Critical Physiology Series #80 Neuroleptic Malignant Syndrome vs Serotonin Syndrome Two patients develop fever, altered consciousness, tachycardia and autonomic instability after medication exposure. One has lead-pipe rigidity. The other has clonus and hyperreflexia. The physiology is different. Neuroleptic Malignant Syndrome = too little dopamine activity NMS usually follows dopamine D₂ blockade from antipsychotics or abrupt withdrawal of dopaminergic therapy. Reduced hypothalamic dopamine signaling disrupts thermoregulation and autonomic control. Reduced nigrostriatal activity produces profound rigidity. Sustained muscle contraction generates: heat → ATP consumption → rhabdomyolysis → CK elevation → hyperkalemia → AKI Typical pattern: rigidity + brady/hyporeflexia + fever + autonomic instability Onset usually evolves over 1to3 days. Serotonin Syndrome = too much serotonin activity Serotonin toxicity results from excessive serotonergic signaling, particularly 5-HT₂A activation. It often follows combinations, overdose or interactions involving serotonergic drugs. Neuromuscular excitation dominates: clonus + hyperreflexia + tremor Lower limbs may show these findings most clearly. Autonomic activation produces diaphoresis, tachycardia, hypertension and hyperthermia. Severe hyperthermia results primarily from excessive muscle activity, not resetting of the hypothalamic thermostat. Therefore antipyretics do little. The bedside distinction NMS → rigid and slow Serotonin toxicity → clonic and hyperexcitable CK may rise in both, so CK alone does not differentiate them. Monitoring Both require close monitoring of: core temperature ECG/hemodynamics CK K⁺, Ca²⁺, Mg²⁺ creatinine/urine output pH/lactate coagulation Management For both: STOP the causative drugs + supportive ICU care + fluids + active cooling when needed. For severe NMS, benzodiazepines are useful; bromocriptine or amantadine may restore dopaminergic activity, while dantrolene is sometimes used in severe rigidity/hyperthermia. Evidence for these specific therapies remains limited. For serotonin toxicity, benzodiazepine sedation reduces muscle activity and autonomic stimulation. Cyproheptadine, a 5-HT₂A antagonist, can be considered, although evidence is limited. Severe hyperthermia requires sedation, paralysis, intubation and active cooling. Both can look like “drug-induced hyperthermia.” But examine the muscles. Rigidity points toward dopamine failure. Clonus and hyperreflexia point toward serotonin excess. Recommended lectures 📚 Wijdicks, 2024, doi.org/10.1056/NEJMra240460… Pileggi, 2016, doi.org/10.1177/106002801665… Scotton, 2019, doi.org/10.1177/117864691987… Dunkley, 2003, doi.org/10.1093/qjmed/hcg109

❤️Bradycardia in the ED: treat the cause, not the number Bradycardia should not be approached simply as a heart rate problem. The real question is whether the slow rate represents intrinsic conduction disease, a reversible physiological response, or the first sign of a life threatening process. A useful review proposes the mnemonic H.I.T.H.I.T. for the dangerous diagnoses we should actively exclude: Hyperkalemia Ischemia Toxin induced Hypoxia Increased intracranial pressure Thyroid dysfunction Hyperkalemia is the classic example Severe hyperkalemia can progressively produce PR prolongation, QRS widening, disappearance of P waves, conduction block and eventually a sine wave pattern. But hyperkalemia can also present with less stereotypical patterns such as junctional rhythms and sinus pauses. In this situation, atropine and even electrical pacing may be ineffective. The same reasoning becomes even more important in BRASH syndrome, where renal dysfunction, AV nodal blockade, hyperkalemia, bradycardia and shock reinforce each other in a vicious cycle. Ischemia can hide behind the slow rhythm Bradycardia may reflect ischemia of the SA or AV nodal circulation, particularly with inferior myocardial infarction and right coronary artery occlusion. And the absence of chest pain does not exclude it. So a new unexplained bradyarrhythmia deserves an ECG interpretation that goes beyond simply identifying the rhythm. Toxicology deserves particular attention Beta blockers, calcium channel blockers, digoxin, cholinergic agents and local anesthetic systemic toxicity can all produce severe bradycardia, but their treatments are fundamentally different. Even more interesting is the changing recreational drug landscape. A patient with CNS depression, respiratory abnormalities and bradycardia may appear to have straightforward opioid toxicity. But xylazine and medetomidine, increasingly encountered as adulterants, can produce profound bradycardia that does not respond to naloxone. Hypoxia may produce bradycardia before arrest. Raised ICP may produce bradycardia as part of the Cushing response, and after traumatic subdural hemorrhage, bradycardia may occasionally precede the complete triad. Severe hypothyroidism may present with bradycardia, hypotension, conduction abnormalities and multiorgan dysfunction. Then comes treatment Hemodynamic stability determines urgency. Atropine remains useful, particularly for vagally mediated bradycardia, but it is not a universal antidote to a slow heart rate. The review emphasizes its limited effectiveness in high grade AV block and metabolic causes such as hyperkalemia. Epinephrine or norepinephrine can provide chronotropic and hemodynamic support, while refractory instability may require transcutaneous or transvenous pacing. Reference 📚 Menon R. The Heartbeat that Hesitates: Quick H.I.T.s for Emergency Bradycardia. Emerg Med Clin North Am. 2026;44:289–307. doi.org/10.1016/j.emc.2025.1…