Cough responsiveness in neurogenic dysphagia

OBJECTIVES

In neurogenic dysphagia a good cough is important for airway protection. If triggering of cough, or its effectiveness, is impaired this might result in an increased aspiration risk. Capsaicin, an agent which induces cough through sensory nerve stimulation, was used to test cough sensitivity in groups of patients with and without neurogenic dysphagia.

METHODS

On the basis of swallowing speed (ml/s) in a validated water test 28 alert neurological inpatients (16 women, aged 22–71 years) were classified into 13 with abnormal and 15 with normal swallowing (median swallowing speed 23% and 99%, median volume/swallow 43% and 106% of that predicted for age and sex respectively: p<0.001). Capsaicin nebulised on air in saline was inhaled via a low resistance valve using a mouthpiece and noseclip. Up to seven incremental concentrations of capsaicin ranging from 0.07–20.0 × 10⁻⁴ mol/l were each inhaled for up to a minute. A pneumotachograph connected to the expiratory limb gave a paper recording of expiratory air flow. Coughs were recorded as high flow expirations of short duration. Capsaicin concentrations at first cough (threshold) were recorded; concentrations at frequencies of 10 and 20 coughs/minute were interpolated from the dose-reponse curve.

RESULTS

Cough threshold tended to be lower in those with abnormal swallowing (non-significant): the (log) concentration of capsaicin producing 10 or 20 coughs/minute also tended to be lower (p=0.12 and 0.07 respectively) in those with abnormal swallowing.

CONCLUSION

Contrary to expectation, these results suggest that cough responsiveness is enhanced in alert patients with neurogenic dysphagia even after allowing for diagnostic category, the possible presence of a bulbar upper motor neuron lesion, or voluntary respiratory capacity. It is concluded that these patients with neurogenic dysphagia do not have a reduced sensitivity of cough triggering.