Magnetic Resonance Evaluation of the Midbrain in Parkinson's Disease

To the Editor. —We commend the investigation by Huber et al 1 of midbrain pars compacta size in Parkinson's disease (PD) using magnetic resonance imaging (MRI). The authors have stimulated interest in an important area of new research, namely the use of MRI to correlate anatomic finding in vivo with clinical disease. Their detailed attention to the MRI methodology adds credence to their findings. However, methodological issues related to the composition and selection of the control group need to be considered carefully and the design of the present study illustrates some of the difficulties frequently encountered. The authors have not reported the range and mean (±SD) age for the mild and moderate/severe PD groups as well as the standard deviation for age in the control and PD groups. We are concerned that the group of mild PD patients may have been significantly younger than the control subjects. Age may have