Mydriatics.
Dilation of the pupil by mydriatics (mydriasis dilation of the pupil) is a most important means of diagnosis, and therefore a knowledge of the action of the different mydriatics is essential. The mydriatics in common use not only dilate the pupil, but also paralyze the ciliary body and the power of accommodation in ratio with the strength of the solution employed. This determines an adaptation of the eye to the farthest point of vision and holds it there until the action of the mydriatic passes off and normal power of accommodation is restored. In short it renders the subject long sighted, during its action.
Atropine the alkaloid of atropa belladonna is the most generally available and persistent of the mydriatics, and is in most common use. It is usually employed as sulphate of atropine, though some prefer the nitrate, the salicylate or the borate to obviate the danger of atropinism. This form of poisoning may show in the occurrence of conjunctivitis and in case of one attack the susceptibility to atropine is greatly to be dreaded, so that it should never again be used on the same subject. The real cause of atropinism is uncertain, it has been variously ascribed to too great acidity or alkalinity, or to micro-organisms growing in the solution. Hence the importance of using the antiseptic salts of atropine, and of testing the solution to see that it is exactly neutral before it is applied.
The strength of the solution of atropine is an important consideration. Donders found that 1:120 of water produced a full effect, while Jaarsma obtained the full effect in one hour from a drop of a solution of one to twelve hundred of water. The action on carnivora (dogs and cats) is equivalent to that on man, while on the herbivora (rabbit, horse, ox, sheep) it is somewhat less, and on birds very slight indeed. On diseased eyes a large amount may be required, and with synechia (adhesion of the iris to the capsule of the lens) dilatation may be impossible. The full effect may last 24 hours, and accommodation may remain very imperfect for 11 days.
The direct action of atropine on the eye is shown in dilatation of the pupil of the frog after the eye has been detached from all connection with heart or brain, by excision. It acts also in the normal system through reflex nervous action, since, after division of the sympathetic trunk going to the eye, that eye does not dilate so much under atropia as the opposite eye.
Atropine is usually employed by lodging a drop in the pouch of the conjunctiva (inside the lower lid), and from this it makes its way into the aqueous humor, for if that liquid is transferred to the conjunctiva of another animal it causes dilatation. Puncture of the cornea with evacuation of the aqueous humor lessens the action of the atropine. Atropine dilatation is increased by following it with cocaine which causes contraction of the iridian vessels, the antithesis of the dilatation of the vessels which occurs when the cornea is perforated and the pressure of the aqueous humor is removed.
Atropine is one of the most potent poisons and must be used with caution especially in the carnivora and omnivora. The danger lies not alone in the absorption from the conjunctiva, but also from the escape of the liquid through the lachrymo-nasal duct, to the nose and later to the actively absorbing mucosæ of the lungs and stomach.
The symptoms of general poisoning are: rapid pulse, vertigo, weakness of posterior limbs, general prostration and thirst or dryness of the throat.
Homatropine is an oily liquid produced by the action of muriatic acid on the cyanate of atropine. With hydrobromic acid it forms a readily crystallizable salt, the solution of which acts on the eye like atropine, but more promptly and transiently. One drop of a solution of one to one hundred and twenty, usually gives in twenty minutes, full pupillary dilation and complete paralysis of accommodation which lasts only for twenty-four hours. Add to this that there is little danger of constitutional disturbance and poisoning, and homatropine must be accepted as a more desirable agent than atropine. It is especially to be preferred in cases of senility with shallow anterior chambers, and in glaucoma, in which atropine tends to aggravate the lesion.
Daturine, the alkaloid of datura stramonium is a potent mydriatic, causing pupillary dilatation in a solution of one to one hundred and sixty thousand of water. It appears to be identical with atropine.
Duboisine the alkaloid of duboisia myoporoides is also a potent mydriatic. Jaarsma found that a solution of the sulphate, of one to three thousand, paralyzed accommodation for twenty-four hours. It acts more promptly than atropine but is more poisonous.
Hyoscyamin, the alkaloid of hyoscyamus niger, is also strongly mydriatic. One drop of an one to three hundred solution of the sulphate paralyzed accommodation for from seventy-five to one hundred hours. Risley found it to act more promptly than atropine, and to be less dangerous than duboisine.