An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald.

Theobald, Samuel, 1846-1930.

Date:: 1874

Credit: An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated : and that the muscular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction / by Samuel Theobald. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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$muscles in myopia he says, “The insufficiency first makes itself known by asthenopia muscularis in binocular vision for near objects. Sometimes we find (at least temporarily) the proper remedy for this state of things in concave glasses, which bring [the furthest point of binocular distinct vision] to from 12 to 14, and we may try these if there is no contra- indication for other reasons against them.”' These statements may be looked upon, I think, as decidedly favourable to the position which I have endeavoured to establish, not only as regards the cause of the muscular asthenopia, but as regards the benefit to be de- rived from the elimination of this cause in correcting the asthenopia and in favourably modifying the development of the myopia. It is, however, from the careful application of prismatic glasses, to be worn whenever the eyes are used for near work, the strength of the prisms to vary, as a rule, with the degree of the myopia, and the distance at which the work is to be done, whereby we are enabled to neutralize the excess of convergence over accommodation; or, where the myopia has reached such a degree that the far point lies nearer than 8° or 10° from the combina- tion of concave and prismatic glasses, that we shall be able to derive the greatest benefit in our treatment of progressive myopia. This plan of treatment, indeed, is that from which Yon Graefe obtained most satisfactory results in the management of myopia, in connection with which he had proved, in accordance with his method (the fallibility of which, under such circumstances, has, I thiid^, been clearly established), the existence of insufficiency of the internal recti. That it has not been more generally adopted, and that it has not been attended with that degree of success which its rationality would seem to bespeak for it, is not the result, I believe, of any defect in the method itself, but of its applicability having been based upon incorrect grounds, which, of course, rendered the construction of correct rules for its application impossible. Our starting-point in the formation of such rules should be the restora- tion of parallelism between convergence and accommodation; but, as in hypermetropia, its complete restoration by entire correction of the error of refraction by convex glasses, though theoretically the proper remedy, is not to be thought of so long as accommodation remains active; so, in myopia, its complete restoration by means of prisms, though equally cor- rect in theory, would be quite as erroneous in practice. As prismatic glasses have been resorted to heretofore in the treatment of myopia upon the theory of the existence of insufficiency of the interni, which it was considered desirable should be completely corrected, the error, I think, has been made of prescribing those of too great strength; which, proving on this account irksome and productive less of comfort than of discomfort, have been forthwith thrown aside.$