Prepulse inhibition of the blink reflex is abnormal in functional movement disorders

Background Patients with functional movement disorders also typically have functional somatic symptoms, including pain, fatigue, and sensory disturbance. A potentially unifying mechanism for such symptoms is a failure in processing of sensory inputs. Prepulse inhibition is a neurophysiological method that allows for the study of preconscious somatosensory processing. Objective The objective of this study was to assess prepulse inhibition in patients with functional movement disorders and healthy control subjects. Methods We analyzed the effect of a weak electrical stimulus to the index finger (prepulse) on the magnitude of the R2 response of the blink reflex induced by electrical stimuli delivered to the supraorbital nerve in 22 patients with clinically established functional movement disorders and 22 matched controls. Pain, depression, anxiety, and obsessive-compulsive symptoms were assessed using self-rated questionnaires. In addition, in patients we assessed motor symptom severity. Results Prepulses suppressed the R2 response of the blink reflex in both groups, by 36.4% (standard deviation: 25.6) in patients and by 67.3% (standard deviation: 16.4) in controls. This difference was significant (P < 0.001). There was no significant correlation between motor and nonmotor symptom measures and prepulse inhibition size. Conclusions Impaired prepulse inhibition of the blink reflex suggests an abnormal preconscious processing of somatosensory inputs, which can be interpreted within predictive coding accounts of both functional movement disorders and functional somatic syndromes. Our results, along with previous findings of a reduced prepulse inhibition in fibromyalgia syndrome, support a possible unified pathophysiology across functional neurological and somatic syndromes with noteworthy implications for diagnostic classification and development of novel biomarkers and treatments. (c) 2019 International Parkinson and Movement Disorder Society