Table 34.3 Open in new tab Clues for the...

Table 34.3

Clues for the differentiation of essential myoclonus and myoclonic dystonia

Criterion	Essential myoclonus (± dystonia – also known as dominantly inherited myoclonic dystonia with dramatic response to alcohol)	Myoclonic (idiopathic torsion) dystonia
Age of onset	Classically childhood, but mild cases can be diagnosed at any age	Variable
Pattern of involvement	Stereotyped distribution, predominantly in proximal arms and neck. Dystonia can rarely involve legs in infants, but then subsequently regresses	Variable. However, childhood onset often involves legs, with subsequent spread of dystonia
Predominance of myoclonus and dystonia	May be pure myoclonus or dystonic myoclonus. Pure dystonia very rare	Dystonia ± myoclonus. Pure myoclonus very rare or never
Character of jerks	Lightning, tic-tac, shock-like	Less brusque
Dominant family history given by patient	+±	±
Response to alcohol	+++	±
EMG	30 ms to 1.5 s bursts, occasionally triphasic agonist-antagonist-agonist bursts	50 ms to 1.5 s bursts
SSEP	Normal	Normal
Routine EEG	Normal	Normal
Back-averaged EEG	Some cases show widespread, symmetrical, and synchronous negative or (less often) positive wave preceding jerk by 25–70 ms	Normal

Criterion	Essential myoclonus (± dystonia – also known as dominantly inherited myoclonic dystonia with dramatic response to alcohol)	Myoclonic (idiopathic torsion) dystonia
Age of onset	Classically childhood, but mild cases can be diagnosed at any age	Variable
Pattern of involvement	Stereotyped distribution, predominantly in proximal arms and neck. Dystonia can rarely involve legs in infants, but then subsequently regresses	Variable. However, childhood onset often involves legs, with subsequent spread of dystonia
Predominance of myoclonus and dystonia	May be pure myoclonus or dystonic myoclonus. Pure dystonia very rare	Dystonia ± myoclonus. Pure myoclonus very rare or never
Character of jerks	Lightning, tic-tac, shock-like	Less brusque
Dominant family history given by patient	+±	±
Response to alcohol	+++	±
EMG	30 ms to 1.5 s bursts, occasionally triphasic agonist-antagonist-agonist bursts	50 ms to 1.5 s bursts
SSEP	Normal	Normal
Routine EEG	Normal	Normal
Back-averaged EEG	Some cases show widespread, symmetrical, and synchronous negative or (less often) positive wave preceding jerk by 25–70 ms	Normal

Reproduced with permission from Quinn NP, Rothwell, JC, Thompson, PD, Marsden CD. Hereditary myoclonic dystonia, hereditary torsion dystonia and hereditary essential myoclonus: an area of confusion. In Dystonia 2 (Advances in Neurology), Eds: Fahn S et al, Raven Press, 1988; 50:391–401. © Lippincott Williams & Wilkins.

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