A determination of the prevalence of<unk>  has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define<unk> , and the boundary between<unk>  and isometropia depend on their definition. Several studies have found that<unk>  occurs more frequently and tends to be more severe for persons with high ametropia, and that this is particularly true for myopes.<unk>  follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teenagers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age. One study estimated that 6% of those between the ages of 6 and 18 have<unk> . Notwithstanding research performed on the biomechanical, structural and optical characteristics of<unk>  eyes, the underlying reasons for<unk>  are still poorly understood.<unk>  persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia. However, there are indications that<unk>  influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery,<unk>  may be one of the risk factors for developing consecutive exotropia and poor binocular function may be a risk factor for<unk>  to develop or increase.
Anisometropia
Hypertropia
Emmetropia
Cyclotropia
Physiological anisocoria
Exotropia
Ocular dysmetria
Heterotopia 