Tears also help protect the eye from bacterial and other types of infections and provide
nourishment for the outer layers of the cornea, the ocular structure most vulnerable to
the effects of dry eye.4
The tear film is composed of 3 layers (Figure 1).4, 5
L
ipid Layer
The lipid layer, the outermost
portion of the tear film, is produced
by the meibomian glands. Its
principal function is to prevent tears
from evaporating too quickly.6
Lipid Layer
Aqueous Layer
Mucin Layer
Aqueous Layer
The aqueous layer, the middle layer
FIGURE 1. THE 3 LAYERS OF
of the tear film, is produced by the
THE TEAR FILM
lacrimal gland. Composed of water
and approximately 60 different proteins
and electrolytes, the aqueous layer helps to maintain the normal ocular surface,
protect against infection, and promote corneal wound healing.
M
ucin Layer
The mucin layer, the innermost layer of the tear film, is produced by goblet cells
in the conjunctiva. Mucin, the chief component in mucus, attracts water, allowing
the aqueous layer to spread evenly over the corneal epithelium. It also moves about
freely in the tear film, picking up dirt, debris, and pathogens to keep the ocular
surface clean. Mucin also provides a smooth lubricating surface on the cornea,
preventing pathogens from adhering and allowing the eyelids to glide over the
cornea with minimal friction during blinking.
The tear film is composed of
3 layers.
• Aqueous layer—watery
component of tears produced
by the lacrimal gland
• Mucin layer—secreted by the
goblet cells of the conjunctiva
Aqueous-Deficient Dry Eye
In aqueous-deficient dry eye, the lacrimal gland does not produce enough tears to keep
the entire conjunctiva and cornea covered by a complete layer of tears.4, 7 This type of
dry eye usually occurs in otherwise healthy individuals and is the most common type
found in postmenopausal women.7 Decreased aqueous production is also associated
with increased age.5
The reduced water volume in aqueous-deficient dry eye causes a relative increase in
concentration of the proteins and electrolytes in the tears, or hyperosmolarity, which
in turn causes hyperosmolarity of the epithelial cells on the ocular surface. This process
triggers a cascade of inflammatory events and the generation of inflammatory mediators
on the ocular surface.1
Aqueous-deficient dry eye has 2 major subclasses—Sjögren’s syndrome dry eye and
non-Sjögren’s syndrome dry eye.1 Sjögren’s syndrome, which we will discuss in depth
in Chapter 2, is a disease in which the lacrimal and salivary glands are targeted by an
autoimmune process; other organs are also affected.
Dry Eye
Aqueous Deficient
Sjögren’s Dry Eye
Combination
Non-Sjögren’s Dry Eye
Lacrimal Deficiency
Primary
Secondary
Lacrimal Gland Duct Obstruction
Reflex Block
Systemic Drugs
Evaporative
Intrinsic
Meibomian Oil
Deficiency
Disorders of Lid
Aperture
Extrinsic
Vitamin A Deficiency
Topical Drugs Preservatives
Contact Lens Wear
Low Blink Rate
Ocular Surface Disease e.g. allergy
Drug Action e.g. isotretinoin
FIGURE 2. MAJOR ETIOLOGICAL CAUSES OF DRY EYE1
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F O R S A L E S T R A I N I N G O N LY. N O T TO B E D I S T R I B U T E D O R U S E D I N S A L E S / P R O M O T I O N A L D E TA I L I N G .
Dry Eye, Sjögren’s Syndrome, and Sjö
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