All types of renal tubular acidosis are associated with:
- hyperchloraemic metabolic acidosis
- normal anion gap
The three main types of renal tubular acidosis are:
Type 1 renal tubular acidosis: Distal
This is due to the kidney not excreting hydrogen ions in the distal tubule
Causes:
- idiopathic
- SLE/RA
- hypercalcaemia
- drugs: lithium, amphotericin
- ricketts
- growth failure
- nephrocalcinosis
- renal calculi - calcium phosphate stones
- low potassium
Diagnosis is by oral acid load with ammonium chloride - the urine should acidify, but in type 1 renal tubular acidosis urine pH will remain >5.5
Treatment is with oral bicarbonate
Type 2 renal tubular acidosis: proximal
This is due to the kidneys failing to reabsorb bicarbonate in the proximal tubule
Causes include:
- Wilson's syndrome
- Fanconi syndrome
- cystinosis
- myeloma
- interstitial nephritis
- drugs - lead, acetazolamide, old tetracycline
Complications include:
- osteomalacia
- low potassium
Diagnosis is by IV bicarbonate loading , which will result in a high fractional excretion of bicarb.
Treatment is with oral bicarbonate.
Type 4 renal tubular acidosis
This is caused by hypoaldosteronism, resulting in hyperkalaemia.
Treatment is to treat cause and control hyperkalaemia
The astute amongst you may have noted the lack of a type 3 - this is because those clever renal physicians decided that, upon reflection, what they had named type 3 was probably just a combination of types 1 and 2.
If you had to boil the above battle down to a set of key facts, I'd go with:
- all cause hyperchloraemic metabolic acidosis with a normal anion gap
- types 1 and 2 both cause hypokalaemia and are both treated with oral bicarb
- type 3 causes a hyperkalaemia and is treated by treating the cause
Now on to a symptom-based battle...
