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We read with interest the recently published article by Assal et al on “Post-traumatic stimulus suppressible myoclonus of peripheral origin”.1 They presented a patient with painless “myoclonus” of the first dorsal interosseous muscle of the foot that was abolished by local anaesthetic block. Electromyography showed bursts of rhythmic spontaneous grouping of motor units of 200 to 400 ms duration occurring at 1.6 Hz frequency. We take issue with the usage of the term “myoclonus” for the abnormal movements described. Myoclonus is a brief, shock-like, involuntary movement with EMG activity usually lasting less than 200 ms (often even 50 ms or less).2 Myoclonic discharges are usually arrhythmic and irregular, with the exception of segmental (brainstem or spinal) myoclonus which presents with rhythmic, long EMG bursts (>l00 ms) involving several spinal or body segments. Descriptions of the electrophysiology of peripheral myoclonus are scarce. In our experience, the EMG bursts in peripheral myoclonus can be >100 ms in duration (rarely more than 200 ms), but are not truly sinusoidal, are irregular in frequency and duration, and have a very variable interburst interval. By contrast, in the case presented by Assal et al, the EMG bursts appear sinusoidal, regular in frequency, and come in trains of EMG activity that are interrupted by periods of silence. Furthermore, even in rhythmic myoclonus with long EMG bursts, the movements still appear brief and shock-like. In contrast, the description by Assalet al of the “myoclonus” in their case was continuous and rhythmic. It also seems unlikely that EMG discharges of 400 ms duration will appear brief or shock-like. We agree that the EMG bursts resemble that of the painful legs and moving toes (PLMT) syndrome, which is often associated with peripheral nerve or nerve root lesions.3 A variant of PLMT, however, can have no associated pain and is referred to as the painless legs and moving toes syndrome.4 The EMG findings presented by Assalet al may very well represent myokymia. Myokymic discharges fire rhythmically at 1–20 Hz and in trains that last a few seconds followed by a short period of silence.5Myokymia is often seen in peripheral nerve disorders, is abolished by local nerve blocks, and is thought to represent ectopically generated potentials in demyelinated nerve fibres. A myokymic EMG pattern has also been described in PLMT due to hypertrophic mononeuritis.6
The authors reply:
We thank Evidente and Caviness for their interest in our paper. Their comments on the descriptive term we used to describe the abnormal movements of the second toe of our patient are well taken. We already partially considered this issue in our discussion.1-1
We do not agree that “myokymia” would adequately describe these movements for the following reasons. Clinically, there were no quivering nor undulating movements of the skin overlying the muscle, and, although myokymia may sometimes lead to a movement of the fingers or toes, this movement is of very limited magnitude, in our experience not nearly as gross as that seen in our patient.1-2 1-3Electrophysiologically, EMG myokymic discharges concern single motor units1-4 whereas many motor units fired synchronously in our patient. In some conditions myokymia may be related to the discharge of numerous single or grouped fasciculation potentials,1-5 but this activity does not manifest gross rhythmic muscle contractions either.1-2 Moreover, when myokymia have a peripheral origin, the ectopic activity originates from the motor nerve supplying the contracting muscle1-2 1-3 1-6 whereas we showed that this was not the case in our patient.1-1
The particular condition in our patient was that the abnormal impulse generator seemed to be located on a sensory nerve (branch of the deep peroneal nerve), distinct from the nerve (tibial nerve) supplying the contracting muscle. Therefore, a spinal cord relay had to be implicated. This might be an explanation for the rhythm and long duration of the myoclonic bursts seen since, as mentioned by Evidente and Caviness, spinal myoclonus may present such characteristics. This last hypothesis reinforces our impression that myoclonus, already used by others in similar conditions,1-7-1-9 was the most adequate term to describe the single painless moving toe of our patient.