Measuring the cortical silent period can increase diagnostic confidence for amyotrophic lateral sclerosis

Single-pulse transcranial magnetic stimulation in amyotrophic lateral sclerosis

INTRODUCTION

Transcranial magnetic stimulation (TMS) is one of the best methods to identify changes in the corticospinal tract. We used single-pulse TMS at the beginning of the disease and in the follow-up in a group of patients with amyotrophic lateral sclerosis (ALS).

METHODS

We evaluated the corticospinal tract in the bulbar, upper, and lower regions in 55 patients with ALS, and we monitored them for a period of 24 months. Data were correlated with clinical scales.

RESULTS

An increase of central motor conduction time (CMCT) was the most sensitive marker of upper motor neuron involvement. The resting motor threshold, CMCT, and the central silent period increased linearly with disease duration and upper/lower motor neuron involvement.

DISCUSSION

Transcranial magnetic stimulation could be an essential neurophysiological technique in the early phase of ALS because it has been shown to be useful in detecting subclinical upper motor neuron involvement. Multiple evaluations of several regions increase TMS sensitivity.

Amyotrophic lateral sclerosis or not: Keys for the diagnosis

Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease (MND) which prognosis is poor. Early diagnosis permit to set up immediately adapted treatment and cares. Available diagnostic criteria are based on the detection of both central and peripheral motor neuron injury in bulbar, cervical, thoracic and lumbar regions. Electrodiagnostic (EDX) tests are the key tools to identify peripheral motor neuron involvement. Needle examination records abnormal activities at rest, and looks for neurogenic pattern during muscle contraction. Motor unit potentials morphology is modified primary to recruitment. Motor evoked potentials remain the test of choice to identify impairment of central motor neurons. In the absence of diagnostic biomarker of ALS and among essential investigations of suspected MND, a careful clinical and neurophysiological work-up is essential to rule out the differential diagnosis.

Quantification of the corticospinal silent period evoked via transcranial magnetic stimulation

A magnetic pulse to the cortex during a muscle contraction produces a motor evoked potential (MEP) followed by electrical quiescence before activity resumes that is indicative of corticospinal inhibition and commonly referred to as the corticospinal slient period (SP). The purpose of the present study was to determine the effect of stimulus intensity and quantification method on the between-visit variability of the SP in healthy individuals. On two occasions we recorded the SP from 9 humans at 3 stimulus intensities (10, 20 and 30% above active motor threshold [AMT]) and quantified the SP based on 8 common criteria. We evaluated the effect of stimulus intensity on reliability by using the limits of agreement, and this analysis revealed that the lower stimulus intensities (10 and 20% AMT) exhibited heteroscedasticity, which indicates the amount of random error increases as the silent period increases. The 30% AMT intensity was homoscedastic. We used both visual and mathematical approaches to quantify the SP, and observed that the between-visit coefficient of variation (CV) was less for the visual methods, and that the CV was reduced when the SP onset was earliest in the temporal occurrence of events (i.e. MEP onset to EMG return CV=12%). Inter-rater reliability for the visual analyses were high (r=0.91-0.99). These results suggest that SPs evoked with a stimulus intensity >or=30% AMT and quantified visually by defining the start of the SP at stimulus delivery or the start of the MEP be utilized to decrease the between visit variability.

Supratentorial ischemic stroke: more than an upper motor neuron disorder

The primary goal of this study was to identify secondary functional changes in the peripheral motor units of the paretic upper extremity (UE) in patients with severe ischemic stroke and to determine how these changes develop during the first weeks after stroke. An inception cohort of 27 consecutive patients with an acute ischemic supratentorial stroke and an initial UE paralysis was compared with 10 healthy control subjects. The ulnar nerve was electrically stimulated proximal to the wrist and electromyographic recordings were obtained from the abductor digiti minimi muscle. Hemiparetic side mean values of the compound muscle action potential (CMAP) 1 and 3 weeks after stroke were compared with the nonparetic side and with CMAP values obtained from healthy control subjects. The mean CMAP amplitude in patients was significantly lower on the paretic side compared with the nonparetic side and with control subjects. Decrease in CMAP amplitude was observed in more than half of the stroke patients, sometimes as early as 4 days after stroke, and persisted in most cases. Whenever present, it was accompanied by absence of motor recovery at that specific time after stroke. Decreased CMAP amplitude in the abductor digiti minimi muscle can be seen already in the very acute phases after stroke unrelated to peripheral neuropathy, radiculopathy, or plexopathy, and it is accompanied by absence of UMN recovery. This knowledge is important for interpreting electrophysiological data in stroke patients.