What are the causes of hypernatremic dehydration in various patient populations, including older adults and those with underlying medical conditions such as diabetes insipidus or impaired renal function?

Causes of Hypernatremic Dehydration

Hypernatremic dehydration results from either inadequate water intake (low-intake dehydration) or excessive water loss relative to sodium loss, with the elderly being particularly vulnerable due to blunted thirst mechanisms and impaired renal concentrating ability. 1, 2

Primary Mechanisms

Low-Intake (Hypertonic) Dehydration

This represents pure water deficit exceeding sodium loss, causing elevated serum sodium and osmolality, with intracellular and extracellular fluid depletion 2:

Physiological factors in older adults:

• Blunted thirst response - aging diminishes the protective thirst mechanism that normally triggers drinking when osmolality rises 1, 3
• Impaired renal concentration - reduced ability of kidneys to concentrate urine and conserve water 1, 3
• Decreased total body water - smaller fluid reserve provides less buffer against dehydration 1, 3
• Reduced ADH responsiveness - baroreceptor sensitivity decreases with age, impairing volume regulation 3

Behavioral and access factors:

• Memory problems causing patients to forget to drink 1, 4
• Voluntary fluid restriction due to continence concerns and fear of incontinence 1, 4
• Social isolation leading to loss of drinking routines (drinking with others is a key trigger) 1
• Physical inability to access fluids independently 1
• Swallowing problems and dysphagia 1

Medication-induced increased losses:

• Diuretics increasing urinary water losses 1, 4
• Laxatives promoting fluid loss 1

Diabetes Insipidus

Central diabetes insipidus results from inadequate ADH production, commonly from:

• Head trauma 5
• Pituitary surgery (transsphenoidal hypophysectomy) 5
• Hypothalamic pathology 6

Nephrogenic diabetes insipidus involves renal unresponsiveness to ADH 5, 6:

• Intrinsic renal disease 6
• Medications affecting renal tubular response 6

Excessive Water Loss (Volume Depletion Contributing to Hypernatremia)

When water loss exceeds sodium loss 2:

• Fever increasing insensible losses 1
• Diarrhea and vomiting 2
• Severe hemorrhage 1
• Renal sodium and water losses 7

High-Risk Populations

All older adults should be considered at risk for hypernatremic dehydration, with specific vulnerability in 1, 4:

• Hospitalized elderly patients who rely on others for water access 8
• Frail nursing home residents 8, 9
• Patients with functional impairment and cognitive decline (more predictive than age alone) 1, 4
• Those on diuretics with multiple converging risk factors 4
• Malnourished or at-risk patients 1

Clinical Significance

Mortality increases significantly with hypernatremia severity 9:

• Overall mortality rate of 29.8% during hospitalization for hypernatremic dehydration 9
• Mortality of 33.3% with sodium 151-153 mEq/L 9
• Mortality of 71.4% with sodium >154 mEq/L 9
• Both serum sodium levels and age are independent risk factors for mortality 9

Raised serum osmolality (>300 mOsm/kg) is associated with:

• Increased mortality risk in older adults 1, 4
• Doubling of 4-year disability risk 1, 4

Diagnostic Threshold

Serum osmolality >300 mOsm/kg defines hypernatremic dehydration in older adults 1, 4, with calculated osmolarity >295 mmol/L as an alternative screening threshold when direct measurement is unavailable 1, 4.