Cardiac sympathetic innervation and vesicular storage in pure autonomic failure

Abstract

Objective: Pure autonomic failure (PAF) is a rare disease characterized by neurogenic orthostatic hypotension (nOH), absence of signs of central neurodegeneration, and profound deficiency of the sympathetic neurotransmitter norepinephrine. Reports have disagreed about mechanisms of the noradrenergic lesion. Neuropathological studies have highlighted denervation, while functional studies have emphasized deficient vesicular sequestration of cytoplasmic catecholamines in extant neurons. We examined both aspects by a combined positron emission tomographic (PET) neuroimaging approach using ¹¹ C-methylreboxetine (¹¹ C-MRB), a selective ligand for the cell membrane norepinephrine transporter, to quantify interventricular septal myocardial noradrenergic innervation and using ¹⁸ F-dopamine (¹⁸ F-DA) to assess intraneuronal vesicular storage in the same subjects.

Methods: Seven comprehensively tested PAF patients and 11 controls underwent ¹¹ C-MRB PET scanning for 45 minutes (dynamic 5X1', 3X5', 1X10', static 15 minutes) and ¹⁸ F-DA scanning for 30 minutes (same dynamic imaging sequence) after 3-minute infusions of the tracers on separate days.

Results: In the PAF group septal ¹¹ C-MRB-derived radioactivity in the static frame was decreased by 26.7% from control (p = 0.012). After adjustment for nonspecific binding of ¹¹ C-MRB, the PAF group had a 41.1% mean decrease in myocardial ¹¹ C-MRB-derived radioactivity (p = 0.015). The PAF patients had five times faster postinfusion loss of ¹⁸ F-DA-derived radioactivity (70 ± 3% vs. 14 ± 8% by 30 minutes, p < 0.0001). At all time points after infusion of ¹⁸ F-DA and ¹¹ C-MRB mean ¹⁸ F/¹¹ C ratios in septal myocardium were lower in the PAF than control group.

Interpretation: PAF entails moderately decreased cardiac sympathetic innervation and a substantial vesicular storage defect in residual nerves.