Keratoconjunctivitis sicca (KCS) is often used synonymously with the general term ‘dry eye’. KCS is applied equally to disorders involving insufficient tear production and those associated with excessive evaporation of tears.
In KCS, the reduction in the aqueous component of the precorneal tear film is due to reduced tear production or increased tear evaporation.
In 2007, the Report of the Dry Eye Workshop (DEWS) assigned Dry Eye to the following major classes:
- Aqueous tear-deficient dry eye (ATDDE), in which there is a disorder of lacrimal function or a failure of transfer of lacrimal fluid into the conjunctival sac. Lacrimal disease is also associated with a reduction in secreted lacrimal proteins. ATDDE is the largest category of dy eye and is more common in women than in men.
- Evaporative dry eye (EDE), in which lacrimal function is normal. This is often the result of an abnormalitiy in the lipid layer. (It has been shown that tear film evaporates 20 times faster if there is no lipid layer).
AQUEOUS TEAR-DEFICIENT DRY EYE
Aqueous tear-deficient dry eye occurs as a result of defective lacrimal function. This is usually demonstrated by showing reduced aqueous tear volume and tear flow. The standard measure is the Schirmer test, but there are other indicators of reduced tear produciton, such as the lacrimal thread test and fluorophotometry. Tests that show reduced secretion of lacrimal proteins (the lysozyme test and lactoferrin test) are also useful, as it has been suggested that deficiency of lacrimal proteins may be the earliest sign of TDDE.
Aqueous tear-deficient dry eye can be further classified into Sjögren's syndrome dry eye (SSDE) and non-Sjögren's dry eye (NSDE). In NSDE, there are none of the systemic signs of clinical manifestations of autoimmune disease, which are the hallmarks of SSDE.
Dry eye Workshop (DEWS) classification of dry eye (2007)*
*Lemp MA. The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry eye Workshop (DEWS). The Ocular Surface, April 2007, Vol. 5, NO. 2: 75-91 (www.theocularsurface.com)
SJÖGREN'S SYNDROME TEAR DEFICIENCY
Sjögren's syndrome is a disease that is characterised by a slow, immune-mediated destruction of exocrine glands. The syndrome is classified into primary or secondary Sjögren's syndrome.
Primary Sjögren's syndrome occurs in isolation and affects the lacrimal and salivary glands. It consists of the features of tear-deficient dry eye in combination with a dry mouth and the presence of auto-antibodies.
Secondary Sjögren's syndrome has the same features as primary Sjögren's syndrome, but in conjunction with the overt clinical manifestations of an autoimmune connective tissue disease such as rheumatoid arthritis or systemic lupus erythematosis (SLE). Polymyositis, scleroderma, primary biliary cirrhosis, chronic active hepatitis and coeliac disease are other less common associations.
Sjögren's syndrome is more common in women than in men. The age of onset is usually in the fourth or fifth decade of life, although it can occur at any age, including childhood. It is not known what causes the condition, but one hypothesis is that a viral infection (such as Epstein Barr virus, hepatitis C virus or cytomegalovirus) may trigger an inappropriate immune response, resulting in a focal infiltrate of destructive lymphocytes in the affected organs. Sjögren's syndrome is diagnosed using the San Diego criteria: KCS, dry mouth (xerostomia) and either a characteristic minor salivary gland biopsy or characteristic auto-antibodies.
NON-SJÖGREN TEAR DEFICIENCY
NSTD is divided into three different forms:
Primary lacrimal deficiency (PLD) is either congenital or acquired. Acquired PLD is sometimes referred to as non-Sjögren KCS. PLD is more common in women and its frequency increases with age. It results from a gradual destruction of lacrimal gland and ductal tissue. Secondary lacrimal deficiency can occur in the presence of diseases such as sarcoidosis, lymphoma and HIV infection, as well as following the removal (ablation) of the lacrimal gland.
Cicatrising conjunctival disease causes aqueous tear deficiency by scarring the orifices of the orbital and accessory lacrimal glands. Disorders that can cause conjunctival scarring and are associated with dry eye are trachoma, cicatricial pemphigoid, erythema multiforme and chemical and thermal burns.
Tear secretion is mainly reflex in origin. Any factor that reduces corneal sensory function facilitates drying by two mechanisms: sensory loss causes decreased tear secretion and, when bilateral, reduces the blink rate. Loss of corneal sensation (hypoesthesia) is a feature of contact lens wear and has been proposed as a mechanism for dry eye associated with long-standing contact lens use, particularly among hard and extended wear contact lens users.
A loss of corneal sensitivity is also experienced following refractive surgery. PRK is performed with a laser that uses a cool ultraviolet light beam to precisely remove (ablate) very tiny pieces of tissue from the surface of the cornea in order to reshape it, so that it can focus light into the eye more effectively. During LASIK, a knife is used to cut a flap in the cornea and a hinge is left at one end. The flap is folded back to reveal the stroma (the middle section of the cornea). Pulses from the computer-controlled laser vaporise a portion of the stroma and the flap is replaced. However, when the flap is made, some of the corneal nerves are severed. In both of these surgical procedures, there is a loss of nerve endings that leads to a decrease in corneal sensitivity and ultimately causes dry eye lasting up to six months following surgery.
Other causes of reflex hyposecretion including neurotrophic keratitis and VII nerve palsy are presented in the table below:
Causes of ocular sensory loss
|Infective||Herpes simplex keratitis
Herpes zoster ophthalmicus
|Corneal surgery||Limbal incision (extra-capsular cataract extraction)
Refractive surgery (PRK, LASIK, RK)
|VII nerve / ganglion section / injection / compression
Chronic contact lens wear