General bulging of the cornea is com monly not uniform and gives rise to irregular astigmatism, the common form being conical cornea; but the most com mon cause is incomplete restoration of the corneal tissue to normal after kera titis.
Treatment. — With irregular astigma tism following keratitis there is always, at first, haziness of the cornea; and prob ably, remedies for corneal opacity im prove vision partly by lessening irregular astigmatism.
Conical cornea, if extreme, may be greatly improved by making a para centesis at the apex of the cone with galvano-eautery needle. In one case vision improved from 'h„,, to with lenses. R. D. Gibson (Trans., Section on Oph., Amer. Med. Assoc., '92).
In a few cases dilatation of the pupil may improve vision by admitting light through a better portion of the cornea or crystalline lens. Iridectomy is appli cable in some cases for the same purpose. Contraction of the pupil often makes vision better by lessening the areas of diffusion. Solutions of pilocarpine, 1 to 500, or eserinc, 1 to 2000, may be in stilled for this purpose. Stenopaic spec tacles improve vision, but interfere too much with the field of vision to be of much practical value. Hyperholoid lenses have been used for conical cornea, but rarely with enough advantage for the patient to continue their use. For the mass of cases the correction of regular astigmatism commonly associated with the irregular, and the use of spherical lenses that will prevent the straining of accommodation, is the only available optical treatment.
Regular Astigmatism.
Definition.—It is the astigmatism that can be corrected by a cylindrical lens.
Symptoms.—It causes the blurring of some or all lines looked at. The eye is able to see with perfect clearness only the lines running in one direction at any one time, although by changing its ac commodation it may be able to see clearly lines running at right angles to the first. These two directions in which lines may be seen clearly, the "principal meridians," may be perceived by the patient, although usually they are only recognized when the eyes are carefully tested. A certain adjustment of the power of accommodation renders lines equally blurred in all directions. Astig matism may thus cause imperfect vision; but very often the imperfection has never been noticed by the patient. Gen
erally some form of eye-strain, from the effort to focus clearly the lines running in the different directions which all ob jects present, or to recognize from im perfect retinal images the real form of an object, gives rise to the symptoms complained of. These are weakness of the eyes, headache, pain in the eyes on use, inability to use them long, excess ive lacrymation, photophobia, nervous ness, twitching of the eyelids, and even more serious nerve disease.
Astigmatism usually co-exists with Hyperopia and myopia, and a portion of the symptoms may be due to one of these.
In the majority of cases with low de grees of hyperopia it is sufficient to cor rect the astigmatism, and the correction of astigmatism removes the troublesome symptoms that were assumed to depend on muscular insufficiency. D. 13. St. John Roosa (Ophthal. Review, Oct., '91; Annals of Oph. and Otol., '92).
The correction of astigmatism alone many times may give marked relief, but it is better to give full correcting glasses so that the refraction of the dioptric system including the glasses show ap proximate emmetropia. S. D. Risley (Annals of Oph. and Otol., July, '92).
The eye-strain caused by astigmatism is probably a very important factor in the development of myopia.
Among 2000 eyes 9 per cent. were simply hyperopic, but only of 1 per cent. showed simple myopia. In the other myopic eyes there was also astig matism. F. W. Marlow (N. Y. Med. Jour., July, '95).
The correction of errors of refraction, particularly astigmatism, has greatly decreased the prevalence of myopia, par ticularly of high myopia. In Philadel phia in the last twenty years the per centage of eyes requiring myopic correc tions is diminished from 25.4 to 15.2 per cent. S. D. Risley (Trans. Amer. Oph. Soc., vol. vii, p. 16S).
Among S69 patients, about 70 per cent. of whom had astigmatism, there were 60 per cent. suffering from headache, 20 per cent. aching and pain in the eyeballs, 11 per cent. twitching or spasm of the lids, and 11 per cent. had inflammatory symptoms. H. Bert Ellis (Trans., Sec tion on Oph., Amer. Med. Assoc., '95).