# Neuroleptic Malignant Syndrome and Serotonin Syndrome

## Introduction

Neuroleptic malignant syndrome (NMS) and serotonin syndrome (SS) are life-threatening, drug-induced emergencies encountered in psychiatric practice. Both require rapid recognition and intervention. Despite overlapping features such as hyperthermia and altered mental status, these syndromes have distinct pathophysiologies, clinical presentations, and management strategies.

## Neuroleptic Malignant Syndrome

### Pathophysiology

Central **dopamine D2 receptor blockade** in the hypothalamus and basal ganglia. Results in thermoregulatory failure and extrapyramidal rigidity. Most commonly triggered by first-generation antipsychotics but can occur with any dopamine antagonist, including antiemetics (metoclopramide)

### Clinical Features — The Classic Tetrad

**Hyperthermia** (temperature often > 40 C) **Lead-pipe muscular rigidity**. **Altered mental status** ranging from agitation to coma. **Autonomic instability**: labile blood pressure, tachycardia, diaphoresis.

### Laboratory Findings

Markedly elevated **creatine kinase (CK)**, often > 1000 IU/L. Leukocytosis, metabolic acidosis, elevated liver transaminases. Myoglobinuria raising the risk of **acute renal failure**.

### Management

Immediate discontinuation of the offending agent. Aggressive IV hydration and cooling measures. **Dantrolene** (skeletal muscle relaxant) for severe rigidity. **Bromocriptine** or **amantadine** to restore dopaminergic tone. ICU admission for hemodynamic monitoring.

![Clinical algorithm for the diagnosis and management of NMS](images/nms-diagnostic-algorithm.png)

## Serotonin Syndrome

### Pathophysiology

Excess **serotonergic activity** at 5-HT1A and 5-HT2A receptors. Typically precipitated by drug combinations: SSRIs + MAOIs, SSRIs + tramadol, SSRIs + linezolid. Can also occur with single-agent overdose.

### Clinical Features — Hunter Criteria

**Clonus** (spontaneous, inducible, or ocular) is the hallmark. Agitation, diaphoresis, diarrhea. Tremor and **hyperreflexia**. Hyperthermia (usually milder than NMS unless severe)

### Differentiating NMS from Serotonin Syndrome

| Feature | NMS | Serotonin Syndrome |
|---|---|---|
| Onset | Days to weeks | Hours |
| Muscle tone | Lead-pipe rigidity | Clonus, hyperreflexia |
| Reflexes | Decreased | Increased |
| Pupils | Normal | Dilated |
| Bowel sounds | Decreased | Increased |
| CK elevation | Markedly elevated | Mildly elevated |

![Side-by-side comparison of NMS vs serotonin syndrome clinical features](images/nms-vs-serotonin-syndrome.png)

### Management

Discontinue all serotonergic agents. Benzodiazepines for agitation and mild cases. **Cyproheptadine** (5-HT2A antagonist) for moderate to severe cases. Active cooling and supportive care. Avoid physical restraints that worsen hyperthermia and rhabdomyolysis.

## Prevention Strategies

Perform thorough medication reconciliation before prescribing serotonergic agents. Allow adequate **washout periods** when switching antidepressants (especially MAOIs: 14 days) Educate patients about over-the-counter serotonergic substances (dextromethorphan, St. John's Wort) Monitor for early signs during antipsychotic initiation or dose escalation.

![Washout period guidelines for safe antidepressant switching](images/antidepressant-washout-periods.png)

## Key Clinical Pearls

NMS is a diagnosis of exclusion; always rule out infection, heatstroke, and malignant hyperthermia. Serotonin syndrome typically resolves within 24 hours of drug discontinuation if recognized early. Both syndromes can be fatal if untreated; mortality for NMS ranges from 5-20%. Rechallenge with antipsychotics after NMS is possible but requires a low-potency agent, low starting dose, and careful monitoring after a minimum 2-week drug-free interval. Clonus is the single most reliable distinguishing feature favoring serotonin syndrome over NMS.

## References

1. Ware MR, Feller DB, Hall KL. Neuroleptic malignant syndrome: diagnosis and management. *Prim Care Companion CNS Disord*. 2018;20(1):17r02185.
2. Boyer EW, Shannon M. The serotonin syndrome. *N Engl J Med*. 2005;352(11):1112-1120.
3. Dunkley EJC, Isbister GK, Sibbritt D, et al. The Hunter Serotonin Toxicity Criteria: simple and accurate diagnostic decision rules for serotonin toxicity. *QJM*. 2003;96(9):635-642.
4. Pileggi DJ, Cook AM. Neuroleptic malignant syndrome. *Ann Pharmacother*. 2016;50(11):973-981.