Anticholinesterase agents have served as the primary means of pharmacological reversal of non-depolarizing neuromuscular block for over seven decades. Their mechanism is indirect: rather than displacing the blocking drug from the receptor, they inhibit acetylcholinesterase (AChE) at the neuromuscular junction (NMJ), increasing the concentration of acetylcholine (ACh) available to compete with the blocking agent for nAChR binding sites. Understanding the ceiling effect and the requirements for spontaneous partial recovery before reversal is administered are the two most clinically essential aspects of anticholinesterase pharmacology.

Neostigmine is a quaternary ammonium carbamate that reversibly inhibits AChE by forming a carbamylated enzyme complex with a half-life of approximately 30 minutes. Because neostigmine does not cross the blood-brain barrier, its actions are confined to peripheral cholinergic synapses. At the NMJ, AChE inhibition increases the concentration and dwell time of ACh in the synaptic cleft, allowing ACh molecules to compete more effectively with the non-depolarizing blocking agent for alpha-1 subunit binding sites on the nAChR.¹ The result is a shift in occupancy from blocking drug toward ACh, increasing the probability of channel opening and restoring EPP amplitude above the threshold required for muscle action potential generation. Neostigmine is the standard anticholinesterase used for reversal and is dosed at 40 to 70 micrograms per kilogram intravenously, with a typical clinical dose of 2.5 to 5 mg in adults. Onset of reversal effect is 7 to 11 minutes and duration of action is 60 to 90 minutes.

The ceiling effect is the single most important pharmacological limitation of neostigmine. Because AChE inhibition can only increase ACh concentration to a finite degree, and because ACh must compete with a drug that is present at concentrations proportional to the depth of block, neostigmine cannot reliably reverse deep block. When the train-of-four (TOF) count is zero or one, the concentration of blocking drug at the receptor is so high that even maximal AChE inhibition cannot generate sufficient ACh competition to restore adequate neuromuscular function.² Neostigmine should not be administered until at least a TOF count of 2 is detectable; more recent evidence and guidelines recommend waiting for a TOF count of 4, at which point reversal is both more reliable and faster. Administering neostigmine at TOF count zero or one not only fails to reverse block but may paradoxically worsen it through a mechanism involving receptor desensitization by excess ACh and direct inhibition of residual nAChR function.

Muscarinic side effects are an inescapable consequence of AChE inhibition at non-NMJ cholinergic synapses. Neostigmine stimulates muscarinic receptors throughout the body, producing bradycardia, increased salivation and bronchial secretions, bronchospasm, miosis, increased bowel motility, and urinary urgency. These effects must be attenuated by co-administration of an antimuscarinic agent. Glycopyrrolate (200 micrograms per 1 mg neostigmine) is preferred because its quaternary structure prevents central nervous system (CNS) penetration, avoiding the tachycardia and CNS effects of atropine. Atropine (10 to 20 micrograms per kilogram) is acceptable and acts more rapidly but produces more pronounced tachycardia.¹ Edrophonium, an older quaternary ammonium AChE inhibitor that acts by electrostatic binding rather than carbamylation, has a faster onset (1 to 2 minutes) but shorter duration and is less widely used than neostigmine in current practice.

Residual neuromuscular blockade (RNMB) at the time of tracheal extubation remains one of the most common and consequential preventable complications of anesthesia involving NMBDs. Its persistence is not a minor inconvenience: impaired pharyngeal function, reduced hypoxic ventilatory response, and aspiration risk at TOF ratios below 0.9 are well-documented and associated with postoperative pulmonary complications that increase length of stay and mortality.

RNMB is defined as a TOF ratio below 0.9 at the adductor pollicis muscle measured by quantitative acceleromyography (AMG) at the time of tracheal extubation. This threshold was established through studies demonstrating that pharyngeal dilator muscle function, upper esophageal sphincter competence, hypoglossal motor activity, and the ventilatory response to hypoxia are all measurably impaired at TOF ratios below 0.9, even in patients who appear clinically awake and cooperative.⁶ The prevalence of RNMB at extubation in studies that use quantitative monitoring ranges from 20 to 64 percent when intermediate-duration NMBDs are used without reversal agents, and from 3 to 26 percent even after neostigmine reversal, depending on the depth of block at the time of neostigmine administration and the interval before extubation.

Qualitative assessment of neuromuscular recovery, including visual or tactile assessment of TOF fade and the five-second head-lift test, cannot reliably detect RNMB. Multiple studies demonstrate that trained anesthesia providers cannot distinguish TOF ratios between 0.4 and 1.0 by tactile assessment alone. The five-second head-lift test requires only 33 percent diaphragm strength and does not assess the pharyngeal and upper airway muscle function that is most vulnerable to residual block. Subjective endpoints such as adequate hand grip, sustained eye opening, and response to commands are similarly insensitive.⁷ The use of these qualitative tests as the sole criterion for extubation decisions guarantees a substantial rate of undetected RNMB at the time of extubation.

Quantitative AMG at the adductor pollicis with the ulnar nerve as the stimulation site is the evidence-based standard for confirming adequate neuromuscular recovery before extubation. The adductor pollicis is chosen because it is the last muscle to recover from non-depolarizing block, making it the most sensitive indicator of residual block in the peripheral musculature; its recovery lags behind the diaphragm, laryngeal adductors, and corrugator supercilii. The ulnar nerve is the preferred stimulation site because it provides reproducible supramaximal stimulation and because the adductor pollicis response is easily transduced by AMG. A confirmed TOF ratio of 0.9 or greater at this site is the minimum criterion for safe extubation; a ratio of 1.0 provides additional assurance. Some evidence supports a threshold of 1.0 in high-risk patients such as the morbidly obese and those with significant obstructive sleep apnea.⁸

The choice of reversal strategy is no longer a binary decision between neostigmine and spontaneous recovery. The availability of sugammadex has created a clinically meaningful decision tree that must account for the blocking agent used, the depth of block at the time reversal is needed, patient-specific risk factors for RNMB, and institutional factors including drug availability and cost. The overarching principle is that no patient should leave the operating room or have a tracheal tube removed without confirmed TOF ratio 0.9 or greater by quantitative monitoring.

For patients who have received aminosteroid NMBDs, sugammadex is the preferred reversal agent in all clinical contexts where it is available, regardless of block depth. At moderate block levels (TOF count 2 or greater), sugammadex 2 mg/kg provides faster, more complete, and more predictable reversal than neostigmine at any dose. At deep block levels where neostigmine is ineffective, sugammadex 4 mg/kg remains fully effective. The pharmacoeconomic argument against routine sugammadex use has weakened as evidence accumulates that RNMB-related postoperative pulmonary complications increase hospital length of stay and costs that exceed the acquisition cost of sugammadex per case.¹² Where sugammadex is unavailable or contraindicated (severe renal impairment), neostigmine with glycopyrrolate remains appropriate when the TOF count is 4 and the depth of block has recovered spontaneously to a moderate level before reversal is attempted.

For patients who have received benzylisoquinolinium NMBDs (atracurium, cisatracurium, mivacurium), sugammadex has no activity and neostigmine with glycopyrrolate is the only pharmacological reversal option. Because cisatracurium and atracurium undergo spontaneous Hofmann elimination, allowing adequate time for spontaneous partial recovery before neostigmine administration is generally more feasible than with aminosteroids, particularly for cases of moderate duration. Mivacurium's plasma pseudocholinesterase-mediated hydrolysis produces such a short duration that reversal agents are rarely needed; however, pseudocholinesterase-deficient patients will require neostigmine for reversal of prolonged mivacurium block, as sugammadex has no activity against it.

Several patient populations warrant heightened attention to reversal completeness. The morbidly obese patient has increased pharyngeal collapsibility and higher aspiration risk, making undetected RNMB particularly dangerous; sugammadex dosed on actual body weight rather than lean body weight is recommended to ensure adequate plasma concentrations. Patients with myasthenia gravis (MG) have a reduced functional nAChR population and may have paradoxical resistance to succinylcholine and sensitivity to non-depolarizing agents; reversal with sugammadex is preferred because neostigmine further impairs neuromuscular reserve by inhibiting AChE at already-depleted receptor populations. Elderly patients have reduced hepatic and renal function that may impair aminosteroid clearance, increasing spontaneous RNMB risk and strengthening the case for quantitative monitoring and proactive reversal with sugammadex.¹³¹⁴¹⁵