Develops over 1-3 days
Tetrad of:
Altered mental status - agitated delirium progressing to stupor/coma
Muscle rigidity - generalized, "lead pipe" rigidity
Hyperthermia >38C (87%); >40C (40%)
Autonomic instability - tachycardia, hypertension, diaphoresis
Altered mental status - agitated delirium progressing to stupor/coma
Muscle rigidity - generalized, "lead pipe" rigidity
Hyperthermia >38C (87%); >40C (40%)
Autonomic instability - tachycardia, hypertension, diaphoresis
Dehydration
Electrolyte imbalance
Acute renal failure (rhabdomyolysis)
Dysrhythmias 
ACS
Respiratory failure
Chest wall rigidity, aspiration pneumonia, PE
DIC
Seizure (hyperthermia, electrolyte derangements)
Hepatic failure
Sepsis
Chest wall rigidity, aspiration pneumonia, PE
Neuroleptic malignant syndrome
Serotonin Syndrome
Limbic encephalitis
Serotonin syndrome more likely to have hyperreflexia, myoclonus, ataxia, nausea and vomiting, diarrhea
Osmotic demyelination syndrome
Rigidity and hyperthermia, if present, is less severe than in NMS; more commonly presents with clonus and hyperreflexia
Posterior reversible encephalopathy syndrome
Malignant Hyperthermia