No relation between sympathetic outflow to muscles and respiratory function in amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), not only impairment of motor neurons but also impairment of the autonomic nervous system has been demonstrated by previous physiological studies. Several investigators have reported a correlation between autonomic dysfunction and respiratory dysfunction in ALS. This study analyzed the relation between parameters of respiratory function and muscle sympathetic nerve activity (MSNA) in a large number of ALS patients.

In 50 patients with ALS (mean age (SD): 62.1 (11.7) years), MSNA, heart rate (HR), and blood pressure (BP) were recorded simultaneously. The arterial oxygen content (PaO2), arterial carbon dioxide content (PaCO2), and forced vital capacity expressed as a percentage of the predicted value for healthy controls (%VC) were determined as parameters of respiratory function.

There were no significant correlations between MSNA and PaO2, PaCO2, %VC, or the disability score. Analysis of chronological changes in 14 patients examined twice showed that the disability score and PaCO2 were significantly increased, and %VC was significantly more decreased at the second examination compared with the first examination (p<0.01 and p<0.05, respectively). In contrast, HR, BP, burst rate of MSNA, and age-adjusted MSNA exhibited no significant changes between the first and second examinations.

These findings show that gradual deterioration of respiratory function in ALS patients is not associated with changes of quantitative MSNA parameters, which may suggest that abnormality of the autonomic nervous system is a primary feature of ALS.