What is glaucoma?
Glaucoma is a condition in which the nerve of the eye is damaged. The eye functions like a camera, and sends signals to the brain via the optic nerve so that we can see. If the nerve is damaged, vision is affected, even if the other parts of the eye remain normal. In most cases, this damage to the nerve is caused by high pressures in the eye.
How is there pressure in the eye?
Fluid is continuously being produced in the eye in a part called the ciliary body. Most of this fluid then slowly flows out of the eye via another part called the trabecular meshwork. If there is an imbalance, ie. too little outflow (blockage) or too much fluid production then the pressure in the eye will increase as the fluid builds up. Most people have eye pressures between 10-21 mmHg.
Types of Glaucoma
In general terms, we can separate glaucoma into acute and chronic types.
In this form of glaucoma, pressure in the eye rises quickly to very high levels. These pressures are often higher than 40mmHg (twice normal). In this situation the patient will experience sudden onset of severe eye pain and headache on the affected side. Vision in that eye will also be very blur. Sometimes nausea and vomiting occur as well.
Chronic glaucoma does not cause any symptoms in the early stages. There is no pain, no redness, not even a little irritation. As the nerve is progressively damaged, however, vision is affected. Eventually, only 'tunnel vision' may be left, meaning that the patient can only see what is directly ahead. In the most advanced stages, even tunnel vision is lost and the patient may become blind.
Causes and Risk Factors
In most cases, glaucoma arises without an obvious cause. However, it is one of the conditions that tends to become more common with age.
Most cases of acute glaucoma and some cases of chronic glaucoma occur because the front part of the eye (called the anterior chamber) is narrow. This narrowness is partly inherited, however it tends to become worse with age because of the development of cataract.
Other less common causes of glaucoma include the sometimes seen side effect of medications like steroids, eye injury, and eye inflammation (uveitis).
How is glaucoma detected?
The most important tests in glaucoma are tonometry (measurement of eye pressure), assessment of the nerve (dilated fundus examination) and visual field testing.
In its early stages, it may be difficult to say for certain that a patient has glaucoma. For example, the eye pressure may be only slightly above 'normal', and the nerve looks healthy and visual fields are normal. These patients may not be started on treatment right away. Instead, they are monitored for a period of time to look for any deterioration in the visual field or increase in the eye pressure.
Other tests may be helpful in monitoring glaucoma. These include optical coherence tomography to assess the nerve fibers.
The main aim of treatment in glaucoma is to lower the eye pressure. Sometimes, the eye pressure may be in the 'normal' range of 10-21mmHg, but the nerve continues to deteriorate. In such cases, the doctor tries to lower the eye pressure further still. Lowering the eye pressure of most glaucoma patients starts with eyedrops. These medicines may either reduce aqueous production, or improve its drainage from the eye.
In cases of glaucoma where the front of the eye is narrow, a laser procedure called a laser peripheral iridotomy may be performed. This is a simple procedure performed in the clinic and may take 15 minutes to complete. It helps to improve fluid flow out of the eye despite the narrowness of the anterior chamber.
In some patients, medical or laser treatment fail to lower the eye pressures, necessitating surgery. Surgery may take one of a few forms.
In the simplest situation, where a patient has narrow anterior chambers, cataract and the condition has only developed recently, cataract surgery alone may solve the problem as that immediately widens the front of the eye.
In other patients where medications fail to lower the eye pressure sufficiently, an operation to create a new drainage channel for fluid in the eye needs to be created. This operation is called a 'trabeculectomy'. In some patients where trabeculectomies have previously been performed but eye pressures have increased again, silicone drainage tubes may be implanted for better fluid flow out of the eye and pressure control.
The ExPRESS Shunt
Recently, the ExPRESS mini glaucoma shunt has become available for glaucoma surgery. This device is a very small metal tube that is permanently implanted under a scleral flap to improve the drainage of fluid from the eye. Compared to traditional trabeculectomies, the extent of fluid drainage is more consistent and the internal opening of this drainage channel cannot close because the metal tube projects slightly into the eye.
Glaucoma In The Long Term
Many people with glaucoma are worried about blindness. However, most glaucoma patients do not become blind. The most important factor is control of the eye pressure, which is achieved either with the use of eyedrops or surgery. If eyedrops are regularly instilled, and eye pressures are well controlled, vision is usually well preserved.
However, patients should realise that damage to the nerve before treatment was started cannot be reversed. The aim in glaucoma treatment is to stabilise the condition, and in most patients vision is maintained for life.