Constipation relief turns to grief!
What’s causing an elderly woman’s kidney failure?
Vol.17, No.02, March 2009

Mrs. L., a 72-year-old woman with an unremarkable medical history, save for a remote background of seizures, fell, hurt her back and came to the ER. She was diagnosed with a T11 compression fracture without any neurovascular compromise, and received Tylenol #3 for pain relief. A few days later, she returned with ongoing vague lower back and abdominal discomfort and was noted to be constipated. Tylenol #3 was stopped and she was given oral sodium phosphate (45 ml) to treat the constipation, plus naproxen 375 mg twice a day for pain control.

Three days later, she returns to the local emergency department (ED) complaining of generalized weakness, numbness around her lips, ongoing vague abdominal discomfort and nausea. She denies vomiting or diarrhea, however. Her food intake has been poor since the fall and she notes decreased urine output. There’s no history of diabetes or hypertension. Her medications are phenytoin, phenobarb and rabeprazole. She’s taken two doses of the naproxen prescribed three days before.

Investigations in the ED

Investigations in the local ED reveal low hemoglobin of 109 g/L, normal WBC of 4.5, elevated BUN of 9.4 mmol/L with serum creatinine of 345 μmol/L, serum potassium of 3.4 mmol/L. In December 2007, her BUN had been 6.1 mmol/L with serum creatinine of 74 μmol/L. She is transferred for further management of acute renal failure.

Physical examination shows a woman of stated age with mildly decreased skin turgor, blood pressure of 106/60 mm Hg without orthostatic changes, and regular rate of 72 BPM. Lungs are clear and heart sounds normal. Abdominal exam reveals a soft abdomen with mild diffuse tenderness without rebound. There are no masses, renal angle fullness or tenderness. There’s mild tenderness in lower thoracic area. There’s no pedal edema and a neurological exam is non-focal. She has a Foley catheter with a small amount of concentrated urine in the bag.

The investigations in our ED reveal a low hemoglobin of 112 g/L, normal WBC of 4.9, elevated BUN of 9.4 mmol/L with serum creatinine of 419 μmol/L, serum potassium of 3.4 mmol/L, low serum calcium of 1.85 mmol/L (normal range 2.02-2.60 mmol/L) with serum albumin of 36 g/L, low ionized calcium of 0.85 mmol/L (normal 1.15-1.29 mmol/L), elevated phosphate of 3.68 mmol/L (normal 0.87-1.45 mmol/L) and creatinine kinase (CK) of 349.

Urinalysis shows concentrated urine with specific gravity of >1.030, 1+ protein and trace of blood with few white and red blood cells and few hyaline casts. Random urine sodium is 64 mmol/L. A urine culture is negative. An abdominal ultrasound next morning is unremarkable and shows normal size kidneys with some increased echogenicity. There’s no obstruction.

What is the cause of Mrs. L.’s acute renal failure?

ANSWER
Acute phosphate nephropathy

Mrs. L. presents with acute renal failure following a fall a few weeks prior. Oral intake has been poor for the past few weeks with ongoing nausea.

The patient received oral sodium phosphate solution for constipation and is noted to have decreased skin turgor, hyperphosphatemia, hypocalcemia and slightly elevated CK, in addition to acute renal failure. The urine sodium isn’t low. The differential diagnosis includes ischemic acute tubular necrosis (poor intake, not low urine sodium), or rhabdomyolysis because of elevated phosphate and CK (although CK is not significantly elevated at present, we might be seeing its tail end, as she presents two weeks after the fall).

The development of significant hyperphosphatemia and hypocalcemia with acute renal failure — within 72 hours of oral administration of sodium phosphate solution — indicates a high probability of acute phosphate nephropathy from deposition of calcium-phosphate crystals in the renal parenchyma (nephrocalcinosis). A kidney biopsy confirms findings of acute phosphate nephropathy with acute tubular necrosis. Her creatinine progressively increases to 620, requiring transient supportive dialysis. It took about 10 days for her renal function to recover.

Call for vigilance

This case report describes a phenomenon now being commonly recognized. It highlights the need for vigilance when using oral sodium phosphate solutions for bowel preparation or to relieve constipation.

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