The dry eye disease is recognised as a disturbance of the lacrimal functional unit, which is an integrated system comprising of the:
- lacrimal glands
- ocular surface (cornea, conjunctiva and meibomian glands)
- sensory and motor nerves that control the blink reflex
For therapeutical and diagnostical reasons the Dry Eye (Keratoconjunctivitis Sicca) is divided into 2 main categories:
1, Hyposecretoric dry eye
The hyposecretoric form of dry eye is characterized of a reduced tearflim secretion. The water compound is the main part of the tearfilm with aproximately 98%. The aqueous layer consists of different peptides and proteins and covers the ocular surface with a homogenous layer. Due to a reduced volume of the aqueous layer a brake up of the tearfilm is resulting. Also an increased osmolarity of the tearfilm may occur, which leads to activation of inflammatory processes and release of cytokines enhancing symptoms and severity of the disease.
A hyposecretoric dry eye can also be the result of an inflammation within the lacrimal gland itself. In this case the inflammatory mediators may affect the ocular surface directly from the lacrimal glands. The hyposecretoric form of dry eye can be divided in an extrinsic and an intrinsic form. The extrinsic form is caused by environmental factors. Intrinsic means due to the condition of eyelid or ocular surface.
2, Hyperevaporative dry eye
The main cause, with an occurance of roundabout 80%, of dry eye disease is called blepharitis leading to an hyperevaporative dry eye. The hyperevaporative form of dry eye disease is characterized of an increased loss of the aqueous layer, with a normal production of the lacrimal glands (watery compound). The most common reason of a hyperevaporative dry eye is a dysfunction of the meibomian glands (due to blepharitis). These glands are producing the lipid compound of the tearfilm. The reasons of a meibomian gland dysfunction can be very diverse but mainly are caused due to rosacea or other skin related diseases. Skin mites like, the very common, "demodex" can play a role as well.
The secret of the meibomian glands consists of polar and apolar lipids, which are characterized of a specific physical melting point. Due to a dysfunctional meibomian gland this melting point changes, resulting in an obstruction of the gland exits and, if not treated properly, a longterm loss of the glands results. Due to inceased evaporation of the tearfilm a hyperosmolarity occures as well, leading to inflammatory processes on the ocular surface and to dry eye symptoms.