Renal tubular acidosis (RTA) refers to defects in the renal tubular transport of hydrogen ions, bicarbonate ions, or both, in the kidneys resulting in a normal anion gap metabolic acidosis.
The exact prevalence of renal tubular acidosis is unknown but the entity is probably under-recognized. However, it is known that primary forms are rarer than acquired forms 1.
Renal tubular acidosis produces a normal anion gap metabolic acidosis and hyperchloremia. It should be suspected in patients with this pattern of acidosis and electrolyte disturbance and the absence of gastrointestinal bicarbonate loss.
Patients may have a relatively normal renal function and glomerular filtration rate 2. Clinical features otherwise depend on the type of renal tubular acidosis:
- type 1: hypercalciuria, polyuria, impaired growth, nephrocalcinosis, urolithiasis and hypokalemia 2
- type 2: rickets and osteomalacia are not usually seen unless hypophosphatemia is also present (e.g. Fanconi syndrome), nephrocalcinosis and urolithiasis are uncommon overall 2
- type 4: nephrocalcinosis and nephrolithiasis are generally absent 2
Renal acid-base homeostasis is achieved primarily through two processes: either reabsorption of bicarbonate ions in the proximal tubule or urinary acidification in the distal tubule.
Renal tubular acidosis has been classified into four types 2,3:
- type 1 renal tubular acidosis: caused by impaired distal tubal acidification. It defined as the inability to acidify urine (pH <5.5) under the stimulus of a systemic academia. Impaired secretion of NH4+ ions occurs secondary to this. Bicarbonate ion resorption is essentially normal. It may result from primary causes (idiopathic or genetic) or secondary causes. Secondary causes may relate to genetic disease (e.g. sickle cell disease), proteinaemic syndromes (e.g. amyloidosis), drugs (eg. lithium), calcium disorders (e.g. primary hyperparathyroidism), autoimmune disease (e.g. SLE), or renal disease (e.g. medullary sponge kidney).
- type 2 renal tubular acidosis (also known as proximal renal tubular acidosis): caused by an impaired ability to reabsorb bicarbonate in the proximal tubule, so the majority is lost and bicarbonaturia occurs resulting in a metabolic acidosis. It may occur as part of a primary disorder (e.g. idiopathic and hereditary forms or as part of Fanconi syndrome). It has also been described as occurring secondary to toxins (e.g. acetazolamide) and in association with other diseases (e.g. hyperparathyroidism).
- type 4 renal tubular acidosis (also known as hyperkalemic renal tubular acidosis): defined as an ability to normally acidify the urine (pH <5.5) after a systemic acid load but with defects in the ability to produce and secrete ammonia ions. It may occur in a primary form or in a secondary form. Secondary forms are due to mineralocorticoid deficiency (both in the absence and presence of renal disease, e.g. Addison disease and diabetic nephropathy), mineralocorticoid resistance (in genetic disease e.g. pseudohypoaldosteronism), chronic interstitial nephropathies (e.g. medullary cystic disease), and drug induced hyperkalemia (e.g. potassium-sparing diuretics).
- a fourth type (type 3) of renal tubular acidosis is also described where patients share features of both type 1 and type 2 renal tubular acidosis. It is now referred to as a sub-type of type 1 disease.3
Radiographic signs are generally only seen in those with type 1 renal tubular acidosis or those with chronic kidney disease from another type of renal tubular acidosis 3. Signs of the underlying disorder causing secondary forms of renal tubular acidosis may also be visible (eg. medullary cystic disease).
The changes seen on radiograph relate in calcium resorption from bone or deposition in the kidneys. Nephrocalcinosis and nephrolithiasis, as well as rickets in children and osteomalacia in adults, may be seen in type 1 disease. Osteopenia may be seen in type 4 disease 3,4.
Nephrocalcinosis, if present, is also often visible on ultrasound.
Treatment and prognosis
The treatment of renal tubular acidosis involves correcting the underlying cause or ceasing the offending drug in secondary forms of the disorder, as well as alkali supplementation in the form of either bicarbonate or citrate. Treatment should begin as early as possible to stop progression to nephrocalcinosis and chronic kidney disease 2.
- 1. Igarashi T, Sekine T, Inatomi J, Seki G. Unraveling the molecular pathogenesis of isolated proximal renal tubular acidosis. (2002) Journal of the American Society of Nephrology : JASN. 13 (8): 2171-7. Pubmed
- 2. Soriano J. Renal Tubular Acidosis: The Clinical Entity. (2002) Journal of the American Society of Nephrology. 13 (8): 2160. doi:10.1097/01.ASN.0000023430.92674.E5 - Pubmed
- 3. Santos F, Gil-Peña H, Alvarez-Alvarez S. Renal tubular acidosis. (2017) Current opinion in pediatrics. 29 (2): 206-210. doi:10.1097/MOP.0000000000000460 - Pubmed
- 4. Brenner RJ, Spring DB, Sebastian A, McSherry EM, Genant HK, Palubinskas AJ, Morris RC. Incidence of radiographically evident bone disease, nephrocalcinosis, and nephrolithiasis in various types of renal tubular acidosis. (1982) The New England journal of medicine. 307 (4): 217-21. doi:10.1056/NEJM198207223070403 - Pubmed