What is myopia?
Myopia, a.k.a. near-sightedness, is a refractory error. Refraction is the process by which the optical system focuses images of objects on the retina, which are then transmitted to the visual cortex of the brain to be interpreted as vision. The major components of refraction are the cornea, lens, and axial eye length (distance from the front to the back of the eye). The problem in myopia is that images are focused anterior to the retina, like this:
Causes of myopia
I don't really understand the exact mechanism responsible for the transition from 1 [ciliary muscle spasm] to 2 [elongation of the eye].
Swelling or spasm of the ciliary body (the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes) can be seen after trauma or in inflammatory conditions. Contraction of the ciliary muscles causes decreased tension in the suspensory ligaments that connect the ciliary body to the lens. The results in the lens becoming more rounded and/or moving anteriorly. Both processes also bring the focal point more anterior, i.e. further in front of the retina, resulting in a configuration as in the picture above. You could call this "elongation of the eye" if you wanted. This 25 second video explains the process succinctly with pretty pictures.
Although interesting to think on, this is not the most common cause of myopia. Myopia is an extremely common condition, occurring in ~25% of the US population. Myopia can be classified as axial or refractive.
Axial myopia: The refracting power of the eye is normal but the length of the eye is too long.
Refractive myopia: The length of the eye is normal, but the lens refracts too much for a given axial length.
Both axial and refractive myopia are usually caused by failure of emmetropization, the process by which a normal eye has coordinated growth of its refractive components. This process takes place mostly during the childhood and adolescent years, which is why myopia most commonly develops during that time. There are certain disease conditions that increase lens power (e.g. osmotic effects in diabetes) or axial length (e.g. posterior staphyloma) that may also cause myopia at other points in life, but these are less common.
1. Neil J. Friedman and Peter K. Kaiser. Essentials of Ophthalmology. © 2007, Elsevier Inc.
2. Albert, Daniel M., MD MS.Albert & Jakobiec's Principles & Practice of Ophthalmology, Third Edition. © 2000, 1994 by W.B Saunders Company, © 2008, Elsevier Inc.