Hypnic jerks: neurophysiological characterization of a new motor pattern.

Hypnic jerks (HJs) are physiological, non-periodic, myoclonic bodily jerks, occurring mainly at sleep onset [1]. HJs occasionally reach abnormal proportions in frequency and degree (intensified HJs) impeding to fall or stay asleep [2]. In a recent video-polysomnographic (VPSG) study, four motor patterns of HJs have been recognized in 10 patients with intensified HJs and comorbid disorders: startle reflex, reticular reflex myoclonus, dystonic myoclonus and pyramidal myoclonus patterns [3]. We describe a further motor pattern of HJs, recorded through a nocturnal VPSG in a healthy 35-year-old man with a long history of sleep-onset insomnia due to involuntary jerks of the trunk and limbs accompanied by a sensation of ‘petrified head’. The patient was diagnosed with intensified HJs and received clonazepam with an improvement of jerks and insomnia.