Trainee Optometrist Danya, gives the technical break-down on the 2 different types of Dry Eye.
Photo by Aleksandr Khomenko
TYPE 1: Evaporative Dry Eye
Evaporative dry eye is the extreme loss of water from the ocular surface. It is split into intrinsic (factors in the eye) and extrinsic (external factors affecting the eye)
Intrinsic factors
Meibomian oil deficiency – this is most common cause of evaporative dry eye and its generally known as meibomian gland dysfunction (MGD). The meibomian gland is found in our eye lids and it secretes essential lipid (ref figure 1) which keeps the eyes lubricated and it also stabilises the tear film. The meibomian glands can become infected, dysfunctional or blocked (the oils solidify in the glands). In these cases, the tear film will lose its stability; causing the aqueous portion of the tear film to evaporate.
Disorders of lid apertures – the palpebral aperture is the opening of the eyes and bigger apertures tend to have higher rates of aqueous evaporation. Lid deformity can also lead to exposure and poor tear film resurfacing.
Low blink rate – this could be because of using screens or due to damaged nerves on the surface of the eye. This means the surface of the eye is exposed for longer periods, causing the tears to evaporate more readily.
Extrinsic factors
Vitamin A deficiency – Vitamin A is essential for the mucous membrane of the eyes, which produces the mucin component of the tear film. Vitamin A deficiency may also cause damage in the lacrimal gland which produces the tears.
Topical drugs and preservatives – many topical drugs contain preservative chemicals (e.g. Benzalkonium chloride BAC) to increase their shelf life. However, these preservatives in high quantities can cause a toxic response, which damages the ocular surface of the eye; causing dry eye.
Contact lens wear – Contact lens wearers are 5x more likely than spectacle wearers to report dry eye symptoms. As developed as contact lenses currently are, they still may interfere with the tear film. They can contribute to changes in the lipid layer which leads on to evaporative dry eye.
Ocular surface disease – Allergic eye disease is most common in this category. It causes allergic conjunctivitis which is the allergic inflammation of a thin membrane that covers the front of the eye and inside layer of eyelids. It can cause irregularities of the ocular surfaces leading to tear film instability. It can also be associated with meibomian gland dysfunction and lid apposition (due to swelling of eyelids), further exacerbating dry eye.
TYPE 2: Aqueous Deficient Dry Eye
Aqueous deficient is essentially not enough tear flow to keep the eyes hydrated. This is separated into Sjogren and non-Sjogren dry eye.
Sjogren syndrome dry eye
This is an autoimmune disorder that affects parts of the body that produce fluids and its commonly characterised by dry eyes and a dry mouth. Dry eye associated with Sjogren is separated into primary (the lacrimal gland which produces tears is affected) and secondary.
Non-Sjogren syndrome dry eye
Lacrimal deficiency – this means not enough tears are produced by the lacrimal gland and can be caused by age.
Lacrimal gland obstructions – scars caused by conditions like trachoma or even chemical burns can obstruct the ducts that produce tears.
Reflex block – this describes a reduction in sensory feedback to the lacrimal duct to produce tears. When the eyes are open, exposed nerves in the eye send sensory feedback to the lacrimal gland to produce tears. However, if these nerves aren’t working correctly then these glands will not produce the tears and the blink rate of the eye will reduce (this causes tears to evaporate – evaporative dry eye).
Systemic drugs – certain systemic drugs can reduce lacrimal secretion:
• Antihistamines
• Betablockers
• Diuretics
• Antispasmodics
• Tricyclic antidepressants
• Selective serotonin reuptake inhibitors
• Psychotropic drugs
References