Abc

prepulse prepulse | ^ ^ prepulse

100 ms prepulse prepulse | ^ ^ prepulse

100 ms

Fig. 5. Prepulse inhibition of the blink reflex in a healthy volunteer and in a patient with PSP. The traces of the upper row show the responses to a stimulus to the supraorbital nerve applied at the vertical line. Ipsilateral recordings (ipsi) show R1 and R2 responses whereas contralateral recordings show only the R2. The traces of the lower row show the responses to the same stimulus when a prepulse is applied 100 ms before (arrows). (A) Normal auditory prepulse inhibition in a healthy volunteer; (B) Absent auditory prepulse inhibition in a patient with PSP. (C) Absent somatosensory prepulse inhibition in the same patient.

frequency spectrum of the signal through fast Fourier transformation. This procedure showed that movements of MSA patients were rather non-rhythmic in comparison to those of patients with other forms of tremor (Fig. 6). Salazar et al. (84) suggested the term minipolymyoclonus to be used to describe these small amplitude, irregular, jerklike abnormal movements. Other forms of myoclonus have been also reported in a few MSA patients (30,85), which might have their origin in a reduced inhibition of the strio-palido-thalamo-cortical circuit (86). Table 3 shows a list of disorders in which activity defined as minipolymyoclonus has been encountered, together with some of the most relevant neurophysiological findings.

Myoclonus is also an apparent feature in patients with CBD (87), in whom they are thought to be of cortical origin in spite of lacking neurophysiological evidence. The expected findings of cortical myoclonus, such as giant somatosensory evoked potentials and jerk-locked EEG potentials, are inconsistent in CBD. The cortical response is occasionally absent, which is attributed to the marked frontoparietal cortical atrophy and neuronal degeneration characteristic of these patients (88,89). Cortical atrophy of inhibitory neurons could lead to the enhanced (disinhibited) motor cortex excitability.

The "C" wave, or focal reflex myoclonus (90), is a response seen in forearm muscles after electrical stimulation of ipsilateral cutaneous nerves of the hand. This response is thought to be mediated by fast-conducting afferent and efferent pathways and might have a latency as short as 43.1+/-3.2 ms (87). In some patients, focal reflex myoclonus might be elicited by stimuli of an intensity below perception threshold, which suggests a direct connection from the thalamic nuclei to the motor cortex (91). The "C" response should not be mistaken for the long latency excitatory response of the cutaneo-muscular reflexes (92,93). The cutaneo-muscular reflex can be elicited during a sustained tonic voluntary contraction of the forearm muscles. The long latency excitatory component of the cutaneo-muscular reflex is abnormally enhanced in patients with IPD or MSA (30). However, the latency of such a response is longer than that of the "C" reflex.

Fig. 6. Surface EMG recording from wrist extensors (A), accelerometric recording of finger movements (B), and FastFourier analysis of the movement recording (C) in a patient with MSA and minipolymyoclonus. See the absence of a dominant frequency peak.

Table 3

Disorders Featuring Minipolymyoclonus

Disorder

Dominant Clinical Sign

EMG Bursts

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