Focal subcortical reflex myoclonus. A clinical and neurophysiological study
R. Cantello, M. Gianelli, C. Civardi and R. Mutani
Department of Neurology, School of Medicine, University of Turin, Novara, Italy.
BACKGROUND: Patients with progressive myoclonus epilepsy or progressive
myoclonus ataxia often show a focal myoclonus, both spontaneous and reflex
to somatosensory stimuli. Myoclonus is time-locked to large ("giant")
electroencephalographic potentials. Previous authors have classified it as
a "cortical reflex myoclonus," with the assumption that it invariably
arises from an abnormal corticifugal neuron discharge. OBJECTIVE: To
identify the myoclonus source, using various neurophysiological techniques,
in 5 patients with progressive myoclonus epilepsy/ataxia. METHODS:
Extensive investigations were performed to ascertain the clinical
diagnosis. Electrophysiologically, the main method was transcranial
cortical stimulation and motor evoked potential measurement. The latency
and amplitude of the spontaneous myoclonus and the premyoclonus cortical
spike, the reflex myoclonus (C-reflex), and the giant somatosensory evoked
potential were also analyzed. The behavior of giant somatosensory evoked
potentials and C-reflexes were then studied on single, consecutive trials.
Finally, the central motor pathway excitability and its changes
attributable to a prior somatosensory input were determined. RESULTS: The
motor evoked potential studies showed that the expected corticomuscular
conduction time (23 milliseconds) of the myoclonic electromyographic
potential was longer than that previously suspected. Considering this, the
premyoclonus cortical spike and the giant somatosensory evoked potential
were so close to the spontaneous/reflex jerks that they could not reflect a
cortical myoclonus source. In 4 patients, the C-reflex latency (< 41.6
milliseconds) was shorter than that often reported in previous studies. The
giant somatosensory evoked potential and the C-reflex showed no simple
cause-effect link. Motor pathways were hyperexcitable only in response to
somatosensory inputs. CONCLUSIONS: The data pointed to a cortical myoclonus
origin only in the patient whose C-reflex had the longest latency (44
milliseconds). In the remaining patients, a subcortical source was far more
likely. In this group of patients, cortical stimulation disclosed a new
myoclonus variety, for which the term focal subcortical reflex myoclonus is
proposed; it mimics cortical reflex myoclonus but has a shorter latency.
