Measurement of the intraocular pressure in a large number of normal subjects reveals a normal distribution extending from pressures of 10-12 mmHg to 25-28mmHg. The pattern of distribution fits a Gaussian curve, so that the majority of subjects have a pressure of about 16mmHg. For clinical purposes, it is necessary to set an arbitrary upper limit of normal. By and large, the eye can stand low pressures remarkably well, but when the pressure is abnormally high, the circulation of blood through the eye becomes jeopardised and serious damage can ensue. For clinical purposes, an upper level of 21 mmHg is often accepted. Above this level, suspicions are raised and further investigations undertaken.
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