A 38-year-old woman with major depressive disorder has been on sertraline 100 mg daily for 8 weeks with good symptom response. She now presents with hyponatraemia (Na⁺ 118 mEq/L) and confusion. Regarding the management and pharmacology of antidepressant-induced hyponatraemia, all of the following are true EXCEPT:

Explanation

## Antidepressant-Induced Hyponatraemia (SIADH) ### Epidemiology and Risk Factors **Key Point:** Antidepressant-induced hyponatraemia is a real and potentially serious adverse effect, particularly with SSRIs. - **Incidence:** 0.5–3% with SSRIs (highest in first 2 weeks); <0.1% with TCAs - **Risk factors:** Age >65 years, female sex, concurrent diuretic use, polypharmacy, hypovolemia - **Mechanism:** Enhanced serotonergic signalling → increased ADH secretion from posterior pituitary → water retention → dilutional hyponatraemia ### Pathophysiology **High-Yield:** Serotonin (5-HT) directly stimulates ADH-secreting neurons in the supraoptic (SON) and paraventricular (PVN) nuclei via 5-HT~1A~ and 5-HT~7~ receptors. SSRIs increase synaptic serotonin, amplifying this effect. ```mermaid flowchart TD A[SSRI initiated]:::action --> B[↑ Synaptic serotonin]:::outcome B --> C[5-HT activation in SON/PVN]:::outcome C --> D[↑ ADH secretion]:::outcome D --> E[↑ Aquaporin-2 expression in collecting duct]:::outcome E --> F[↑ Water reabsorption]:::outcome F --> G[Dilutional hyponatraemia]:::urgent G --> H{Symptoms?}:::decision H -->|Mild Na+ 125-130| I[Fluid restriction]:::action H -->|Severe Na+ <120 + confusion| J[Hypertonic saline + fluid restriction]:::action ``` ### Why the Correct Answer (Option 4) Is Wrong **Warning:** Option 4 is a dangerous misconception. Fluid restriction ALONE is inadequate for symptomatic hyponatraemia (Na⁺ <120 mEq/L with altered mental status). **Clinical Pearl:** Symptomatic hyponatraemia is a medical emergency: 1. **Mild–moderate (Na⁺ 125–130 mEq/L, asymptomatic or mild symptoms):** - Fluid restriction (500–800 mL/day) - Monitor sodium daily - Discontinue SSRI if possible 2. **Severe (Na⁺ <120 mEq/L with seizures, confusion, coma):** - **3% hypertonic saline** (3 mL/kg bolus over 10–20 minutes) - Goal: Raise sodium by 4–6 mEq/L acutely to stop seizures - Then slower correction (8–10 mEq/L per 24 hours) to avoid osmotic demyelination - ICU monitoring - Concurrent fluid restriction **High-Yield:** The fear of osmotic demyelination syndrome (ODS) should NOT prevent treatment of acute symptomatic hyponatraemia. ODS occurs with **rapid overcorrection** (>12 mEq/L per 24 hours), not with controlled hypertonic saline use. In this case, the patient has confusion (Na⁺ 118) — she requires hypertonic saline + fluid restriction, not fluid restriction alone. ### Correct Statements (Options 1, 2, 3) | Statement | Accuracy | Notes | |-----------|----------|-------| | Option 1: SSRI-SIADH epidemiology | ✓ | Incidence 0.5–3% SSRIs vs <0.1% TCAs; peak risk first 2 weeks; elderly > young | | Option 2: Serotonergic mechanism | ✓ | 5-HT~1A~ and 5-HT~7~ receptors on ADH neurons in SON/PVN; well-established | | Option 3: TCA safety advantage | ✓ | TCAs have minimal serotonergic activity on hypothalamic ADH neurons; much lower risk | **Mnemonic for antidepressant hyponatraemia risk:** **SSRI > SNRI > TCA** (serotonin selectivity = hyponatraemia risk) [cite:Harrison 21e Ch 470; KD Tripathi 8e Ch 18]