Neuroleptic Malignant Syndrome (NMS)

Separating NMS from malignant hyperthermia - Malignant hyperthermia (MH) is almost clinically indistinguishable from NMS, so exposure history is the discriminator. MH follows a halogenated volatile or succinylcholine and announces itself inside the anesthetic, with rising EtCO2 and tachycardia as the earliest signs. NMS builds over one to three days on a dopamine blocker, or after a dopaminergic drug was stopped. If the trigger was an anesthetic agent, treat it as MH — dantrolene 2.5 mg/kg IV, stop the triggers, hyperventilate with 100% oxygen.

Separating NMS from serotonin syndrome - Onset speed and the neuromuscular exam do it. Serotonin syndrome moves fast: 30% of cases within an hour of the offending dose, 60% within 6 hours, nearly all within 24. Clonus is its signature — spontaneous, inducible or ocular — along with hyperreflexia, myoclonus, dilated pupils, hyperactive bowel sounds and diarrhea, and the findings are more pronounced in the legs. NMS instead gives severe rigidity and hypertonia over days, with both hyperthermia and rigidity more severe than in serotonin syndrome. See the serotonin syndrome entry.

Also consider malignant catatonia - Fever, rigidity, akinesia and altered mental status again, but with prominent positive symptoms and a behavioral prodrome of automatisms, agitation or psychosis. Worth separating because up to 80% recover fully with prompt benzodiazepines and electroconvulsive therapy.

Labs that support the diagnosis - Comprehensive metabolic panel with electrolytes and creatinine, creatine kinase, urinalysis for myoglobinuria, and an arterial or venous blood gas for metabolic acidosis. Expect a profoundly elevated CK, acutely declining renal function, leukocytosis of 10,000 to 40,000 with a left shift, and mildly raised transaminases. When the picture is unclear, neuroimaging and lumbar puncture to exclude CNS infection.

Treatment - Stopping the offending agent is the step that matters most. Then aggressive cooling, volume resuscitation and correction of electrolytes. Bromocriptine, given orally or through a gastric tube, reverses the hypodopaminergic state. Dantrolene is used intravenously in severe cases and orally in milder ones, though the evidence behind it is thin enough that some case series question both its efficacy and its safety. Benzodiazepines control agitation. If the cause was dopaminergic withdrawal, restarting that drug may resolve the syndrome. Admit to the ICU.

Airway and rhythm - Chest wall rigidity can produce respiratory failure, so plan on controlled ventilation rather than assuming the patient will keep their own. Dysrhythmias are common and are a leading cause of death — treat them conventionally while you correct temperature, volume and electrolytes.

Antiemetic choice with an NMS history - Skip the dopamine antagonists. Metoclopramide, prochlorperazine, droperidol and haloperidol all block D2, and antiemetics are documented NMS triggers. Use a 5-HT3 antagonist or dexamethasone instead.

Restarting a neuroleptic - Guidance is to wait at least two weeks after symptoms resolve, use a lower-potency agent, start low and titrate slowly, and avoid pairing it with lithium. Patients should be told to avoid dehydration, which is a documented precipitant, and watched for recurrence.

Recovery timeline - With early recognition and aggressive treatment most patients recover fully in 2 to 14 days. Delay leaves residual catatonia, parkinsonism, or renal and cardiopulmonary damage.

Neuroleptic malignant syndrome (NMS) is a life-threatening reaction to dopamine D2 receptor blockade — usually an antipsychotic, but also antiemetics, tricyclic antidepressants and lithium — or to abrupt withdrawal of a dopaminergic drug such as levodopa or amantadine. The sudden fall in central dopaminergic activity produces the four cardinal features: muscle rigidity, hyperthermia, altered mental status and autonomic instability.

Rigidity drives rhabdomyolysis, so creatine kinase rises steeply and tracks severity, and myoglobinuria threatens the kidneys. NMS is reported most with high-potency first-generation agents such as haloperidol and fluphenazine and with depot forms, and the main trigger is starting the drug or raising the dose. Incidence runs 0.01% to 3.2% of patients on neuroleptics. Mortality was once above 30% and is now under 10%.