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Renal tubular acidosis refers to a group of disorders in which metabolic acidosis develops as a result of defects in the kidney's ability to acidify the urine.
Renal tubular acidosis is classified into four forms whereby all these are characterized by a normal anion gap and hyperchloremic metabolic acidosis.
Renal tubular acidosis type 1 is characterized by impaired secretion of H+ by A-type intercalated cells of the collecting ducts and therefore ammonium. It is suspected in any patient with metabolic acidosis with a normal anion gap(non-anion gap metabolic acidosis) and high urine pH greater than 5.0.
A negative gap indicates ammonium is present, and a positive gap indicates ammonium is not present.
Major causes of distal RTA in adults are;
Hereditary distal RTA is the most common cause of RTA in children.
Treatment is bicarbonate and citrate.
Proximal renal tubular acidosis results from defective bicarbonate reabsorption in the proximal tubule, causing bicarbonate wasting in the urine.
In a normal person HCO- excretion occurs when the levels exceed 24 to 28 mEq/L. In patients with proximal RTA, there is a lower threshold leading to loss of filtered bicarbonate until serum bicarbonate is very low.
This wasting continues until serum bicarbonate concentration reaches a lower threshold enough to allow all of the filtered bicarbonate to be reabsorbed.
It can be present as an isolated disorder or in association with a generalized proximal tubular dysfunction known as Fanconi syndrome, in which reabsorption of other solutes such as phosphate, glucose, uric acid, and amino acids is impaired resulting in bone demineralization due to phosphate wasting.
Most commonly due to increased excretion of light chains in multiple myeloma and other monoclonal gammopathies.
Causes of proximal RTA include
Variable urine pH, depending on whether the patient is treated with alkali therapy or not
Low serum bicarbonate concentration (12–20 mEq/L) with hyperchloremic acidosis.
Intravenous (IV) infusion of sodium bicarbonate (0.5–1.0 mEq/kg/ hour) causes an increase in serum bicarbonate concentration toward normal (18–20 mEq/L) and a rapid increase in urine pH (>7.5) and fractional excretion of bicarbonate (>15–20%).
Mixed RTA is most often applied to a rare autosomal recessive syndrome that results from carbonic anhydrase II deficiency and has features of both proximal and distal RTA.
Renal tubular acidosis (RTA) type 4, also called hyperkalemic renal tubular acidosis, is characterized by hyperchloremic metabolic acidosis, hyperkalemia, and decreased urinary NH4+ excretion, usually due to aldosterone deficiency or aldosterone resistance.
This type results from either aldosterone deficiency (e.g., primary adrenal insufficiency, ACE inhibitors, severe illness, inherited disorders) or tubular resistance to aldosterone action (e.g., pseudohypoaldosteronism).
Aldosterone resistance due to drugs such as pentamidine, spironolactone,diuretics,calcineurium inhibitors.
Genetic aldosterone resistance ie pseudoaldosteronism
Hypoaldosteronism due to high renin, ie in Addison disease, inhibition of aldosterone secretion by heparin, ACE inhibitors, AT1 receptor.
Hypoaldosteronism due to low renin due to diabetes mellitus, NSAIDs, beta-blockers.
You should suspect type 4 RTA in patients with non-AG metabolic acidosis and hyperkalemia.
Characterized by
Note that, type 4 is the common form in adults.
Because of the normal functioning of the H+ ATPase, the urine is approximately acidic.