Regulation of Pupil Dilation
Pupil dilation is mediated by the dilator pupillae muscle (m. dilatator pupillae). Its muscle fibers are arranged radially (like spokes of a wheel) within the stroma of the iris. This muscle is innervated by the sympathetic pathway, which consists of a three-neuron chain:
- First-order neuron (Central pathway): Descends from the hypothalamus through the brainstem to the spinal cord.
- Second-order neuron (Spinal center): Located in the intermediolateral cell column of the spinal cord at the Th1 segment (ciliospinal center of Budge).
- Preganglionic fibers: Leave the spinal cord and ascend via the sympathetic trunk (truncus sympathicus).
- Synapse: Occurs in the superior cervical ganglion (ganglion cervicale superius).
- Postganglionic fibers: Form a plexus around the internal carotid artery (a. carotis interna), enter the cranial cavity and the orbit, and reach the radial muscle of the iris.
Neurological Causes of Mydriasis
Mydriasis can result from a complete lesion of the oculomotor nerve (n. oculomotorius, CN III). In this case, pupil dilation is caused by weakness of the sphincter pupillae muscle, which is normally innervated by the parasympathetic fibers carried within this nerve.
When the oculomotor nerve is damaged, mydriasis is accompanied by the absence of the pupillary light reflex and loss of accommodation (due to ciliary muscle insufficiency). A characteristic triad of symptoms is also observed:
- Ptosis (drooping of the upper eyelid) — results from paralysis of the levator palpebrae superioris muscle and unopposed action of the orbicularis oculi muscle.
- «Down and out» eye position — caused by the unopposed action of the lateral rectus and superior oblique muscles.
- Pupil dilation (mydriasis).
An isolated paresis of the intraocular muscles (sphincter pupillae and ciliary muscle) results in internal ophthalmoplegia, in which extraocular movements are fully preserved, but the pupil remains "fixed" and unreactive to light.
Toxic and Pharmacological Significance
Evaluating pupillary responses is a critical diagnostic tool in intoxications and adverse drug reactions.
- Psychoactive substances: Marked mydriasis is characteristic of acute intoxication with cocaine, psychostimulants, and cannabinoids. Conversely, acute opioid intoxication causes pinpoint pupils (miosis); however, during opioid withdrawal (abstinence syndrome), the opposite reaction occurs — marked mydriasis accompanied by lacrimation, tachycardia, chills, and myalgia.
- Medications: Antimuscarinics (atropine, trihexyphenidyl, antiasthmatic drugs) induce mydriasis. In overdose, pupil dilation is combined with agitation, tachycardia, tremor, and delirium. Antipsychotics and tricyclic antidepressants (TCAs), which possess anticholinergic properties, produce similar side effects (along with dry mouth, tachycardia, and urinary retention).
Associated Diseases
Mydriasis is one of the classic clinical signs of rabies (hydrophobia) in humans. In rabies, pupil dilation occurs alongside marked hypersalivation, hydrophobia, psychomotor agitation, hallucinations, and convulsions.