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Q1: Mechanism and clinical recognition (3 min)
Examiner: A bipolar patient on lithium presents with a coarse tremor, ataxia and confusion, and a serum lithium of 2.0 mmol/L. Walk me through the mechanism of lithium toxicity and why the level is only mildly raised.
Expected answer:
- Lithium is a small monovalent cation handled by the kidney like sodium — freely filtered, proximally reabsorbed, almost entirely renally excreted; narrow therapeutic index (0.6-1.2 mmol/L).
- Therapeutically it inhibits inositol monophosphatase (inositol depletion) and GSK-3β — the antimanic mechanism. In excess these, plus direct cation-substitution, disrupt neuronal and especially cerebellar function.
- Three patterns: acute overdose (GI early, neuro evolves as lithium distributes slowly into the CNS — brain levels lag serum by hours); acute-on-chronic; and chronic accumulation (commonest, most dangerous) — tissue already saturated, so toxicity occurs at LOWER serum levels and neurology dominates. That is why a level of 2.0 mmol/L with chronic tissue saturation and severe neurology is more dangerous than 4.0 mmol/L an hour after an acute overdose.
- Clinical ladder: fine tremor (earliest) -> coarse tremor, fasciculations, hyperreflexia, ataxia, dysarthria, nystagmus -> seizures, coma. Plus nephrogenic diabetes insipidus.
Follow-up: Why does the thiazide diuretic precipitate toxicity? Thiazide causes sodium/volume loss -> the proximal tubule increases sodium (and therefore lithium) reabsorption -> serum lithium rises — the kidney handles lithium like sodium.
Q2: The renal mechanism and nephrogenic DI (3 min)
Examiner: Explain lithium's renal handling and the mechanism of lithium-induced nephrogenic diabetes insipidus. How would you treat it?
Expected answer:
- Lithium is freely filtered at the glomerulus and reabsorbed proximally like sodium; almost all filtered lithium is excreted in urine — no other clearance route. So reduced GFR, dehydration, NSAIDs and thiazides all raise the level.
- Nephrogenic DI: lithium enters collecting-duct principal cells via ENaC, accumulates, and blocks the vasopressin V2-receptor / Gs / cAMP cascade, producing downregulation and mistargeting of aquaporin-2 water channels -> polyuria of dilute urine.
- Because the defect is downstream of the V2 receptor, desmopressin (DDAVP) does NOT work — distinguishing it from central DI (which responds to desmopressin).
- AMILORIDE is the specific treatment — it blocks ENaC, reducing lithium entry into principal cells and ameliorating aquaporin-2 downregulation.
Q3: Management ladder (3 min)
Examiner: How do you manage lithium toxicity? Give drugs and doses.
Expected answer — stepwise:
- Stop lithium AND the precipitants (NSAIDs, ACE-i/ARBs, thiazide/loop diuretics); ABCDE, continuous cardiac monitoring.
- Aggressive isotonic saline (0.9% NaCl) — the cornerstone; restores euvolaemia and filtered sodium -> enhances renal lithium excretion.
- Treat seizures — lorazepam 4 mg IV (2 mg in elderly), or diazepam/midazolam.
- Haemodialysis for severe (EXTRIP criteria).
- Whole-bowel irrigation (polyethylene glycol 1-2 L/h via NG) for sustained-release / large recent ingestion.
Follow-up — what to AVOID and why:
- Activated charcoal — does NOT adsorb lithium (a small metal cation); useless for isolated lithium overdose.
- Loop and thiazide diuretics — worsen renal lithium retention.
- NSAIDs and ACE-inhibitors/ARBs — reduce renal lithium clearance.
- Dehydration.
Q4: Haemodialysis — EXTRIP criteria, rebound (2 min)
Examiner: When do you dialyse a lithium-toxic patient, and what is rebound?
EXTRIP RECOMMENDED indications (Grade 1D):
- Impaired kidney function AND serum lithium over 4.0 mEq/L; OR
- Decreased level of consciousness, seizures, or life-threatening dysrhythmia at ANY serum lithium level.
- (SUGGESTED if over 5.0 mEq/L, significant confusion, or expected time below 1.0 mEq/L exceeds 36 hours.)
- Continue until clinical improvement OR level below 1.0 mEq/L.
Rebound: after haemodialysis, lithium redistributes from saturated tissues back into plasma and the level RISES again — recheck at 4-6 hours and use SLED or CRRT as a follow-on to prevent it. Lithium is fully dialysable (small ion, negligible protein binding), which is why dialysis works.
Pearl: lithium is preferred by haemodialysis (clears fastest) for the index treatment, with CRRT/SLED as an adjunct to prevent rebound.
Q5: Complications, special populations and pitfalls (2 min)
Expected answer:
- SILENT — Syndrome of Irreversible Lithium-Effectuated Neurotoxicity: persistent cerebellar/cognitive deficits lasting weeks-to-months after the level normalises; more common after chronic or severe poisoning.
- Chronic endocrine: hypothyroidism (goitre) and hypercalcaemia (lithium-induced hyperparathyroidism) — always check calcium and TFTs.
- Renal: nephrogenic DI; chronic tubulointerstitial nephritis with long-term therapy (monitor eGFR).
- Teratogenic: Ebstein's anomaly (right-heart malformation) — pregnancy is doubly high-stakes; management of toxicity unchanged.
- Elderly: highest-risk chronic-toxicity group; use a lower target level (0.6-0.8 mmol/L) and monitor closely.
- Pitfalls: treating the number not the patient (chronic toxic at lower levels); relying on a single early level after acute overdose (repeat every 2-4 h); giving charcoal, a loop/thiazide, or an NSAID; missing rebound after dialysis; missing the precipitant interaction at discharge.