Thursday, November 29, 2007

1.8 Other kinds of myopia

In addition to school myopia, there are several other kinds of myopia, mostly secondary. To complete the discussion on myopia, I will also mention them.

Usually this type of myopia drives everybody nuts. Because the refractive error does not stablize, it is very difficult to know if the endpoint of refraction is reached. Often the patients complain. The opticians blame the doctors for "measurement errors". The doctors re-refract ending with minimal changes and try to explain the underlying factors to the patients, usually in vain. And the patients are still quite unhappy because they expect 20/20 vision with a new pair of expensive yet now apparently useless spectacles.

It is actually quite simple, any condition that increases the overall refractive power of the eye will lead to myopia, sometimes transient, often part of permanent or semi-permanent ocular pathological changes. The possibilities are (1) increase in the refractive index in the nucleus of the crystalline lens; (2) steepening of corneal curvature and/or corneal edema; (3) sustained accommodation - prolonged accommodation precipitates pseudomyopia which usually recover once the stress factor is removed; (4) medicine-induced: drugs such as sulphonamides can cause acute transient myopia; (5) ocular surgery, e.g., retinal detachment repair in which scleral buckling around the eye globe can deform the eye and lengthen the axial length; (6) tumor-related increase in the vitreous volume or the anterior-posterior length of the eye, even though intraocular and intraorbital tumors are rare.

Most commonly encountered are actually (1) and (2):

(1) Lenticular change: The refractive index of the nucleus can increase as a result of aging or diabetes, often part of nuclear sclerosis that eventually leads to nuclear opacities (i.e., cataracts). Diabetes actually causes more than just lenticular changes (more when we discuss cataracts). In both cases, myopia induced by nuclear cataracts frequently occur. The rate of myopia increase is, however, unpredictable. This type of myopia is often accompanied by deteriorating vision.

(2) Corneal change: Typically long-term contact lens wear can result in corneal warpage. And chronic oxygen deprivation can result in corneal edema with changes in both thickness and refractive index. Again, these manifest as gradual myopic increase throughout the contact lens wear period, known as the "myopia creep". If the patients stop wearing contacts, or switch to high-oxygen transmission lenses, the corneas tend to return to the baseline albeit slowly. This type of change is also a major concern prior to refractive surgery.

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