Low-pressure glaucoma and the concept of the IOP tolerance distribution curve.

Although glaucoma is considered to be caused by an elevated IOP, many eyes with an IOP within the normal range develop manifest glaucoma. The concept of an IOP tolerance distribution may clarify this paradox. The frequency of low-pressure glaucoma is a result of the constellation of the IOP distribution and the distribution of the IOP tolerance limit in the population. The low risk of glaucoma in the high normal IOP range is counteracted by the large number of eyes in this IOP range. It is possible to calculate the prevalence of glaucoma and the distribution of IOP in the untreated glaucoma population, resulting from different alternative distributions of IOP tolerance limit. Credible results were obtained by assuming a normal distribution of the IOP tolerance limit with an average = 26 mmHg and a standard deviation = 2.5 mmHg. In the development of glaucoma, the importance of the IOP relative to other factors, increases continually from low importance at low IOP to high importance at high IOP.