A Bereitschaftspotential, or readiness potential, is a slow negative shift over the central electrodes that begins one to two seconds before a self-initiated voluntary movement. Demonstrating it before an apparently involuntary jerk, using jerk-locked back averaging, shows that the same cortical preparatory machinery that precedes voluntary movement is active, which supports functional myoclonus. The clinical picture reinforces this: jerks that vanish with distraction and during sleep, variable latency and a normal examination.
The distinguishing feature is timing and morphology. Cortical myoclonus shows a brief time-locked cortical spike 20 milliseconds or less before the electromyographic burst, with giant somatosensory evoked potentials and enhanced long-latency reflexes. Propriospinal myoclonus classically produces axial flexion jerks with slow rostrocaudal propagation along the cord at a few metres per second; however, systematic reappraisal has shown that the large majority of patients labelled with propriospinal myoclonus in fact have a Bereitschaftspotential and a functional disorder, so a readiness potential should always be sought before accepting a spinal generator.
Jerk-locked back averaging and readiness potential interpretation are fellowship-level clinical neurophysiology, so this item sits at the Subspecialty level of RITE content. Note the asymmetry of the evidence: presence of a Bereitschaftspotential supports a functional origin, but its absence does not prove the movement is organic.
Incorrect Answers
- B. Cortical reflex myoclonus is supported by a brief spike within about 20 milliseconds of the jerk, giant somatosensory evoked potentials and enhanced C-reflexes, not by a slow potential lasting seconds.
- C. A true propriospinal generator produces no cortical premovement potential; the presence of a readiness potential specifically argues against that mechanism.
- D. Reticular reflex myoclonus is identified by electromyographic recruitment beginning in sternocleidomastoid with rostral spread to cranial muscles, not by a slow cortical shift.
- E. Epileptic myoclonus would show a time-locked epileptiform transient or would be supported by polyspike discharges, and would not typically abate with distraction.
Testing Pearls
- A Bereitschaftspotential 1 to 2 seconds before a jerk supports functional myoclonus.
- Absence of a readiness potential does not establish that a movement is organic.
- Cortical myoclonus shows a spike within about 20 ms of the jerk plus giant somatosensory evoked potentials.
- Most patients labelled as having propriospinal myoclonus have a functional disorder.
- Distractibility, variability and disappearance in sleep are clinical clues to functional jerks.
References
- Jankovic J, Mazziotta JC, Newman NJ, Pomeroy SL, editors. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022. p. 2082.
- Terada K, Ikeda A, Van Ness PC, Nagamine T, Kaji R, Kimura J, et al. Presence of Bereitschaftspotential preceding psychogenic myoclonus: clinical application of jerk-locked back averaging. J Neurol Neurosurg Psychiatry. 1995;58(6):745-747.
- van der Salm SM, Erro R, Cordivari C, Edwards MJ, Koelman JH, van den Ende T, et al. Propriospinal myoclonus: clinical reappraisal and review of literature. Neurology. 2014;83(20):1862-1870.
