Monday, 8 November 2010

MRCP revision battle 43.5: Renal calculi

"Good god" exclaimed my medical student "that man looks like he's in labour!"  Indeed, glancing into cubicle 4 there was a man who resembled a woman trying to give birth - he was puffing away at the entonox the ambulance crew had given him, pacing as if he just couldn't get comfortable and every now and then was hit by a pain that caused him to yelp out.  "I think he's got renal stones" I said.  The medical student looked hugely impressed at my confident diagnosis from a distance; I'm sure however you aren't and the MRCP examiners wouldn't be either, giving a classical description of renal colic just to lull you into a false sense of security before asking you a random question on the minuitae, so lets dive into the small print...

Location of stone

Classical renal colic ("loin to groin pain") suggests the stone is in the ureter
Stones in the bladder/urethra may cause pain on passing water

Type of stone

Commonest stone = calcium oxalate = 75%
  • Calcium oxalate stones are spikey and radioopaque
  •  Foods associated with increased levels of oxalate include chocolate, tea, rhubarb and spinach
  • However, medical conditions which result in excessive colonic absorption of oxalate are more likely to predispose, for example:
    • Crohns disease
    • post ileal resection
    • chronic pancreatitis
    • short bowel syndrome
  •  Hypercalciuria/hypercalcaemia also predispose to calcium oxalate stones:
    • hyperparathyroidism
    • sarcoidosis
    • hyperthyroidism
    • vit D excess
    • drugs: lithium, loop diuretics
  • Prevention of calcium oxalate stones includes drinking more water and possibly thiazide diuretics  or pyridoxine

Next commonest stone = magnesium ammonium phosphate (=struvite, =triple phosphate) =10-20%
  • radiologically tend to be 'large' with a 'staghorn' appearence.  Radioopaque
  • tend to form in alkaline urine so UTIs with ureaplasma urealyticum or proteus predispose

Other radioopaque stones include calcium phosphate.

Cysteine stones (1% total) are semi-radio opaque and form when renal tubular defects are present.

Uric acid stones (5-10%) are radiolucent.  They form when there is excess uric acid.  Prevention includes allopurinol and urinary alkalinisation.

The other radiolucent stone is xanthine, but this accounts for <1% of all stones.

Blood on urine dipstick is often seen as a prerequisite for renal stones, however the reality is that 50% of those with loin to groin pain and a positive blood on dipstick won't have renal stones while 20% of those with loin to groin pain and real renal colic won't have any blood on their dipstick....

Lets now move on to continue our slow battle through the various forms of glomerulonephritis...