Cyst of the septum pellucidum presenting as hemiparesis.

An 11-year-old boy with Down syndrome is presented who suffered progressive hemiparesis on the left side for a period of 5 years. Computed tomography demonstrated a large cyst of the septum pellucidum and a calcified spot in the head of caudate nucleus on the right side. By penetrating the cyst wall to create a communication into the lateral ventricle, shrinkage of the cyst and improvement of the hemiparesis were obtained. The pathogenesis of the hemiparesis was presumed to be attributed to circulatory compromise in the deep cerebral veins, secondary to the cyst.