Pathological myopia is a rare form of nearsightedness in which the back of the eyeball continues to grow longer after reaching normal adult size. It causes very rapid changes in vision, often requiring a change in eyeglass or contact lens prescriptions every 4 to 6 months.
The cause of pathological myopia is not clear. The condition is usually progressive. It does not stabilize within normal limits and causes gradual breakdown and thinning of tissues at the back of the eye and the growth of new blood vessels under the retina that may cause serious complications and loss of vision.
Most people who have severe nearsightedness (more than 6 to 8 diopters) have some degree of retinal changes. But not everyone who has severe nearsightedness has pathological myopia.
WebMD Medical Reference from Healthwise