Correct Answer: B. Serum lithium level
Lithium is the gold-standard first-line mood stabilizer for bipolar disorder in India, with a narrow therapeutic window (0.6–1.2 mEq/L). The clinical presentation—seizures, coarse tremors, confusion, and limb weakness—is classic for lithium toxicity. Fasting for several days causes dehydration and reduced sodium intake, which decreases renal lithium clearance (lithium is filtered and reabsorbed in the proximal tubule in competition with sodium). When sodium depletion occurs, the kidneys reabsorb more lithium to maintain osmotic balance, leading to accumulation and toxicity. Acute lithium toxicity manifests as tremor, confusion, ataxia, and seizures; chronic toxicity adds polyuria and nephrogenic diabetes insipidus. The discriminating feature here is the temporal relationship: fasting → dehydration → reduced renal clearance → toxicity. Serum lithium level is the definitive diagnostic test. Levels >1.5 mEq/L are toxic; >2.0 mEq/L cause severe toxicity with seizures and arrhythmias. Management includes stopping lithium, aggressive hydration with normal saline (to restore sodium and increase glomerular filtration), and supportive care. In severe cases, hemodialysis may be needed. This is a classic Indian board question testing knowledge of lithium pharmacokinetics and the sodium-lithium interaction.
Why the other options are wrong
A. MRI — MRI is a structural neuroimaging tool used to rule out intracranial lesions (tumors, stroke, encephalitis) in seizure workup. However, the clinical context—patient on lithium with acute onset after fasting—makes drug toxicity the primary diagnosis. MRI does not measure drug levels and would delay critical management (stopping lithium, hydration). This is an NBE trap: students may think 'seizures = neuroimaging' without considering the pharmacological context. C. ECG — ECG is indicated in lithium toxicity to detect arrhythmias (flattened T waves, prolonged QT, bradycardia) and assess cardiac safety before and during treatment. However, ECG is a secondary investigation for monitoring; it does not diagnose lithium toxicity or measure serum levels. The question asks for the diagnostic investigation, not monitoring. ECG would be done after serum lithium level confirms toxicity. D. Serum potassium level — Hypokalemia can cause muscle weakness and arrhythmias, but it does not explain tremor, confusion, or seizures in this context. Lithium toxicity directly causes CNS symptoms via effects on second-messenger systems (inhibition of inositol monophosphatase). Serum potassium may be checked as part of supportive care, but it is not the diagnostic test for lithium toxicity. This is a distractor for students who confuse electrolyte imbalance with drug toxicity.
High-Yield Facts
- Lithium therapeutic window: 0.6–1.2 mEq/L; toxicity >1.5 mEq/L (tremor, confusion); severe toxicity >2.0 mEq/L (seizures, arrhythmias).
- Sodium depletion increases lithium reabsorption in the proximal tubule, reducing renal clearance and causing accumulation—key mechanism in fasting/dehydration.
- Lithium toxicity triad: coarse tremor, confusion, ataxia; seizures and arrhythmias indicate severe toxicity.
- Management of lithium toxicity: stop lithium immediately, aggressive IV normal saline (0.9% NaCl) to restore sodium and GFR, hemodialysis if severe.
- Lithium monitoring: baseline serum level 5 days after initiation, then every 3–6 months; check renal function (creatinine, eGFR) and TSH annually.
Mnemonics
LITHIUM TOXICITY SIGNS Tremor (coarse), Ataxia/confusion, Seizures, Hyperreflexia, Incontinence, Uremia (polyuria), Muscle weakness. Use when you see tremor + confusion + seizures in a bipolar patient on lithium. SODIUM ↓ = LITHIUM ↑ Fasting/dehydration → sodium depletion → kidneys reabsorb more lithium (competing with Na+ in proximal tubule) → lithium accumulation. Remember: low Na+ = high Li+ levels.
NBE Trap
NBE pairs seizures with neuroimaging (MRI) to lure students into structural diagnosis, when the clinical context (lithium use + fasting) makes pharmacological toxicity the primary concern. Students must recognize that acute drug toxicity is more likely than a new intracranial lesion in this scenario.
Clinical Pearl
In Indian clinical practice, lithium remains the gold standard for bipolar disorder despite newer agents, due to cost and efficacy. Patients fasting for religious reasons (Ramadan, Navratri, Karva Chauth) are at high risk for lithium toxicity—counsel them to maintain hydration and sodium intake, or temporarily switch to alternative mood stabilizers during fasting periods.
_Reference: KD Tripathi Pharmacology Ch. 12 (CNS Agents); Harrison Ch. 397 (Mood Disorders); Robbins Ch. 28 (Environmental/Nutritional Pathology)_