Acute neuromuscular disorders occasionally occur in the pediatric intensive care unit. Many of these are primary motor unit disorders that present acutely and require admission to a critical care unit.1 We will discuss these disorders by typically presenting age and by anatomic localization.2,3 A third category relates to disorders developing during an acute systemic illness requiring intensive care management causing secondary acute motor unit dysfunction.

The differential diagnosis for disorders presenting with lesions at each level of the peripheral motor unit may vary by age, with some specific to newborns and toddlers. Spinal muscular atrophy (SMA), both the classic “5q” form due to deletions/variants in SMN1 and “non-5q” forms such as SMA with respiratory distress (SMARD) can present in the neonatal or early infantile period with an acute decompensation, sometimes in the setting of illness. Neonatal congenital polyneuropathies (e.g. Dejerne-Sottas syndrome), congenital myasthenic syndromes, and congenital muscle disorders are all genetic disorders often presenting with profound hypotonia in the neonatal period. Neonatal myasthenia gravis presents with transient neonatal bulbar, respiratory and limb weakness due to maternal antibodies. Infantile botulism may develop in previously healthy infants with the classic presentation of acute bulbar and limb weakness, dilated pupils and constipation. Most of these disorders typically present during the first 3-6 months of life, although some may not occur until the infant is almost one year old.

Electrodiagnostic testing in the neonatal period can be technically challenging but may be useful for evaluation of the “floppy neonate” with a suspected neuromuscular disorder. Normative values exist for nerve conduction studies (NCS).4 Neonates and toddlers generally have lower response amplitudes and slower conduction velocities since peripheral myelination is still developing over the first 3–5 years of life. Repetitive nerve stimulation for neuromuscular junction assessment is useful in the early diagnosis of congenital myasthenic syndromes, neonatal myasthenia gravis, and infantile botulism before confirmatory testing (e.g. genetic testing, antibody testing) is obtained. Needle electromyogram (EMG) requires an experienced pediatric electromyographer. In infants and toddlers, MUPs are smaller and can even appear myopathic. In general, NCS and EMG are the least useful for diagnosing myopathic disorders in this early period.3 This is partially due to the decreased sensitivity of EMG in general for myopathies, but also because confidently identifying the hallmark features of myopathic disorders (early recruitment and small MUPs) is a challenge.

In comparison, the older child and adolescent are predisposed to another set of critical care peripheral motor unit disorders that are quite like those in the adult. These include Guillain-Barre Syndrome (GBS) and myasthenia gravis (MG) most commonly. The neuromuscular complications of extended intubation and sepsis affecting either the peripheral nerve, or muscle cell, also occur in children. The differential diagnosis of these various motor unit disorders is significantly aided using clinical neurophysiology. These studies provide an objective means to assign a specific patho-anatomic or neurophysiologic localization for the child's illness. Electrodiagnostic testing in the older child is technically like adults.

The acute care motor unit disorders discussed in this chapter illustrate a broad anatomic spectrum from the anterior horn cell to the muscle cell. This review is based on our 44-year experience (1979 to 2023) evaluating children, ages newborn through 18 years, at the Boston Children's Hospital (BCH) Neuromuscular Program, intensive care units, and EMG laboratory. Given the major advances in neuromuscular disorders, this chapter is updated from a previous chapter in Seminars in Pediatric Neurology.5 We also acknowledge the major contributions of H. Royden Jones, MD (deceased) to this field and to our Neuromuscular Program.