Impact of different stimulation types on orthostatic tremor
Introduction
Primary orthostatic tremor (OT) is defined as a clinical syndrome with unsteadiness when standing caused by a fast regular leg tremor at frequencies of 13–18 Hz (Deuschl et al., 1998). Tremor activities in arm and leg muscles of both sides are highly coherent (Lauk et al., 1999, Köster et al., 1999, McAuley et al., 2000). Furthermore, cranial muscles exhibit the same tremor frequency as limb muscles (Köster et al., 1999). Thus, OT is thought to be driven by a unique supraspinal tremor generator (Lauk et al., 1999, Köster et al., 1999), in contrast to other tremors with independent tremor frequencies in different limbs (Lauk et al., 1999, Raethjen et al., 2000). Further differences between OT and other tremors have been revealed by resetting studies: Tsai et al., 1998, Pfeiffer et al., 1999 observed in a total of three patients that OT can be reset by single transcranial magnetic stimuli (TMS). On the other hand, peripheral nerve stimuli do not modify OT (Thompson et al., 1986, Britton et al., 1992a, Sander et al., 1998, Wu et al., 2001). In contrast to OT, parkinsonian tremor and essential tremor are reset by both TMS (Britton et al., 1993b, Pascual-Leone et al., 1994) and peripheral nerve stimuli (Bathien et al., 1980, Britton et al., 1993a, Spiegel et al., 2002).
So far, only the stimulation effect on OT in the stimulated contralateral leg has been studied. The impact of stimulation on OT in the ipsilateral leg is unclear. However, in the case of a unique supraspinal tremor generator one would expect similar stimulation effects on both the contra- and ipsilateral leg. To answer this question, we stimulated different levels of the central and peripheral nervous system in OT patients and examined the stimulation effects on OT in the contralateral as well as the ipsilateral leg. In detail, we used TMS, lumbar magnetic stimulation and supramaximal peripheral electrical nerve stimulation analogous to previous resetting stimulation protocols in different tremor types (Bathien et al., 1980, Thompson et al., 1986, Britton et al., 1992a, Britton et al., 1993a, Britton et al., 1993b, Pascual-Leone et al., 1994, Tsai et al., 1998, Sander et al., 1998, Pfeiffer et al., 1999, Wu et al., 2001). Further, we applied rhythmical submaximal nerve stimulation to study the impact of proprioceptive input on OT.
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Patients
All seven patients (3 women, 4 men, age 44–76 years, median 70 years) fulfilled the diagnostic criteria of primary OT (Deuschl et al., 1998). The duration of illness amounted to 3–12 years (median 5 years). At the time of investigation, two patients were on a medication with primidone, one patient on a medication with valproate, but OT was still present in these patients even during medication. Four patients had no medication. In all patients, sitting or walking relieved the symptoms
Orthostatic tremor during transcranial magnetic stimulation
The transcranial magnetic stimulation (TMS) evoked a motor evoked potential (MEP) in the contralateral tibialis anterior muscle (TA; Fig. 2). After this MEP, tremor was suppressed for 226±65 ms (mean±SD). Then OT reappeared in the contralateral TA but did not follow the predicted tremor rhythm: there was a clear difference between the actual time and the predicted time of appearance of tremor bursts (Δt) in all patients (Δt for the first post-stimulus burst 42.2±11.9 ms). This indicates that OT
Discussion
In our study, the transcranial magnetic stimulation (TMS) was the only stimulation type that influenced OT significantly: the tremor rhythm post-stimulus differed from the predicted tremor rhythm. The burst-to-burst intervals were significantly shorter after the magnetic stimulus. Our results are in line with Pfeiffer et al., 1999, Tsai et al., 1998 who applied a stimulus intensity sufficient to evoke an EMG response in the relaxed muscle. Both studies were done in a small number of patients
References (34)
- et al.
Effect of stimulation of an upper limb on motor evoked potentials in lower limb muscles to transcranial magnetic stimulation in normal subjects and patients with thalamic infarction
Clin Neurophysiol
(1999) - et al.
Phase resetting and frequency entrainment of essential tremor
Exp Neurol
(1992) - et al.
Side-to-side correlation of muscle activity in physiological and pathological human tremors
Clin Neurophysiol
(1999) - et al.
The silent period after transcranial magnetic stimulation is of exclusive cortical origin: evidence from isolated cortical ischemic lesions in man
Neurosci Lett
(1994) - et al.
Cortico-motor excitability of the lower limb motor representation: a comparative study in Parkinson's disease and healthy controls
Clin Neurophysiol
(2002) - et al.
Inhibition and synchronisation of tremor induced by a muscle twitch
J Neurol Neurosurg Psychiatry
(1980) - et al.
Primary orthostatic tremor: further observations in six cases
J Neurol
(1992) - et al.
‘Resetting’ of postural tremors at the wrist with mechanical stretches in Parkinson's disease, essential tremor, and normal subjects mimicking tremor
Ann Neurol
(1992) - et al.
Modulation of postural tremors at the wrist by supramaximal electrical median nerve shocks in essential tremor, Parkinson's disease and normal subjects mimicking tremor
J Neurol Neurosurg Psychiatry
(1993) - et al.
Modulation of postural wrist tremors by magnetic stimulation of the motor cortex in patients with Parkinson's disease or essential tremor and in normal subjects mimicking tremor
Ann Neurol
(1993)