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Nerves involved in the resizing of the pupil connect to the pretectal nucleus of the high midbrain, bypassing the lateral geniculate nucleus and the primary visual cortex. From the pretectal nucleus neurons send axons to neurons of the Edinger-Westphal nucleus whose visceromotor axons run along both the left and right oculomotor nerves .
Mydriasis is frequently induced by drugs for certain ophthalmic examinations and procedures, particularly those requiring visual access to the retina. Fixed, unilateral mydriasis could be a symptom of raised intracranial pressure. The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological.
Pupillary response is a physiological response that varies the size of the pupil between 1.5 mm and 8 mm, [1] via the optic and oculomotor cranial nerve. A constriction response , [2] is the narrowing of the pupil, which may be caused by scleral buckles or drugs such as opiates/opioids or anti-hypertension medications.
Demecarium causes constriction of the pupil , which improves the drainage of the fluid in the eye (aqueous humour). [2] As demecarium reversibly inhibits cholinesterase , it can be administered less frequently than other parasympathomimetic drugs, such as carbachol .
If the pupil does not dilate, the lesion is postganglionic. [7] Hydroxyamphetamine has some limitations to its use as a diagnostic tool. If it is intended as an immediate follow up to another mydriatic drug (cocaine or apraclonidine), then the patient must wait anywhere from a day to a week before hydroxyamphetamine can be administered.
Pilocarpine is a drug that acts as a muscarinic receptor agonist. It acts on a subtype of muscarinic receptor (M 3) found on the iris sphincter muscle, causing the muscle to contract - resulting in pupil constriction . Pilocarpine also acts on the ciliary muscle and causes it to contract.
When light is shone into right eye, right pupil constricts. Direct reflex of the right pupil is unaffected, The right afferent limb, right CN II, and the right efferent limb, right CN III, are both intact. The left consensual reflex is lost. When the right eye is stimulated by light, left pupil does not constrict consensually.
All cycloplegics are also mydriatic (pupil dilating) agents and are used as such during eye examination to better visualize the retina. When cycloplegic drugs are used as a mydriatic to dilate the pupil, the pupil in the normal eye regains its function when the drugs are metabolized or carried away.