Muscle-Specific Differences in Neuromuscular Block and Quantitative Neuromuscular Monitoring: A Narrative Review

Neuromuscular blocking agents are an essential component of contemporary anesthesiology. Adequate intraoperative neuromuscular monitoring is recommended to ensure sufficient depth of blockade and prevent postoperative residual neuromuscular block. Several quantitative techniques have been developed to assess neuromuscular transmission at the neuromuscular junction, and the number of clinically relevant monitoring sites has expanded beyond conventional peripheral muscles. In routine practice, train-of-four (TOF) stimulation combined with acceleromyography is most commonly used; the adductor pollicis serves as the standard monitoring site. However, neuromuscular responses measured at the adductor pollicis may differ from those observed in muscles critical for airway management, including the diaphragm and pharyngeal and laryngeal muscles. These muscles typically demonstrate faster onset and recovery, along with earlier recovery to TOF ratios of at least 0.9 by several minutes relative to the adductor pollicis; such dynamics have important clinical implications for intubation conditions and safe extubation. This narrative review summarizes physiologic and pharmacodynamic differences in neuromuscular block among various muscle groups and evaluates clinical evidence regarding alternative monitoring sites (eg, facial, laryngeal, pharyngeal, and foot muscles). The clinical performance and limitations of the TOF-Cuff technique applied to both the upper and lower limbs are also discussed. The review is based on clinically relevant experimental and clinical literature identified through searches of PubMed and Google Scholar, supplemented by reference screening of key papers.