maladaptive plasticity in the form of excessive inhibition of the affected hemisphere by the unaffected hemisphere can negatively impact recovery of the paretic hand after stroke
. a single session of low - frequency repetitive transcranial magnetic stimulation of the unaffected hemisphere ( uh - lf - rtms ) can downregulate excitability of the motor cortex of the unaffected hemisphere ( m1uh ) , reflected by decrease in amplitude of motor evoked potentials ( meps ) recorded from the nonparetic hand in patients in the chronic phase after stroke .
decrease in mep amplitude may be related to long - term - depression - like ( ltd - like ) plasticity .
in addition , uh - lf - rtms decreases the duration and amplitude of the ipsilateral silent period , a measure of interhemispheric inhibition recorded by delivering transcranial magnetic stimulation ( tms ) to m1uh during voluntary muscle contraction of the paretic hand .
importantly , improvements in motor function of the paretic hand were reported after single [ 1 , 3 , 4 ] or several [ 511 ] sessions of uh - lf - rtms , applied either alone or in combination with motor training , in the subacute or chronic stages after stroke .
these encouraging results suggest that uh - lf - rtms has potential as an adjuvant therapeutic strategy in stroke rehabilitation . despite these exciting findings , mechanisms underlying effects of uh - lf - rtms are not entirely clear .
there is virtually no information about effects of uh - lf - rtms on m1uh short - interval intracortical inhibition ( sici ) or facilitation ( icf ) , in the subacute stage after stroke .
sici and icf probably reflect intracortical excitability in separate excitatory and inhibitory neurons , with a relative contribution of spinal mechanisms to icf .
sici is likely mediated by gabaa receptors activity , while icf is mainly influenced by the glutamatergic system ( nmda receptors ) [ 1214 ] .
the objective of this pilot , double blinded , randomized proof - of - principle study was to explore , for the first time , effects of uh - lf - rtms as an add - on therapy to standard - of - care rehabilitation , on sici and icf of m1uh , in patients within 545 days after ischemic stroke and mild to severe hand paresis .
the protocol was conducted as part of a study that compared effects of uh - lf - rtms or sham rtms as add - on therapies to standard - of - care rehabilitation . between february 2008 and october 2012 , 1083 patients with ischemic stroke
inclusion criteria were age , 1880 years ; first - ever symptomatic ischemic stroke involving up to 50% of the internal carotid artery territory , confirmed by ct or mri ; time from stroke , 545 days ; and mild to severe hand paresis ( medical research council scale , 4 - 0 in finger flexion or extension ) .
exclusion criteria were previous symptomatic strokes ; history of seizures ; other neurologic diseases and uncontrolled chronic diseases , such as congestive heart failure or cancer ; shoulder pain ; joint deformity of the paretic upper limb ; and relative or absolute contraindications to tms : use of implantable infusion pumps , pacemaker , pregnancy , intracranial hypertension , intracranial metal , bone skull defects , and use of drugs that can interfere on cortical excitability such as tricyclic antidepressants , neuroleptics , or benzodiazepines . in addition , an implanted stent in the carotid or vertebral arteries less than two months was considered a relative contraindication to tms .
other exclusion criteria were inability to provide informed consent due to severe aphasia , anosognosia , or cognitive impairment as well as inability to attend to treatment sessions . the protocol was approved by the local ethics committee , and all patients provided informed consent to participate . the protocol was registered in clinicaltrials.gov ( nct01333579 ) .
one patient in the sham group had a recurrent stroke on the day before the second session of treatment and did not complete the study .
due to technical problems ( malfunction of the paired - pulse tms stimulator ) , cortical excitability data in 7 patients in the active group and 7 patients in the sham group were not collected .
one patient in the active group coughed repeatedly during one of the tms experiments . because contraction of facial muscles can interfere on cortical excitability , data from this patient were excluded .
the following characteristics were evaluated at baseline : age , gender , handedness before stroke ( oldfield inventory ) , days after stroke , infarct side / location , nih stroke scale , and modified rankin scale .
infarcts were classified as corticosubcortical or subcortical according to involvement of primary motor , primary somatosensory , supplementary motor , or premotor cortices on mri ( 16 ) or ct ( 2 ) by a radiologist blinded to group assignment .
patients were randomized in blocks by the principal investigator with a basic random number computerized generator in a 1  :  1 ratio to receive immediately before rehabilitation treatment lasting for 60 minutes , either active or sham rtms , five days per week , during two weeks ( total , 10 treatment sessions ) .
patients were unaware of group assignment . in each treatment session , the optimal site of motor stimulation of the uh (  hot spot  )
was defined as the location where tms elicited the largest meps in the nonparetic abductor pollicis brevis muscle ( apb ) with surface electrodes .
the signal was amplified and filtered ( 10  hz to 2  khz ) with an electromyography ( emg ) and evoked potential measuring unit ( meb-9104 j , nihon kohden , japan ) . in both groups , 1  hz rtms was administered with a figure - of - eight coil ( mcf b-65 ) at 90% of the nonparetic apb muscle resting motor threshold ( rmt ) with a biphasic magpro compact stimulator ( alpine biomed ) , for 25 minutes ( 1500 pulses ) . for the active intervention
, tms coil was tangentially positioned by an investigator on the nonparetic apb  hot spot  in the uh with the intersection of both wings at 45 angle with the midline . in the sham intervention
all patients were comfortably seated , with their arms in a constant relaxed position , wore earplugs , and were instructed to remain at rest during tms .
tms was delivered to the uh at the nonparetic apb  hot spot  through a figure - of - eight shaped coil ( mean diameter 70  mm ) , connected to a bi - stim 200 module ( magstim , uk ) .
emg responses were amplified ( 1000 ) , filtered ( 2  hz2  khz ) , sampled at 5  khz and recorded on a computerized data acquisition system built with the labview graphical programming language .
its conditional triggering feature was used to deliver tms stimuli only when the apb muscle was relaxed .
relaxation of apb muscle was defined as emg activity at baseline < 50  v peak - to - peak amplitude for at least 1 second .
trials showing emg activity 50  v peak - to - peak amplitude were excluded during recordings .
this investigator was also in charge of running the labview acquisition system and saving data files throughout the experiment .
the following tms measurements were performed before the first ( d0 ) session of treatment and after the last ( d10 ) session of treatment in patients in both groups : rmt , defined as the minimum tms intensity required to elicit at least three out of six meps  50 microv in consecutive trials [ 20 , 21];sici and icf measured with paired - pulse tms [ 22 , 23 ] . the conditioning stimulus ( cs ) intensity was set to 80% of the unaffected apb rmt .
the intensity of the test stimulus ( ts ) was that required to evoke meps of approximately 0.5 to 1  mv .
the order of presentation of inhibitory ( 2  ms ) , excitatory ( 10  ms ) , and control ( test stimulus alone ) trials was randomized .
results are expressed as average percentages of mep amplitudes in conditioning trials and in test trials .
rmt , defined as the minimum tms intensity required to elicit at least three out of six meps  50 microv in consecutive trials [ 20 , 21 ] ; sici and icf measured with paired - pulse tms [ 22 , 23 ] .
the conditioning stimulus ( cs ) intensity was set to 80% of the unaffected apb rmt .
the intensity of the test stimulus ( ts ) was that required to evoke meps of approximately 0.5 to 1  mv .
the order of presentation of inhibitory ( 2  ms ) , excitatory ( 10  ms ) , and control ( test stimulus alone ) trials was randomized .
results are expressed as average percentages of mep amplitudes in conditioning trials and in test trials .
raw data from paired - pulse experiments were inspected by an investigator blinded to group assignment with a playback application in system built with the labiview graphical programming language ( sampling rate 5000  khz ) .
this was an exploratory study and determination of the sample size was not formally determined to evaluate tms outcomes .
between - group differences in changes in rmt , sici , and icf ( baseline  after treatment , d0
there were no significant differences in baseline characteristics between the active and sham groups ( table 1 ) .
there were no serious adverse events . in the active group , two patients reported headache or local pain ( 22.2% ) , two patients reported nuchal pain ( 22.2% ) , and five patients reported drowsiness ( 55.6% ) . in the sham group , three patients reported headache or local pain ( 33.3% ) , four patients reported nuchal pain ( 44.4% ) , and six patients reported drowsiness ( 66.7% ) .
there was a significant increase in icf in the active group compared to the sham group after treatment ( p = 0.038 ) .
five patients in the active group and only one patient in sham group presented increase in icf after treatment .
there were no significant differences in changes in rmt ( p = 0.489 ) or sici ( p = 0.145 ) after treatment between the two groups .
post hoc analysis showed that the power to find significant differences between the two groups was 20% for rmt and 34% for sici .
one of the subjects ( patient 4 of the sham group in figure 2 ) presented a higher level of icf ( 571.8% ) compared to other subjects , at baseline .
however , post hoc analysis was performed without this subject , and the difference in change in icf between the two groups was no longer statistically significant ( p = 0.074 ) .
the main result of this study was the increase in icf in m1uh after 10 sessions of uh - lf - rtms in patients with hand paresis , in the subacute phase after ischemic stroke .
this is the first time that a change in icf in the uh is reported after uh - lf - rtms .
icf is a measure of intracortical synaptic excitability , with a relative contribution of spinal mechanisms .
icf is usually upregulated by glutamatergic agonists . because icf has a spinal component , a spinal mechanism underlying the observed change
the observed increase in icf in m1uh in the active group is consistent with results obtained in patients at a chronic stage ( average , 31 months after stroke ) , after 10 sessions of constraint - induced therapy ( cit ) .
cit consists of restraint of the unaffected upper limb during waking hours and intensive and repetitive task - oriented therapy of the affected upper extremity , to overcome reliance on the nonparetic upper limb to perform daily activities , and consequent  learned nonuse  of the paretic limb .
it is possible that changes in the m1uh synaptic excitability may reflect reorganization induced by specific interventions such as rtms or cit .
reorganization in m1uh does not necessarily translate into measurable , relevant motor improvement of the paretic upper limb .
for instance , modified cit ( shaping training of the paretic hand , with or without restraint of the upper limb for two weeks ) is associated with increased cortical thickness in m1uh . however , increased thickness does not correlate with enhancement of motor function evaluated with the wolf motor function test or the motor activity log .
the link between structural or functional changes in m1uh and behavioral improvements remains to be clarified .
a single session of lf - rtms was reported to decrease icf in the stimulated hemisphere in healthy subjects , when a similar paired - pulse protocol was applied by romero et al . .
the opposite result observed in patients with stroke may reflect a difference in experimental paradigm ( one rtms session in the study by romero et al .
, in contrast with ten sessions in the present study ) or a state - dependent phenomenon ; that is , effects of rtms may vary according to baseline excitability of the stimulated cortex .
we did not include a control group to investigate differences in baseline excitability between patients with stroke and controls .
the two studies that compared icf in the uh of patients in the subacute phase after stroke , and icf in hemispheres of healthy subjects , did not report significant differences [ 4 , 28 ] .
still , comparable icf at baseline should not necessarily translate into comparable responsiveness to ten sessions of rtms .
the lack of significant changes in rmt or sici may reflect absence of changes in membrane excitability mediated by ion channels , as well as absence of modulation of gabaergic synapses mediating sici , by 1  hz rtms of m1uh .
alternatively , it may reflect insufficient power given the study 's sample size . still , the main limitation of this study is the number of patients .
this low rate of inclusion is consistent with those reported in other studies of tms in stroke .
then , because of technical problems with the stimulator , data from 14 patients were not available .
post hoc analysis showed that if results from one patient were excluded , the results would no longer be statistically significant .
furthermore , the number of subjects included in the analysis was not substantially different from those reported by other studies that evaluated excitability to tms in patients with stroke [ 4 , 28 ] . considering
that sici and icf vary considerably even in healthy subjects , multicenter studies may be necessary to overcome challenges in this field of research .
another limitation of this study is the lack of evaluation of interhemispheric inhibition after uh - lf - rtms . in patients with mild hand paresis in the chronic phase after stroke , interhemispheric inhibition of the affected hemisphere by
the uh , evaluated by the ipsilateral silent period , decreases after uh - lf - rtms .
recording of the ipsilateral silent period in this context involves stimulation of the uh during voluntary contraction of the paretic hand .
we could not record the ipsilateral silent period because we included patients with severe hand motor deficits , unable to perform hand muscle contractions in the paretic hand , as required to record the ipsilateral silent period while applying tms to the m1uh .
finally , spinal excitability was not evaluated . in summary , the increase in icf in m1uh after ten sessions of active uh - lf - rtms indicates that modulation of synaptic excitability in the uh or in the spinal cord is associated with this intervention .
further studies are required to confirm this result in a larger sample of patients with stroke and to investigate the functional role of this phenomenon , possibly related to reorganization , in stroke recovery .
 low - frequency repetitive transcranial magnetic stimulation of the unaffected hemisphere ( uh - lf - rtms ) in patients with stroke can decrease interhemispheric inhibition from the unaffected to the affected hemisphere and improve hand dexterity and strength of the paretic hand . 
 the objective of this proof - of - principle study was to explore , for the first time , effects of uh - lf - rtms as add - on therapy to motor rehabilitation on short - term intracortical inhibition ( sici ) and intracortical facilitation ( icf ) of the motor cortex of the unaffected hemisphere ( m1uh ) in patients with ischemic stroke . 
 eighteen patients were randomized to receive , immediately before rehabilitation treatment , either active or sham uh - lf - rtms , during two weeks . 
 resting motor threshold ( rmt ) , sici , and icf were measured in m1uh before the first session and after the last session of treatment . 
 there was a significant increase in icf in the active group compared to the sham group after treatment , and there was no significant differences in changes in rmt or sici . 
 icf is a measure of intracortical synaptic excitability , with a relative contribution of spinal mechanisms . 
 icf is typically upregulated by glutamatergic agonists and downregulated by gabaergic antagonists . the observed increase in icf in the active group , in this hypothesis - generating study , may be related to m1uh reorganization induced by uh - lf - rtms . 
1. Introduction
2. Methods
3. Results
4. Discussion