Genitourinary and Renal Systems
Core concepts
Classify acute kidney injury before treating it. Prerenal injury shows a blood urea nitrogen to creatinine ratio above 20, urine sodium under 20, and a fractional excretion of sodium below 1 percent. Intrinsic tubular injury shows muddy brown casts and a fractional excretion above 2 percent. Postrenal injury requires bladder scanning and relief of obstruction.
Nephrotic syndrome means heavy proteinuria, hypoalbuminemia, edema, and hyperlipidemia; nephritic syndrome means hematuria with red cell casts, hypertension, and oliguria. Post-streptococcal glomerulonephritis follows pharyngitis by one to three weeks with low complement; IgA nephropathy occurs during the infection itself.
Stones: calcium oxalate predominates, noncontrast CT is the imaging study of choice, and stones larger than 10 millimeters rarely pass spontaneously. Infection plus obstruction is an emergency requiring decompression.
Hyperkalemia is the electrolyte emergency to nail down: peaked T waves, widened QRS, then sine wave. Give intravenous calcium first to stabilize the myocardium, then shift potassium with insulin and glucose or a beta agonist, then remove it with a binder or dialysis. Dialysis indications include acidosis, electrolytes, intoxications, overload, and uremia.
Traps
- Relying on fractional excretion of sodium in a patient on diuretics; use fractional excretion of urea or the urine sodium trend instead.
- Missing obstructive uropathy in a patient with anuria and a palpable bladder.
- Treating hyponatremia rapidly and causing osmotic demyelination.
Rule of thumb
Hypotension, a rising creatinine, and a bland urine sediment with a fractional excretion of sodium below 1 percent is prerenal disease until proven otherwise; the first treatment is volume, and the first thing to stop is the nephrotoxin.