Optimization of the recording of the CMAP scan and assessment of its reproducibility

Most neuromuscular disorders have in common that they affect motor units (MUs).
Motor units are the functional units of the peripheral motor system. They
consist of a single motoneuron together with the muscle fibres that this neuron
innervates.
Considering the MU in a pathological context, a basic distinction can be made
between the neurogenic disorders that primarily affect the cell bodies in the
spinal cord and/or their nerve fibres (axons), and the myogenic diseases with a
major impact on the muscle fibres. In neurogenic disorders that affect the
nerve cells, loss of functioning motoneurons leads to a reduced number of MUs.
Loss of motoneurons, even when severe, may be masked by sprouting of other,
still intact nerve fibres.
In myogenic disorders, the number of axons tends to be preserved, at least
initially. However, the number of muscle fibers that are innervated by each
axon decreases, resulting in a smaller MU size and a reduction of the force
that can be delivered by the affected muscles.

The CMAP scan
The CMAP scan is a noninvasive neurophysiological tool that records the
electrical activity of a muscle in response to repetitive transcutaneous
stimulation of the motor nerve. It is based on the fact that MUs differ with
respect to stimulus intensity that is required to activate them (differing
thresholds). If stimulus intensity is gradually increased from subthreshold to
supramaximal values, all MUs in the muscle are successively activated. Thus, by
plotting response size versus stimulus intensity, a visual assessment of the
entire muscle can be obtained.
Patterns in or properties of the CMAP scan (steps, maximum, variability,
decrements, stimulus intensities used) provide clinically relevant information
regarding motor unit number, size and stability, and neuromuscular transmission
and axonal excitability. The scan can be recorded noninvasively in about 5
minutes and is fairly easy to interpret. Because it is built up from
contributions of all functioning motor units, the scan shows if and how many
large motor units are present. There is no sample bias.
De CMAP scan can provide important adjuvant information to the standard
neurophysiological investigations. It helps to interpret the results of
investigations and can help to decide if more extended neurophisiological
investigation has additional value.

This study aims to determine choices for stimulus number, frequency and
duration that cause minimal discomfort to subjects. In addition, it will assess
reproducibility and the effect of going up or down in stimulus intensity for
the APB.

In this multicenter study (3 centers) 36 subjects in total will be measured.
Each lab studies 4 subjects at each of 20-39, 40-59, 60-80 yrs. This yields
data on 12 subjects per lab. The department of Clinical Neurophysiology of the
Erasmus MC in Rotterdam will study 2 variables: stimulus duration and stimulus
frequency. On the first day, the CMAP scan will be measured with different
stimulus frequencies. The second day, the CMAP scan will be measured with
different stimulus durations.
The Mayo Clinic Rochester and the Royal Brisbane and Women's Hospital will
study the effect of going up and down in stimulus intensity and the optimal
choice for stimulus number, respectively.

The investigations are noninvasive. There are no risks, nor are there immediate
benefits for individual subjects.