dry eyes

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Dry eye or dry eye syndrome (DES) (also known as keratoconjunctivitis sicca , or more recently dysfunctional tear syndrome) is a multifactorial disorder of the tear film and ocular surface which is associated with symptoms of ocular discomfort.

  • definition of the condition has changed over time with the progress of  understanding the disease process e.g. –
    • in 1995 the National Eye Institute defined DES as

    “a disorder of the tear film due to tear deficiency or excessive tear evaporation which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort.”

    • in 2007, the International Dry Eye Work Shop defined DES as

    “a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, 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” (1,2)

Tear film is an essential component of the ocular surface which covers the cornea and exposed conjunctiva. It consists of 3 components or layers:

  • innermost mucin or mucus layer
    • thinnest layer produced by cells of conjunctiva
    • helps the overlying watery layer to spread evenly over the eye
  • middle or aqueous layer  
    • largest and thickest layer produced by the glands of upper lids and the accessory tear glands
    • keeps the eye moist and helps in the removal of any dust, debris, or foreign particles
    • defects of this layer cause DES in most cases
  • uppermost lipid layer
    • produced by the meibomian glands and the glands of Zeis (oil glands in the eyelids)
    • helps to decrease evaporation of the watery layer beneath it.

A disturbance of the Lacrimal Functional Unit (LFU) results in dry eye syndrome.

  • LFU consists of  - lachrymal glands, ocular surface including cornea, conjunctiva, eyelids, meibomian glands, ocular nerves, and goblet cells
  • dysfunction of this unit cause a change of the composition of the tear fluid and tear film stability leading to inlammation of ocular surface (2)


Last reviewed 11/2020