Recommended placements of stimulating and recording electrodes for routine pediatric studies of the limbs and trunk are presented in Table 5.1 and Figs. 5.1–5.7. In all cases an effort has been made to provide anatomical surface markings and distances to guide electrode placement. These are based on standard placement protocols in adults [27, 28]. However, the described positions utilize landmarks rather than fixed linear measures to allow for the size variability in children.

It is important to accord equal attention to placement of both the active (E1, also known as G1) and the reference (E2, also known as G2) electrodes. Although the reference is sometimes referred to as the “indifferent” electrode, it is well established that the tendon is not in fact electrically inactive. The placement of the reference has a major role in determining the morphology (whether bifid or simple) of the recorded CMAP [29].

As presented in Table 5.1, it is sometimes beneficial to choose an off-tendon site for the reference in order to achieve a sharp negative take-off for the CMAP. This is notably the case for ulnar motor recording from the first dorsal interosseous muscle. It is not infrequent to observe a prominent positive deflection in FDI CMAP recordings, even when the reference is placed over the second and not the first MCP joint, which many authors report to be preferable [30]. In my experience the trapezoid bone, an off-tendon site proposed by Seror, produces sharper take off for FDI than either of the MCP sites and it is therefore my preference [31].

The importance of standardizing placement of surface recording electrodes is emphasized by the work of Phongsamart and colleagues, which demonstrates that positioning of the reference influences not only CMAP morphology but also distal motor latencies [32].

The presence of an initial positive deflection in the CMAP wave-form at conventional gain implies technical error and may be explained by one of two things. Firstly there may be mal positioning of either E1 (G1) relative to the motor-point (end-plate region) of the target muscle or there may be mal positioning of E2 (G2) leading to an abnormal electrical contribution from the reference. The second possibility is that positioning of the recording electrodes is accurate but that there is volume conduction from another source that is contaminating the recording; this occurs in situations of over-stimulation with subsequent radial spread of the stimulus to adjacent nerves (e.g., a median motor study causing co-excitation of ulnar-innervated thenar muscles because of spread of stimulus to the ulnar nerve at the wrist) [2].

Another effect of over-stimulation is longitudinal spread of the stimulus along the nerve beyond the site of the surface cathode. It is suggested that supramaximal stimulation of 15–20 mA results in longitudinal spread of stimulus current by some 3 mm, which produces depolarization at a more distal node of Ranvier. At stimuli of 60 mA (which may be required to elicit CMAPs in come demyelinating neuropathies), the extent of longitudinal spread can be as much as 12 mm; this affects latencies and alters calculations of motor conduction velocity [2].

The dangers of both radial and longitudinal spread of stimulus away from the site of the surface cathode are particularly real in children because of their smaller limbs. It is therefore required to strike a balance between using stimuli that are truly supramaximal and using stimuli that exceed this and which spread elsewhere. As a general rule, in the absence of demyelinating neuropathy, stimulus intensities of >50 mA can be avoided in almost all children unless it is required to stimulate very proximal sites such as Erb’s point, or the phrenic nerve in the neck; neither is a routine site of stimulation in children.

The possibility of under-stimulation in pediatric NCS is another potential pitfall, as in adults, but the risk of this occurring is higher in some pediatric situations when the child becomes uncomfortable and the neurophysiologist is tempted to rush through the nerve conduction studies and avoid escalating the distress. Under-stimulation can occur when the given stimulus is too low or when it is off-target with respect to the underlying motor nerve as may happen, for example, if the child is moving during the test. The consequence of under-stimulation is to give the impression of abnormally low CMAP amplitudes or to create an impression of motor conduction block (see below).

As discussed in Chap. 4, the small distances involved in pediatric EMG will also increase the likelihood and scale of error related to over- or under-measurement of distances for calculation of velocities. For this reason, the examiner must be consistent with positioning of the limbs (e.g., right angled measurement of ulnar motor nerve conduction around the elbow) in all cases.

It is important to be aware that marked drops (21–43%) in tibial CMAP amplitude from abductor hallucis normally occur between distal and proximal stimulation sites of stimulation. This is a product of phase cancellation and physiological temporal dispersion and should not be misinterpreted as motor conduction block without evidence of demyelination from other nerves [33]. The phenomenon is less marked in children, particularly in the very young where the distances between proximal and distal stimulation are relatively short. The second point is that for lower limb F-wave measurement it is best to choose the tibial and not the peroneal motor nerve, in which F-waves are harder to elicit without recourse to very high supramaximal stimuli.