Neuroanatomy, Pupillary Dilation Pathway

The pupillary dilation pathway is a sympathetically driven response beginning in the hypothalamus and ending with the contraction of the dilator pupillae muscle. It is for this reason that pupillary dilation may result from any physical or emotional stress that triggers the autonomic sympathetic nervous system, which is mediated by the hypothalamus.

The pupillary dilation pathway is a three-neuron pathway; disruptions anywhere along the oculosympathetic pathway may cause Horner syndrome, which is a classic triad of ptosis, miosis, and anhydrosis of the face due to loss of sympathetic innervation. It is essential to avoid damaging the superior cervical ganglion during cervical surgery, as such damage could prompt the development of Horner syndrome.

Although Horner syndrome is predominantly a clinical diagnosis, in more subtle cases, administration of topical cocaine to the eye for confirmation secondary to cocaine's sympathetically activating qualities is an option. One of cocaine's mechanisms of action is to block the reuptake of norepinephrine at the neuromuscular junction. Topical cocaine in a person without Horner's syndrome will cause pupillary dilatation. Topical cocaine in a person with Horner syndrome will lead to minimal or no dilation at all. In addition to Horner syndrome, chronic opiate abuse can cause miosis.