Renal tubular acidosis

Today we discussed a case of acidosis. A young patient presenting with a significant metabolic acidosis of mixed origin (both anion gap and non-anion gap). This was determined by carefully examining the patient electrolytes and blood gas results, identifying the primary acid-base abnormality and calculating what is called the delta-delta.

This refers to the difference between the calculated and expected anion gap and the difference in the bicarbonate from normal serum values. When the bicarbonate value is lower than what would be expected from the anion gap difference (a 1:1 change with respect to AG and bicarbonate) and additional non-anion gap acidosis is identified. When the bicarbonate is higher than expected, a concomitant metabolic alkalosis is diagnosed.

Anion gap acidosis is a commonly encountered scenario, and there are widespread mnemonics to help you consider the differential diagnosis (ex. MUDPILES). Non-anion gap metabolic acidosis seem to be less commonly discussed, but are worth reviewing. The differential diagnosis includes:

Resuscitation with hyperchloremic solution (NS)/hyperalimentation
Medications: acetazolimide
Renal tubular acidosis
Diarrhea
Anatomic fistulas: uretoenteric fistuls and pancreaticoduodeneal fistulas

The patient today was known to have chronic metabolic acidosis, raising a RTA as a likely cause. These can be divided anatomically into four categories:

a) proximal RTA (type 2) - here there is an abnormality resulting in the reabsorption of bicarbonate. Patient tend to have mild to moderate acidosis with a relative ability to excrete acid in the form of NH4Cl-, and acidify the urine.

Proximal RTA can be thought of as primary and secondary. There are congenital syndromes associated with this, some of which have associated visual and cognitive deficits. One must look for evidence of the Fanconi's syndrome, where in addition to inability reabsorb bicarbonate, glucose, phosphate and potassium are lost in the urine. When present, paraproteinemia (multiple myeloma), should be considered, given the light chains can block re-absorption of electrolytes in the tubules. Drugs and other metabolic and congenital diseases may also present with this and should be considered (See attached article for details). Other things to watch for include bone disease and renal stones with increased loss of electrolytes in the urine.

b)distal RTA (type 1) - results from the inability to excrete protons at the level of the distal tubules. This results in severe acidosis and inability to acidify the urine. Again, this can be a result of congenital disease (often associated with sensorineural hearing loss because of shared gene expression in the cochlea and the kidney) and acquired. A detailed history for connective tissue and autoimmune disease should be performed, with rheumatoid arthritis and Sjogrens syndrome as common entities identified.

c) Mixed proximal/distal (type 3) - these patients have features of both distal and proximal RTA. Rare mutations in the carbonic anhydrase gene have been attributed to mixed RTA. Other clinical features include osteopetrosis, cerebral calcification and mental retardation.

d) Hyperkalemia RTA (type 4) - is due to impaired ammoniagenesis. Most common causes include hyporeninemic states as a result of primary renal disease (diabetic nephropathy) or medications that cause hypoaldosternonism (spironolactone). Congenital syndromes do exist causing pseudohypoaldosteronism however these are uncommon.

For additional details regarding the diagnosis, investigations and treatment of RTA please see the below review article which provides a good overview.

RTA review