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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- HHS = severe hyperglycemia + high serum osmolality + profound dehydration with minimal ketosis
- Typical labs: glucose >600 mg/dL, effective osmolality >320 mOsm/kg, pH >7.30, bicarbonate >18
- Mental status changes correlate with osmolality; coma is uncommon unless osmolality is very high
- Treatment begins with aggressive IV fluids; insulin is usually lower-dose and after initial volume resuscitation
- Mortality is higher than DKA because patients are often older with infection, MI, stroke, or renal disease
Overview
Hyperosmolar hyperglycemic state is a life-threatening complication of diabetes characterized by extreme hyperglycemia, severe dehydration, and hyperosmolarity without substantial ketoacidosis. Enough endogenous insulin is present to suppress lipolysis and ketogenesis, but not enough to prevent hyperglycemia. The resulting osmotic diuresis may cause 8-12 liters of free water deficit. HHS typically evolves over days to weeks, unlike the more rapid onset of DKA.
Epidemiology
HHS most often occurs in older adults with type 2 diabetes, particularly those with limited access to water, impaired thirst, dementia, nursing home residence, infection, or poor medication access. Common precipitants include pneumonia, urinary tract infection, sepsis, myocardial infarction, stroke, corticosteroids, thiazides, atypical antipsychotics, and dehydration. Mortality is commonly quoted around 10-20%, higher than DKA because comorbidity burden is greater.
Clinical Features
Symptoms
Polyuria, polydipsia, progressive weakness, and weight loss over days to weeks
Severe thirst or inability to maintain hydration
Confusion, lethargy, seizures, or coma from hyperosmolality
Fever, cough, dysuria, chest pain, or focal neurologic symptoms from precipitating illness
Minimal abdominal pain compared with DKA
Signs
Profound dehydration: dry mucosa, poor skin turgor, tachycardia, hypotension
Altered mental status proportional to osmolality
Focal neurologic deficits or seizures that may mimic stroke
Usually no Kussmaul respirations and no fruity breath odor
Signs of infection such as fever may be absent in older adults
Investigations
First-line
Basic metabolic panel and glucoseGlucose usually >600 mg/dL; assess sodium, potassium, bicarbonate, BUN/Cr. Correct sodium for hyperglycemia
Serum osmolalityEffective osmolality >320 mOsm/kg supports HHS; effective osmolality = 2 x Na + glucose/18
Venous blood gas and ketonespH usually >7.30 and bicarbonate >18; mild ketones may occur but severe ketosis suggests DKA overlap
Urinalysis and infection workupUTI and pneumonia are common precipitants
Second-line
ECG and troponinScreen for MI as precipitant, especially in older adults or those with atypical symptoms
CBC, cultures, lactate if septicEvaluate infection and hypoperfusion
Chest X-rayIf pulmonary symptoms, fever, hypoxia, or unexplained trigger
Specialist
CT headIf persistent focal neurologic deficits, seizure, trauma, or concern for stroke after osmolality begins correcting
Frequent electrolyte monitoringBMP every 2-4 hours; monitor potassium closely after insulin and fluids
1
Initial management
- Airway, breathing, circulation; cardiac monitoring if unstable or potassium abnormality
- Aggressive isotonic crystalloid resuscitation, often 15-20 mL/kg in the first hour
- Estimate large free water deficit; replace gradually over 24-48 hours
2
Electrolytes and insulin
- Correct potassium before insulin using the same principle as DKA: do not start insulin if K+ <3.3 mEq/L
- After initial fluids, use IV regular insulin at lower dose than DKA if glucose does not fall adequately with fluids alone
- Avoid rapid osmolality correction; overly rapid shifts may precipitate cerebral edema or osmotic injury
3
Dextrose and resolution
- When glucose approaches ~300 mg/dL, add dextrose-containing fluid and continue insulin until osmolality and mental status improve
- Resolution: normalized osmolality, improved mentation, adequate oral intake, stable electrolytes, and controlled glucose
4
Treat precipitant and prevent recurrence
- Treat infection, MI, stroke, medication trigger, or dehydration source
- Review diabetes regimen, hydration access, home support, and sick-day plan before discharge
Complications
- Thrombosis: Severe dehydration and hyperviscosity increase risk of stroke, MI, and venous thromboembolism
- Acute kidney injury: Prerenal azotemia from profound volume depletion
- Seizures and coma: From severe hyperosmolality
- Hypokalemia and arrhythmia: Insulin and correction of acidosis shift potassium intracellularly
- Cerebral edema: Rare but associated with overly rapid correction of osmolality
USMLE Step 2 CK Exam Tips
- 1HHS vs DKA: HHS has much higher glucose and osmolality, minimal ketones, pH usually >7.30
- 2Older type 2 diabetic patient with confusion and glucose >600 = HHS until proven otherwise
- 3Mental status change is driven by osmolality, not the glucose number alone
- 4Initial answer is IV fluids; glucose often falls substantially before insulin because renal perfusion improves
- 5If significant ketones and acidosis are present, treat as mixed DKA/HHS
- 6Search aggressively for infection, MI, or stroke as the precipitant
- 7HHS has higher mortality than DKA despite less acidosis
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