Spectacle Correction of Ametropias

Excerpt

Ametropia refers to any refractive condition that results in the image of the object in view, which does not allow for a properly focused image on the retina. As such, hyperopia, myopia, and astigmatism are all considered in these abnormal refractive disorders. Hyperopia refers to a condition in which the refractive power of the eye is less than expected, causing objects in view to come to a focus at a hypothetical point behind the eye. This results in a blurred image being formed on the retina.

Hyperopia can be further classified into axial, curvature, index, or pathological. Myopia or short-sightedness is a condition where the image of the objects being viewed is formed in front of the retina toward the vitreous body. Myopia can also be defined as axial or curvature. Myopes are commonly known to bring print close to their faces when reading. Astigmatism refers to a situation in which the eye has different powers along the same eye.

Astigmatism, therefore, results in various parts of the viewed objects being formed at different points on the retina. Astigmatism can be present with hyperopia (hyperopic astigmatism) or myopia (myopic astigmatism). An eye may also possess only astigmatism. The opposite of an ametropic state is an emmetropic state or emmetropia. Ametropias can be corrected/altered inherently by normal physiological processes called emmetropization. Ametropias may be congenital or acquired.

Epidemiology of the Ametropias

Varying data exist on the prevalence and incidence of ametropias across the world. A group of researchers conducted an extensive meta-analysis of publications on refractive errors and came up with a pooled prevalence of myopia, hyperopia, and astigmatism of 11.7%, 4.6%, and 14.9%, respectively. These numbers can change and differ according to the age group, gender, race, the severity of ametropia, type of daily work and activities, year of the data collection, etc.

Environmental factors such as prolonged and excessive near-distanced work and habitual style of life are factors in the progression of ametropia, especially myopia.

Diagnosis

The preliminary detection of most ametropias is possible using a vision test with a simple pinhole aperture. The patient is measured for visual acuity monocularly with no lens in place. The procedure is then repeated with a pinhole. An improvement in visual acuity with the pinhole over the unaided values indicates the presence of ametropia.

Ametropias are diagnosed by a process called refraction. This process is further split into objective and subjective refractions. Objective refraction involves a series of testing in which the examiner assesses the amount and type of ametropia without any input from the patient. Objective refraction can be carried out by retinoscopy and auto-refraction. Subjective refraction involves fine-tuning an objective refraction result based on the responses given by the patient.

It is worth noting that ametropias may mimic more severe underlying pathologies; thus, abnormal results should be confirmed and thoroughly assessed. Refraction results must include the type and severity of ametropia in the eye.