Pathophysiology of Signs Associated with a Tonic Pupil

Pathophysiology of signs associated with a tonic pupil. Normally, all parasympathetic fibers of the third cranial nerve synapse in the ciliary ganglion (top). Most postganglionic fibers innervate the ciliary muscle (dashed lines). After injury to the ciliary ganglion, the pupil becomes denervated and larger (bottom). Denervation supersensitivity of the iris sphincter develops (depicted as an increase in the number of oval-shaped receptors). Some fibers never reinnervate the pupil (depicted as open circle neurons within the ciliary ganglion), whereas others reinnervate segments of the iris sphincter they had not innervated before injury. This haphazard process of denervation and reinnervation results in a poorly reactive pupil with segmental palsies of the iris sphincter. A greater proportion of postganglionic fibers that originally innervated the ciliary muscle become miswired and aberrantly reinnervate the iris sphincter muscle, resulting in more efficient pupillary constriction during accommodation than in response to light (i.e., light-near dissociation).