Primary open angle glaucoma is the most common form of glaucoma. It usually develops slowly and often causes no noticeable symptoms during its early stages.

In this condition, the drainage angle between the iris and cornea remains open but the eye's drainage system becomes less efficient. Fluid gradually builds up, which can increase intraocular pressure and place stress on the optic nerve.

Peripheral vision is usually affected first. Because central vision may remain clear for a long time, many people do not realise they have glaucoma until significant optic nerve damage has already occurred.

Regular eye pressure checks, optic nerve examination, OCT imaging and visual field testing are therefore important for early detection.

Angle closure glaucoma occurs when the drainage angle of the eye becomes narrow or completely blocked, preventing fluid from leaving the eye normally.

It can develop gradually but in some cases the angle closes suddenly and causes a rapid rise in eye pressure. This is known as an acute angle closure attack and requires urgent treatment.

Symptoms may include:

• Severe eye pain
• Sudden blurred vision
• Headache
• Redness of the eye
• Halos around lights
• Nausea or vomiting
• Sudden reduction in vision

Unlike open angle glaucoma, acute angle closure may produce dramatic symptoms and should be treated as an eye emergency.

In normal tension glaucoma, characteristic optic nerve damage and visual field loss occur even though the measured eye pressure remains within what is traditionally considered the normal range.

The exact reason is not always clear. Some people may have an optic nerve that is particularly sensitive to pressure, while reduced blood supply to the optic nerve may also contribute.

Because the eye pressure may appear normal, diagnosis relies heavily on careful examination of the optic nerve, retinal nerve fibre layer and visual field.

Treatment usually focuses on lowering eye pressure further to reduce additional stress on the optic nerve.

Ocular hypertension is not technically glaucoma but it is closely related and deserves monitoring.

In ocular hypertension, eye pressure is higher than normal but there is no detectable optic nerve damage or visual field loss.

Not everyone with raised eye pressure develops glaucoma. The risk depends on several factors, including:

• Level of eye pressure
• Corneal thickness
• Age
• Family history
• Optic nerve appearance
• Other eye and medical conditions

Regular monitoring helps identify whether glaucoma begins to develop over time.

Secondary glaucoma develops because another eye condition, injury, medication or disease interferes with the eye's normal fluid drainage.

Treatment usually involves controlling both the glaucoma and the underlying cause.

Several important forms fall within this category.

Neovascular glaucoma develops when abnormal new blood vessels grow over the iris and drainage angle.

These vessels can block the eye's drainage system and cause a significant increase in eye pressure.

It is often associated with conditions that reduce blood supply to the retina, particularly:

• Advanced diabetic retinopathy
• Retinal vein occlusion
• Other severe retinal vascular disorders

Treatment may involve controlling eye pressure while also treating the underlying retinal disease with injections, retinal laser or other procedures.

Uveitis is inflammation inside the eye. In some patients, inflammation can interfere with the drainage system and increase eye pressure.

Glaucoma may develop because of:

• The inflammation itself
• Scarring inside the drainage angle
• Long term use of steroid medicines used to control inflammation

Managing uveitic glaucoma can be complex because doctors need to control both eye inflammation and intraocular pressure.

Some people experience an increase in eye pressure after using steroid medication.

This may occur with:

• Steroid eye drops
• Oral steroids
• Injections
• Inhaled steroids
• Skin preparations used around the eyes

The response varies from person to person. Patients requiring long term steroid treatment may therefore need regular eye pressure monitoring.

Pigmentary glaucoma occurs when pigment granules from the back surface of the iris are released into the eye.

These particles can accumulate within the drainage system and make it harder for fluid to leave the eye, resulting in increased pressure.

It may develop from a condition known as pigment dispersion syndrome. Not everyone with pigment dispersion develops glaucoma but regular monitoring is important when eye pressure begins to rise.

Pseudoexfoliation syndrome causes abnormal microscopic material to accumulate on structures inside the eye, including the natural lens and drainage system.

When this material blocks fluid drainage and raises eye pressure enough to damage the optic nerve, it is called pseudoexfoliative glaucoma.

Eye pressure in this condition can sometimes fluctuate considerably, making regular monitoring particularly important.

An eye injury can damage the drainage system and cause glaucoma either immediately or many years later.

It may occur after:

• Blunt eye trauma
• Penetrating injuries
• Internal bleeding
• Damage to the drainage angle
• Previous eye surgery

Because glaucoma may develop long after the original injury, anyone with a history of significant eye trauma should inform their ophthalmologist.

Congenital glaucoma is a rare form that develops in infants or young children because the drainage system of the eye has not formed normally.

Possible signs include:

• Excessive tearing
• Light sensitivity
• Cloudiness of the cornea
• Enlarged appearance of the eye
• Frequent eye squeezing or blinking

Unlike most adult glaucoma, childhood glaucoma often requires surgical treatment to improve drainage and protect the developing optic nerve.

Not all angle closure glaucoma develops suddenly. In chronic angle closure, the drainage angle gradually becomes narrower and closes over time.

Patients may have few or no symptoms while optic nerve damage slowly progresses. Gonioscopy and detailed examination of the drainage angle are therefore important in people with narrow angles.

Some patients have more than one mechanism contributing to glaucoma. For example, a person may have both narrowing of the drainage angle and reduced function of the remaining open drainage system.

These cases are described as mixed mechanism glaucoma and treatment is planned according to the combination of factors affecting eye pressure and optic nerve health.