A 48-year-old woman on lithium 800 mg daily presents with tremor, confusion, and ataxia. Serum lithium level is 2.8 mEq/L. Which is the most common organ system affected in acute lithium toxicity at this level?

Explanation

## Acute Lithium Toxicity: Organ System Involvement **Key Point:** The **central nervous system (CNS)** is the most commonly and severely affected organ system in acute lithium toxicity, especially at serum levels >2.0 mEq/L. ### Serum Lithium Level Classification | Serum Level (mEq/L) | Clinical Presentation | Severity | |---------------------|----------------------|----------| | **0.6–1.2** | Therapeutic (no toxicity) | Safe | | **1.5–2.0** | Early toxicity: fine tremor, nausea, polyuria | Mild | | **2.0–3.0** | **Moderate toxicity: coarse tremor, confusion, ataxia, slurred speech** | **Most common acute presentation** | | **>3.0** | Severe toxicity: seizures, coma, cardiac arrhythmias, renal failure | Life-threatening | ### CNS Manifestations (Most Common) **Mnemonic: LITHIUM TOXICITY CNS** — early to late signs: - **L**ethargy, confusion - **I**ncreased reflexes - **T**remor (coarse, not fine) - **H**ypertonia, hyperreflexia - **I**ncontinence (urinary) - **U**nsteadiness (ataxia) - **M**emory impairment **T**witching, tremor **O**phthalmoplegia (rare, severe) **X**enophobia (not typical) **I**ncreased intracranial pressure (cerebral edema in severe cases) **C**oma (severe toxicity) **I**nvoluntary movements (choreoathetosis, rare) **T**emporal lobe seizures **Y**awning (early sign) ### Why CNS Is Most Commonly Affected 1. **Lithium crosses the blood–brain barrier** — accumulates in CSF and brain tissue 2. **Interferes with second messenger systems** — inhibits inositol monophosphatase, depleting phosphatidylinositol (PI) cycle 3. **Alters Na^+^/K^+^ ATPase function** — disrupts neuronal excitability 4. **Affects neurotransmitter release** — particularly acetylcholine and norepinephrine **Clinical Pearl:** At the serum level of 2.8 mEq/L in this case, CNS toxicity (tremor, confusion, ataxia) is the expected and most common presentation. Cardiovascular and renal complications are more common at higher levels (>3.5 mEq/L) or in chronic toxicity. ### Differential Organ Involvement by Toxicity Severity ```mermaid flowchart TD A["Acute Lithium Toxicity<br/>Serum Level >1.5 mEq/L"]:::outcome A --> B{"Severity Level?"}:::decision B -->|"1.5–2.0 mEq/L<br/>Mild"|C["GI: nausea, diarrhea<br/>Neuro: fine tremor, polyuria"]:::action B -->|"2.0–3.0 mEq/L<br/>Moderate"|D["CNS: confusion, ataxia,<br/>coarse tremor, slurred speech<br/>(MOST COMMON)"]:::action B -->|"3.0–3.5 mEq/L<br/>Severe"|E["CNS: seizures, coma<br/>CV: arrhythmias, hypotension<br/>Renal: AKI"]:::urgent B -->|">3.5 mEq/L<br/>Life-threatening"|F["Multi-organ failure<br/>Cerebral edema<br/>Cardiogenic shock"]:::urgent ``` **High-Yield:** In acute toxicity, **CNS symptoms dominate** the clinical picture. GI symptoms (nausea, vomiting, diarrhea) are early and mild; cardiovascular and renal complications are late and severe. ### Management Approach 1. **Stop lithium immediately** 2. **Hydration**: IV normal saline to enhance renal clearance 3. **Dialysis**: if serum level >3.5 mEq/L or severe CNS/cardiac symptoms 4. **Supportive care**: seizure prophylaxis, cardiac monitoring 5. **Renal function monitoring**: lithium can cause acute tubular necrosis [cite:KD Tripathi 8e Ch 12; Harrison 21e Ch 396]