Neural correlates of cough hypersensitivity in humans: evidence for central sensitisation and dysfunctional inhibitory control

INTRODUCTION: Chronic non-productive coughing is a major complication of pulmonary disease and can also occur in many individuals without identifiable underlying pathology. The common clinical link in patients with cough is an enhanced sensitivity of the respiratory system to stimuli that subsequently evoke excessive coughing. The aetiology of this 'cough hypersensitivity syndrome' is unclear but believed to involve hypersensitivity of the sensory neural pathways that innervate the airways and lungs. METHODS: In the present study, we used functional brain imaging to compare central neural responses to airway stimulation using inhaled capsaicin in healthy people and patients with cough hypersensitivity. RESULTS: Hypersensitivity in response to inhaled capsaicin coincided with elevated neural activity in the midbrain in a region encompassing the nucleus cuneiformis (left: p<0.001; right: p<0.001) and periaqueductal gray (p=0.008) in comparison to normal sensitivity in controls. The enhanced activity noted in the midbrain is similar to that occurring in patients with chronic pain, thus providing empirical evidence to support the notion that cough and pain share neurobiological similarities. Furthermore, patients with cough hypersensitivity displayed difficulty controlling their cough, which manifested as a failure to suppress cough during capsaicin challenge (ie, reduced cough frequency) in controls compared with patients with cough hypersensitivity (p=0.046). Cough suppression was associated with reduced activity in a forebrain network that included the dorsomedial prefrontal and anterior mid-cingulate cortices. Additionally, cough frequency was correlated with activity in the right inferior frontal gyrus (R(2)=0.6, p<0.001) and right anterior insula (R(2)=0.6, p<0.001), regions previously implicated in voluntary cough suppression. CONCLUSIONS: These findings provide insight into the central neurobiology of cough hypersensitivity and suggest that both central amplification of cough sensory inputs and reduced capacity to suppress cough motor behaviours define patients with problematic cough.