Interoceptive processing deficit: A behavioral marker for subtyping Parkinson's disease

Background: Non-motor symptoms in Parkinson's disease (PD), such as cognitive, emotional, autonomic and somatosensory alterations, are not ubiquitous but vary between the tremor dominant (TD) and postural instability/gait difficulty (PIGD) subtypes of the syndrome. Non-motor phenomena (e.g., anxiety, depression and apathy) have been related to representation of autonomic and somatosensory sensations (interoception), and recent findings suggest interoceptive deficits in PD. Objectives: To test whether interoceptive processing is differently affected in TD and PIGD phenotypes, by assessing both interoceptive accuracy and sensibility in PD patients with TD and PIGD subtypes, and in healthy controls. Methods: Interoceptive accuracy was measured by the heartbeat perception task requiring participants to count their own heartbeats in a given time interval. A time-estimation, control task was also administered asking participants to count the seconds in a set period of time. Interoceptive sensibility was assessed by a questionnaire of subjective interoception. Finally, the patients underwent measures of anxiety, depression, apathy and anhedonia, and impulsive-compulsive disturbances. Results: The main results showed reduced interoceptive accuracy and sensibility in TD patients relative to both PIGD patients and healthy controls. Reduced interoceptive accuracy of TD group was a reliable result since their performance on the time estimation control task was comparable to that of both PIGD patients and healthy controls. Conclusions: These findings demonstrate that the behavioural assessment of different aspects of interoceptive processing can provide with a further marker for subtyping patients with PD.