When administered to both eyes, if a preganglionic lesion is present, it causes (relative to the normal eye) rapid pupil constriction.Topical administration differentiates between preganglionic and postganglionic parasympathetic lesions.Physostigmine (0.5%) is a cholinesterase inhibitor and, thus, requires an intact postganglionic neuron to induce miosis. Pharmacologic testing can aid the practitioner with lesion localization within the sympathetic or parasympathetic system. The third order neuron exits the cranial cervical ganglion, runs through the middle ear, then alongside the ophthalmic branch of the trigeminal nerve (CN V), and ends in the periorbital muscles, third eyelid, and iris dilator muscles.The second order neuron exits the spinal cord between T1 and T3, courses cranially through the thoracic cavity, out the thoracic inlet, and along the jugular groove to the cranial cervical ganglion.Sympathetic function originates in the hypothalamus of the brain and courses as the first order neuron through the brainstem and cervical spinal cord to thoracic spinal cord segments T1 to T3.

The sympathetic pathway ( Figure 4) is a 3-neuron pathway that takes a longer course to the eye compared with the parasympathetic system. The opposing system is the sympathetic system, which is responsible for pupillary dilation. The degree of constriction is lesser in the opposite eye. Clinically, this can be observed when a bright light is shone in one eye and the opposite eye also constricts.A relay between the paired PSN CN III in the midbrain results in indirect (or consensual) PLR.These parasympathetic fibers transmit this information to the eye, resulting in pupillary constriction.When a bright light enters the eye, a proportion of crossed CN II fibers enter the pretectal nucleus in the midbrain to synapse with neurons which, in turn, synapse with efferent parasympathetic fibers in PSN CN III.The parasympathetic pathway is best assessed using the pupillary light reflex (PLR): The first order neuron is depicted in orange and the second order neuron is depicted in yellow.

Pinpoint eyes