As readers of Glaucoma News are well aware, it is the level of pressure inside the eye (the intra-ocular pressure or IOP) that is usually the major risk factor for the development of damage to the optic nerve fibres. It is damage to these nerve fibres that leads to the loss of vision from glaucoma. In the normal population, the level of this intra-ocular pressure is within the range of 10 to 20 millimeters of mercury above atmospheric pressure. In the majority of eyes with glaucoma, the pressure rises higher than this, and it is this raised pressure that leads to nerve destruction

There are eyes, however, which suffer nerve damage and loss of sight even though the IOP lies within the normal range. Such eyes are said to suffer from a special type of open angle glaucoma known as "normal tension glaucoma". In the past, terms like "low tension": or "normal pressure" or "pseudo-glaucoma" were used to describe this condition.

Because the pressure in this group of eyes is within the so-called "normal" range, the presence of glaucoma cannot be detected by a simple eye-pressure test. Unless the glaucoma is revealed by further tests to examine the optic nerve and to check the field of vision, considerable damage may occur before the diagnosis is made. Other factors which may predispose to glaucoma, (such as family history of glaucoma, short-sightedness or diabetes), are no different in people with normal tension glaucoma compared with other glaucoma patients.

Dr Mark Loane estimated that up to 30% of patients with open angle glaucoma have normal tension glaucoma (see Glaucoma News Issue 8). They form a most important group. Their evaluation needs to be especially thorough, both to confirm the diagnosis and to exclude other possible conditions that may mimic glaucomatous damage to the optic nerves. These patients are of research interest because they demonstrate that factors other than a very raised eye pressure can cause glaucomatous nerve damage. Much research is going on round the world to identify these factors so they can be treated, and in this issue of Glaucoma News Dr Stuart Graham reports on current research to establish the connection between the supply of blood to the nerves of the eye and the ability of the nerves to withstand rising intra-ocular pressure. It may be that a decrease in the blood flow to the optic nerve in certain people results in glaucomatous damage to the nerve in spite of a "normal" IOP. Diseases such as hypertension and diabetes may have an adverse effect on the blood supply to the optic nerve head.

Some interesting research is currently being undertaken in America into the possibility that normal tension glaucoma may be an auto-immune disease in some patients. An auto-immune disease is when the antibodies of a person’s immune system, which usually protect the body, turn against it. The study has found that some NTG patients have higher blood levels of certain specific antibodies, compared with non-glaucoma or high pressure glaucoma patients of the same age.

At present, the principles of treatment for normal tension glaucoma are the same as for other types of glaucoma - medications alone or in combination, laser techniques and/or surgery are used to reduce the eye pressure as much as possible in an attempt to halt the progress of the disease. As with all glaucomas, early detection and prompt, effective treatment is vital to reduce visual damage, but NTG patients require an extra degree of caution in monitoring the disease, because eye pressure levels likely to be safe in other glaucoma patients may not be safe for them. They pose a greater challenge for the treating ophthalmologist

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