[A case of monoballism in unilateral lower extremity--somatotopic relation in subthalamic nucleus].

A 62-year-old man with monoballism in the right lower extremity was reported. The cranial MRI showed a small lesion affecting the posterior portion of the contralateral subthalamic nucleus. Superficial EMG recording revealed 1-2 Hz rhythmic grouping discharges in right quadriceps femoris, hamstrings, tibialis anterior and gastrocnemius-soleus. In the vast majority of cases, ballistic movements involved both upper and lower extremities of one side (hemiballism), but the present case showed monoballism in the lower extremity only. Previous reports suggested the somatotopy mapping subthalamic nucleus; the posterior portion being associated with the lower extremity, middle part with the upper extremity and the oral pole with the face. In the present case, the affected part of subthalamic nucleus was towards its posterior portion and it seemed legitimate to associate such topographic location of the lesion with the absence of involuntary movements in the upper extremity and the face. Coronal and axial sections of high-field MR scans were useful for the detection of the responsible lesion.