Case 14 – A 68-year-old man with lower abdominal pain

A 68-year-old man presents to the emergency department with severe lower abdominal pain. He has a constant desire to urinate but can only produce a small amount of urine. His past medical history is unremarkable, but he says he has experienced hesitancy before the onset of urination and decreased strength of his urinary stream for several months. Physical examination is significant for trace pedal edema, and his blood pressure is 140/86 mm Hg, with no orthostatic changes. He has marked tenderness in the suprapubic region and dullness to percussion to the level of the umbilicus. Rectal examination reveals a large, smooth, firm, midline mass located anteriorly. Laboratory tests show:

Sodium: 142 mEq/L
Potassium: 6.0 mEq/L
Chloride: 113 mEq/L
Bicarbonate: 17 mmol/L
Blood urea nitrogen: 110 mg/dL
Creatinine: 7 mg/dL
Hemoglobin: 15 g/dL
Hematocrit: 45%

What is the most likely cause of this patient’s lower abdominal pain?

This is an example of post-renal (obstructive) acute renal failure. The patient’s early symptoms are suggestive of a partial bladder outlet obstruction. His current symptoms suggest that the obstruction is essentially complete. The suprapubic tenderness and dullness to percussion are due to a distended bladder. These signs and symptoms in elderly men are most commonly caused by benign prostatic hyperplasia, but prostate cancer must also be considered.

How do the laboratory findings help to confirm the diagnosis?

The elevations in blood urea nitrogen (BUN) and creatinine levels indicate a significant reduction in glomerular filtration rate (GFR). This is due to the increased tubular hydrostatic pressure from the obstruction, which offsets glomerular capillary hydrostatic pressure. The BUN:creatinine ratio is increased because of decreased flow through the collecting duct, which causes enhanced urea reabsorption. The normal hemoglobin and hematocrit levels indicate that this is acute and not chronic renal failure, which almost always causes a hypoproliferative anemia.

How could this diagnosis be confirmed?

A catheter bypassing the obstruction may be placed in the bladder. This is sometimes difficult to do when the prostate is very enlarged. An ultrasound of the kidneys would show dilated renal collecting ducts and bilateral ureteral dilatation. A bladder scan would show a markedly enlarged bladder.

What is the most appropriate management for this patient?

Temporary relief may be achieved with a transurethral catheter, if possible. A suprapubic catheter should be considered if the prostate is too large to pass a catheter by. A transurethral prostatectomy is the definitive procedure. In addition, a work-up to distinguish benign from malignant prostatic disease should be conducted.

What are potential consequences of the treatment?

A postobstructive diuresis is typical after the relief of prolonged urinary tract obstruction. The magnitude of the diuresis can reach several liters per day. One mechanism for this is due to the elimination of sodium and water retained during the period of obstruction. Once the obstruction is removed, GFR increases and the excess sodium and water can be eliminated, which is a self-limiting process. Another mechanism is osmotic diuresis from retained urea, which is eliminated as GFR recovers.

By Kristen Vierregger, University of Pennsylvania School of Medicine, Class of 2008; in association with Le TT, Schabelman E, Shivaram A, and Klein J, eds: First Aid Cases for the USMLE Step 2 CK. New York: McGraw-Hill, 2007.