A 48-year-old man with bipolar I disorder on lithium carbonate 1200 mg daily presents to the emergency department with tremor, confusion, ataxia, and diarrhea. Serum lithium level is 3.2 mEq/L. What is the most common organ system affected in acute lithium toxicity?

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, with manifestations ranging from fine tremor to seizures, coma, and death. ### Pathophysiology of Lithium Toxicity Lithium toxicity occurs when serum levels exceed 1.5 mEq/L (therapeutic range: 0.6–1.2 mEq/L). Lithium: 1. Inhibits inositol monophosphatase → depletes phosphatidylinositol (PI) cycle intermediates → impairs cellular signaling 2. Inhibits glycogen synthase kinase-3β (GSK-3β) → alters protein synthesis and neuronal function 3. Displaces Na^+^ and K^+^ → disrupts membrane potential and neurotransmitter release 4. Accumulates in the CNS due to slow blood–brain barrier clearance ### CNS Manifestations of Acute Lithium Toxicity (Dose-Dependent) | Severity | Serum Level | Clinical Features | |----------|-------------|-------------------| | **Mild** | 1.5–2.0 mEq/L | Fine tremor, nausea, vomiting, diarrhea, polyuria | | **Moderate** | 2.0–3.0 mEq/L | Coarse tremor, confusion, ataxia, dysarthria, muscle weakness, hyperreflexia | | **Severe** | >3.0 mEq/L | Seizures, coma, arrhythmias, acute renal failure, death | **Clinical Pearl:** In this case, the serum lithium level of 3.2 mEq/L (severe toxicity) correlates with the patient's CNS symptoms: tremor (coarse), confusion, and ataxia. These are hallmark signs of acute lithium neurotoxicity. ### Why CNS is Most Commonly Affected 1. **Slow CNS clearance:** Lithium crosses the blood–brain barrier slowly and accumulates in cerebrospinal fluid (CSF) and brain tissue. 2. **High sensitivity:** Neurons are exquisitely sensitive to lithium's effects on second-messenger systems (PI cycle, GSK-3β inhibition). 3. **Irreversible damage risk:** Severe acute toxicity can cause persistent neurological sequelae ("lithium neurotoxicity syndrome") even after serum levels normalize. ### Other Organ Systems in Acute Lithium Toxicity **Gastrointestinal tract:** Early sign (nausea, vomiting, diarrhea) but not the most severe or characteristic manifestation. **Cardiovascular system:** Benign ST-segment depression, T-wave flattening, and arrhythmias (rare in acute toxicity; more common in chronic toxicity). **Renal system:** Acute tubular necrosis and acute kidney injury can occur in severe toxicity, but are secondary to systemic effects rather than primary CNS involvement. **High-Yield:** The **triad of acute lithium toxicity** is tremor + confusion + ataxia — all CNS manifestations. This triad should immediately raise suspicion for lithium toxicity and prompt serum level measurement. ### Management of Acute Lithium Toxicity 1. **Discontinue lithium immediately** 2. **Supportive care:** IV fluids, maintain urine output (target >200 mL/hr) 3. **Enhance renal clearance:** - **Hemodialysis:** Gold standard for severe toxicity (level >3.0 mEq/L, symptomatic, renal failure) - **Osmotic diuretics:** Mannitol or normal saline loading (increases GFR) 4. **Symptomatic management:** Anticonvulsants for seizures, correction of electrolyte abnormalities 5. **Monitor:** Serial serum lithium levels, ECG, renal function, neurological status **Mnemonic:** **LICE** — **L**ithium toxicity → **I**nositol depletion → **C**entral nervous system effects → **E**mergency hemodialysis (if severe).