Mydriatics

MYDRIATICS, Mydriatica, substances that dilate the pupil. The width of the pupil - i.e., the opening formed by the inner edge of the iris - is determined by the interaction, or relative tone, of the circular (sphincter) and radial (dilator) muscles of the iris. With increased tone of the circular muscles, the pupil constricts (miosis), and with increased tone of the radial muscles, it dilates (mydriasis). The tone of these muscles can be changed by both nervous and humoral influences; the latter in turn can act either on the muscular or on the nervous elements of the iris. Nervous influences are transmitted through the oculomotor nerve (parasympathetic) system, which excites the sphincter, and the sympathetic nerve system, which excites the dilator. Thus, dilation of the pupil can be caused either by excitation of the (cervical) sympathetic nerve system or by paralysis, or decreased tone, of the oculomotor nerve. According to this, two groups of mydriatics are distinguished: one acts through the sympathetic nerve, the other through the oculomotor nerve; the former includes adrenaline and cocaine, the latter includes atropine, homatropine, scopolamine, euphthalmine, mydrin. - Ophthalmic agents are applied locally to the conjunctiva, either as solutions or as ointments, or sometimes by introducing minimal amounts in substance into the conjunctival sac; adrenaline with such application, due to insufficient absorption, usually does not lead to dilation of the pupil; therefore, it is preferable to use it as subconjunctival injections, which produces a very pronounced effect. - The mechanism of action of substances of the first group: adrenaline dilates the pupil by exciting the sympathetic nerve peripherically; more specifically, the point of its physiological application is localized in the 'myoneural' intermediate substance, and not in the endings of the sympathetic nerve, because after degeneration of the latter, the pupil-dilating effect of adrenaline not only does not disappear but even increases (Meltzer, Auer); cocaine under these conditions no longer produces pupil dilation; therefore, the action of cocaine is localized in the endings of the sympathetic nerve, and not in the myoneural intermediate substance. In addition, it is believed that cocaine sensitizes the dilator pupillae to the action of adrenaline (a special case of sensitization of the action of adrenaline by cocaine). The mechanism of action of substances of the atropine group consists in paralysis of the myoneural intermediate substance of the iris sphincter. This is evident first from the fact that after atropinization, irritation of both pre- and postganglionic fibers of the ciliary nerves (of the oculomotor nerve system) has no effect; second, the muscle itself remains excitable to direct irritation by induction current; third, pupil constriction from pilocarpine, which occurs even after degeneration of the fibers of the oculomotor nerve (see Autonomic Nervous System), turns into dilation under the influence of atropine. With local application to the eye, atropine, in addition to pupil dilation, produces paralysis (or weakening) of accommodation; the point of application of atropine is again seen in the myoneural intermediate substance of the ciliary muscle; the latter moves to the periphery, flattens the lens, and the eye is set for the far point of clear vision; mydriasis and paralysis of accommodation are accompanied by phenomena of blinding and micropsia, i.e., objects appear smaller. Paralysis of accommodation occurs later and passes earlier than pupil dilation; with very small doses of atropine, paralysis of accommodation is not noted. - In the normal eye, atropine has no noticeable effect on intraocular pressure; in an eye predisposed to glaucoma or glaucomatous, atropine can cause an acute attack of glaucoma and is therefore absolutely contraindicated. The increase in intraocular pressure is explained by the difficulty of lymph outflow from the internal media of the eye due to the decrease in the angle between the cornea and iris and the compression of Fontana's space by the displaced and thickened iris. Adrenaline and cocaine usually do not lead to changes in intraocular pressure in the normal eye. The atropine group is widely used in ophthalmic practice both for diagnostic purposes (e.g., for determining refraction, examination of the fundus of the eye, etc.) and for therapeutic purposes. Thus, atropine finds wide application (often in combination with cocaine and adrenaline), when it is first necessary to give functional rest to the eye, in cases of acute inflammations, eye injuries; its application is especially frequent in acute iritis, for rupturing or preventing adhesions of the iris, in threatening 'occlusion of the pupil' and others. - For the purpose of pupil dilation, 0.01-0.01% solutions of atropine sulfate are used, and for pupil dilation and paralysis of accommodation, 0.5-1% solutions. The latter cause complete paralysis of accommodation for several hours, which is fully restored only after 5-6 days; the pupil returns to normal only after 10 and even 14 days. Pupil dilation passes much more quickly with scopolamine, homatropine, and others. These latter agents are preferred if a relatively short-term effect on the eye is needed. - In recent times, to enhance the action of mydriatic (as well as miotic) agents, attempts are being made to use them not in neutral, but in slightly alkaline solutions; the stronger effect is probably explained by the fact that in an alkaline environment, there occurs a kind of 'liberation' of the alkaloid - base from its salts, which allows it to penetrate more easily and quickly into the cell. - The following preparations are most frequently used as mydriatics: Atropinum sulfuricum (atropine sulfate), Homatropinum hydrobromicum (homatropine hydrobromide), Mydrin, Scopolaminum hydrobromicum (scopolamine hydrobromide), Euphtalminum hydrochloricum, Cocainum hydrochloricum, Adrenalinum hydrochloricum.