Altered Excitability of the Motor Cortex after Minor Head Injury Revealed by Transcranial Magnetic Stimulation

Abstract

This study attempts to find out whether the motor evoked potential (MEP) elicited by single pulse and slow-rate (1 Hz) repetitive transcranial magnetic stimulation (TMS) can disclose concealed subclinical impairments in the cerebral motor system of patients with minor head injury. The motor response to single pulse TMS (STMS) of the patient group was characterized by significantly higher threshold compared with that of the control group. The central motor conduction time, as well as the peripheral conduction time were normal in all patients pointing to cortical impairment. Two main patterns of MEP changes in response to repetitive TMS (RTMS) were observed in the patient group. A.--progressive decrease of the MEP amplitude throughout the stimulation session to a near complete abolition. B.--irregularity of the amplitude and the waveform of the MEP in a chaotic form. The MEP latency remained stable during the whole stimulation session. The MEP abnormalities recovered gradually over the period of a few months. The higher threshold of the motor response to STMS and the abnormal patterns of the MEP to RTMS seem to reflect transient impairment of cortical excitability or "cortical fatigue" in patients who sustained minor head injures. Further study is needed to evaluated the extent and the pathophysiological mechanisms of the central nervous system fatigue phenomenon following head injury.

Transcranial Magnetic Stimulation-Electroencephalography Responses in Recovered and Symptomatic Mild Traumatic Brain Injury

“…While this hypothesis is based on studies in M1 nTMS (see Introduction), similar relation with peak latency and balance of inhibition-facilitation could be expected on DLPFC. Our findings would also be compatible with previous TMS studies in mTBI, where acutely decreased M1 excitability 17,18 and a persistently prolonged cortical silent period have been found, 41 suggesting lowered excitability and a stronger inhibitory response.…”

“…12,15 Higher rMT has previously been associated with TBI of varying severity, [16][17][18] possibly also correlating with recovery, at least in mild injuries. 17 nTMS-evoked electroencephalographic responses (nTMS-EEG) can be used to study cortical excitability and connectivity. In nTMS-EEG, certain individually highly reproducible deflections are found, including P30, N45, P60, N100, and P200.…”

Corticospinal excitability in patients with anoxic, traumatic, and non-traumatic diffuse brain injury

“…For the detection of the failure of corticospinal tract conduction, the triple stimulation technique, which consists of one transcranial magnetic and two peripheral electric stimuli (plexus and nerve) delivered along the motor pathways, might have higher sensitivity than conventional MEP or CMCT testing [64]. In contrast to these results, a higher RMT is reported in patients with moderate focal TBI, while no difference was found between patients with minor or moderate diffuse TBI [13,19]. The authors have also described some improvement in the observed high RMT in the three-months follow-up study [19].…”

“…In contrast to these results, a higher RMT is reported in patients with moderate focal TBI, while no difference was found between patients with minor or moderate diffuse TBI [13,19]. The authors have also described some improvement in the observed high RMT in the three-months follow-up study [19]. Our findings of normal RMT in brain injury patients could firstly be explained by that fact, that the period after the insult was longer in our study (on average 4 months); secondly, that the majority of the included patients suffered from a non-traumatic brain injury with possibly less pronounced axonal damage.…”

“…Paired TMS has been proposed as a tool for predicting outcome after stroke [17] as well as predicting functional outcome after rehabilitation interventions [18]. Normalization of amplitude and waveform of MEPs and motor threshold (MT) was related to the clinical recovery in mild to moderate TBI [19]. Despite this, studies identifying changes in cortical excitability in patients with diffuse non-traumatic brain injury due to subarachnoidal haemorrhage and anoxia are rare, and the extent of motor cortical reorganization in these patients remains largely unknown.…”