Neuromuscular Blockers
The neuromuscular blockers used in neonates are vecuronium and pancuronium. They act on the nicotinic receptors and produce competitive nondepolarizing blockage. Neuromuscular blockers have an expected duration of action. The action of vecuronium lasts for 25 to 40 minutes in older patients. In neonates, the action of pancuronium usually lasts for 35 to 55 minutes. The action of neuromuscular blockers sometimes lasts longer than anticipated in neonates, leading to persistent hypotonia. This possibility should be considered prior to embarking on a complete neurophysiological evaluation in neonates that have previously received neuromuscular blockers.

Hypotonia due to muscle involvement is associated with normal or decreased dynamic tone. Cranial nerves are usually involved. Myopathic hypotonia may course with elevated serum creatine phosphokinase. Electromyography pattern in muscle disease is characterized by low amplitude, brief duration, abundant motor unit potentials. Muscle biopsy may be normal or may show dystrophic, inflammatory, or structural changes.

Figure 142.1.— Salient features of generalized hypotonia due to muscle disease. Arrow indicates the anatomical location of the injury (muscle). CPK: creatinine phophokinase; EMG: electromyogram; MUP: motor unit potentials.