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intraoperative monitoring ( iom ) of the motor pathways is a routine procedure for ensuring integrity of corticospinal tracts during scoliosis surgery . in combination with somatosensory evoked potentials ,
motor evoked potential ( mep ) monitoring is widely utilized in operations with significant risks of spinal cord damage [ 2 , 14 ] .
however , anesthetic agents , which cause suppression of cortical and spinal motor neuron excitability , affect them [ 4 , 5 ] . while most iom protocols involving meps utilize total intravenous anesthesia ( tiva ) , we have previously reported success with desflurane as a halogenated inhalational anesthetic agent . in iom , meps are elicited mostly with contralateral cortical electrical stimulation .
our paper describes observations of ipsilateral and contralateral meps with bilateral recordings , in conjunction with tiva or desflurane during iom of scoliosis surgery .
we studied nine patients ( mean age 16.2  years ; range 1417  years ; 1 male ) over a 6-month period in a prospective manner .
multi - pulse transcranial electrical stimulation was performed using two constant - current stimulators connected in parallel configuration from a dantec keypoint emg machine ( dantec , skovlunde , denmark ) .
a train of five square wave stimuli 0.5  ms in duration was delivered at 4  ms ( 250  hz ) interstimulus intervals .
stimulating electrodes consisted of 9  mm gold - plated disc electrodes at c3c4 ( international 1020 system ) affixed with collodion .
stimulation output was increased from 50  ma in steps of 5  ma until a reproducible mep was elicited .
the intensity was then increased and fixed at 10% above this threshold intensity to obtain a supramaximal mep response .
each stimulator was capable of delivering a maximum output of 100  ma ( 200  ma in total ) .
mep recordings were obtained with 13  mm disposable subdermal needles ( technomed europe , beek , netherlands ) in the tibialis anterior ( ta ) bilaterally .
input impedance of stimulating and recording electrodes were maintained below 5  k. for induction of anesthesia , sodium thiopentone at 4  mg / kg and fentanyl at 2  mcg / kg was administered ; 0.8  mg / kg of intravenous atracurium was used to facilitate endotracheal intubation .
no further doses of neuromuscular blocking agents were used subsequently . in the desflurane group , anesthesia
desflurane was introduced through a calibrated vaporizer up to an end - tidal concentration of 2.14.3 % , with a mean concentration of 3.4% ( approximately 0.5 maximum alveolar concentration ) .
this was measured using an ohmeda respiratory gas monitor 5250 ( boc group , louisville , usa ) .
closed circuit mechanical ventilation was adjusted to maintain end - tidal carbon dioxide levels between 32 and 35  mmhg . in the tiva group , anesthesia
was maintained using the regime of 10  mg / kg of propofol for the first 10  min , 8  mg / kg for the next 10  min and 5  mg / kg for the subsequent length of operation ; 50% air in oxygen was administered . in both groups , morphine was titrated as required for pain relief .
after approximately 45  min post - induction , a train of four - twitch assessment was performed using a nerve stimulator ( fischer paykel ns242 , uk ) .
cortical stimulation was commenced only when the amplitude of the fourth was visibly similar to the first .
peak to peak amplitudes ( between two largest peaks opposite in polarity ) and onset latency was utilized for all mep responses recorded bilaterally .
hence , ipsilateral meps refer to meps recorded from the ta on the same side as cortical stimulation . within each patient
, ten consecutive supramaximal meps obtained before insertion of pedicle screws used as a baseline were averaged to obtain the first two parameters .
the initial stimulation intensity was defined as the minimal intensity required to obtain five consistent visible mep responses at a vertical gain of 20  v per division . during insertion of pedicle screws and instrumentation , a 50% reduction of mep amplitude or 10% prolongation of latency
statistical analyses using student s t - tests were obtained with microsoft spss for windows version 10.1 .
there were no complaints of headache , seizures or skin burns postoperatively ; all patients had normal neurological examination . meps were successfully obtained from all patients with ta recordings bilaterally .
mean ages for desflurane ( 16.2 ) and tiva ( 15.7 ) groups were not significantly different ( p  =  0.6 ) .
so as to require immediate surgical attention during and after pedicel screw insertion and spinal instrumentation .
the ipsilateral mep amplitudes ( standard deviation ) were significantly larger than contralateral mep amplitudes [ 68.9 ( 27.1 ) vs. 52.5 ( 15.7 )  v , p  <  0.01 , paired t - test ] . the initial stimulation intensity to obtain ipsilateral meps was significantly lower than for contralateral meps [ 66.9 ( 12.3 ) vs. 74.4 ( 10.1 )  ma , p  <  0.05 , paired t - test ] .
however , there were no significant latency differences for ipsilateral and contralateral meps [ 32.0 ( 2.1 ) vs. 31.5 ( 2.0 )  ms , p  =  0.3 , paired t - test ) .
there was no significant difference between use of tiva or desflurane anesthesia for mep amplitudes obtained , with ipsilateral ( p  =  0.06 , unpaired t - test ) and contralateral ( p  =  0.09 , unpaired t - test ) stimulation .
additionally , there was also no significant difference between right and left sided mep amplitudes , either with ipsilateral ( p  =  0.9 , paired t - test ) or contralateral ( p  =  0.7 , paired t - test ) stimulation .
we consecutively studied an additional 17 subjects monitored for scoliosis surgery using an identical protocol ( 1 men , mean age 16.1  years , range 1422 ) . all
with right cortex stimulation , mean initial stimulation intensity to obtain ipsilateral meps [ 39.7 ( 9.9 )  ma ] was significantly lower than to obtain contralateral meps [ 47.1 ( 11.3 )  ma , paired t - test , p  <  0.0005 ] . with left cortex stimulation ,
similar findings were obtained [ 40.6 ( 10.7 ) vs. 50.3 ( 11.8 )  ma , paired t - test , p  <  0.0005 ] . with right cortex stimulation , mean ipsilateral mep amplitudes [ 107.1 ( 35.7 )  v ] were significantly larger than mean contralateral mep amplitudes [ 90 ( 37.1 )  v , paired t - test , p  =  0.01 ] . with left cortex stimulation ,
similar findings were again observed [ 112.1 ( 37 ) vs. 82.3 ( 30.9 )  v , paired t - test , p  =  0.0004 ] . with right cortex stimulation , mean ipsilateral mep latencies [ 30.2 ( 2.4 )  ms ] were not significantly different from mean contralateral mep latencies [ 30 ( 2.3 )  ms , paired t - test , p  =  0.3 ) . with left cortex stimulation ,
similar findings were again observed [ 30 ( 2.1 ) vs. 30.4 ( 2.3 )  ms , paired t - test , p  =  0.2 ] .
examples of meps obtained with both ipsilateral and contralateral stimulation are shown in schematically in fig . 
1schematic diagram showing right cortical stimulation , resulting in ipsilaterally and transcallosally conducted corticospinal impulses activating the spinal cord anterior horn cell .
summation of ipsilaterally conducted and transcallosally generated descending impulses may thus result in larger ipsilateral meps from right cortical stimulationfig . 
2actual consecutive meps obtained from a patient , showing larger amplitude responses with ipsilateral stimulation .
both recordings were made from the ta at 70% stimulation intensity schematic diagram showing right cortical stimulation , resulting in ipsilaterally and transcallosally conducted corticospinal impulses activating the spinal cord anterior horn cell .
summation of ipsilaterally conducted and transcallosally generated descending impulses may thus result in larger ipsilateral meps from right cortical stimulation actual consecutive meps obtained from a patient , showing larger amplitude responses with ipsilateral stimulation .
the present study showed that ipsilateral meps have significantly larger amplitudes and were elicited with significantly lower stimulation intensities than contralateral meps .
the origin of ipsilateral meps in humans is not well understood . in animal studies , cat corticospinal neurons
have been shown to evoke ipsilateral actions via ipsilaterally descending reticulospinal tracts , as well as via contralaterally descending reticulospinal neurons , both by synapsing spinal interneurons .
tracer studies in rhesus monkeys have quantified ipsilateral corticospinal fibers as approximately 912% of the total descending corticospinal projections .
thus , current evidence points to contralateral corticospinal fibers as the predominant pathway for spinal motorneuron activation .
patients with congenital mirror movements , schizencephaly , and kallmann s syndrome   show ipsilateral meps , which likely result from abnormal structure and function of ipsilateral corticospinal fibers .
however , functional reorganization and unmasking of ipsilateral corticospinal pathways may contribute to the generation of ipsilateral meps after adult stroke   and congenital hemiparesis .
one study involving 50 normal children suggested that presence of ipsilateral meps might be a normal state of ontogeny .
their disappearance after 10 years old is likely due to increasing transcallosal inhibitory influences . in our study , all patients were above 10  years of age , and did not have clinical features to suggest presence of underlying conditions mentioned above .
another study comparing healthy adults with stroke patients has suggested ipsilateral meps may be conducted via corticoreticulospinal or corticopropriospinal pathways in normal subjects .
firstly , it is possible that ipsilateral meps may be solely due to transcallosal stimulation of the contralateral motor cortex . additionally , the effects of anesthesia on corticospinal excitability may facilitate this , hence resulting in significantly lower initial stimulation intensity to obtain ipsilateral meps . while evidence to suggest this is scarce , rat brain studies have demonstrated widespread action of anesthesia at multiple binding sites .
magnetic resonance brain imaging has also demonstrated increased callosal t2 changes with anesthesia , suggesting structural alterations at a molecular level .
motor pathway reorganization and spinal cord plasticity have been well documented in response to cord injury   in an activity - dependent manner .
thus , structural and postural changes of longstanding scoliosis may have resulted in reorganization of cortical or subcortical motor pathways , including ipsilateral corticoreticular fibres leading to our observations . however , lack of lateralization of mep amplitudes with ipsilateral or contralateral stimulation was not supportive of this hypothesis . additionally , lack of significant ipsilateral and contralateral latency differences suggest bilateral motor cortex stimulation has resulted in ipsilateral meps , which may have comprised early ipsilaterally conducted components and late transcallosally stimulated components ( fig
this might also explain the larger amplitudes of ipsilateral meps obtained than meps derived from contralateral motor cortex stimulation .
while the relative contributions of ipsilaterally and transcallosal conducted meps remain uncertain , bilateral mep recordings during spinal surgery iom may provide additional information regarding the integrity of descending motor tracts .
together with the electrophysiological findings presented here , future studies clarifying these aspects would be justified .
it thus may be feasible to routinely monitor meps bilaterally in future iom protocols for spinal surgery .
ipsilateral motor evoked potentials ( meps ) in spinal cord surgery intraoperative monitoring is not well studied . 
 we show that ipsilateral meps have significantly larger amplitudes and were elicited with lower stimulation intensities than contralateral meps . 
 the possible underlying mechanisms are discussed based on current knowledge of corticospinal pathways . 
 ipsilateral meps may provide additional information on the integrity of descending motor tracts during spinal surgery monitoring .