Pediatric EMG

Developmental neuromuscular physiology

At term birth, peripheral conduction velocities are roughly half adult values — typically near ~25–30 m/s in the major limb nerves — because the axons are thin and only partially myelinated and the internodal distances are short. Maturation is then rapid: velocities climb steeply through infancy as myelination proceeds and axon calibre increases, reach approximately half-to-two- thirds of adult values by the first year, and attain full adult values by roughly 3–5 years of age, after which internodal lengthening with limb growth produces only minor further change. Distal latencies, late-response latencies, and amplitudes follow their own maturational trajectories, the latter also shaped by the changing distance between stimulating and recording sites as the limb grows.

The clinical consequence is absolute: a velocity of 30 m/s denotes severe demyelination in an adult but is physiologically normal in a neonate. Every paediatric nerve-conduction value must be compared against a table stratified by age, and ideally by the specific nerve and laboratory, or the interpretation is meaningless.

Congenital myopathies

The structural congenital myopathies — central core, nemaline, and centronuclear/myotubular disease — present with the "floppy infant": hypotonia, weakness, and delayed motor milestones. Their electrodiagnostic signature is paradoxically subdued. The needle examination is often electrically bland or shows only mildly myopathic motor-unit potentials — short-duration, low-amplitude, polyphasic units with early recruitment — and spontaneous activity is usually absent. Because the EMG abnormality may be subtle or non-specific, these disorders are frequently indistinguishable on electrophysiology alone, and the diagnosis is driven by muscle biopsy ultrastructure and increasingly by genetic testing. The role of EMG is to establish that the process is myopathic rather than neurogenic and to exclude a treatable junctional mimic, not to subclassify the myopathy.

Hereditary neuropathies and spinal muscular atrophy

Charcot–Marie–Tooth disease is the prototypical inherited neuropathy of childhood. The common demyelinating form (CMT1) produces uniform, length-independent slowing of conduction velocity across all nerves — a homogeneity that distinguishes it from the multifocal, patchy slowing of acquired inflammatory demyelination. Because the slowing is uniform and symmetric, even a limited study sampling one or two nerves can be strongly suggestive, and the finding of markedly and equally reduced velocities in a child with pes cavus and areflexia is nearly diagnostic.

Spinal muscular atrophy is the anterior-horn-cell disease that every paediatric electromyographer must recognise. Conduction velocities are relatively preserved (it is an axonal, motor-neuron process) while motor amplitudes fall. The needle examination shows diffuse denervation and reinnervation — fibrillations and positive sharp waves, large reinnervated potentials, and reduced recruitment — across multiple myotomes and limbs. A characteristic and highly suggestive finding is a tremulous, spontaneously firing motor-unit baseline at rest, reflecting the spontaneous rhythmic discharge of diseased motor neurons. As with CMT, genetic confirmation (here SMN1 deletion) has reduced but not eliminated the diagnostic role of EMG, which remains valuable when presentation is atypical or testing is pending.

Technical and sedation adaptations

Paediatric studies are an exercise in maximising diagnostic yield per unit of distress. The examination must be focused and short: a precise clinical question is formulated in advance, a minimal montage of nerves and muscles is selected to answer it, and superfluous needling is avoided. Practical adaptations include warming the limb (small limbs cool quickly, spuriously slowing conduction), using shorter inter-electrode distances appropriate to the limb, allowing parental presence for comfort, child-life distraction, and topical anaesthetic over stimulation sites. Conscious sedation or, for extensive studies in very young children, general anaesthesia may be required — with the crucial caveat that sedation does not alter nerve-conduction or spontaneous-activity findings but precludes assessment of voluntary motor-unit recruitment, which depends on the child's cooperation. The art lies in obtaining the few measurements that change management while inflicting the least possible distress.