All head traumas can exist with varying levels of severity and can shift unexpectedly toward improvement or deterioration. Consequently, accurate assessment and careful monitoring is essential, especially in the early post-trauma period. This assessment necessitates application of basic physiological principles. Pupillary Reflex Monitoring the pupil s extent of dilation or constriction as well as their reactivity to light can be a valuable assessment tool if the underlying physiology is understood. The iris contains both circular smooth muscles, which constrict the pupil when active, and radial smooth muscles, which dilate the pupil when active.
Parasympathetic activation constricts the pupil and sympathetic activity dilates it.
Parasympathetic supply to the iris is from the midbrain where the third nerve originates out to the circular muscles of the iris. Sympathetic innervation originates in the hypothalamus and courses down the brainstem and cord emerging within the first three thoracic nerves. Consequently, the pupillary reflex can be a valuable assessment tool. For example, damage to the hypothalamus destroys only the sympathetic branch allowing the parasympathetic to predominate. Parasympathetic nerve supply causes constriction with reaction to light.