All patients have frequent (near-continuous), fragmentary erratic myoclonus. This migrates from body part to body part in an asynchronous, asymmetric, random pattern. The face and limbs are commonly affected, and the myoclonus can be highly localized e.g. to a finger, toe, eyelid or lip.
After onset of the myoclonus, focal seizures and later epileptic spasms or tonic (generalized tonic seizures or focal tonic) seizures may occur. Focal seizures are often subtle and may be accompanied by tonic eye version and autonomic features (such as apnoea or facial flushing). Massive, usually bisynchronous, axial myoclonic jerks may occur.