Clinical Presentation Differences Between SIADH and Diabetes Insipidus
SIADH and diabetes insipidus present with opposite clinical pictures: SIADH causes water retention leading to hyponatremia with concentrated urine, while diabetes insipidus causes massive water loss leading to hypernatremia with dilute urine. 1
Volume Status and Fluid Balance
SIADH:
- Euvolemic to mildly hypervolemic presentation with no peripheral edema 2
- Water retention without proportional sodium retention 3
- No signs of dehydration (normal skin turgor, moist mucous membranes) 2
- Absence of orthostatic hypotension or tachycardia 4
Diabetes Insipidus:
- Polyuria (excessive urine output) and polydipsia (excessive thirst) as predominant symptoms 5
- Risk of hypertonic dehydration, particularly in infants who lack free access to fluids 5
- Signs of volume depletion: orthostatic hypotension, dry mucous membranes, decreased skin turgor 5
- In infants: failure to thrive, gastro-oesophageal reflux, and vomiting from large fluid volumes 5
Serum Sodium and Osmolality
SIADH:
- Hyponatremia (serum sodium <134-135 mEq/L) 3
- Plasma hypoosmolality (<275 mOsm/kg) 3
- Serum uric acid typically <4 mg/dL (73-100% positive predictive value for SIADH) 2, 4
Diabetes Insipidus:
- Hypernatremia (serum osmolality usually >300 mOsm/kg H2O) 5
- Elevated serum sodium due to free water loss 5
- Mean age at diagnosis approximately 4 months in congenital forms 5
Urine Characteristics
SIADH:
- Inappropriately concentrated urine despite hyponatremia 3
- Urine osmolality >500 mOsm/kg (inappropriately high relative to plasma) 3
- Elevated urinary sodium (>20-40 mEq/L) despite low serum sodium 4, 3
- Urine osmolality greater than plasma osmolality 3
Diabetes Insipidus:
- Inappropriately dilute urine despite hypernatremia 5
- Urine osmolality usually <200 mOsm/kg H2O (can be higher in milder cases) 5
- Urine osmolality less than plasma osmolality 5
- Inability to concentrate urine due to insensitivity to ADH (nephrogenic) or lack of ADH (central) 5
Symptom Presentation
SIADH:
- Often asymptomatic with mild hyponatremia (130-135 mEq/L) 2
- Moderate symptoms: nausea, vomiting, headache 2
- Severe symptoms (sodium <120 mEq/L): confusion, seizures, altered mental status, coma 2, 3
- Increased fall risk (21% vs 5% in normonatremic patients) 2
- Neurocognitive problems including attention deficits 2
Diabetes Insipidus:
- Polydipsia is the predominant symptom in older children and adults 5
- Polyuria with passage of large volumes of dilute urine 5, 6
- Extreme thirst and craving for cold water 6
- In infants: irritability, failure to thrive, dehydration signs 5
- Potential for rapid deterioration if fluid intake cannot match losses 5
Underlying Pathophysiology
SIADH:
- Inappropriate ADH activity causing water retention 3
- Physiologic natriuresis occurs to maintain fluid balance, resulting in elevated urine sodium despite euvolemia 4
- Common causes: malignancy (especially small cell lung cancer), CNS disorders, pulmonary disease, medications 2, 3
Diabetes Insipidus:
- Central DI: deficiency of posterior pituitary to release ADH 6
- Nephrogenic DI: kidney insensitivity to ADH at the distal nephron level 5
- Loss of ability to concentrate urine leads to massive free water loss 5, 7
Key Diagnostic Distinctions
The critical differentiating feature is that SIADH presents with water retention and concentrated urine in the setting of hyponatremia, while diabetes insipidus presents with water loss and dilute urine in the setting of hypernatremia or risk thereof 1. Volume status assessment is essential: SIADH patients are euvolemic, while DI patients show signs of volume depletion unless they can maintain adequate oral intake 5, 2, 4.