A 2.5-year-old boy with Prader-Willi syndrome and a history of neonatal superior sagittal sinus thrombosis developed a febrile seizure and new onset atonic drop seizures originating in the parasagittal region, as reported from Children’s Hospital, Boston. A video-EEG captured 11 electroclinical seizures associated with head drops and loss of postural tone. EEG showed seizure onset at Cz, followed by generalized bursts of slowing with embedded spikes. At 4 and 5 months after levetiracetam initiation and normalized EEG, he developed breakthrough seizures with a viral illness, controlled with increased dose. Video-EEG with EMG electrodes is recommended for Prader-Willi syndrome patients with drop seizures, to differentiate cataplexy from seizures. 
COMMENT. Prader-Willi syndrome caused by absence of expression of the paternal active genes in chromosome 15q11-q13 is associated with seizures in 26% patients, whereas Angelman syndrome, caused by absence of expression of the same region, but from maternally inherited chromosome, is associated with seizures in 89% patients. Febrile seizures may occur in both syndromes, and in Prader-Willi patients, afebrile seizures are varied in type, mostly generalized tonic-clonic, less frequently absence, partial, myoclonic, and now, atonic. (Wang PJ et al, 2005; Kumada T et al, 2005; Fan Z et al, 2009; cited by authors).