Introduction and module overview
Introduction
At first sight – no pun intended – glaucoma appears a simple disease: a build-up of pressure in the eye due to fluid not being able to drain properly. This damages the optic nerve, which in turn affects vision.
Look more closely, however, and matters are more complicated. Glaucoma is actually a group of eye diseases and diagnosis is not always easy, particularly because many people experience no symptoms in the early stages. This has an impact on its management.
Left undetected and untreated, consequences can be life-changing. Glaucoma has the potential to cause irreversible sight loss and even blindness.
The charity Glaucoma UK says that between 2015 and 2035, the number of people living with glaucoma is expected to increase by 44 per cent – so this is a condition that should be on the radar of all pharmacists and their teams.
With this in mind, this CPD module looks at glaucoma, covering risk factors, symptoms, diagnosis and management, and is intended as a companion to the First Prescription CPD module on the same condition, which will feature information on patient counselling including advice on administering eye drops.
Glaucoma is classified in a variety of ways:
- Age of onset (congenital, infantile, juvenile or adult)
- Cause (primary or secondary)
- Rate of progression (acute, sub-acte or chronic).
The most common distinction between glaucoma types is in the description of the angle between the iris and cornea in the anterior chamber, where they join the sclera.
Types of glaucoma
Angle closure glaucoma arises from the angle between the iris and cornea in the anterior chamber partially or completely closing.
Primary angle closure glaucoma (PACG) can be chronic – accounting for the majority of cases, which are usually asymptomatic for a significant amount of time – or acute.
The latter is rare but it is also a medical emergency, with complete closure of the angle causing an abrupt increase in intraocular pressure (IOP), which is not only painful but can cause complete sight loss in the affected eye.
Anyone who experiences acute PACG has a 40-80 per cent chance of developing acute angle closure of the other eye over the next five to 10 years. PACG can be intermittent, presenting in the acute form but self-limiting and often recurrent.
Secondary PACG is rare and only occurs as a result of eye conditions such as chronic anterior uveitis, which are themselves very uncommon.
Risk factors for PACG include age, female, Asian ethnicity, family history and hypermetropia.
In open angle glaucoma, the anterior chamber angle is normal. Primary chronic open angle glaucoma (COAG) comprises the majority of glaucoma cases, mainly affecting people over 40 years of age and usually involving both eyes, although one eye is often affected to a greater extent.
The condition is usually insidious in onset and chronic in nature. Most patients have raised intraocular pressure (IOP; see panel) due to reduced aqueous humour drainage as a result of age-related changes to the trabecular meshwork structure.
COAG causes visual field loss because the elevated IOP physically presses on retinal ganglion cells as they pass over the head of the optic nerve, although ischaemic damage to the blood vessels supplying the cells also contributes.
Some patients with COAG have normal IOP, which is referred to as normal tension glaucoma (NTG). In these cases, visual loss stems from having optic nerve fibres that are more susceptible to damage. It is also possible to have raised IOP without any signs of glaucoma. This is known as ocular hypertension (OHT) and is a risk factor
for the future development of glaucoma.
Secondary chronic open angle glaucoma is relatively uncommon, but can occur as a result of anything that impedes aqueous humour outflow via the trabecular meshwork, such as diabetic retinopathy or inflammation of the uvea, iris or ciliary body, which causes more fluid to accumulate. Corticosteroid use can also be a cause, more commonly from topical products, but sometimes as a result of other routes of administration.
Risk factors for COAG include raised IOP, age, family history, black ethnicity, corticosteroid use, type 2 diabetes, hypertension, cardiovascular disease and myopia.
What exactly is raised intraocular pressure?
Raised intraocular pressure (IOP) is the most common cause of glaucoma. It refers to the force exerted by the fluid within the eye, which maintains its spherical shape. This fluid – aqueous humour – is produced from plasma by the ciliary body, which is located just behind the iris, a process aided by the enzyme carbonic anhydrase.
Aqueous humour is secreted into the posterior chamber of the eye, the small space between the lens and iris, and then flows into the anterior chamber (between the iris and cornea). A small amount of aqueous humour goes into the venous circulation of the iris, ciliary body, choroid and sclera, but most drains through the trabecular meshwork of the anterior chamber.
Most cases in which IOP increases are due to the eye not being able to get rid of fluid fast enough – often referred to as poor outflow – which results in it building up and hence the pressure rising. An IOP between 11-21mmHg is considered normal.