The causes of asthenopia / by D. B. St. John Roosa.

  • Daniel Bennett St. John Roosa
Date:
[1891]
Catalogue details

Licence: Public Domain Mark

Credit: The causes of asthenopia / by D. B. St. John Roosa. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    4/6 (page 4)
    the causatiou of astheaopia. Probably the human race can tolerate less of astigmatism without asthenopia, and especially of hyperme- tropic astigmatism, than they can of any other ocular defect, but even here we find a great deal of tolerance to abnormal condi- tions ; that is to say, conditions that are technically abnormal. If we were to examine a large number of people who had never suf- fered from asthenopia, we should find a large proportion of them hypermetropic, or having hypermetropic astigmatism. The inves- tigations that I have made and published in this direction show this. The profession, in my opinion, has fallen into an error in seeking the causes of asthenopia too exclusively in errors of refrac- tion, but they have made a much greater error in assuming that con- ditions of the muscles produced entirely by errors of refraction are causes of asthenopia. An enormous degree of energy has been ex- pended in determining the relations of the external muscles of the eye to each other. Numerous interesting machines for the measure- ment of the power of these muscles liave been invented—but the machines are unnecessary, because we are merely measuring a rela- tive power; we are determining merely how much effect an error of refraction has had upon disturbing the relation of the muscles to each other. Strabismus, or squint, is a striking example of this. Whatever else causes strabismus, one of the most important factors in its etiology is the refraction. At the bottom of most cases of strabismus convergens will be found hypermetropia, or hyperme- tropic astigmatism, together with the unequal refraction of the two eves. Whenever it is possible to secure binocular vision by glasses, it is often very easy to correct the strabismus without operation. In my opinion, the matter of strabismus explains the whole subject of insufficiencies—waut of coordination of the ocular mus- cles ; that is to say, in strabismus such a marked deviation is in- variably found to be associated with a decided error of refraction, and is corrected in very many cases simply by correcting the error of refraction. It is certainly proper to infer that latent insufficien- cies of the muscle, latent squints if you please, are also caused by the errors of refraction—the greater includes the less. After opera- tion for the deformity which we call strabismus, we correct the remaining insufficiency by glasses, not by prisms, but by spherical or cylindrical or spherico-cylindrical glasses as the case may require.