Original articleThe Ocular Surface and Tear Film and Their Dysfunction in Dry Eye Disease
Section snippets
Regulation of Tear Production
The production of tears is regulated through a reflex loop (Fig. 1). The ocular surface (conjunctiva, cornea, accessory lacrimal glands, and meibomian glands) and main lacrimal glands act with this neuronal loop to regulate the production of tears necessary for ocular surface homeostasis and repair.64 Stimulation of nerves at the ocular surface or in the nasal mucosa sends impulses to the brain via the fifth cranial nerve, which generate a reflex response via nerves passing to the lacrimal
Composition of the Tear Film
Tears are a complex solution composed of water, enzymes, proteins, immunoglobulins, lipids, various metabolites, and exfoliated epithelial and polymorphonuclear cells. Because of the highly dynamic characteristics of the tear film, defining its exact composition at a particular point in time is impossible. Its specific content will vary depending upon the challenges with which the ocular surface has to deal.
The tear film is composed of three main components (Fig. 2), each of which has been
The Conjunctiva and Cornea
Although the tear film provides the primary source of protection for the eye against chemical, mechanical, bacterial, and viral attack, the corneo-conjunctival surface represents the last barrier preventing penetration of the deeper ocular structures. The epithelium of the conjunctiva and cornea is, therefore, a key component of the defense mechanisms of the eye. Although the cornea and conjunctiva are exposed in a similar manner to the outside environment, the defensive mechanisms of the two
Dysfunction of the Ocular Surface and Tear Film
Deficiencies in any of the tear film layers, defective spreading of the tear film, systemic diseases, and some systemic and topical medications can disturb the ocular surface or tear film and cause dry eye disease.
Collagen Vascular Disorders (Collagenoses)
This category contains various autoimmune disorders, including the following.
Rheumatoid arthritis
Most patients with rheumatoid arthritis have some degree of dry eye disease. It is thought that lymphocytic infiltration of the lacrimal gland results in reduced tear secretion.
Scleroderma
Sicca syndrome has been described in 70% of patients with scleroderma. Patients with the limited form of the disease (CREST syndrome [calcinosis, Raynaud's phenomenon, esophageal hypomotility, sclerodactyly, and telangiectasia]) tend to have
Allergies
Dry eye is a common consequence of chronic allergic conjunctivitis due to continuing activation of the local immune system. A vicious cycle of irritation and ocular surface damage may cause dry eye symptoms to persist for months after the allergic episode.5
Mucocutaneous syndromes
One of the most severe forms of dry eye is seen in the course of Stevens–Johnson syndrome. Chronic mucocutaneous syndromes, such as cicatricial pemphigoid, also often lead to severe loss of goblet cells and dry eye in late stages of the
The Vicious Cycle of Increased Evaporation and Ocular Surface Damage
The causes of increased evaporation of water from the tear film can be found in any of the quantitative or qualitative changes occurring on the ocular surface or in the tear film itself that affects the formation and the spreading of a normal oily layer. This feature does not indicate that a primary abnormality of the oily layer is the only cause of tear film disruption.
As indicated, many other factors that are able to destroy the delicate architecture of the tear film will increase
Acknowledgements
The authors have no proprietary or commercial interest in any product or concept discussed in this article.
References (76)
- et al.
Correlation of tear fluorescein clearance and Schirmer test scores with ocular irritation symptoms
Ophthalmology
(1999) The pathology of dry eye
Surv Ophthalmol
(2001)- et al.
Immunologic defense mechanisms of the ocular surface
Ophthalmology
(1983) - et al.
Biosynthesis and partial characterization of tear film glycoproteins. Incorporation of radioactive precursors by human lacrimal gland explants in vitro
Exp Eye Res
(1980) - et al.
Management of conjunctival cicatrizing diseases and severe ocular surface dysfunction
Surv Ophthalmol
(1998) Blinking and the mechanics of the lacrimal drainage system
Ophthalmology
(1981)Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink
Am J Ophthalmol
(1980)- et al.
The effects of experimental tear film removal on corneal surface regularity and barrier function
Ophthalmology
(2000) - et al.
Morphologic effect of hyperosmolarity on rabbit corneal epithelium
Ophthalmology
(1984) Formation and rupture of the tear film
Exp Eye Res
(1973)
Basic tear flow. Does it exist?
Ophthalmology
A standardized visual scale for evaluation of tear fluorescein clearance
Ophthalmology
The oily layer of the tear film and evaporation from the corneal surface
Exp Eye Res
Autoimmune phenomena of the external eye
Ophthalmology
Conjunctival cytologic features of primary Sjogrens syndrome
Ophthalmology
Tear evaporimeter for measuring water evaporation rate from the tear film under controlled conditions in humans
Exp Eye Res
Tear production in vitamin A-responsive xerophthalmia
Am J Ophthalmol
Oculocutaneous manifestations of the erythema multiforme/Stevens-Johnson syndrome/toxic epidermal necrolysis spectrum
Dermatol Clin
Sjogrens syndrome in rheumatoid arthritis and progressive systemic sclerosis. A comparative study
Clin Exp Rheumatol
Sjogrens syndrome in systemic lupus erythematosus
J Rheumatol
Hyaluronan, CD44 and fibronectin in rabbit corneal epithelial wound healing
Jpn J Ophthalmol
Flow cytometric analysis of inflammatory markers in conjunctival epithelial cells of patients with dry eyes
Invest Ophthalmol Vis Sci
Non-Sjogren dry eye. Pathogenesis and diagnosis and animal models
Measurement of centripetal migration of normal corneal epithelial cells in the mouse
Invest Ophthalmol Vis Sci
Cell migration in repair of mouse corneal epithelium
Invest Ophthalmol Vis Sci
Effects of conjunctival hyperosmolar challenge in allergic subjects and normal controls
Int Arch Allergy Immunol
Alteration of mucin in human conjunctival epithelia in dry eye
Invest Ophthalmol Vis Sci
Structure and function of the tear film
Adv Exp Med Biol
Contribution of the epithelium to the stability of the tear film
Trans Ophthalmol Soc UK
The dry eyeits mechanisms and therapy, with evidence that contact lens is a cause
CLAO J
Fatty layer of the precorneal film in the ‘office eye syndrome’
Acta Ophthalmol (Copenh)
Prevalence of objective eye manifestations in people working in office buildings with different prevalences of the sick building syndrome compared with the general population
Int Arch Occup Environ Health
Changes in rabbit lacrimal gland fluid osmolarity with flow rate
Invest Ophthalmol Vis Sci
Ocular surface drying and tear film osmolarity in thyroid eye disease
Acta Ophthalmol (Copenh)
Osmolarity of tear microvolumes in keratoconjunctivitis sicca
Arch Ophthalmol
Cellular origin of mucins of the ocular surface tear film
Adv Exp Med Biol
Expression of human beta-defensins in intraocular tissues
Invest Ophthalmol Vis Sci
The eye in bone marrow transplantation. I. Clinical study
Arch Ophthalmol
Cited by (292)
Establishment of an LC-MS/MS method for quantification of lifitegrast in rabbit plasma and ocular tissues and its application to pharmacokinetic study
2023, Journal of Chromatography B: Analytical Technologies in the Biomedical and Life SciencesAn injectable thermo-sensitive hydrogel of PNICL-PEG-PNICL block copolymer as a sustained release carrier of EGCG
2023, European Polymer JournalCurrent progress in preservative-free topical ophthalmic formulations
2023, Journal of Drug Delivery Science and TechnologyRecent advances in superlubricity of liposomes for biomedical applications
2022, Colloids and Surfaces B: Biointerfaces