[123I]Metaiodobenzylguanidine (MIBG) is a norepinephrine analog, which is transported into and stored in the terminals of sympathetic nerve endings. MIBG uptake is expressed as a ratio of single photon emission computed tomography signal in heart to that of the upper mediastinum. The lower the ratio, the fewer are the functioning sympathetic nerve terminals in the heart. A number of studies have looked at the sympathetic innervation of cardiac muscle in PD patients using this technique, and all have shown that MIBG uptake in myocardium is significantly less in PD than in matched controls (4). Courbon et al. (5) compared MIBG uptake in two groups of patients with PD: those with normal autonomic function tests and those with overt dysautonomia. They found that patients without dysautonomia had impaired MIBG uptake, just like patients with overt autonomic symptoms. They concluded that MIBG scintigraphy is a sensitive marker of PD, even in patients without autonomic symptoms. Orimo et al. (6) used this technique to compare MIBG uptake in PD patients, normal controls, neurological controls (essential tremor, vascular parkinsonism), and in patients with MSA. They found that the ratio of MIBG uptake in heart to mediastinum was decreased in 84% of PD patients, compared with normal controls. This uptake ratio was lower in more advanced compared with earlier stage PD patients, a finding which has been replicated using fluorodopa positron emission tomography (PET) scanning (7). The MIBG uptake ratio was not significantly different from controls in patients with essential tremor, vascular parkinsonism, or MSA (6). Thus, although MSA can be difficult to differentiate from PD with autonomic failure on clinical grounds, a number of studies have suggested that MIBG scintigraphy may be useful in making this distinction; PD patients have decreased uptake with MSA patients having relatively normal uptake.
In addition to being helpful at differentiating PD from mimicking conditions, autonomic involvement of the heart appears to be an early finding in PD. Spiegel et al. (8) performed MIBG scintigraphy in 18 PD patients with Hoehn and Yahr stage I disease, who were demonstrated to be levodopa-responsive. They found that 13 patients (72%) had significantly reduced cardiac tracer accumulation indicating that in most PD patients, autonomic involvement of the myocardium is an early finding and might be useful in helping to differentiate PD from other conditions.
Several authors have performed pathologic studies on postmortem cardiac tissue in an effort to visualize the sympathetic denervation associated with PD. These studies have shown a near complete loss of sympathetic axons in nerve fascicles of the epicardium (9,10), with Lewy bodies being found in the cardiac plexus (11,12). In one study, five of nine patients with Lewy bodies in the sinoatrial node had atrial cardiac arrhythmias during their life, suggesting that the sympathetic involvement of the heart in PD might predispose to cardiac arrhythmias (12).
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