Serotonin toxicity and bipolar presentations

Serotonin Syndrome (Serotonin Toxicity)

Serotonin toxicity can follow a serotonergic overdose, dose change or interacting drugs. Multiple agents raise risk, but a single agent can cause toxicity.

High-Risk Drug Combinations

• SSRI / SNRI + Monoamine Oxidase Inhibitors (MAOIs: phenelzine, tranylcypromine, moclobemide).
• SSRI / SNRI + Synthetic Opioids with serotonin reuptake inhibition (tramadol, pethidine, fentanyl, methadone, dextromethorphan).
• SSRI / SNRI + Herbal / Illicit Serotonergics (St John's Wort, MDMA / ecstasy, amphetamines, cocaine).
• SSRI / SNRI + Antibiotics with MAOI activity (linezolid).
• Washout Rules: Switching from an SSRI to an irreversible MAOI requires a 2-week washout (except fluoxetine, which requires a 5-week washout due to the long half-life of its active metabolite norfluoxetine).

Clinical Presentation & Hunter Criteria

Serotonin syndrome manifests with a rapid onset (often within 6 to 24 hours of drug exposure or dose escalation) characterized by a clinical triad:

1. Neuromuscular Excitation: Spontaneous, inducible, or ocular clonus; hyperreflexia (pronounced in lower limbs); tremor; myoclonus; and hypertonia.
2. Autonomic Instability: Diaphoresis, hyperthermia ($> 38.5^\circ\text{C}$ in severe cases), tachycardia, labile blood pressure, flushing, and diarrhoea.
3. Altered Mental State: Agitation, delirium, confusion, restlessness, and hypervigilance.

Differential Diagnosis: Serotonin Syndrome vs NMS

• Precipitating Drugs: Serotonin Syndrome: Serotonergic agents (SSRIs, SNRIs, MAOIs, tramadol); Neuroleptic Malignant Syndrome (NMS): Dopamine antagonists (antipsychotics, metoclopramide)
• Onset Velocity: Serotonin Syndrome: Rapid (hours to 24 hours post-dose); Neuroleptic Malignant Syndrome (NMS): Gradual (develops over days to 2 weeks)
• Neuromuscular Tone: Serotonin Syndrome: Hyperreflexia, clonus (ocular/inducible), tremor; Neuroleptic Malignant Syndrome (NMS): "Lead-pipe" rigidity, hyporeflexia, bradykinesia
• Pupils: Serotonin Syndrome: Dilated (mydriasis); Neuroleptic Malignant Syndrome (NMS): Normal
• Gastrointestinal: Serotonin Syndrome: Hyperactive bowel sounds, diarrhoea; Neuroleptic Malignant Syndrome (NMS): Normal or decreased bowel sounds
• Specific Antidote: Serotonin Syndrome: Cyproheptadine (5-HT2A antagonist); Neuroleptic Malignant Syndrome (NMS): Bromocriptine (dopamine agonist) / Dantrolene

Emergency Management

1. Immediate Cessation: Discontinue all serotonergic medications.
2. Supportive Care: Intravenous fluid resuscitation with $0.9\%$ sodium chloride; active external cooling (cooling blankets, ice packs; avoid antipyretics like paracetamol as fever is driven by muscular rigidity, not hypothalamic set-point reset).
3. Sedation & Muscle Relaxation: Intravenous benzodiazepines (e.g., diazepam $5\text{ to }10\text{ mg}$ IV every 10–15 minutes) to alleviate agitation, dampen hyperadrenergic drive, and control muscular hyperactivity.
4. Specialist treatment: Supportive care, benzodiazepines, temperature control and organ support are primary. Cyproheptadine may be considered in selected cases after toxicology advice; evidence is limited, and it is not a definitive antidote that replaces resuscitation.
5. Intensive Care & Neuromuscular Blockade: If core temperature exceeds $39.5^\circ\text{C}$ or severe rigidity threatens rhabdomyolysis and metabolic acidosis, emergency endotracheal intubation, mechanical ventilation, and non-depolarizing neuromuscular paralysis (vecuronium) are lifesaving.

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Bipolar Affective Disorder

Bipolar disorder is characterized by pathological oscillations between depressive episodes and states of abnormally elevated, irritable, or energized mood.

Diagnostic Distinction: Bipolar I vs Bipolar II

• Bipolar I Disorder: Mandates the occurrence of at least one lifetime manic episode. A depressive episode is typical but not strictly required for formal diagnosis.
  • Manic Episode: An abnormally and persistently elevated, expansive, or irritable mood, accompanied by persistently increased goal-directed activity or energy, lasting at least 7 consecutive days (or any duration if psychiatric hospitalization is required).
  • Symptoms (must have $\ge 3$, or $\ge 4$ if mood is only irritable): Grandiosity/inflated self-esteem, decreased need for sleep (feels fully rested after 2–3 hours), pressured speech, flight of ideas/racing thoughts, extreme distractibility, increased goal-directed activity or psychomotor agitation, and excessive involvement in high-risk pleasurable activities (spending sprees, sexual indiscretions, reckless driving).
  • Severity: Causes marked impairment in social or occupational functioning, requires hospitalization to prevent harm, or features psychotic delusions/hallucinations.
• Bipolar II Disorder: Requires at least one hypomanic episode AND at least one major depressive episode. There has never been a lifetime manic episode.
  • Hypomanic Episode: Symptoms identical to mania lasting at least 4 consecutive days; represents an unequivocal change in functioning uncharacteristic of the individual, observable by others.
  • Crucial Boundaries: Hypomania does not cause marked functional impairment, does not require hospitalization, and never features psychotic symptoms. The presence of psychosis automatically reclassifies the episode as Bipolar I mania.

Clinical Warning: Antidepressant Monotherapy in Bipolar Disorder

Antidepressant monotherapy is not recommended for Bipolar I depression because it can provoke mania, mixed symptoms or cycling. Do not describe Bipolar II treatment as an identical absolute rule: selected pure-depression presentations may have specialist antidepressant options, while mixed features or a switch history argue strongly against them. Assess the illness course and use the relevant specialist pathway. If antidepressants are ever utilized for refractory bipolar depression, they must always be co-prescribed with a therapeutic antimanic mood stabiliser (lithium, valproate, or an atypical antipsychotic).

Management of Acute Mania

• First-Line Pharmacotherapy: Atypical antipsychotics (olanzapine $10\text{ to }20\text{ mg}$, quetiapine $400\text{ to }800\text{ mg}$, risperidone $2\text{ to }6\text{ mg}$) or mood stabilisers (lithium, sodium valproate).
• Severe Agitation & Psychosis: Combination therapy with an atypical antipsychotic plus a mood stabiliser (e.g., olanzapine + lithium) is superior to monotherapy. Short-term oral or intramuscular benzodiazepines (clonazepam $1\text{ to }2\text{ mg}$ or lorazepam) provide rapid behavioural de-escalation.

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Primary references (checked 7 October 2026): EXTRIP lithium recommendations.