BACKGROUND: Bobble-head doll syndrome is a rare and surgically treatable 
movement disorder characterized by up-and-down (yes-yes) head bobbing occurring 
at a rate of 2-3 Hz. Side-to-side (no-no) head bobbing is less frequently 
described. Bobble-head doll syndrome is usually associated with dilation of the 
third ventricle, but is rarely associated with posterior fossa disease.
PATIENT: We describe an infant with fetal hydrocephalus and an arachnoid cyst of 
the posterior fossa. Endoscopic fenestration of the arachnoid cyst was performed 
on postnatal day 12. A routine examination at 4 months indicated the infant 
showed "no-no" type head bobbing, but no other neurological disorder was 
observed. The third ventricle was dilated during the perioperative period, but 
not at 2-4 months. In contrast, cerebellar compression decreased gradually and 
persisted at 4 months.
CONCLUSION: Although few patients with bobble-head doll syndrome do not have 
third ventricle dilation, these patients typically show cerebellar dysfunction. 
Our findings support the hypothesis that cerebellar dysfunction is present in 
bobble-head doll syndrome when third ventricle dilation is absent.