Hypnic Jerking has had many names and has been hard to detect on testing. The diagnosis is easily overlooked because the involved muscle groups are small. This article sheds light on how to view evidence of the condition.
"Excessive fragmentary myoclonus (EFM), previously called excessive fragmentary hypnic myoclonus, is defined by the American Academy of Sleep Medicine (AASM) as at least five characteristic fragmentary myoclonus electromyographic potentials per minute during at least 20 minutes of NREM sleep.
EFM is scored using an optional rule (VII Movement Rules, Section 4) in the AASM scoring manual (Table 1).1 These electromyographic potentials are usually seen as muscle surface potentials of the tibialis anterior muscle with an amplitude of 50 to 200 microvolts and a duration of less than 150 milliseconds.2,3 Patients may describe these feelings as twitches or cramps, or they may be oblivious to any physical discomfort.
There are few published reports on the topic. In 1984, Broughton and colleagues first described this polysomnographic finding in a patient with excessive daytime somnolence.2 The clinical significance of EFM is not clear, but the polysomnographic findings are interesting, and the diagnosis can be overlooked since the muscle groups involved are small and do not usually lead to visible limb motion. "