RTA

Type 1 RTA

  • pt: sjogren, RA, kid, amphotericin B
  • cause: distal tubule cannot excrete H or absorb K
  • sx:
    • recurrent bilateral stones
    • rickets disease
    • growth failure in children
  • dx:
    • non AG metabolic acidosis but high urine pH (can’t excrete H)
    • acidosis increases Ca excretion (stones)
    • hypokalemia (can’t absorb K)
    • hyperchloremia
    • positive urine AG
  • rx: Na bicarb

Type 2 RTA

  • pt
  • cause: can’t absorb HCO3 absorption in proximal tubule. Fanconi syndrome
  • sx
    • no stones
  • dx
    • milder metabolic acidosis bicarb 12-20
    • hypokalemia from increased aldosterone from volume contraction
    • urine pH < 5.3

Type 4 RTA

  • pt:
  • cause: aldosterone deficiency or resistance
    • diabetes, ace, arb, nsaids, adrenal insufficiency
    • bactrim, K sparing diuretics
  • sx
  • dx
    • hyperkalemia from decreased K excretion
    • pH < 5.4
  • rx: mineralocorticoid or fludrocortisone

The patient clearly has risk factors for developing renal tubular acidosis (RTA). She has new, acute renal failure and takes a number of medications that can cause type 4 RTA, including angiotensin-converting enzyme inhibitors and spironolactone. The primary problem in type 4 RTA, however, is lack of aldosterone effect on the kidney, either through decreased aldosterone secretion or aldosterone resistance. This lack of aldosterone effect on the kidney leads to hyperkalemia, which is characteristic of type 4 RTAs and which she does not have. In addition, when the normal gap acidosis is due to RTA, the malfunctioning kidneys have impaired secretion of ammonium cations (and thus chloride anions). In these situations, the urine chloride level remains relatively low, and the urine anion gap is positive.

Hypokalemia is seen in both type 1 (distal) and type 2 (proximal) RTAs, although both are relatively uncommon in adults. Distal RTA results from impaired acidification in the distal tubules of the kidney and can be seen with autoimmune disorders such as Sjögren syndrome, rheumatoid arthritis, and lupus. In addition, toluene inhalation, or “huffing,” can lead to distal (type 1) RTA with normal gap metabolic acidosis and profound hypokalemia. Proximal RTAs (type 2) are characterized by a reduced ability of the proximal kidney to reabsorb bicarbonate, which leads to bicarbonate wasting in the urine. Multiple myeloma and amyloidosis can cause proximal RTAs. Like proximal RTA, carbonic anhydrase inhibitors, like acetazolamide, also cause normal gap metabolic acidosis through bicarbonate wasting in the urine. Both keto acid and lactic acid are organic acids that result in an increased anion gap metabolic acidosis. This patient has a normal anion gap of 10, suggesting that organic anions, such as ketones and lactate are not abnormally present and not the cause of her metabolic acidosis.