Types of Glaucoma
The term Glaucoma refers to a characteristic pattern of damage to the optic nerve which currently cannot be reversed. For all types of glaucoma, early detection and prompt effective treatment is vital to reduce visual damage.
Clinicians will refer to someone as a ‘glaucoma suspect’ if they think the person might be showing early signs of glaucoma but they are not yet sure. Many people suspected of having glaucoma at this stage turn out not to have it at all, but some do develop it in time and it is these people who can benefit the most from timely treatment. If someone has a very high intraocular pressure or very advanced optic nerve damage then the diagnosis of glaucoma is usually straightforward. However sometimes it is not entirely clear whether someone has glaucoma or not. The early signs of glaucoma can be subtle, and many glaucoma patients have a normal pressure.
Primary Open-Angle Glaucoma (POAG) is the most common form of glaucoma in Australia. It is a condition in which the optic nerve is damaged, in most cases by high eye pressure, leading to loss of peripheral vision. The rise in pressure and subsequent optic nerve damage is usually due to impaired drainage of fluid out of the eye. Most patients have no symptoms of the condition as there is no pain and in the early stages, vision seems normal.
Normal tension (or Low tension) glaucoma
Optic nerve damage can occur in people even though they have low or normal eye pressure. Even though eye pressure is in the normal range, there is good evidence to show that lowering the pressure with eye drops and sometimes laser or surgery can help preserve patients’ vision. It is also widely recognised that in about 1/3rd of cases of glaucoma the characteristic optic nerve changes and visual field loss can develop in an eye with normal pressure – this is termed normal tension glaucoma.
Chronic angle-closure glaucoma is the result of an inherited narrowness of the drainage angle of the eye. The angle is located between the iris (the coloured part of the eye) and the trabecular meshwork through which aqueous fluid must drain to leave the eye. It is a common form of glaucoma particularly in Chinese and South East Asians and more often needs laser treatment as well as eye drops. Some people with chronic angle-closure will experience an acute attack.
Acute angle-closure glaucoma is when the pressure inside the eye rapidly increases due to the iris blocking the drain through which fluid travels out from the eye. An attack of acute angle-closure is often severe. Symptoms are pain, nausea, blurred vision and redness of the eye. Immediate medical help should be sought. If treatment is delayed there can be permanent visual damage in a short time. Usually, laser surgery performed promptly can clear the blockage and protect against visual impairment. The narrow angles that lead to angle-closure can be opened by laser surgery, allowing fluid to leave the eye without obstruction.
Childhood glaucoma is a rare form of glaucoma caused by an abnormal drainage system. It can exist at birth or develop later. Parents may note that the child is sensitive to light, has enlarged and cloudy eyes which water excessively. Surgery is usually needed. There are two types of childhood glaucoma:
Secondary glaucomas can develop as a result of other disorders of the eye such as injuries and previous eye conditions. The prolonged use of cortisone (steroid) medication has a tendency to raise eye pressure and therefore can lead to secondary glaucoma.
Traumatic glaucoma refers to cases in which a direct injury to the eye has led to the development of glaucoma. Blunt or penetrating trauma may cause a raise in pressure inside the eye. This raised pressure may go on to cause glaucoma straight away or take some time. Glaucoma seems to occur in two peak periods, around 3 and 10 years after a traumatic injury. A blow to the eye such as a punch, car accident or head injury can cause damage to the drainage mechanism in the eye.
Steroid Induced Glaucoma
Treatment with steroids can elevate the pressure in the eye. This is termed Steroid-Induced Ocular Hypertension. If there is persistent eye pressure elevation then damage to the optic nerve of the eye can occur with corresponding damage to the peripheral vision, this is termed Steroid Induced Glaucoma. Children, the elderly, persons with glaucoma, glaucoma suspects and immediate relatives of persons with glaucoma are more likely to have a pressure elevation with steroid use. The type of steroid (how potent it is), dosage and mode of administration can affect the steroid response.
Pseudo Exfoliation Glaucoma (PXF)
In some eyes with raised intra-ocular pressure and glaucoma, many tiny white flakes can be seen when viewed through a slit lamp microscope, lying on the edge of the pupil, and on the front surface of the crystalline lens. These white flakes have the appearance of microscopic dandruff and are usually accompanied by a mild dispersion of pigment granules from the back surface of the iris, with accumulation of this pigment in the tissues of the trabecular meshwork. About 50% of the time, only one eye of a patient is affected by the pseudo-exfoliation syndrome. This deposit of white flakes and pigment granules on the trabecular meshwork interferes with the drainage of aqueous fluid from the inside of the eye to the blood vessels on the surface of the globe. The eye pressure then rises. It may do so rapidly and to very high levels. If this occurs damage to the optic nerve fibres and thus to the vision, may occur rapidly.
Pigmentary Glaucoma (PDS)
Pressure can be elevated if the trabecular drainage tissues become blocked by particles. Pigment dispersion syndrome is one example of an underlying eye condition that can lead to such a block. The focusing lens of the eye is held in position by taut scaffolding called the zonule. If the iris (the coloured part of the eye) bows backwards, it can rub against these zonules. The pigment granules, which give the iris its colour, may be dislodged. They flow with the aqueous fluid until the trabecular mesh traps them as the fluid leaves the eye -much like a strainer traps tea leaves. As more and more pigment granules become caught, the trabecular drain works less and less efficiently. The eye pressure starts to rise.
Uveitis is the term used to describe a group of conditions where there is inflammation of one or more layers of the vascular tissues within the eye. The most common form of uveitis, where the front part of the eye experiences the inflammation is also termed ‘iritis’, because it involves the iris (coloured tissue at the front of the eye). Other structures such as the ciliary body (where the clear aqueous fluid comes into the eye) and the choroid (which lies behind the retina) can also be inflamed. Sometimes the inflammation is caused by an infection but more commonly the cause is unknown. The type of uveitis is named based on where the inflammation occurs and if it is in the ciliary body it is called cyclitis and in the choroid it is called choroiditis
Rubeotic (or Neovascular) Glaucoma
Some diseases may result in the eye being starved of blood and oxygen which may lead to a cascade of processes finally resulting in neovascular glaucoma. In Australia probably the most common causes of this are retinal vein occlusion, advanced diabetic eye disease and ocular ischaemic syndrome. In retinal vein occlusion there is a blockage in the main venous drainage from the retina, resulting in very poor circulation. Severe narrowing of retinal arterioles in advanced diabetic retinopathy may also lead to poor delivery of blood and oxygen to the retina. Ocular ischaemic syndrome describes a condition where there is poor delivery of blood to the eye as a whole, usually due to severe narrowing of the carotid artery in the neck.
Mixed Mechanism Glaucoma
Combined mechanism glaucomas refers to an eye with elevated pressure in which both open�angle and angle-closure glaucoma mechanisms are present. Usually the structure of the eye is similar to that found in eyes with angle-closure glaucoma – that is; the globe is somewhat smaller than ideal, with the result that the internal ocular components are crowded together. This means the coloured part of the eye (the iris) is very close to the trabecular drainage tissues, making it difficult for the aqueous fluid to gain access to the drain, and raising the possibility that the iris tissues can completely block the drain.
Iridocorneal Endothelial (ICE) syndrome
The cornea is the clear window in front of the coloured part of the eye (the iris). It has five layers to it and the inner layer is called the endothelium. The endothelium is a single layer of cells that does not regenerate. Its purpose is to pump fluid out of the cornea, effectively preventing it from becoming waterlogged.
In iridocorneal endothelial (ICE) syndrome, there is a dysfunction with this inner layer of cells. Instead of being a single layer of non-regenerating cells, it starts to replicate and spread. This has three main effects in the eye:
Conditions associated with glaucoma
Sturge-Weber Syndrome involves a port-wine birthmark over the face, including the eye. This mark consists of abnormal dilated veins, and those with this syndrome are prone to developing high eye pressure, increasing the risk of glaucoma.
Developmental glaucoma is an umbrella term for a glaucoma due to a relatively rare group of eye disorders characterised by abnormal development of the front part of the eye known as the anterior chamber. People with this underlying anterior segment dysgenesis (ASD) have a significantly increased risk of glaucoma developing. Developmental glaucoma includes the following conditions:
Charles Bonnet Syndrome – It is thought that nearly one third of people with low vision (of any cause) will develop Charles Bonnet Syndrome. In studies of glaucoma, it was found that nearly 1 in 4 (23%) glaucoma patients experienced this phenomenon, which involves visual hallucinations.
View web site www.charlesbonnetsyndrome.org