Primary open angle glaucoma
Primary open angle glaucoma is the most common form of glaucoma and is the third most common cause of registration of blindness in the United Kingdom. The resistance to outflow through the trabecular meshwork gradually increases, for reasons not fully understood, and the pressure in the eye slowly increases, causing damage to the nerve. The level of IOP is the major risk factor for visual loss. There may be other damage mechanisms, particularly ischaemia of the optic nerve head.
Symptoms
Because the visual loss is gradual, patients do not usually present until severe damage has occurred. The disease can be detected by screening high risk groups for the signs of glaucoma. At present most patients with primary open angle glaucoma are detected by optometrists at routine examinations.
Groups at risk
The prevalence increases with age from 0.02% in the 40-49 age group to 10% in those aged over 80. Those with an increased risk include first degree relatives of patients (1 in 10), patients with ocular hypertension (particularly those with thin corneas, larger cup to disc ratios and higher IOPs), people with myopia, and people of African-Caribbean origin (X5 risk in Caucasians). Recently, genetic mutations have been identified that account for 3-4% of primary open angle glaucomas.
Signs
The eye is white and on superficial examination looks normal. The best signs for the purpose of detection are the optic disc changes. The cup to disc ratio increases as the nerve fibres atrophy. Asymmetry of disc cupping is also important, as the disease often is more advanced in one eye than the other. Haemorrhages on the optic disc are a poor prognostic sign. Longer term changes in disc cupping are best detected by serial photography, and the more recently introduced scanning laser ophthalmoscope may be able to detect structural changes in the nerve at an early stage of the disease.
Visual field loss is difficult to pick up clinically without specialised equipment until considerable damage (loss of up to 50% of the nerve fibres) has occurred. Computerised field testing equipment may detect nerve fibre damage earlier, particularly if certain types of stimuli such as fine motion or blue on yellow targets are used. Computer assisted field testing is also the best method for detecting long term change and deterioration of visual fields.
The classical signs of glaucoma (field loss and optic disc cupping) often are seen in patients who have pressures lower than the statistical upper limit of normal (21 mm Hg).
However, many clinicians now feel that these two glaucomas are part of the same spectrum of pressure dependent optic neuropathies, although these patients are sometimes referred to as having normal tension glaucoma. For an accurate measurement of IOP, intraocular pressure phasing, taking multiple measurements throughout the day is useful, so that any spikes can be detected.
