It’s common knowledge that the pupils of your eyes dilate in darkness—to absorb as much light as is available—while they contract in bright light so as not to overwhelm the system. Pupils also dilate in response to various stressors, including the muscle activity involved in clenching your teeth. These eye responses are regulated by branches of the autonomous (also called the autonomic) nervous system (ANS). The sympathetic branch dominates in “fright or flight” stress responses, while the parasympathetic branch dominates in vegetative states, for example, when you are digesting your dinner. “Dominates” is the key verb here, since in the case of the eyes, the two branches interact through complex excitatory and inhibitory pathways to the nerves controlling pupillary muscles. Based on reports that some TMD patients suffer deregulation of the ANS, particularly the sympathetic branch, a team of Italian scientists led by Annalisa Monaco at the University of l’Aquila, conducted a pilot study comparing eye responses of Temporomandibular Disorders (TMD) patients and controls in response to various conditions of light and darkness.
The scientists recruited 20 TMD women patients under 30 and matched them with 20 healthy controls also under 30. Their study used a pupillometer to measure pupil size in the volunteers under four conditions, tested in random order: 1) when the jaw was at rest and the eyes were exposed to infrared light (simulating darkness); 2) when the jaw was at rest and the eyes were exposed to yellow-green light; 3) when the teeth were put into forced occlusion (simulating clenching) and the eyes exposed to infrared light and 4) when teeth were clenched and eyes exposed to yellow-green light.
The results showed some interesting differences between TMD patients and controls. Under infrared conditions, the control group showed significantly larger pupil sizes when teeth were clenched compared to when their jaws were at rest, while TMD patients showed just the opposite: their pupil sizes were significantly smaller when teeth were clenched compared to when their jaws were at rest. Under infrared conditions the scientists also found a significant difference in the ratio of clenched-to-resting position pupil sizes between the groups, again illustrating that controls respond to the forced muscle condition by enlarging pupil size while the TMD patients’ pupils decreased in size.
The scientists also found significant differences under resting conditions when they looked at the ratio of pupil sizes in light compared to dark conditions. The figure was 0.662 for the control group (the pupil was about 2/3 as large for the controls in light conditions compared to darkness) while for the patients the light/dark ratio was 0.485 (their pupils under light conditions were less than half the size they were in darkness.
The Interpretation. The authors speculate that TMD subjects show greater activation (or less inhibition) of pupillary muscles under conditions of specific stimulation (presence or absence of light) and less activation or greater inhibition of contraction under the stressful condition of teeth clenching. They suggest that this could mean an impairment of the sympathetic branch of the ANS under conditions of stress. Bolstering this speculation are findings that some TMD symptoms (including pain) are improved by drugs aimed at stimulating selective parts of the sympathetic branch. The authors readily admit that their study was small and should be replicated along with further research. If additional studies confirm these results, pupillometry testing could emerge as a safe non-invasive means of diagnosing dysregulation of the ANS.
by Joan Wilentz
Source: Monaco A., Cattaneo R., Mesin L, Ciarrocchi I., Sgolastra F., et al. (2012) Dysregulation of the Autonomous Nervous System in Patients with Temporomandibular Disorders: A Pupillometric Study. PloS ONE 7(9): e45424. doi:10.137/journal.pone.0045424.