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This page intentionally left blank CHAPTER 5 Electrophysiological Approaches to the Movement Disorder Patient Carla Cordivari Introduction Neurophysiological techniques are an objective way of investigat- ing movement disorders and can support the clinical diagnosis as well as monitoring their severity and the eff ects of treatment. Over the past 25 years there have been enormous advances in our understanding of the pathophysiology of movement disorders, and this has been paralleled by the broadening range of investiga- tive techniques available to clinicians. Many of these techniques, particularly transcranial magnetic stimulation (TMS), have been extremely usefu l in supporting clinical fi ndings. However, the inter-individual variability in the respon ses, particularly for TMS, has resulted in diffi culties in translating this method into a defi nite clinically applicable diagnostic use. As a result, many neurophysio- logical meth ods are still only used for research purposes. Tremor can arise from diff erent sources. It is possible to sepa- rate a central tremor fro m peripheral tremor by using an external loading of the limb (500 \200\2231000 g in the han d). External loading typically reduces the tremor frequency in peripheral tremor (physi- ol ogical tremor, exaggerated physiological tremor (EPT) , neuro- pathic tremor), but not in c entral tremor (Parkinson \200\231s disease (PD), essential tremor (ET), dystonic tremor (DT ) , cerebellar tremor). However, most pathological tremors are generated from a central osc illator and this test is not always necessary. Th e amplitude of the tremor is actually of very little diagnostic value, but can be useful for objectively monitoring the response to treatment. Th e pattern of muscle activation between agonist and antagonist muscles (synchronous versu s alternating) is not pathognomonic for a particular subtype of tremor. Both synchronous an d alternating patterns of muscle activity can be seen in several tremor disorders. Furtherm ore, the pattern of muscle activity is oft en infl uenced by \200\230crosstalk \200\231 ac tivity from agonist \200\223antagonist muscles which further limits its diagnostic utility. In this chapter we will discuss the most common neurophysio- logical tests. Th ese should a lways be interpreted in conjunction with clinical features. Th ese techniques may disclose information that cannot be obtained by clinical observation alone. Tremor results in bursts of muscle activity, and the duration of the burst is directly rela ted to the tremor frequency. Faster tremors, such as orthostatic tremor and tremor of corti cal origin (rhythmic cortical myoclonus), produce shorter bursts of mus- cle activity. Tremor Tremor is the most common movement disorder and the most fre- quently evaluated by n europhysiological studies (1). For diagnostic purposes, the frequency of the tremor is the most useful piece of informatio n that can be gained neurophysiologically. Tremor occurring at frequencies higher than 11 H z is defi nitely pathological and usually represented by orthostatic tremor with a frequenc y of 13 \200\22318 Hz. Patients with orthostatic tremor present clinically with gait unstea diness and a trembling sensation that occurs on standing. Th e clinical examination is unre markable and tremor is frequently palpable but not seen because of the very high frequency. Th e pathognomonic neurophysiological feature is a \200\230coherent \200\231 tremor activ ity of the lower limbs at 13 \200\22318 Hz that is absent when lying or sitting (3). Th is tremor can be studied by simply positioning one surface electrode or accelerometer on the q uadriceps or tibial muscles and recording the frequency of muscle bursts (see Fig. 5.1). Tremor is a rhythmical oscillatory movement of a body part. It may be a symptom of many dis eases aff ecting the nervous sys- tem, but may also occur in normal subjects as a manifesta tion of physiological tremor. More than with any other movement disor- ders, the neurophysi ological fi ndings of tremor cannot be evaluated in isolation, as a proper diagnosis oft en requires a close correlation with the clinical features. Th e conditions favoring tremor ( rest, postural, action) and the presence of associated neurological signs provide important guidance in the electrophysiological diagnosis of tremor. Neurophysiological tests using multichannel electromyographic (EMG) recording and accelerom eter data identify the type of tremor on the basis of its frequency, pattern of muscle acti vation, duration, and amplitude of the muscle bursts (2). Oxford Textbook of Movement Disorders 34 250 Coherence between R VM and L VM 0.85 0.8 \200\223250 250 0.75 0.7 0.65 0.6 \200\223250 250 0.55 0.5 e c n e r e h o C 0.45 0.4 \200\223250 250 0.35 0.3 0.25 0.2 \200\223250 250 0.15 0.1 0.05 100mS/div \200\223250 0 10 20 30 40 50 Freq. (Hz) 60 70 80 90 100 Fig. 5.1 13 Hz orthostatic tremor and right and left vastus medialis coherence at 13 Hz. (a) (b) (c) 200 ms/div 50 ms/div 20 ms/div Fig. 5.2 (a) Negative myoclonus in 20 Hz tremor. (b) Short duration muscle bursts of 30 ms . (c) Rostro-caudal progression. Fast rhythmical cortical myoclonus can also produce a tremor in this or a higher frequency range. Cortical myoclonus may present with a fast and rhythmic activity resembling a tremor (4, 5). Hereditary forms have recently been identifi ed, and individuals fre- quently pres ent with cortical myoclonus and epilepsy (6 \200\2239). It is a postural \200\230tremor \200\231 with a jerky action. Surface EMG recording reveals a high frequency rhythmic activ ity up to 20 Hz with muscle bursts of very short duration (less than 50 ms). When such a hi gh frequency is recorded a multichannel EMG recording may be necessary to show the characte ristic pattern of muscle recruitment, with rostro-caudal progression refl ecting propagatio n along the pyramidal pathway. Rhythmic cessation of muscle tone, which is characteristic o f nega- tive cortical myoclonus or asterixis, is also seen. Th e predominant involvement of distal muscles of the hands and face simply refl ects the disproportionately large represe ntation of these body parts within the sensorimotor cortex. Simultanous EEG recording may b e useful to detect cortical abnormalities such as spikes, polyspikes, or runs of rhythmic s low activity (10) (see Fig. 5.2). For those parkinsonian patients with a combination of rest and postural/kinetic tremors, th e frequency of the latter may be similar to or more than 1.5 Hz higher than the rest tremor frequency. Physiological and enhanced physiological tremors are character- ized by relatively high fre quencies (7 \200\22312 Hz). Physiological trem- ors are present in many normal individuals and generally have no clinical signifi cance. Th e enhanced physiological tremor is mainly postural, with a high frequency between 7 and 12 Hz and relatively low amplitude. Typically , it is aggravated by drugs, alcohol with- drawal, and hyperthyroidism. It occurs also unde r conditions of stress, fatigue, anxiety, or hypoglycaemia (1). In classic essential tremor (ET) , the rate of tremor may vary between 4 and 12 Hz, with ol der patients typically demonstrating tremor frequencies towards the lower range. In contras t, younger patients with mild ET tend to have tremor frequencies that extend into the 7 \200\22312 Hz range of physiological tremor. A kinetic compo- nent, when present, is genera lly at the same frequency. Functional disabilities include problems with handwriting, drink ing, eating, dressing, and speaking. It is still debated whether ET can be defi ned as a \200\230bilateral, largely symmetrical postural or kinetic tremor involving hands and forea rms that is visible and persistent \200\231. It is also accepted that it can be unilateral at onset but will ultimately become bilateral in all suff erers (11). Tremors occurring at frequencies lower than 4 Hz are rare and represent midbrain (rubral or Holmes) tremor, cerebellar tremor (2 \200\2234 Hz), and palatal tremor (1 \200\2233 Hz). T h ey are easily distinguished on clinical grounds (11). Th e pure rest tremor seen in some patients with Parkinson \200\231s dis- ease (PD) is char acteristically greater than 4 Hz, although the upper frequency limit of these tremors has n ot been fi rmly established. Neuropathic tremor is relatively uncommon and generally con- sists of irregular postural or kinetic tremors of variable frequency CHAPTER 5 Electrophysiological Approaches to the Movement Disorder Patient 35 ranging from 4 to 11 Hz. It occurs in the context of an established peripheral neuropathy ( 12). central origin or reduce in physiological tremor (15, 16). In PsyT, amplitude and frequency decrease during distraction (counting, tapping). Furthermore, tapping diff erent frequenci es with the unaf- fected hand \200\230entrains \200\231 the same frequency on the tremulou s side. Ballistic movements performed on the contralateral limb can also signifi cantly red uce the tremor amplitude or even produce a pause in the tremor (18 \200\22321). Increased c oherence between limbs in bilat- eral tremor is also a pointer to a psychogenic etiology. M uscle coactivation between agonist and antagonist muscles is oft en recorded (coactivation sign). Dystonic tremor is an irregular tremor (jerky tremor), usually below 7 Hz, which is associa ted with dystonic postures in the aff ected extremity or elsewhere. It is subject to positi on- and task-specifi c worsening, and increases with attempts to move the body part in the opposite direction to the dystonic pattern (11). Th is may be associated with head dystonia . Although classifi ed separately, some tremors such as \200\230primary writing tremor \200\231 (frequency 5 \200\2237 Hz) and \200\230vocal tremor \200\231, which produces a qu ivering of the speech at a fre- quency of approximately 3 \200\2238 Hz, actually represent a form of dys- tonic tremor. Dystonic tremor is a mainly postural or action tremor but may also be present at rest and resemble PD (13). When the onset of tremor can be recorded, a tonic coactiva- tion occurring in the antagonis t muscles is seen 300 ms before the onset of the tremor burst in PsyT (16). A set of scorin g criteria using the sum results from all these individual tests has recently been reported to be both sensitive and specifi c in the diagnosis of PysT (22). Most pathological tremors occur at intermediate frequencies (4 \200\2239 Hz), and therefore it is not possible to distinguish those entities on the basis of the frequency of the trem or alone as in individual cases their frequencies oft en overlap. Additional tests and neur ological fi ndings are useful in classifying these tremors. Myoclonus Myoclonus is an involuntary movement disorder which occurs in many diff erent con ditions with a wide variety of diff erent aeti- ologies and neurophysiological phenomena, b ut with a common semiological element: sudden, brief, shock-like, involuntary move- ments c aused by muscular contractions or inhibitions (23). Recent studies with patients suspected to have tremulous dominant early PD but no evidence of nigrostriatal denerva- tion on dopamine transporter scanning (scans without evidence of dopaminergic defi cit (SWEDDs)), suggest that these subjects may in fact have dystonic trem or (13). It is very important to distinguish these two forms of tremor as the prognosis is diff er- ent; SWEDDs are not likely to progress in severity or to develop long-term motor a nd non-motor complications, as seen in PD. Furthermore, a correct diagnosis will avoid inap propriate use of PD drug treatments (14). Muscle contraction produces positive myoclonus, and muscle inhibition causes negative myocl onus. It can be focal, multifocal, generalized, spontaneous, or refl ex. It may occur at re st, when maintaining a posture, or during action. Myoclonus is classifi ed according to its physiopathological basis as cortical, reticular, or spinal (10, 24). In SWEDDs and patients with dystonic or essential tremor, no re-emerging tremor is seen as it occurs immediately aft er changing from rest to a position with the arm outstretched at shoulder level. In contrast, patients with PD with a postural tremor have a \200\230re-eme rgent \200\231 postural tremor, starting on average aft er 3 \200\2234 s. SWEDDs and those with segmental dystonia exhibit an abnor- mally exaggerated response to the paired associat ive stimulation (PAS) protocol which involves repeated pairing of median nerve and motor co rtical magnetic stimulation given at inter-stimulus intervals of 25 ms (14). Cortical myoclonus is characterized by jerks of short duration, involving many muscles and usually synchronously in agonists and antagonists. Myoclonus may be positive at rest or tri ggered by movements, or negative when the limbs are outstretched against gravity. Cortical myoclonus is most marked in the distal limb, and in focal forms is usually confi ned to thi s site. If widespread, myo- clonus is multifocal, and in some cases there are additional bi lateral or generalized jerks. It may aff ect speech and gait. Th ere are features that can help to distinguish between PD patients and SWEDDs which may b e useful in clinical practice: lack of true bradykinesia, presence of dystonia, and head tr emor favored a diagnosis of SWEDDs, whereas re-emergent tremor, true fatiguing or decrement , good response to dopaminergic drugs, and presence of non-motor symptoms favor PD (13, 14) . Refl ex jerks may be elicited by touch and tap, or visual stimuli (refl ex myoclonus). When there is sensitivity to auditory stimuli, this suggests a startle syndrome, although some patients may have a combination of cortical myoclonus and pathological startle. Multifocal jerks occurring with voluntary action are very suggest- ive of a cortical origin for the my oclonus. Jerks are usually arrhyth- mic but can also be rhythmic (cortical tremor) or perio dic. Patients with essential tremor can easily be confused with a dys- tonic tremor when a minim al dystonic component is present; lack of an exaggerated response to the PAS protocol suppo rts the diag- nosis of essential tremor. However, the role of TMS in improving diagnostic a ccuracy has not been fully validated (15). Th e myoclonus is usually due to a lesion of the sensorimotor cor- tex which results in an abnormal discharge that is conducted rap- idly down the corticospinal pathways to produce a brupt excitation or inhibition of spinal motor neurons and cause brief contraction or inhib ition of innervated muscles (jerk). Psychogenic tremor Psychogenic tremor (PsyT) is the most commonly reported psy- chogenic mo vement disorder. Neurophysiological studies are help- ful in distinguishing between PsyT an d organic tremor (OrgT). Positive myoclonus is represented by brief jerks of electromyo- graphic (EMG) activity with a duration usually less than 70 ms, while negative myoclonus is produced by a sudden inter ruption of a tonic muscle contraction (between 50 and 400 ms). EMG record- ing from an extr emity can also demonstrate spread of jerks (or periods of \200\230muscle silence \200\231) from the proximal to the distal muscle with a velocity corresponding to that of alpha moto r fi bres. For this reason muscle activity is recorded from several limb and cranial muscle s using multichannel EMG (25). Variation in the frequency of the tremor in response to weight loading can assist in diff e rentiating PsyT from OrgT (16, 17). In the former, the frequency of tremor increases with w eight load owing to an increase in coactivation to maintain the oscillation. With OrgT the frequency tends to remain the same in tremors of Oxford Textbook of Movement Disorders 36 Th e simultanous recording of EMG and EEG allows analysis of the relationship between myocl onic and cortical events. Cortical activities are recorded by classical EEG using a 10 \200 \22320 montage. In cortical myoclonus the EEG oft en shows multifocal or generalized spike and wave discharges, or multiple spike and wave discharges, which are usually time-locked t o the muscle jerks. While the EEG may not show any time-locked abnormality in some cases, E EG back-averaging can disclose myoclonus-related EEG activity that may not be recognized on the conventional polygraph. Th is tech- nique can determine the precise time interval from the EEG activ- ity to the myoclonus. It also identifi es the scalp distribution of the myo clonus-related EEG activity based on simultanous multichan- nel recordings. and recorded over the parietal cortex. Th ese SEPs are obtained by averaging EEGs over seve ral hundreds of peripheral nerve stimuli. Th e initial components, a post-central negative peak (N20) and a pre-central positive peak (P20), are not enhanced; however, the subsequent components (P25, P30, N35) are three to ten times larger than normal. It is generally agre ed that 10 V represents the threshold above which cortical responses are considered enla rged, yielding so-called giant SEPs. Giant SEPs are commonly found in patients with cortica l myoclonus, in particular those with progres- sive myoclonic epilepsy, and are presumably related to a hyperexcit- able sensorimotor cortex (25). Not all patients with cortical myoclonus display giant SEPs. In this case it is useful to a ssess cortical excitability by studying the recov- ery cycle of SEPs (SEP-R) using paired-p ulse stimuli. In normal subjects the second cortical SEP is attenuated at an inter-stimulus interval below 100 ms. Th e fi nding of SEPs that are larger aft er a paired-pulse stimula tion than aft er a single stimulation is strongly suggestive of cortical hyperexcitability and thus, in the context of myoclonus, of a cortical origin (10). EEG \200\223EMG back-averaging is performed by analyzing the record- ing offl ine and placi ng a marker at the onset of each myoclonic jerk. Th en the EEG is averaged around the marke r, for example 200 ms before and 200 ms aft er (see Fig. 5.3). Back-averaging analysis shows a positive \200\223negative biphasic spike at the central ele ctrode somatotopically representing the muscle from which the myoclonus is recorded. Th e i nitial positive precedes the onset of myoclonic EMG discharge in a hand mus- cle by approxi mately 20 ms. Th e more distal the muscle the myo- clonus is recorded from, the longer is t he EEG \200\223EMG time interval. Myoclonus-related discharge spreads through the motor cor tex within one hemisphere, and also transcallosally to the homolo- gous area of the contral ateral motor cortex (10 \200\22315 ms) (26). Unfortunately, EEG back-averaging analysis is limited by muscle activity artifacts on the scalp, and also in cases where the jerks are of high frequency or are infrequent. Further supportive evidence for a cortical origin of the jerks can be obtained by also asse ssing the long-latency refl ex (C-refl ex) which can be simultaneously recorded with SEPs. Th e long-loop refl ex (C-refl ex) is obtained by sub-threshold motor stimulation of the me dian nerve at 2 \200\2233 Hz and is recorded from both thenar mus- cles. It has an ipsilate ral latency of around 45 ms, and is 10 \200\22315 ms longer contralaterally because of the transcallosal transit time (26). Muscles should be recorded at rest and then during a weak tonic contraction. Th e presence of a C-refl ex at rest is always pathological and indicates cortical hyperexc itability. It should be noted that a C-refl ex can be found during weak muscle activation e ven in healthy subjects. Recording the muscle activity during a weak contraction is useful for detecting any stimulus-related EMG silent periods which refl ect an abnormal negative l ong latency (negative C-refl ex). In subjects in whom myoclonic EMG bursts are of small ampli- tude, or are repeated rhythmic ally at high frequency, frequency analysis has advantages over back-averaging. Frequency an alysis of both EMG \200\223EMG and EEG \200\223EMG coherence can detect a patho- logically exaggerated common drive in distal limb muscles, show- ing signifi cant coherence in the ph ysiological range (15 \200\22360 Hz) and also, in some cases, at much higher frequencies (2 7). It must be remembered that the presence of a C-refl ex indicates only that the myoclonus is refl ex and so sensitive to somatosensory stimuli. As refl ex myoclonus can also be subcor tical (i.e. reticular Another feature of cortical myoclonus is the enlarged somatosen- sory evoked potential (SEP ) obtained by median nerve stimulation (a) (b) Fig. 5.3 (a) Routine EEG does not show signi \201 cant abnormalities in the left frontocen tral area. Myoclonic jerks on the right \201 rst dorsal interosseous. (b) Back-averaging of 50 jerks shows a cortical correlate with a positive \200\223negative sharp wave (20 ms) before the jerks. CHAPTER 5 Electrophysiological Approaches to the Movement Disorder Patient 37 1 s/div 5 ms/div Fig. 5.5 Patient with myoclonus dystonia ( -sarcoglycan gene). is the trapezius or sternocleidomastoid (SCM). Subsequently, there is spread to cranial and caudal muscles with diff erent veloci- ties of propagation but with a constant pattern of muscle activa- tion. Myoclonic jerks may be associated with cortical spikes, but the lack o f correlation between the two suggests that spikes are projected to, but do not originate i n, the cortex. Evoked poten- tials are not increased in amplitude, but there may be an enha nced C-refl ex. Fig. 5.4 Giant evoked potential with P25/N35 of 53 V, C-re \202 ex at latency of 40 ms. refl ex myoclonus), its presence does not defi nitely indicate a corti- cal origin for the myoclonus (see Fig. 5.4). A C-refl ex is frequently found in corticobasal degeneration (CBD), a disease in which SEPs are not enlarged and EEG back-averaging fails to reveal a pre-myoclonus spike (28). In Creutzfeldt \200\223Jakob disease (CJD), myoclonus is not stimulus- sensitive and occurs continuously and quasi-periodically in the resting condition every 600 \200\2231500 ms. Th ere may be accompa- nying dystonic posturing. EMG bursts are similar to, or slightly longe r than, those of classical cortical myoclonus. Periodic sharp discharges (PSDs) are usually associated with muscle jerks, but can occur independently. EEG spike wave and EMG activity cor- relate loosely. On back-averaging, the negative spike wave is much smaller than the P SD recorded with the raw EEG. Th e time interval between EEG and jerks is 50 \200\22385 ms (much longer than required for conduction through the pyramidal tract). Typical cortical re fl ex myoclonus may be seen in the late stages of the disease. Th e normal human startle response consists of a brief fl exion response, most marked in th e upper half of the body, elicited by unexpected auditory, and sometimes somaesthetic, visu al or vestibular stimuli. Th e nucleus reticularis pontis caudalis seems to be particularly important. Conduction of eff erent impulses both upwards and downwards from the generator, possibly in the medial reticular formation, is slow. Th e shortest latencies are 20 \200 \22350 ms for the orbicularis oculi muscle, while in the quadriceps the latency of the resp onses reaches 100 \200\223150 ms. EMG responses in the intrinsic hand and foot muscles are particularly delayed. Auditory startle refl ex EMG latencies are rather variable. Th e con- stant refl ex EMG activity in orbicularis oculi is the most impor- tant event in the norma l auditory startle refl ex. Considering the auditory blink refl ex in orbicularis oculi as separated from the startle response, the earliest muscle recorded is the SCM with a latency of <100ms. Th e activity then spreads up the brainstem from cranial nerve XI to cranial ne rve V and down the spi- nal cord (refl ex brainstem myoclonus). In physiological startle th e stimulus-induced response tends to habituate, and disap- pears aft er four to six stimuli . Exaggerated startle responses are seen in hyperekplexia, neuropsychiatric startle syndrom e, and stimulus-induced epilepsy or other movement disorders such as stiff person syndrome or tics (30) (see Fig. 5.6). In subacute sclerosing panencephalitis (SSPE) sudden move- ments followed by a tonic phase ( \200\230hung-up jerks \200\231) can be related to periodic high amplitude EEG discharges occurring every 4 \200\22313 s. Myoclonus dystonia is a movement disorder characterized by a combination of rapid brief mus cle contraction (myoclonus) and/ or sustained twisting and repetitive movements that result in an abnormal posture (dystonia). Myoclonic jerks typically aff ect the neck, trunk, and upper limbs, with less common involvement of the legs (29). About 50% of patients have addi tional focal or seg- mental dystonia, presenting as cervical dystonia or writer \200\231s c ramp. Myoclonus dystonia is characterized by multifocal, focal, and gen- eralized jerks of brief and long duration, and spasms. Oft en they are stimulus-sensitive and may present a s tartle response. No cortical origin has been found for the absence of enlarged SEPs and pos itive cortical event on EEG back-averaging (see Fig. 5.5). Two diff erent patterns of spinal myoclonus are recognized: prop- riospinal myoclonus and s egmental myoclonus. In the latter, myo- clonus is limited to muscles innervated by one to t wo contiguous spinal segments. It is oft en caused by a structural lesion of the spi- nal c ord, although in some cases no lesion is found. Th e myoclonus is oft en rhythmic and may p ersist during sleep. Stimulus sensitivity is unusual. Reticular myoclonus (brainstem myoclonus) is characterized by generalized jerks with promin ent involvement of proximal and fl exor muscles. Jerks may be spontaneous or stimulus-induc ed, particularly with respect to sound (30). Th e jerks originate from the brainstem reticu lar formation. Th e fi rst muscle to be activated When segmental myoclonus is suspected a multichannel EMG recording should be performed in d istant muscles belonging to the Oxford Textbook of Movement Disorders 38 (a) (b) 200 ms/div 100 ms/div Fig. 5.6 (a) Acoustic stimulation. (b) Tapping on the nose. same myotome, and it is useful to study at least two myotomes. Th e frequency of the jerks is highly variable, and EMG burst can have a duration of up to 1000 ms (31). Propriospinal myoclonus is characterized by axial muscle con- tractions usually causing fl exion of the trunk and typically induced by lying down. Rhythmicity is not common in propri ospinal myo- clonus, although \200\230runs \200\231 or clusters of jerks are seen. It is h ypoth- esized that propriospinal myoclonus arises from a spinal myoclonic generator recruit ing axial muscles up and down the spinal cord via long propriospinal pathways (31, 32). Th e propagation of the myo- clonic discharge within the spinal cord can be studied by recordi ng proximal upper limb, trunk (cervical, thoracic paraspinal, and rec- tus abdominis), and lower limb muscles. is characterized by arrhythmic sequences or runs of axial jerks producing fl exion or exten sion of the trunk. Bursts of muscle activity vary from 50 ms to 4 s. EMG jerks arise from a bdominal or cervical spinal segments and slowly spread rostrally and caudally at <10 m/s. C ranial muscles are not involved, with the exception of the neck. Th is form of myoclonus ca n be stimulus-sensitive (see Fig. 5.7). Propriospinal myoclonus Fig. 5.7 Multichannel EMG recording in propriospinal myoclonus. 2 s and the latter 400 ms before the jerk onset. Both the components have a cortical genera tor: the early BP originates from the supple- mentary motor area (SMA) and the late BP from the primary motor cortex (M1). Cervical trauma, albeit mild, is reported in some idiopathic cases, and some have viral mye litis. Most cases are labeled as idiopathic as no structural lesion is found. As the \200 \230typical \200\231 clinical and electrophysiological features of propriospinal myoclonus can be voluntarily mimicked, and in some cases an immediate response to a placebo dose of b otulinum toxin has been reported, a psycho- genic etiology should be considered in idiopath ic cases (33, 34). Interestingly, however, ultrastructural lesions in the spinal tracts hav e recently been reported using diff usion tensor magnetic reso- nance imaging (35). BPs can be recorded prior to self-paced voluntary movements. A \200\230typical \200\231 BP has never been recorded prior to proven organic myoclonus. Its presence in patients with j erks or other intermit- tent movements indicates that there is a (conscious or unconscious) volition component in the origin of these jerks.