Defective motor control of coughing in Parkinson's disease

The high incidence of serious chest infections in patients with Parkinson's disease is unexplained, but an impairment in cough reflex may have a role. Maximal voluntary cough (MVC) and reflex cough (RC) to inhalation of ultrasonically nebulized distilled water were analyzed in patients with Parkinson's disease and age-matched control subjects by monitoring the integrated electromyographic activity (IEMG) of abdominal muscles. The peak amplitude of IEMG activity (IEMG(p)) was expressed as a fraction of the highest IEMG(p) value observed during MVC corrected to account for possible losses in abdominal muscle force due to reduced central muscle activation. Cough intensity was indexed in terms of both the IEMG(p) and the ratio of IEMG(p) to the duration of the expiratory ramp (TEC), i.e., the rate of rise of IEMG activity. Cough threshold was slightly higher in patients than in control subjects, but the difference failed to reach statistical significance. Compared with control subjects, patients displayed a lower IEMG(p) during maximal expiratory pressure maneuvers (PEmax), MVC, and RC (p always < 0.01); TEC during RC was longer (p < 0.01) than in controls. Consequently, the rate of rise of IEMG activity during cough was always lower in patients (p < 0.01), especially during RC. Finally, PEmax, and both the peak and rate of rise of IEMG activity during RC were inversely related to the level of clinical disability (Spearman rank correlation coefficient, r(s) = -0.88, -0.86, and -0.85, respectively, p always < 0.01). The results indicate that the central neural mechanisms subserving the recruitment of motor units and/or the increase in their frequency of discharge during voluntary and, even more markedly, RC are impaired in patients with Parkinson's disease.