The mechanism of mydriasis depends on the agent being used. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. The sphincter is controlled by nerves of the parasympathetic nervous system, and the dilator by the sympathetic nervous system. There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. ( August 2014) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources in this section. This section needs additional citations for verification. Anisocoria is the condition of one pupil being more dilated than the other. Both mydriasis and miosis can be physiological. The opposite, constriction of the pupil, is referred to as miosis. Mydriasis is frequently induced by drugs for certain ophthalmic examinations and procedures, particularly those requiring visual access to the retina.įixed, unilateral mydriasis could be a symptom of raised intracranial pressure.
More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis. A mydriatic pupil will remain excessively large even in a bright environment. Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drug. Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response.
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