-
Journal of Neurology, Neurosurgery, and Psychiatry
1991;54:505-510 5
Central motor pathways in patients with mirrormovements
T C Britton, B-U Meyer, R Benecke
AbstractCentral motor pathways were inves-tigated in three
patients with congenitalmirror movements using magneticmotor cortex
stimulation. Responsethresholds, amplitudes and latencieswere
normal. The projection of the cor-ticomotoneuronal pathways was
assessedby placing the coil over the vertex andcomparing the size
of responses in thefirst dorsal interosseous (FDI) musclesevoked by
clockwise and anticlockwisecoil currents. In normal subjects,
rightFDI responses are larger with clockwisecurrents than with
anticlockwisecurrents at the same stimulationstrength and vice
versa. In two out ofthree patients with congenital mirrormovements,
this sensitivity of responseamplitude to coil current direction
wasreversed. The third patient with con-genital mirror movements
and a fourthpatient with acquired mirror move-ments had responses
which werenormally sensitive to current direction.These findings
support the hypothesisthat some cases of congenital mirrormovements
may be due to abnormalprojection of corticomotoneuronal
path-ways.
Department ofNeurology, Universityof Dusseldorf,GermanyT C
Britton*B-U MeyerR BeneckeCorrespondence to:Professor
Benecke,Neurologische Klinik,Universitat Dusseldorf,Moorenstr. 5,
4000Dusseldorf, Germany*MRC Human Movementand Balance Unit,
Instituteof Neurology, Queen Square,London, UKReceived 8 December
1989and in final revised form2 August 1990.Accepted 10 August
1990
Mirror movements are a special type ofassociated movement in
which voluntarymovements performed by one part of thebody,
particularly the arm or hand, areinvoluntarily and symmetrically
performed bythe other side as a result of bilateral activationof
homologous muscles.'2 Mirror movementsoccur in children during
development butnormally disappear with maturation. Persis-tence of
mirror movements into adult life,termed congenital mirror
movements, is con-sidered pathological and may be associatedwith
other developmental abnormalitiesincluding the Klippel-Feil
syndrome'4 andKallmann's syndrome.5 Mirror movementscan also appear
with a variety of acquiredneurological conditions.' 2
Anatomically, congenital mirror movementsare often associated
with midline fusion dis-orders affecting the motor pathways.
Incom-plete pyramidal tract decussation has beenreported in the
necropsy examinations of twocases that had congenital mirror
movementsassociated with the Klippel-Feil syndrome."4Dysraphic
defects at different levels within thecentral nervous system are
thought to underlie
congenital mirror movements associated withthe syndrome of
anosmia and hypophysealdysfunction.5
Neurophysiological evidence for abnormalipsilateral
corticomotoneuronal projectionshas recently been reported in brief
abstractform in two patients with congenital mirrormovements' and
in one patient with mirrormovements associated with Klippel-Feil
syn-drome7 using the technique of trans-cranialelectrical motor
cortex stimulation. Thisstudy on three patients with
congenitalmirrror movements reports that
abnormalcorticomotoneuronal projections can bedemonstrated with the
technique of magneticmotor cortex stimulation' in two of the
threecases. However, the third patient with con-genital mirror
movements and a patient withacquired mirror movements as a result
ofinternal capsular infarction had normallyprojecting
corticomotoneuronal pathways.These findings have implications for
thepathophysiological significance of non-decussated pyramidal
pathways in congenitalmirror movements.
MethodsPATIENTS AND SUBJECTSFour patients with mirror movements
wereinvestigated. Three had had mirror movementssince childhood.
The fourth developed mirrormovements one week following a left
internalcapsular infarction.
Case 1 A 52 year old right handed man hadnoticed mirror
movements of his hands sincechildhood. He was moderately disabled
in finemotor tasks of the hands. Pregnancy and earlydevelopment
were otherwise normal. Onexamination there were
non-suppressiblemirror movements on finger or wristmovements of
either side. Simultaneous sym-metrical flexion-extension movements
of thewrists could be made normally, but the patientexperienced
considerable difficulty in produc-ing simultaneous flexion ofone
wrist and exten-sion of the other. Sequential opposition of
thethumb to each of the fingers was slow. Nomirror movements were
observed in the legs.Muscle strength, tone and reflexes in the
upperand lower limbs were normal. The neck was ofnormal length and
a cervical spine radiographwas normal. Sense of smell was
normal.
Case 2 A 26 year old man, with a mild rightsided hemiplegia
since birth, had noticedinvoluntary movements of his left hand
when
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Britton, Meyer, Benecke
moving with his right hand and, additionally,involuntary
movements of his right hand whenattempting to move only his left
hand. Onexamination he had a right hemiparetic gait anda mild right
hemiparesis with increased muscletone and brisk reflexes on the
right. Mirrormovements of the left hand were observedwhen he
attempted to move only the right handand mirror movements of the
right hand wereseen when he attempted to move only the lefthand. CT
of the head showed hemi-atrophy ofthe left cerebral cortex. There
was no abnor-mality of the neck and sense of smell wasnormal.
Case 3 A 38 year old right handed woman hadbeen aware of mirror
movements in her handsall her life. She was the second of
identicaltwins. Pregnancy and development were other-wise normal.
On examination there were non-suppressible mirror movements of both
hands,which were present with self initiatedmovements as well as
with externally triggered(reaction time paradigm) movements.
Finefinger movements were mildly impaired. Shehad considerable
difficulty in producing alter-nating flexion-extension movements of
onewrist and extension-flexion movements of theother. Muscle
strength, tone and tendonreflexes were normal. There was no
abnor-mality of the neck and sense of smell wasnormal.
Case 4 A 59 year old right handed hyperten-sive man suddenly
developed a severe righthemiparesis. CT revealed a hyperdense
lesionin the region of the left internal capsule com-patible with
cerebral haemorrhage. Threeweeks later, after considerable
functionalrecovery, he noticed that attempts to move hisright hand
resulted in associated mirrormovements in the left hand; movements
of hisleft hand, however, were not accompanied bymovements of the
right.A fifth patient, aged 48, with a left hemiplegia
as a result of embolic occlusion of the rightmiddle cerebral
artery was also studied.
Five male normal subjects (age range 27 to 31years, mean 28
years) were studied. Ethicalcommittee approval for these
investigationshad been obtained.
RECORDING AND ANALYSISElectromyographic (EMG) recordings
weretaken from both first dorsal interosseous (FDI)musces -using.
Ag/AgCl -surface electrodesplaced over the muscles. Signals were
amplifiedby a Tonnies Myograph II (TonniesMedizinische Elecktronik,
Freiburg, Ger-many) with bandpass filtering between 20 Hzand 3000
Hz. Data was then collected andstored on floppy disks using a
Tandon personalcomputer (IBM PC compatible) andAUTOLAB data
collection programmes(sampling frequency 8000 Hz/channel;AUTOLAB,
Kunze Software, Dusseldorf,Germany). Onset latencies of responses
weremeasured from the first deviation from thebaseline by visual
inspection of the signals on
the computer screen. Response amplitudes(peak to peak) were
measured by computer.The motor cortex was stimulated using the
commercially available MAGSTIM 200(Novametrix Medical Systems,
Connecticut,USA). The coil (consisting of 19 turns ofcopper wire;
inner diameter 5 5 cm, outerdiameter 12 cm) was laid flat against
the scalpand was centred carefully over the vertex.
It has previously been noted that, with thismake of stimulator
(not necessarily with oth-ers9) and with the coil centred over the
vertex,currents flowing clockwise around the coil (asviewed from
above) normally produce largerresponses in the right FDI muscle
than the leftFDI, whereas anticlockwise currents evokelarger
responses in the left FDI than rightFDI.'° This was confirmed in
our normalsubjects. Given the known contralateralemphasis
(particularly for hand muscles) of thecorticomotoneuronal
projection, the mostlikely explanation for this result is that
clock-wise currents stimulate the left cerebral hemis-phere more
effectively (at least in terms ofevoking hand muscle responses)
than the rightcerebral hemisphere and that anticlockwise
coilcurrents stimulate the right cerebral hemis-phere more than the
left.Thus one method of studying the cortico-
motoneuronal projections in normal subjectsand in patients with
mirror movements wouldbe to compare directly the size of
responsesevoked in the two FDI muscles with clockwisecurrents
(when, in normal subjects, right FDIresponses are larger than left
FDI responses) orwith anticlockwise currents (when, in
normalsubjects, left FDI responses are larger thanright FDI
responses). The occurrence of rever-sed responses (for example,
clockwise currentsproducing larger responses in the left FDI
thanthe right FDI, or vice versa) could then beinterpreted as
indicating a pathological,ipsilaterally projecting
corticomotoneuronaltract. There is, however, a problem with
suchreasoning in that the size of an individual FDIresponse may be
significantly influenced notonly by the relative proportion of
contra- andipsilaterally projecting corticomotoneuronalaxons but
also by other factors (electrodeplacement, peripheral nerve
lesions, lesionsof central motor pathways such as
lacunarinfarction).To avoid these confounding factors, the size
of responses evoked by clockwise coil currentswas compared with
the size of responses evokedby anticlockwise currents at the same
stimula-tion strength and in the same FDI muscle. Innormal
subjects, right FDI responses are largerwith clockwise coil
currents than anticlockwisecoil currents at the same stimulation
strength(within the range of stimulation strengths fromthreshold to
1 5 times threshold): for left FDImuscles, anticlockwise currents
produce largerresponses than clockwise currents. Theimplications of
these normal findings with thecorticomotoneuronal projections and
the inter-pretation of reversed response patterns areraised in the
discussion.With the coil position carefully centred over
the vertex, responses were recorded in FDI
506
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Central motor pathways in patients with mirror movements
Clockwise Anticlockwise
L FDIl
R FDI
Figure I Electromyographic recordingsfrom the left (upper
traces) and right (traces) first dorsal interosseous (FDI) muscles
in a normal subjectfollowing magbrain stimulation with clockwise
coil currents (left side offigure) and anticlockwicurrents (right
side offigure). The coil was centred over the vertex and the
stimudstrength was 75% of the maximum stimulator output. Note how
in the left FDI aresponse is produced by an anticlockwise coil
current, while in the right FDI a larresponse is produced by a
clockwise coil current. Calibration bars 20 ms and I mTV
Left
I ToTI
1
30% -
T
0
T.
_
50
Right
T
T
100 0Stimulator output (%)
50
muscles with clockwise and anticlockwisecurrents (direction of
coil current was changedby simply turning over the coil) using a
series ofstimulation strengths between 30 and 100% ofmaximum output
ofthe stimulator. At least fiveresponses were obtained for each
stimulationstrength. During stimulation, subjects wereasked to
relax completely and this was checkedby constant display of the
surface EMG on anoscilloscope.To quantify the difference in
response size
with different coil currents, the mean peak tolower pleak
amplitude of the evoked responses was
neticoi calculated for each stimulation strength andlation then
plotted on a graph (fig 2). A line was drawnlarger across the graph
at a height equal to halfthe size
,yer of the largest response obtained at rest. Thestimulation
strength (expressed as percent ofmaximal stimulator output) needed
to producesuch a half maximal response when the currentflowed
anticlockwise around the coil was sub-tracted from the stimulation
strength neededwhen the current flowed clockwise. If responseof
half maximal size were not evoked when thecoil current flowed in
the non-preferentialdirection, then the highest stimulation
strengththat was used in the particular subject was
T employed in this calculation. For right FDImuscles of normal
subjects this results in a
l negative value, and for left FDI muscles apositive value.
~5% For the purposes of obtaining corticallyevoked responses
with the shortest latency,magnetic stimulation of the motor cortex
was
T repeated with pre-activation of the muscle and° with the
stimulator output set at 1-5 times
° . threshold for responses in the relaxed100 muscle." 12 Peak
to peak size of responses
evoked with pre-activation of the muscle wereaf half also
measured.'rgure 6 LyrruFr UJ a'XVerUgeSyM UpUK 31zt; UJ
{-e3pflu*& etmit"iU tri; tt tJ Lty truJ
offigure) and right (right half offigure) first dorsal
interosseous (FDI) muscles of anormal subject following magnetic
brain stimulation with clockwise (filled circles) andanticlockwise
(open circles) coil currents plotted against stimulation strength
expressedas percent of maximal output of the stimulator. Note how
clockwise coil currents producelarger responses in the right FDI
than anticlockwise currents at the same stimulationstrength and
vice versa. This difference in the size of responses with currents
of differentorientation has been quantified by subtracting the
stimulation strength required to producea half maximal response
with anticlockwise currentsfrom that required with
clockwisecurrents.
Clockwise
LLFDI
R FDI
Anticlockwise
Figure 3 Electromyographic recordings from the left (upper
traces) and right (lowertraces) first dorsal interosseous (FDI)
muscles in a patient with congenital mirrormovements (case 1)
following magnetic brain stimulation with clockwise coil
currents(left side offigure) and anticlockwise coil currents (right
side offigure). The coil wascentred over the vertex and the
stimulation strength was 90% of the maximumstimulator output. Note
how in the left FDI a larger response is produced by a
clockwisecoil current, while in the right FDI a larger response is
produced by an anticlockwise coilcurrent. This is the opposite
pattern from normal (see fig 1). Calibration bars 20 ms and05
mV.
ResultsNormal subjectsResponses in relaxed FDI muscles could
beobtained in all normal subjects with the coilcurrent in either
direction. Threshold for res-ponses in right FDI muscles (mean 46%
ofmaximum stimulator output; range 40%-60%)was always lower with
clockwise coil currents(when viewed from above) than with
anticlock-wise currents, whereas threshold for responsesin left FDI
muscles (mean 48%; range 40-65%) was always lower with
anticlockwisecurrents. Amplitude of the responses in an FDImuscle
was dependent on stimulation strengthand current direction (figs 1,
2). For the samestimulation strength, the peak to peakamplitude of
responses in right FDI muscleswere always greater with clockwise
coilcurrents than with anticlockwise currents. Theopposite was true
for the left FDI muscles. Toproduce a response of half maximal size
in theright FDI, the stimulation strength requiredwith clockwise
coil currents was 22% (range18% to 30%) less than that for
anticlockwisecurrents, while for half maximal responses inthe left
FDI the stimulation strength requiredwith clockwise currents was
18% (range 10%to 32%) more than that for anticlockwisecurrents.
5
4
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Britton, Meyer, Benecke
0
Left
I
I
1 z
11
IIi
1,0* 06*
3
2
1
Rigi
a
6f
4
II
. n I
ar
6
II
I
4
2
0 50
I
I
50
____ O
100 0Stimulator output (%)
Figure 4 Graphs of average peak to peak size of responses
elicited in theoffigure) and right (right half offigure) first
dorsal interosseous (FDI)patients with congenital mirror
movements,following magnetic brain stimclockwise (filled circles)
and anticlockwise (open circles) coil currents plcstimulation
strength expressed as percent of maximal output of the stimulin A
(case 1) and B (case 2) responses in the right FDI were larger or
oJwhen the coil current was anticlockwise compared to when the coil
currentthis is the opposite of the normalfindings. Conversely
responses in the leftlarger or of equal size when the coil current
was clockwise compared to wicurrent was anticlockwise, which is
again an abnormalfinding. These restthe pyramidal pathways in these
two patients are not normally decussatecthird patient with
congenital mirror movements (C; case 3) had response.dependent on
coil current direction in a normalfashion.
With pre-activation of the mu,tion strength at 1-5 times
threshopreferential coil current directio:ponse latencies were 21-2
ms (rangms) for the right FDI and 21-3 mto 23-0 ms) for the left
FDI.
ht Patients with congenital mirror movementsResponses could be
obtained in both FDImuscles in each patient following magneticbrain
stimulation. Threshold for responses(range 45-60%) lay within the
normal range,but in three out of six sides the threshold
forclockwise and anticlockwise coil currents was
I equal, in contrast to the normal findings (fig 3).t The
amplitude of responses is plotted against0l stimulation strength in
figure 4. The graphs for
, case 1 and case 2 show qualitative and quan-titative
differences from the findings in normal
r a subjects. Qualitatively, the size of responses in*04 an FDI
muscle was not dependent on coil
current direction in the same way that it was fornormals. Thus
for right FDI muscles anti-clockwise coil currents produced equally
sizedor larger responses than clockwise currents at
the same stimulation strength (which is theopposite of that
found in normals). Further-
1'T 1 more, in these two patients, left FDI responseswere
preferentially evoked by clockwise
currents (in normal subjects anticlockwisecurrents produced
larger responses). Quan-titatively, in order to produce responses
of halfmaximal size in the right FDI muscles, thestimulation
strength required with clockwisecoil currents was 0% (case 1) and
3% more
4jl (case 2; mean for normal subjects 22% less)61
than that for anticlockwise currents, while forhalf maximal
responses in the left FDI thestimulation strength required with
clockwisecurrents was 16% less (case 1) and 0% (case 2;mean for
normals 18% more) than that foranticlockwise currents.
In cases 1 and 2, we made the additionalobservation that the
size of responses in theright FDI muscles could be increased
bymoving the coil over the right (ipsilateral)cerebral hemisphere,
while the size of left FDI
t 1 responses could be increased by moving thecoil towards the
left cerebral hemisphere.
In case 3, .response size varied with coilcurrent direction in a
normal manner. That is,clockwise currents evoked larger responses
in
6the right FDI than anticlockwise currents at
00o ~ the same stimulation strength and vice versa.'*100 With
pre-activation of the muscle, stimula-
tion strength at 1 5 times threshold and use ofpreferential coil
current direction, corticallyevoked response latencies and
amplitudes in all
e left (left half three patients with congenital mirrorulaston
woithree movements were found to be within our normalotted against
range. Response configurations also appearedator. Note how normal
although this was not studied(equal size sytmicl.t was clockwise:
systematically.FDI wereien the coil Patient with acquired mirror
movementstHowevr that Responses could be obtained in the
relaxed
s which were FDI muscles of both sides, although theresponses on
the hemiparetic side were muchsmaller than those on the non-paretic
side.Threshold for responses were within our nor-mal range (right
65%; left 50%). Amplitudes of
scle, stimula- responses were dependent on stimulationold and
use of strength and current direction in a normaln, mean res-
manner (fig 5). Responses in the right FDIre 18-8 to 22-4 muscle
were larger with clockwise coil currentsks (range 20 7 than with
anticlockwise currents for the same
stimulation strength. With muscle pre-activa-
3 -
2
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6 -
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C.w0cr
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8 _
6 -
4
508
I 0%I
I In
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Central motor pathways in patients with mirror movements
5
4
31-
2
>ECDN
CoCD
1
0
Left 5'I0
I
01~I
1'T0
.T
0J
"it
0 50
4
3
2
T
100
1
r509
Right
51
4
3
No responses2
1
0
Stimulator output (%)
T
I
;0
I
011I I
IIi~Ti0
fI T,1
o
50
Figure S Graphs of average peak to peak size of cortically
evoked responses in the lej(left half offigure) and right (right
halfoffigure) first dorsal interosseous (FDI) mustof one patient
with acquired mirror movements (upper graphs) and of one patient
withcomplete right hemiplegia (lower graphs) elicited with
clockwise (filled circles) andanticlockwise (open circles) coil
currents plotted against stimulation strength expressesas percent
of maximal output of the stimulator. Note how the responses are
dependent icoil current direction in a normal manner: that is
clockwise coil currents produced largresponses than anticlockwise
currents at the same stimulation strength in the right FDTwhile
anticlockwise currents were better at stimulating the left FDI. No
responses coulbe obtained in the completely paralysed right
FDI.
tion, cortically evoked response latencies w(within normal
limits, although somew}slower in the right FDI (22-4 ms) comparwith
the left (21-7 ms). Response amplituwas also reduced on the right
(1 -6 ncompared to 6 7 mV).
Patient with right CVAResponses following magnetic brain
stimution could be obtained in the right FDI musiwith both
clockwise and anticlockwise ccurrents, but no responses could be
evokedthe left FDI. Responses in the right FDI w(normal for
threshold (45%) and for 1direction of current that produced the
larnresponses for a given stimulation strength. Tstrength of
stimulation required to produchalf maximal response was 60% with a
clocwise coil current and 70% with an anticlocwise current. The
presence of responses in tright FDI muscle with both clockwise
aanticlockwise coil currents in this paticindicates that
bilaterally projecting c(ticomotoneuronal pathways are not
necessi
to explain the finding of bilateral responses innormal subjects
when the coil is centred overthe vertex.
DiscussionWe have demonstrated that hand muscle res-ponses
evoked by magnetic motor cortexstimulation in two out of three
patients withcongenital mirror movements were preferen-tially
elicited by using the opposite direction ofcoil current to that
which produces the largestresponses in normal subjects. This
suggests
* that either: 1) their primary motor corticeswere different
from normal and were more
.-i sensitive to stimulation with coil currents in theopposite
direction to normal (that is, clockwisecoil currents stimulated the
right cerebralhemispheremore effectively than the left hemis-phere)
or 2) the corticomotoneuronal com-ponent of the pyramidal pathways
projected tospinal alpha motor neurons abnormally. Wefavour the
latter hypothesis in view of ourobservation that moving the coil
over theipsilateral hemisphere in these cases oftenfurther
increased the size of the response. Inaddition, given that the
threshold, latency andsize of responses evoked by magnetic
stimula-tion were normal, a major abnormality in thearchitecture of
the motor cortices seems un-likely. Furthermore, such a hypothesis
wouldbe in keeping with the results of electricalmotor cortex
stimulation.67
100 This study has also shown that congenitalmirror movements
are not invariablyassociated with abnormal corticomotoneuronal
fles projections. The third patient, whose congen-i a ital
mirror movements were clinically indistin-
guishable from the mirror movements of thed first and second
patients, had entirely normalon,er motor responses to magnetic
stimulation,l, including responses which were appropriatelyd
sensitive to the direction of current. The fast
corticomotoneuronal component of thepyramidal tracts therefore
projected normallyto spinal alpha motor neurons, although this
ere does not exclude the possibility that the projec-hat tion of
slower components of the pyramidalred pathway might have been
abnormal.ide What can be said of the pathophysiologicalnV basis of
congenital mirror movements? The
appearance of mirror movements on attempt-ing to move only one
side of the body dependsupon the original command from high
centres
ila- within the brain to move only one side of theIcle body
inappropriately reaching the spinal alpha-oil motor neurons of both
sides. Clearly, severalin different routes, via pyramidal or
extra-ere pyramidal pathways, could be suggested tothe account for
this abnormal transmission.'3 Theger initial command might be
incorrectly sent to'he motor centres on both sides of the brain
(fore a example, to the motor cortices of both cerebralck-
hemispheres), or signals in the descendingck- motor pathways from
motor centres ofone sidethe of the brain (for example, pyramidal
pathways)nd might activate spinal structures bilaterally as aent
result of the abnormal functional/anatomicalor- projection of such
pathways, or activity on oneary side of the spinal cord might cross
to the other
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Britton, Meyer, Benecke
side as a result of abnormal interneuronalconnections.Could the
abnormal ipsilateral cortico-
motoneuronal projections as revealed in twoout ofour three
patients explain the appearanceof congenital mirror movements? In
normalsubjects making unilateral hand movementsonly the
contralateral motor cortex isactivated.4 15 A totally
non-decussated(ipsilaterally projecting)
corticomotoneuronalprojection, by itself, could not
thereforeaccount for mirror movements. Assuming thatmotor cortex
activation occurs normally, whatis required is that activity in the
motor neuronsof one cerebral motor cortex reaches the alphamotor
neurons on both sides of the spinal cord.This could be effected
either by individualpyramidal axons branching to synapse
directlywith alpha motor neurons on both sides of thespinal cord or
by a proportion of activatedcortical motor neurons projecting
ipsilaterallyand a proportion projecting contralaterally.Farmer et
al,7 on the basis of a narrow peak oncross-correlograms between
motor units in thehand muscles of a patient with
Klippel-Feilsyndrome, showed that the spinal motorneurons
subserving homologous hand muscleson each side receive synaptic
input from acommon neuron and they suggested that thecommon neuron
was a cortical motor neuronwith an abnormally branched axon.The
finding that one of our patients with
congenital mirror movements had normalcorticomotoneuronal
pathways as assessed bymagnetic motor cortex stimulation is
importantsince it implies either that there is more thanone
pathophysiological cause of congenitalmirror movements or that the
abnormalcorticomotoneuronal projections seen in theother patients
is not the pathological cause ofthe mirror movements, merely an
associatedfeature. That mirror movements are also seenwith a
variety of acquired disorders,' in whomthere would be no reason to
suspect an abnor-mal pyramidal pathway projection does suggestthat
an abnormal corticomotoneuronal projec-tion is not a necessary
requirement for theappearance of mirror movements. This is
supported by the normal projection found inour patient with
acquired mirror movements asa result of internal capsular
infarction. Wherethe abnormality lies in these patients
isuncertain.
ErratumThroughout the text (and in the table) the word
"anticlockwise"should be replaced by "clockwise" and vice versa.
Previouspublished studies using the Novametric magnetic
stimulatormay also contain this error (see JNNP 1990;53:707).
This study was supported by the DFG and EC. TCB is an ECResearch
Fellow.
I Zulch KJ, Muller N. Associated movements in man. In:Vinken PJ,
Bruyn GW, eds. Handbook of clinicalneurology, Vol 1. Amsterdam:
North Holland, 1969:404-26.
2 Myrianthopoulos NC. Mirror movements. In: Vinken PJ,Bruyn GW,
eds. Handbook of clinical neurology, Vol 42.Amsterdam: North
Holland, 1982:233-234.
3 Avery LN, Rentfro CC. The Klippel-Feil syndrome: apathologic
report. Arch Neurol Psychiatr 1936;36:1068-76.
4 Gunderson CH, Solitare GB. Mirror movements in patientswith
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