How rare is pigmentary glaucoma? What is it, and what causes it?

See eye doctor. Pigmentary glaucoma is a result of the colored part of the eye, the iris, rubbing on the lens behind it, releasing pigment that clogs the pressure channels resulting in higher pressure that damages the optic nerve which can result in loss of vision.
Fairly common. Pigmentary glaucoma is due to blockage of the normal internal drainage system of the eye due to pigments from the iris. These pigments block the trabecular meshwork where the aqueous humor exits the front of the eye. When it can drain, it builds up pressure in the eye, causing damage to the optic nerve fibers entering the eye. It is commonly seen in younger men and may be worse after exercise.