Renal · Year 2 · from Renal
Case 2: Hypovolemic Hyponatremia from Diarrhea
Patient Presentation
A 28-year-old female presents with 3 days of profuse watery diarrhea, vomiting, and progressive weakness. She has been drinking large amounts of water to stay hydrated.
History of Present Illness
- 10-12 episodes of watery diarrhea daily for 3 days
- Multiple episodes of vomiting
- Drinking 3-4 liters of water daily but minimal food intake
- Lightheadedness when standing
- Recent travel to Mexico
Physical Examination
- Blood pressure: 92/58 mmHg supine, 76/50 standing
- Heart rate: 108 bpm supine, 128 standing
- Dry mucous membranes
- Decreased skin turgor
- Flat neck veins
- Abdomen: Hyperactive bowel sounds, diffuse mild tenderness
Workup
Laboratory Studies:
- Serum sodium: 124 mEq/L
- Serum osmolality: 258 mOsm/kg
- Potassium: 2.9 mEq/L
- Bicarbonate: 16 mEq/L (metabolic acidosis - GI bicarbonate loss)
- Creatinine: 1.6 mg/dL (AKI from volume depletion)
- Urine sodium: 6 mEq/L
- Urine osmolality: 680 mOsm/kg
Diagnosis
Hypovolemic Hypotonic Hyponatremia
Discussion
This case demonstrates sodium and water balance principles:
- Mechanism: Diarrhea causes loss of both sodium and water. The patient replaced losses with pure water, leading to hyponatremia. The GI losses also caused hypokalemia and bicarbonate loss (non-anion gap metabolic acidosis).
- RAAS and ADH Response: Volume depletion activates RAAS (causing sodium retention, hence low urine sodium) and ADH (causing water retention despite hypoosmolality - a non-osmotic stimulus). The concentrated urine reflects ADH activity.
- Orthostatic Hypotension: The marked orthostatic changes confirm significant volume depletion.
Treatment
- IV normal saline (isotonic) for volume resuscitation
- As volume is repleted, ADH stimulus will be removed, and the patient will excrete dilute urine and self-correct the hyponatremia
- Potassium replacement (IV and oral)
- Bicarbonate typically corrects with volume repletion
- Antiemetics for symptom control
- Stool studies for infectious etiology
Clinical Pearl
In hypovolemic hyponatremia, giving isotonic saline (154 mEq/L Na) corrects both volume depletion AND hyponatremia. Once volume is restored, ADH suppresses, allowing excretion of the excess water. Monitor for overly rapid correction.