Genetics in ophthalmology / Arnold Sorsby.

  • Sorsby, Arnold, 1900-1980.
Date:
1951
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in ophthalmology / Arnold Sorsby. Source: Wellcome Collection.

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    REFRACTION AND ITS ABERRATIONS The correlating mechanism is of further significance in studying the actual course of events in the developing eye. The eye at birth is about two-thirds of its normal diameters and its refraction would appear to conform to binomial dis¬ tribution (Fig. 64). During the process of growth, probably completed by about the age of 8 years, the eye therefore lengthens by about 8 millimetres. As there is general enlargement of the globe, myopia does not result. Myopia normally develops late in school-life when the growth of the eye is presumably complete. The assumption that this myopia is axial in character is hardly tenable now that it is known that these cases are generally examples of combination ametropia. The development of myopia could readily arise from a disturbance in the correlating mechanism which aims at emmetropia and racial differences in the refraction of adults conform to differences already observable in children (Fig. 65). Studies on the individual components are needed to clarify this issue. The present position The following crucial points may be taken as established: (1) There are no optical constants in the eye. There are instead at least four variable components—corneal refraction, depth of anterior chamber, refraction of lens, and axial length—each with a wide range of values. The total refraction is the result of the combination of these different components. (2) Neither the emmetropic nor the ametropic eye has specific values for the individual components, except that on the whole the axial length tends to be onger for myopic eyes than for hypermetropic eyes. In individual cases this does not necessarily apply. (3) Most ametropias are of the combination type: the ametropia results from the combination of the variable normal components of total refraction. (4) In both high hypermetropia and high myopia, the axial length falls outside the range of normal variation, but the degree of ametropia is not dependent on the axial length. On the basis of these findings, the ametropias, excluding high myopia and high hypermetropia, are physiological variants in a continuous curve of refraction in which refractions approaching emmetropia are the most frequent and con¬ siderably in excess of the theoretical expectation. Further evidence of the unity of the different types of combination ametropia is supplied by the relative value of the axial length to the transverse and vertical diameters of the eye, measured radiologically. The emmetropic eye tends to be spherical, the hypermetropic eye rather ñattened, and the myopic eye rather lengthened. Here again, however, individual cases do not conform to type. The axial length may be the longest diameter in both emmetropic and hypermetropic eyes. Furthermore, in anisometropia it is the myopic eye which may show the more normal values; in two such pairs of eyes, it was the emmetropic eye which showed marked flattening and the myopic eye was almost spherical. Genetic implications These considerations emphasize two points: (i) It is fallacious to seek genetic interpretation of refraction and its anomalies by taking into account only the total refraction of the eye or the clinical refraction. 85