The test consists of simultaneous recording of EMG activity (which detects myoclonic jerks) and EEG.

Back-averaging refers to the fact the EEG requires averaging in order to demonstrate the “spike” which precedes the myoclonic jerk, since the signal making up the spike is of low amplitude and signal to noise ratio is poor.

Performing the test therefore needs:

  1. EMG and a trigger line: every time there is a myoclonic jerk of sufficient size, the discharge crosses the trigger line.
  2. Back-averaged EEG: when the trigger line is crossed, this results in the averaging of an EEG epoch that took place immediately before the myoclonic jerk.
  3. Averaging: after a certain number of myoclonic jerks, typically 50, there should be sufficient EEG averaged in order that the spike is visible.

The recorded “spike” should have a latency of 20 ms for a distal hand muscle (a latency approximately equal to that seen with cortical stimulation of the hand area and recording from the hand). The “spike” is typically electropositive.

The usual site of recording is from the C3' and C4' electrode sites, corresponding to the hand area of the two hemispheres. Electrodes are typically linked to the central and frontal vertex electrodes, Cz and Fz.        Filter settings for EEG are set at 50 Hz and 0.05 Hz for high and low cut respectively.  EMG filter settings are typically 3 KHz and 5 Hz for high and low cut respectively.

 

Figure (E)F-1

A: Bipolar parasagittal montage.  JLBA recording from right thenar eminence. 

Electrodes labelled according to the International 10-20 system.  EMG record corresponds to X1-X2.  Vertical division of 5 μV/division.  Horizontal division of 100 msec/division.

Patient with Lance-Adams syndrome.  Figure shows JLBA with spike and myoclonic jerks. 

 

 

Deuschl G, Schenck E, Lucking CH, Ebner A. Cortical reflex myoclonus and its relation to normal long-latency reflexes. In: Benecke B, Conrad B, eds. Motor Disturbances I. London: Academic Press 1987: 305-19.

Allison T, Mccarthy G, Wood CC, Jones SJ. Potentials evoked in human and monkey cerebral cortex by stimulation of the median nerve. A review of scalp and intracranial recordings. Brain. 1991;114 ( Pt 6):2465-503.

Carr, JA. (2009). Familial Adult Myoclonus Epilepsy: A clinical, neurophysiological and genetic study of a familial form of myoclonic epilepsy (PhD thesis, University of Stellenbosch, Cape Town, South Africa).