Motor asymmetry and neuromelanin imaging: Concordance in Parkinson's disease

The onset of motor symptoms in Parkinson's disease (PD) is characteristically asymmetric and correlates with dopaminergic deficit of contralateral basal ganglia. This study explored the concordance between motor asymmetry and changes of substantia nigra pars compacta (SNc) using neuromelanin-sensitive imaging.

Forty-four subjects with PD and fifteen healthy controls were included in this study. Clinical laterality (CL) was based on the motor sub scores of the Unified Parkinson's Disease Rating Scale-Part III. All subjects underwent neuromelanin-sensitive imaging and imaging laterality (IL) was based on the differences between contrast ratios of the right and left lateral SNc. Concordance was evaluated by correlating CL and IL.

Motor asymmetry at disease onset was reported in 97.72% of subjects with PD, of which 65.90% reported right-sided onset. Forty-three subjects were right-handed and 68.18% reported onset of symptoms on dominant side. Right CL was observed in 59.09%, left CL in 40.90%, right IL in 11.36% and left IL in 88.63%. Concordance was established in 61.36%, majority of whom had a combination of right CL and left IL. In healthy controls, a significantly lower contrast ratio of the left lateral SNc was also noted.

Handedness may correlate with motor asymmetry and left hemisphere may have a predilection for neurodegeneration. The finding of significant neuronal loss of left SNc in controls warrants further evaluation for better understanding of motor asymmetry in PD. Neuromelanin sensitive imaging can be a useful tool to study the relationship between motor asymmetry and nigrostriatal dysfunction.