Upper camptocormia in Parkinson's disease: Neurophysiological and imaging findings of both central and peripheral pathophysiological mechanisms

Background: Camptocormia is a disabling complication of Parkinson's disease (PD), but its pathophysiology is poorly elucidated. Depending on the fulcrum of forward trunk flexion, two subtypes have been defined, upper (UCC) and lower camptocormia, the former being much more frequent. The aim of the study was to explore possible pathophysiological mechanisms of PD-related UCC. Methods: Ten PD patients with UCC (UCC-PD) and ten PD patients without camptocormia (NoUCC-PD) underwent simultaneous electromyography (EMG) of thoracic paraspinal (TPS), obliquus externus abdominis (OEA), rectus abdominis (RA), and iliopsoas (IP) muscles during relaxed standing (both groups) and trunk realignment (UCC-PD group). Quantitative EMG and magnetic resonance imaging (MRI) of TPS muscles were also performed. Results: UCC-PD patients showed hyperactivity of TPS and OEA muscles in quiet stance. During voluntary trunk extension, hyperactivity of OEA muscles persisted, thus revealing a co-contraction of flexor and extensor trunk muscles. Motor unit potentials (MUP) of TPS muscles showed shorter duration (p = 0.005) and lower amplitude (p = 0.004) in UCC-PD than in NoUCC-PD patients. MRI did not detect significant between-group differences in the cross-sectional area and fat fraction of TPS muscles, although the latter was higher in the UCC-PD than in the NoUCC-PD group at all thoracic levels. Conclusion: Our findings suggest that hyperactivity of OEA might sustain UCC in PD. Concurrent mild myopathic changes in TPS muscles in PD with UCC may be secondary to muscle disuse but nevertheless may contribute to abnormal trunk posture.