Electromyographic studies of congenital mirror movements

We studied two patients with congenital mirror movements. In one, the movements were associated with impaired sensation and a cervicodorsal meningocele. In the other, no abnormality of the nervous system was found. In an EMG study, the normal temporal characteristics, response latency, duration, and amount of EMG on the normal and mirror sides suggest that similar motor commands are responsible for both voluntary and mirror movements.     

Dominance of ipsilateral corticospinal pathway in congenital mirror movements

OBJECTIVE: To clarify the mechanism of congenital mirror movements. DESIGN: The triple stimulation technique (TST) and the silent period were used to investigate a patient with congenital mirror movements. The TST was used to calculate the ratio of ipsilateral to contralateral corticospinal tracts from the two hemispheres to the spinal motor neurones. RESULTS: Transcranial magnetic stimulation over unilateral M1 induced larger ipsilateral than contralateral motor evoked potentials on both sides. Only 9% of spinal motor neurones innervating the abductor digitorum minimi were excited by contralateral primary motor cortex (M1) stimulation, while 94% were excited by the ipsilateral M1 stimulation. The silent period was examined during mirror movements and with voluntary contraction of the right first dorsal interosseus mimicking mirror movements. Left M1 stimulation (through the crossed corticospinal tract) did not show any difference in silent period between the two conditions, while right M1 stimulation (through the uncrossed tract) caused a longer silent period during mirror movements than during voluntary contractions. CONCLUSIONS: The results suggest that mirror movements may be caused by a strong connection between ipsilateral M1 and the mirror movements conveyed through a dominant ipsilateral corticospinal pathway.     

Pre-movement planning processes in people with congenital mirror movements

Objective

Pre-movement processes were investigated in people with Congenital mirror movement (CMM), a rare disorder in which bilateral movement (mirroring) occurs in the upper distal extremities (primarily the hands and fingers) during intended unilateral movements. Abnormal density of ipsilateral corticospinal projections is an established hallmark of CMM. This study tested whether the Lateralised Readiness Potential (LRP), which reflects movement planning and readiness, is also abnormal in people with CMM.

Abnormal projection of corticospinal tracts in a patient with congenital mirror movements

A 31 year-old woman with familial congenital mirror movements not associated with other neurological defects underwent a detailed neurophysiological evaluation including: voluntary electromyographic activity recorded from upper limbs in response to acoustic stimuli, motor evoked potentials from the thenar muscles to focal transcranial magnetic stimulation, F waves from upper extremities, scalp somatosensory evoked potentials and long-latency responses from thenar muscles to electric stimulation of the median nerve. The results were consistent with the presence of fast-conducting pathways connecting each hand motor cortex with both contra- and ipsilateral spinal motoneurones.

Résumé

Une femme de 31 ans, qui présentait des mouvements « en miroird'origine familiale congénitale, en l'absence d'autres déficits neurologiques, a été soumise à une évaluation neurophysiologique détaillée comprenant: l'activité électromyographique (EMG) dans le muscle court abducteur du pouce en réponse à des stimulations auditives; les réponses EMG dans les muscles de l'éminence thénar par stimulation magnétique transcrânienne focale du cortex moteur; les potentiels évoqués somes-thésiques corticaux; les ondes F et les réponses EMG à longue latence dans l'éminence thénar par stimulation du nerf médian. Les résultats sont compatibles avec la présence de voies motrices à conduction rapide qui joignent l'aire de représentation de la main dans le cortex moteur primaire aux motoneurones spinaux aussi bien contralatéraux que ipsilatéraux.     

Bilateral motor cortex output with intended unimanual contraction in congenital mirror movements

In congenital mirror movements (MM), it is unclear whether the "mirror" motor cortex (M1) produces output during intended unimanual movements. In two patients with MM, the cortical silent period (CSP) was abnormally short after focal transcranial magnetic stimulation (TMS) of either M1, but simultaneous bilateral TMS led to significant CSP lengthening. Thus, it is likely that the shortened CSP after unilateral TMS is caused by output from the nonstimulated M1, suggesting that both M1 produce output with intended unimanual movements in patients with MM.     

Ipsilateral corticospinal projections do not predict congenital mirror movements: a case report

Congenital mirror movements (CMMs) are involuntary, symmetric movements of one hand during the production of voluntary movements with the other. CMMs have been attributed to a range of physiological mechanisms, including excessive ipsilateral projections from each motor cortex to distal extremities. We examined this hypothesis with an individual showing pronounced CMMs. Mirror movements were characterized for a set of hand muscles during a simple contraction task. Transcranial magnetic stimulation (TMS) was then used to map the relative input to each muscle from both motor cortices. Contrary to our expectations, CMMs were most prominent for muscles with the strongest contralateral representation rather than in muscles that were activated by stimulation of either hemisphere. These findings support a bilateral control hypothesis whereby CMMs result from the recruitment of both motor cortices during intended unimanual movements. Consistent with this hypothesis, bilateral motor cortex activity was evident during intended unimanual movements in an fMRI study. To assess the level at which bilateral recruitment occurs, motor cortex excitability during imagined unimanual movements was assessed with TMS. Facilitory excitation was only observed in the contralateral motor cortex. Thus, the bilateral recruitment of the hemispheres for unilateral actions in individuals with CMMs appears to occur during movement execution rather than motor planning.     

Congenital mirror movements.

In this report are described seven patients assessed clinically and neuropsychologically in whom mirror movements affecting predominantly the hands occurred as a congenital disorder. These mirror movements, representing a specific type of abnormal synkinesia, may arise as a hereditary condition, in the presence of a recognisable underlying neurological abnormality, and sporadically, and the seven patients provide more or less satisfactory examples of each of these three groups. Despite the apparent uniformity of the disorder, the heterogeneity and variability may be marked, examples in some of our patients including the pronounced increase in tone that developed with arm movement, and the capacity for modulation of the associated movement by alteration of neck position and bio-feedback. Various possible mechanisms are considered; these include impaired cerebral inhibition of unwanted movements, and functioning of abnormal motor pathways. Emphasis has been placed on the putative role of the direct, crossed corticomotoneurone pathways and on the unilateral and bilateral cerebral events that precede movement.     

Reorganization of the motor cortex in a patient with congenital hemiparesis and mirror movements

Abnormal branching of corticospinal fibers from the unaffected motor cortex is responsible for mirror movements in patients with congenital hemiparesis, but it is unknown which mechanisms enable these patients to lateralize motor activity. Using multiunit electromyographic analysis and transcranial magnetic stimulation, the authors provide evidence for nonbranched crossed and uncrossed corticospinal projections and intracortical inhibition of the mirror hand. They propose that this remarkable reorganization of the unaffected motor cortex helps these patients to reduce mirror movements.     

Neurophysiological investigation of congenital mirror movements in a patient with agenesis of the corpus callosum

We describe a patient with complete agenesis of the corpus callosum and congenital mirror movements in which primary motor cortex (M1) excitability of both hemispheres was assessed with transcranial magnetic stimulation. Voluntary contraction of the index finger was associated with bilateral electromyographic activity in the first dorsal interosseus muscle. Motor-evoked potentials of identical latencies were produced bilaterally after unilateral M1 stimulation. Measures of intracortical inhibition and facilitation were within normal limits bilaterally although a shorter contralateral silent period was found for both hemispheres. Taken together, the current data suggest a pattern of M1 excitability very similar to that found in patients with congenital mirror movements and no other motor abnormality.