Pocket Medicine

NEPHROLOGY

NEPHROLITHIASIS

Types of stones and risk factors (J Clin Endocrinol Metabol 2012;97:1847)

•  Calcium (Ca oxalate > Ca phosphate): 70–90% of kidney stones

Urine findings: ↑ Ca, ↑ oxalate (Ca-ox only), ↑ pH (Ca-phos only), ↓ citrate, ↓ volume

2° hypercalciuria: 1° hyperparathyroidism, distal RTA, sarcoid

2° hyperoxaluria: Crohn’s, ileal disease w/ intact colon, gastric bypass

Diet: ↑ animal protein, ↑ sucrose, ↑ Na, ↓ K, ↓ fluid, ↓ fruits/vegetables, ↑ vit. C, ↓ Ca

•  Uric acid: 5–10% of kidney stones, radiolucent on plain film

Urine findings: ↑ uric acid, ↓ pH (eg, from chronic diarrhea)

•  Magnesium ammonium phosphate (“struvite” or “triple phosphate”)

Chronic upper UTI w/ urea-splitting organisms (eg, Proteus, Klebs) → ↑ urine NH3, pH >7

•  Cystine: inherited defects of tubular amino acid reabsorption

Clinical manifestations

•  Hematuria (absence does not exclude diagnosis), flank pain, N/V, dysuria, frequency

•  Ureteral obstruction (stones >5 mm unlikely to pass spont.) → AKI if solitary kidney

•  UTI: ↑ risk of infection proximal to stone; urinalysis of distal urine may be normal

Workup

•  Noncontrast helical CT scan (ureteral dilation w/o stone suggests recent passage) 97% sens. 96% spec. (AJR 2008;191:396)

•  Strain urine for stone to analyze; U/A & UCx; electrolytes, BUN/Cr, Ca, PO4, PTH

•  24-h urine × 2 (>6 wk after acute setting) for Ca, PO4, oxalate, citrate, Na, Cr, pH, K, vol.

Acute treatment (NEJM 2004;350:684)

•  Analgesia (narcotics ± NSAIDs; combination superior, Ann Emerg Med 2006;48:173), ensure adequate fluid repletion, antibiotics if UTI

•  Consider alpha blocker > CCB to promote ureteral relaxation (Lancet 2006;368:1171)

•  Indications for immediate urologic eval and/or hosp: obstruction (esp. solitary or transplant kidney), urosepsis, intractable pain or vomiting, significant AKI

•  Urologic Rx: lithotripsy (NEJM 2012:367:50), stent, perc nephrostomy, ureteroscopic removal

Chronic treatment (J Clin Endocrinol Metabol 2012;97:1847)

•  Increase fluid intake (>2 L/d) for goal UOP 2 L/d

•  Calcium stones: 24-h urine identifies specific urinary risk factors to treat

↓ Na and meat intake (NEJM 2002;346:77), thiazides: decrease urine Ca

Depending on 24-h urine: K-citrate, dietary oxalate restriction, allopurinol

High dietary Ca is likely beneficial by ↓ oxalate absorp., unclear role of Ca supplements

•  Uric acid: urine alkalinization (K-citrate), allopurinol

•  Magnesium ammonium phosphate: antibiotics to treat UTI, urologic intervention, acetohydroxamic acid: urease inhibitor, reserve for experienced clinician, poorly tolerated

•  Cystine: urine alkalinization (K-citrate), D-penicillamine, tiopronin