Hypermetropia Longsightedness

Most young, normal, eyes have a very low degree of longsightedness (hyper-metropia or hyperopia), and young eyes have an ability to compensate for low to moderate degrees of longsightedness. The eyes compensate by using the power of ocular accommodation (described above) to "over focus" the eyes and correct the longsightedness. This is a misuse of accommodation, which usually serves to keep the image clear during near vision. A very different function during near vision is convergence, which is a turning in of the two eyes so that they both point at an approaching object. Accommodation keeps near objects clear, and convergence keeps them single (if the two eyes were not pointing at the object of regard, then it would be perceived as double). In the natural world, accommodation and convergence always act together and these two functions are linked in the brain. In other words, a given amount of convergence induces an equivalent amount of accommodation and vice versa.

This relationship between accommodation and convergence causes problems in significant degrees of longsightedness. In this condition, accommodation can be used (or, really, misused) to overcome the longsightedness rather than to focus on near objects. But this use of the accommodation will usually trigger an associated convergence. Hence, when accommodation is used to compensate for excessive hypermetropia, it can induce a turning inward of the eyes and uncorrected longsightedness is a common cause of convergent strabismus (squint, see below). In this case, the correct treatment for the strabismus is to correct the longsightedness, and surgery would be an inappropriate treatment.

A constant accommodative effort to overcome excessive longsightedness may also cause eyestrain, headaches, and blurred vision. Longsightedness in children would not usually be detected by a distance letter chart test, and frequently does not even affect performance at brief near vision tests. Longsightedness in one eye will not be detected by binocular (both eye) acuity tests and, although unlikely to affect reading, can result in a permanent visual loss if not detected in the first 5 or 6 years of life. Some studies have found a weak correlation between longsightedness and reading difficulties, others have suggested that this may result from a weak negative correlation between hypermetropia and IQ (Evans, 2001).

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