The glaucomas can be classified into two main types, namely, open angle glaucoma and angle closure glaucoma. As the names imply, in open angle glaucoma the anterior chamber angle is not manifestly occluded while in angle closure glaucoma the anterior chamber angle is obstructed, usually by the iris.
Open angle and angle closure glaucoma are each divided into primary and secondary types: primary open angle glaucoma, primary angle closure glaucoma, secondary open angle glaucoma and secondary angle closure glaucoma. Primary means there is no identifiable cause for the glaucoma or the cause is inherent in the eye. The term primary is used here in the same way that it is used for primary hypertension. Secondary glaucoma means the glaucoma developed due to an initial ocular or systemic disease. Let us take angle recession glaucoma as an example. It is a secondary open angle type of glaucoma because it only occurs if the eye sustained blunt force trauma first.
The primary glaucomas (primary open angle and primary angle closure) are by far the most common of the glaucomas. They are ranked first and second most common types of glaucoma in almost all populations studied worldwide. In studies done on Caucasian, Black, and Afro-Caribbean populations, primary open angle glaucoma ranks first and primary angle closure glaucoma is a somewhat distant second. On the other hand, in studies done on Chinese and South African populations, primary angle closure ranks first while primary open angle ranks second. In the Philippines there have been no population-based studies conducted that have distinguished the different types of glaucoma from each other, but it is estimated that the incidences of primary open angle glaucoma and primary angle closure glaucoma are almost equal.