The Pupillary Response

After learning about the way the eyes react to light by either constricting or dilating and their and their consecutive nervous system stimulation, I wondered what the mechanism of action was that made the pupils un-reactive to light (“blown pupils”) resulting from head injury. Traumatic brain injuries (TBIs) affect more than 1.4 million Americans annually (Adoni & McNett, 2007). EMTs, nurses and doctors perform a battery of neurological tests when a patient presents with a possible head injury, including pupillary examinations. This exam is especially important because it is easy to perform, non-invasive and can be done whether or not the patient is conscious or under sedation. Three components are taken into consideration, pupil size (normally between 2 and 5 mm), shape (round, irregular or oval) and reactivity to light (brisk, sluggish or non-reactive). The physiologic basis of the pupillary response, as we know, is controlled by smooth muscle of both parasympathetic and sympathetic control and is innervated by three cranial nerves (III, IV, VI). When an abnormality is detected during a pupillary exam and it is determined that it was not a pre-existing problem, it is often indicative of increasing inter-cranial pressure (ICP) due to progression of a hematoma/hemorrhage or cerebral edema. However there are clinical factors, such as drugs or infection that can contribute as well.

When answering the question of whether it is the failure to send impulses either to the eye, away from the eye or compression/damage to the entire nerve, it actually proves difficult to give a solid answer, as there are many causes of different abnormalities. This study observed abnormal pupils in four different categories; unequal pupils, constricted pupils, dilated pupils, and equal pupils with abnormal response. In the cases where the abnormality in the affected eye is dilation with abnormal reactivity, the cause was either a defect in the efferent pathway, compression of cranial nerve III, loss of parasympathetic nerve supply to the sphincter muscle in the iris or in the worst cases, brain death. Complete bilateral dilation with no response to light (“blown pupils”), called Anoxia Mydriasis, is found when transtentorial herniation is evident (Adoni & McNett, 2007) where brain tissue of the midbrain is displaced by edema or hemorrhage into the tentorial notch, putting pressure on the brain stem (ACEP, 2009). In the case that the affected pupil was constricted, the usual cause was disruption in sympathetic innervation due to lesion in the brainstem or damage to the hypothalamus, direct orbital trauma, opiate or narcotic use or viral infection. So as physicians, the pupillary response will be a very important and useful tool in assessing neurological function. But as to the level of difficulty of analyzing the results, don’t be fooled by the simplicity of the test.

Adoni, A. McNett, M. “The Pupillary Response in Traumatic Brain Injury”. The Journal of Trauma Nursing. October-December 2007. Vol. 14, Number 4. Page 190-197. Web. October 28^th 2011.

American College of Emergency Physicians. Critical Care Transport. American Academy of Orthopaedic Surgeons. 2009. Pg 1007-1015

tentorial notch. (n.d.). Merriam-Webster's Medical Dictionary. Retrieved October 29, 2011, from Dictionary.com website: http://dictionary.reference.com/browse/tentorial notch