A 62-year-old woman with type 2 diabetes mellitus presents with a 5-day history of progressive confusion, weakness, and polydipsia. She lives alone and has not taken her medications for 2 weeks. On examination, she is alert but disoriented, skin turgor is poor, mucous membranes are dry. Vital signs: BP 95/60 mmHg, HR 108/min, RR 18/min. Laboratory results: blood glucose 680 mg/dL, arterial pH 7.32, HCO₃⁻ 18 mEq/L, anion gap 12 mEq/L, serum osmolality 340 mOsm/kg, serum ketones negative, serum sodium 128 mEq/L. What is the primary pathophysiological mechanism underlying this patient's altered mental status?

Explanation

## Diagnosis: Hyperglycaemic Hyperosmolar State (HHS) **Key Point:** This patient has HHS, not DKA. The distinguishing features are: - Extreme hyperglycaemia (680 mg/dL) with **very high serum osmolality (340 mOsm/kg)** - **Absent or minimal ketonaemia** (serum ketones negative) - **Mild acidosis** (pH 7.32, HCO₃⁻ 18) — NOT severe metabolic acidosis - **Anion gap 12** — normal anion gap (no high anion gap metabolic acidosis) - **Hyponatraemia (128 mEq/L)** — pseudohyponatraemia from hyperglycaemia ## Pathophysiology of Altered Mental Status in HHS ### Osmotic Gradient and Cellular Dehydration **High-Yield:** The mechanism of altered mental status in HHS is **hyperosmolality-induced cellular dehydration**: 1. **Severe hyperglycaemia** (680 mg/dL) creates an osmotic gradient across cell membranes 2. **Water shifts extracellularly** from the intracellular space (including brain cells) to maintain osmotic equilibrium 3. **Neuronal dehydration** → impaired synaptic transmission, altered consciousness, confusion 4. **Reduced cerebral blood flow** from intravascular volume depletion (5–15 L fluid loss) compounds hypoperfusion 5. **Result:** Altered mental status, lethargy, seizures (in severe cases) **Clinical Pearl:** The degree of mental status change correlates with serum osmolality, not glucose alone. In this patient, osmolality of 340 mOsm/kg is very high (normal ~290); this explains her disorientation despite only mild acidosis. ### Comparison: DKA vs. HHS | Feature | DKA | HHS | |---------|-----|-----| | **Glucose** | Usually 250–600 mg/dL | Usually 600–1200 mg/dL | | **Osmolality** | 310–320 mOsm/kg | 330–380 mOsm/kg | | **pH** | <7.30 (often <7.15) | >7.30 (mild acidosis) | | **HCO₃⁻** | <15 mEq/L | >15 mEq/L | | **Anion gap** | High (>12) | Normal (<12) | | **Ketones** | **Strongly positive** | **Absent/minimal** | | **Mental status** | Lethargy, Kussmaul breathing | Confusion, seizures, coma | | **Mortality** | 1–5% | 5–15% (higher) | **Mnemonic: OSMOLALITY is the KEY in HHS** — Osmotic gradient, Severe hyperglycaemia, Minimal ketones, Osmolality >330, Loss of water, Altered mentation, Lethargy, Intracellular dehydration, Thirst (polydipsia), Yolk-like (pseudohyponatraemia). ### Why Pseudohyponatraemia? **Key Point:** The serum sodium of 128 mEq/L is **falsely low** (pseudohyponatraemia). For every 100 mg/dL rise in glucose above 100 mg/dL, sodium drops ~1.6–2 mEq/L due to osmotic water shift. Corrected sodium = measured Na⁺ + (glucose − 100) × 0.016. In this case, corrected Na⁺ ≈ 128 + (680 − 100) × 0.016 ≈ 137 mEq/L (normal). [cite:Harrison 21e Ch 397] ## Clinical Implications **High-Yield:** Treatment of HHS focuses on: 1. **Gradual fluid rehydration** (slower than DKA) to avoid cerebral oedema 2. **Insulin** (lower doses than DKA, ~0.05 units/kg/hour) — the goal is to lower glucose slowly 3. **Osmolality monitoring** — target reduction of ~3–8 mOsm/kg/hour 4. **Electrolyte replacement** — especially potassium and phosphate