Imaging for autonomic dysfunction
ABSTRACT
Direct visualization of heart-brain interactions is the goal when assessing autonomic nervous system function. Cortical topology relevant to neuroimaging consists of the cingulate, insula, and amygdala, all of which share proximity to the basal ganglia. Significant cardiac effects stemming from brain injury are well known, including alteration of cardiac rhythms, cardiac variability, and blood pressure regulation; in some instances, these effects may correlate with neuroimaging, depending on the region of the brain involved. It is difficult to achieve visualization of areas within the brainstem that govern autonomic responses, although investigators have identified brain correlates of autonomic function with the use of functional magnetic resonance imaging and electrocardiographic data obtained simultaneously. The potential utility of brain imaging in sick patients may be limited because of challenges such as the magnetic resonance imaging environment and blunted autonomic responses, but continued investigation is warranted.
FUNCTIONAL BRAIN IMAGING IN DISEASE STATES
There are few studies of functional brain imaging in patients with disease because of the challenges involved. The studies are difficult to perform on sick patients because of the unfriendly MRI environment, with struct requirements for attention and participation. Furthermore, autonomic responses may be blunted, making physiologic comparisons difficult. In addition, there is evidence that BOLD may be intrinsically impaired in disease states. Unlike fMRI studies to locate brain regions involved in simple tasks such as finger tapping, which can be performed in a single subject, detecting changes in autonomic responses in disease states requires averaging over studies of multiple patients.
Woo et al17 used fMRI to compare brain regions of activation in six patients with heart failure and 16 controls upon a forehead cold pressor challenge. Increases in heart rate were measured in the patients with heart failure with application of the cold stimulus. Larger neural fMRI signal responses in patients with heart failure were observed in 14 brain regions, whereas reduced fMRI activity was observed in 15 other brain regions in the heart failure patients. Based on the results, the investigators suggested that heart failure may be associated with altered sympathetic and parasympathetic activity, and that these dysfunctions might contribute to the progression of heart failure.
Gianaros et al18 found fMRI evidence for a correlation between carotid artery intima-media thickness, a surrogate measure for carotid artery or coronary artery disease, and altered ANS reaction to fear using a fearful faces paradigm.
CONCLUSION
Functional MRI of heart-brain interactions has strong potential for normal subjects, in whom the BOLD effect is small, within the limits of motion and susceptibility artifacts. Typically, such applications require averaging results over multiple subjects. Its potential utility in disease states is less significant because of the additional limitations of MRI with sick patients (the MRI environment, blunting of autonomic response in disease, possible impairment of BOLD), but continued investigation is warranted.
