Heart rate variability and neurological disorders

Abstract

Heart rate variability (HRV) can generally be used to evaluate cardiac autonomic function in patients with neurological disorders. The low-frequency (LF) component of the RR interval on spectral analysis is influenced by both the sympathetic and parasympathetic nervous systems, whereas the high-frequency (HF) component of the RR interval mainly reflects cardiovagal tone. The ratio of the power at LF to that at HF (LF/HF) has been suggested to be an indicator of the sympathetic nervous system. LF and LF/HF were reduced even in early-stage de novo Parkinson’s disease (PD) without orthostatic hypotension, despite no significant difference in HF. This finding indicated that sympathetic nerve dysfunction in the sinus node occurs in patients who have early-stage PD without parasympathetic nerve dysfunction. Cardiovascular function as evaluated by HRV was also associated with olfactory impairment. Reduced LF and HF components were found in multiple system atrophy. It seems that the impairment was more severe than that in PD. Although autonomic dysfunction in patients with progressive supranuclear palsy (PSP) has been controversial in previous studies, no clinically significant autonomic dysfunction was found in PSP patients who underwent evaluation of HRV. In dementias, the evaluation of HRV revealed reduced cardiac vagal denervation in dementia with Lewy bodies (DLB) and Parkinson’s disease with dementia (PDD) and partial impairment of sympathovagal balance and abnormal postural heart modulation in Alzheimer disease (AD). It is thought that patients with ischemic stroke have depressed parasympathetic activity in the acute or chronic stage, while the impairment of sympathetic activity remains conflicting. HRV revealed that both sympathetic and parasympathetic functions were significantly decreased in patients with early-stage familial amyloid polyneuropathy (FAP), while sympathetic activity was more decreased in patients with advanced-stage FAP. Cardiac autonomic neuropathy (CAN) was evaluated by spectral analyses of the RR interval in diabetes mellitus. Parasympathetic dysfunction was apparently present in patients with mild autonomic dysfunction, while sympathetic dysfunction was observed in patients with moderate or severe autonomic neuropathy. A few studies that used a 24-hour (24-h) heart rate power spectrum to evaluate autonomic dysfunction in Guillain-Barré syndrome have been reported. The HF component was significantly decreased at the height of the disease, and the LF/HF ratio increased as compared with the follow-up value after 1 year. Spectral analysis of HRV showed significantly reduced LF and HF components, and LF/HF was significantly greater in ALS patients than in healthy subjects. HRV is a useful marker of cardiovascular autonomic dysfunction in neurological disorders because it can be determined simply and noninvasively.