Renal tubular acidosis

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Renal tubular acidosis
Nephrocalcinosis.jpg
Significant bilateral nephrocalcinosis (calcification of the kidneys) on a frontal X-ray (radiopacities (white) in the right upper and left upper quadrant of the image), as seen in distal renal tubular acidosis
Specialty Nephrology   OOjs UI icon edit-ltr-progressive.svg

Renal tubular acidosis (RTA) is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine. [1] In renal physiology, when blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid equivalents, and other solutes before it drains into the bladder as urine. The metabolic acidosis that results from RTA may be caused either by insufficient secretion of hydrogen ions (which are acidic) into the latter portions of the nephron (the distal tubule) or by failure to reabsorb sufficient bicarbonate ions (which are alkaline) from the filtrate in the early portion of the nephron (the proximal tubule). Although a metabolic acidosis also occurs in those with chronic kidney disease, the term RTA is reserved for individuals with poor urinary acidification in otherwise well-functioning kidneys. Several different types of RTA exist, which all have different syndromes and different causes. RTA is usually an incidental finding based on routine blood draws that show abnormal results. Clinically, patients may present with vague symptoms such as dehydration, mental status changes, or delayed growth in adolescents. [2]

Contents

The word acidosis refers to the tendency for RTA to cause an excess of acid, which lowers the blood's pH. When the blood pH is below normal (7.35), this is called acidemia. The metabolic acidosis caused by RTA is a normal anion gap acidosis.

Types

An overview of types 1, 2, and 4 is presented below (type 3 is usually excluded from modern classifications):

TypeType 1Type 2Type 4
LocationCollecting Tubules, distal tubulesProximal tubulesAdrenal
AcidemiaYes (very severe)YesMild when present
Potassium Hypokalaemia Hypokalemia Hyperkalemia
PathophysiologyFailure of α intercalated cells to secrete H+ and reclaim K+Failure of proximal tubular cells to reabsorb H C O −
3		Deficiency of aldosterone, or a resistance to its effects, (hypoaldosteronism or pseudohypoaldosteronism)
Prevalence/IncidencePrevalence of 46 per 1 million people [3] Autosomal dominant: 1 family described [4]

Type 1: distal

Distal RTA (dRTA) is the classical form of RTA, being the first described. Distal RTA is characterized by a failure of H+ secretion into lumen of nephron by the alpha intercalated cells of the medullary collecting duct of the distal nephron.[ citation needed ]

This failure of acid secretion may be due to a number of causes, and it leads to an inability to acidify the urine to a pH of less than 5.3. Because renal excretion is the primary means of eliminating H+
 from the body, there is consequently a tendency towards acidemia. There is an inability to excrete H+ while K+
 cannot be reclaimed by the cell, leading to acidemia (as H+
 builds up in the body) and hypokalemia (as K+
 cannot be reabsorbed by the alpha cell).[ citation needed ]

This leads to the clinical features of dRTA; [1] In other words, the intercalated cells' apical H+/K+ antiporter is non-functional, resulting in proton retention and potassium excretion. Since calcium phosphate stones demonstrate a proclivity for deposition at higher pHs (alkaline), the substance of the kidney develops stones bilaterally; this does not occur in the other RTA types[ citation needed ].

Distal RTA has also been linked to specific genetic mutations that will alter when the disease will present in the patient's life. Patient's with mutations in ATP6V1B1 and ATP6V0A4 will present with symptoms within the first year of life, while those with mutation of the SLC4A1 have delayed onset around 10 years of age. [6] Electrolyte imbalances remain the same, while in severe cases symptoms can advance to amino aciduria and hyperammonemia. [7] In a large Asian series of Distal renal Tubular Acidosis in Sjogren's Syndrome, late diagnosis is a rule in spite of overt hypokalemic periodic paralysis in a vast majority of them [8]

dRTA is the most common form of RTA diagnosed in Western countries, and can be classified as either hereditary (primary) or acquired (secondary). Primary RTA generally results from systemic and autoimmune diseases [9] or drug and toxin exposure in adults, whereas pediatric RTA results from genetic defects in the proteins that facilitate urine acidification at the distal tubule. Hereditary dRTA generally presents as failure to thrive during the first several months of life. Other common clinical manifestations in children include a variety of gastrointestinal and urinary symptoms, including polyuria, polydipsia, constipation, diarrhea, bouts of dehydration, and decreased appetite. [10]

Type 2: proximal

Radiograph of a child with rickets, a complication of both proximal and, less commonly, distal RTA. XrayRicketsLegssmall.jpg
Radiograph of a child with rickets, a complication of both proximal and, less commonly, distal RTA.

Proximal RTA (pRTA) is caused by a failure of the proximal tubular cells to reabsorb filtered bicarbonate from the urine, leading to urinary bicarbonate wasting and subsequent acidemia. Reabsorption of bicarbonate is typically 80-90% in the proximal tubule and failure of this process leads to decreased systemic buffer and metabolic acidosis. [11] The distal intercalated cells function normally, so the acidemia is less severe than dRTA and the alpha intercalated cells can produce H+ to acidify the urine to a pH of less than 5.3. [12] pRTA also has several causes, and may occasionally be present as a solitary defect, but is usually associated with a more generalized dysfunction of the proximal tubular cells called Fanconi syndrome, in which there is also phosphaturia, glycosuria, aminoaciduria, uricosuria, and tubular proteinuria.[ citation needed ]

The principal feature of Fanconi syndrome is bone demineralization (osteomalacia or rickets) due to phosphate wasting.[ citation needed ]

Type 3: combined proximal and distal

In some patients, RTA shares features of both dRTA and pRTA. This rare pattern was observed in the 1960s and 1970s as a transient phenomenon in infants and children with dRTA (possibly in relation with some exogenous factor such as high salt intake) and is no longer observed. [13] This form of RTA has also been referred to as juvenile RTA. [14]

Combined dRTA and pRTA is also observed as the result of inherited carbonic anhydrase II deficiency. Mutations in the gene encoding this enzyme give rise to an autosomal recessive syndrome of osteopetrosis, renal tubular acidosis, cerebral calcification, and mental retardation. [15] [16] [17] It is very rare and cases from all over the world have been reported, of which about 70% are from the Maghreb region of North Africa, possibly due to the high prevalence of consanguinity there. [18] The kidney problems are treated as described above. There is no treatment for the osteopetrosis or cerebral calcification.

Type 3 is rarely discussed. [19] Most comparisons of RTA are limited to a comparison of types 1, 2, and 4.

Type 4: absolute hypoaldosteronism or aldosterone insensitivity

Type 4 RTA is due either to a deficiency of Aldosterone, or to a resistance to its effects. Aldosterone-2D-skeletal.svg
Type 4 RTA is due either to a deficiency of Aldosterone, or to a resistance to its effects.

Type 4 RTA is not actually a tubular disorder at all nor does it have a clinical syndrome similar to the other types of RTA described above. It was included in the classification of renal tubular acidoses as it is associated with a mild (normal anion gap) metabolic acidosis due to a physiological reduction in proximal tubular ammonium excretion (impaired ammoniagenesis), which is secondary to hypoaldosteronism, and results in a decrease in urine buffering capacity. Its cardinal feature is hyperkalemia, and measured urinary acidification is normal, hence it is often called hyperkalemic RTA or tubular hyperkalemia. [19]

Causes include:

  1. Drugs: NSAIDs, ACE inhibitors and ARBs, Eplerenone, Spironolactone, Trimethoprim, Pentamidine
  2. Pseudohypoaldosteronism

History

Renal tubular acidosis was first described in 1935 by Lightwood and 1936 by Butler et al. in children. [20] [21] Baines et al. first described it in adults in 1945. [22]

Donald L. Lewis postulated the character Tiny Tim, of A Christmas Carol , was suffering from renal tubular acidosis. [23]

Researchers published in PLOS ONE in 2009 speculated that the infamously afflicted Charles II of Spain may have suffered from renal tubular acidosis in tandem with combined pituitary hormone deficiency. [24]

See also

Related Research Articles

<span class="mw-page-title-main">Kidney</span> Organ that filters blood and produces urine

In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retroperitoneal space, and in adult humans are about 12 centimetres in length. They receive blood from the paired renal arteries; blood exits into the paired renal veins. Each kidney is attached to a ureter, a tube that carries excreted urine to the bladder.

<span class="mw-page-title-main">Nephron</span> Microscopic structural and functional unit of the kidney

The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman's capsule. The renal tubule extends from the capsule. The capsule and tubule are connected and are composed of epithelial cells with a lumen. A healthy adult has 1 to 1.5 million nephrons in each kidney. Blood is filtered as it passes through three layers: the endothelial cells of the capillary wall, its basement membrane, and between the foot processes of the podocytes of the lining of the capsule. The tubule has adjacent peritubular capillaries that run between the descending and ascending portions of the tubule. As the fluid from the capsule flows down into the tubule, it is processed by the epithelial cells lining the tubule: water is reabsorbed and substances are exchanged ; first with the interstitial fluid outside the tubules, and then into the plasma in the adjacent peritubular capillaries through the endothelial cells lining that capillary. This process regulates the volume of body fluid as well as levels of many body substances. At the end of the tubule, the remaining fluid—urine—exits: it is composed of water, metabolic waste, and toxins.

<span class="mw-page-title-main">Acetazolamide</span> Chemical compound

Acetazolamide, sold under the trade name Diamox among others, is a medication used to treat glaucoma, epilepsy, acute mountain sickness, periodic paralysis, idiopathic intracranial hypertension, heart failure and to alkalinize urine. It may be used long term for the treatment of open angle glaucoma and short term for acute angle closure glaucoma until surgery can be carried out. It is taken by mouth or injection into a vein. Acetazolamide is a first generation carbonic anhydrase inhibitor and it decreases the ocular fluid and osmolality in the eye to decrease intraocular pressure.

<span class="mw-page-title-main">Collecting duct system</span> Kidney system

The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct system is the last part of nephron and participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin.

<span class="mw-page-title-main">Renal physiology</span> Study of the physiology of the kidney

Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, and other electrolytes; clearance of toxins; absorption of glucose, amino acids, and other small molecules; regulation of blood pressure; production of various hormones, such as erythropoietin; and activation of vitamin D.

<span class="mw-page-title-main">Proximal tubule</span> Segment of nephron in kidneys

The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s capsule abruptly transition to proximal tubule epithelial cells (PTECs). The proximal tubule can be further classified into the proximal convoluted tubule (PCT) and the proximal straight tubule (PST).

<span class="mw-page-title-main">Metabolic alkalosis</span> Medical condition

Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35–7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations. The condition typically cannot last long if the kidneys are functioning properly.

<span class="mw-page-title-main">Band 3 anion transport protein</span> Mammalian protein found in Homo sapiens

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Hyperchloremic acidosis is a form of metabolic acidosis associated with a normal anion gap, a decrease in plasma bicarbonate concentration, and an increase in plasma chloride concentration. Although plasma anion gap is normal, this condition is often associated with an increased urine anion gap, due to the kidney's inability to secrete ammonia.

<span class="mw-page-title-main">Bartter syndrome</span> Medical condition

Bartter syndrome (BS) is a rare inherited disease characterised by a defect in the thick ascending limb of the loop of Henle, which results in low potassium levels (hypokalemia), increased blood pH (alkalosis), and normal to low blood pressure. There are two types of Bartter syndrome: neonatal and classic. A closely associated disorder, Gitelman syndrome, is milder than both subtypes of Bartter syndrome.

<span class="mw-page-title-main">V-ATPase</span> Family of transport protein complexes

Vacuolar-type ATPase (V-ATPase) is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pumps protons across the plasma membranes of numerous cell types. V-ATPases couple the energy of ATP hydrolysis to proton transport across intracellular and plasma membranes of eukaryotic cells. It is generally seen as the polar opposite of ATP synthase because ATP synthase is a proton channel that uses the energy from a proton gradient to produce ATP. V-ATPase however, is a proton pump that uses the energy from ATP hydrolysis to produce a proton gradient.

<span class="mw-page-title-main">Dent's disease</span> Medical condition

Dent's disease is a rare X-linked recessive inherited condition that affects the proximal renal tubules of the kidney. It is one cause of Fanconi syndrome, and is characterized by tubular proteinuria, excess calcium in the urine, formation of calcium kidney stones, nephrocalcinosis, and chronic kidney failure.

<span class="mw-page-title-main">Nephrocalcinosis</span> Medical condition caused by the deposition of calcium salts in the kidneys

Nephrocalcinosis, once known as Albright's calcinosis after Fuller Albright, is a term originally used to describe the deposition of poorly soluble calcium salts in the renal parenchyma due to hyperparathyroidism. The term nephrocalcinosis is used to describe the deposition of both calcium oxalate and calcium phosphate. It may cause acute kidney injury. It is now more commonly used to describe diffuse, fine, renal parenchymal calcification in radiology. It is caused by multiple different conditions and is determined by progressive kidney dysfunction. These outlines eventually come together to form a dense mass. During its early stages, nephrocalcinosis is visible on x-ray, and appears as a fine granular mottling over the renal outlines. It is most commonly seen as an incidental finding with medullary sponge kidney on an abdominal x-ray. It may be severe enough to cause renal tubular acidosis or even end stage kidney disease, due to disruption of the kidney tissue by the deposited calcium salts.

<span class="mw-page-title-main">Carbonic anhydrase II</span> Protein-coding gene in the species Homo sapiens

Carbonic anhydrase II, is one of sixteen forms of human α carbonic anhydrases. Carbonic anhydrase catalyzes reversible hydration of carbon dioxide. Defects in this enzyme are associated with osteopetrosis and renal tubular acidosis. Renal carbonic anhydrase allows the reabsorption of bicarbonate ions in the proximal tubule. Loss of carbonic anhydrase activity in bones impairs the ability of osteoclasts to promote bone resorption, leading to osteopetrosis.

<span class="mw-page-title-main">Electrogenic sodium bicarbonate cotransporter 1</span> Protein-coding gene in the species Homo sapiens

Electrogenic sodium bicarbonate cotransporter 1, sodium bicarbonate cotransporter is a membrane transport protein that in humans is encoded by the SLC4A4 gene.

Lightwood–Albright syndrome is a neonatal form of renal tubular acidosis. It is characterized by distal renal tubular acidosis that occurs as a result of bicarbonate wasting and the inability to excrete hydrogen ions. By definition, it is a transient process and has no particular disease course. If untreated, it may lead to nephrocalcinosis and failure to thrive.

Fanconi syndrome or Fanconi's syndrome is a syndrome of inadequate reabsorption in the proximal renal tubules of the kidney. The syndrome can be caused by various underlying congenital or acquired diseases, by toxicity, or by adverse drug reactions. It results in various small molecules of metabolism being passed into the urine instead of being reabsorbed from the tubular fluid. Fanconi syndrome affects the proximal tubules, namely, the proximal convoluted tubule (PCT), which is the first part of the tubule to process fluid after it is filtered through the glomerulus, and the proximal straight tubule, which leads to the descending limb of loop of Henle.

<span class="mw-page-title-main">Distal renal tubular acidosis</span> Medical condition

Distal renal tubular acidosis (dRTA) is the classical form of RTA, being the first described. Distal RTA is characterized by a failure of acid secretion by the alpha intercalated cells of the distal tubule and cortical collecting duct of the distal nephron. This failure of acid secretion may be due to a number of causes. It leads to relatively alkaline urine, due to the kidney's inability to acidify the urine to a pH of less than 5.3.

Proximal renal tubular acidosis (pRTA) or type 2 renal tubular acidosis (RTA) is a type of RTA caused by a failure of the proximal tubular cells to reabsorb filtered bicarbonate from the urine, leading to urinary bicarbonate wasting and subsequent acidemia. The distal intercalated cells function normally, so the acidemia is less severe than dRTA and the urine can acidify to a pH of less than 5.3. pRTA also has several causes, and may occasionally be present as a solitary defect, but is usually associated with a more generalised dysfunction of the proximal tubular cells called Fanconi syndrome where there is also phosphaturia, glycosuria, aminoaciduria, uricosuria and tubular proteinuria.

<span class="mw-page-title-main">Oliver Wrong</span>

Professor Oliver Murray Wrong was an eminent academic nephrologist and one of the founders of the speciality in the United Kingdom. From a background as a "salt and water" physician, he made detailed clinical observations and scientifically imaginative connections which were the basis of numerous advances in the molecular biology of the human kidney. Wrong himself contributed to much of the molecular work after his own "retirement". He dictated amendments to his final paper during his final illness in his own teaching hospital, University College Hospital (UCH), London. Though academic in his leanings, he was a compassionate physician who established a warm rapport with patients, a link he regarded as the keystone of his research. He belonged to a generation of idealistic young doctors responsible for the establishment of the UK's National Health Service in the post-War years.

References

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  2. ↑ Yaxley J, Pirrone C (2016-12-21). "Review of the Diagnostic Evaluation of Renal Tubular Acidosis". The Ochsner Journal. 16 (4): 525–530. PMC   5158160 . PMID   27999512.
  3. ↑ Bianic F, Guelfucci F, Robin L, Martre C, Game D, Bockenhauer D (2021). "Epidemiology of Distal Renal Tubular Acidosis: A Study Using Linked UK Primary Care and Hospital Data" (PDF). Nephron. 145 (5): 486–495. doi:10.1159/000516876. PMID   34198293. S2CID   235713623.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  8. ↑ Sandhya, Pulukool; Danda, Debashish; Rajaratnam, Simon; Thomas, Nihal (2014-12-19). "Sjögren's, Renal Tubular Acidosis And Osteomalacia - An Asian Indian Series". The Open Rheumatology Journal. 8: 103–109. doi: 10.2174/1874312901408010103 . ISSN   1874-3129. PMC   4286932 . PMID   25584094.
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  10. ↑ Soleimani M, Rastegar A (September 2016). "Pathophysiology of Renal Tubular Acidosis: Core Curriculum 2016". American Journal of Kidney Diseases. 68 (3): 488–98. doi: 10.1053/j.ajkd.2016.03.422 . PMID   27188519.
  11. ↑ Pereira PC, Miranda DM, Oliveira EA, Silva AC (March 2009). "Molecular pathophysiology of renal tubular acidosis". Current Genomics. 10 (1): 51–9. doi:10.2174/138920209787581262. PMC   2699831 . PMID   19721811.
  12. ↑ Rodriguez Soriano J, Boichis H, Stark H, Edelmann CM (March 1967). "Proximal renal tubular acidosis. A defect in bicarbonate reabsorption with normal urinary acidification". Pediatric Research. 1 (2): 81–98. doi: 10.1203/00006450-196703000-00001 . PMID   6029811.
  13. ↑ Rodríguez Soriano J (August 2002). "Renal tubular acidosis: the clinical entity". Journal of the American Society of Nephrology. 13 (8): 2160–70. doi: 10.1097/01.ASN.0000023430.92674.E5 . PMID   12138150.
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  16. ↑ Shah GN, Bonapace G, Hu PY, Strisciuglio P, Sly WS (September 2004). "Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation". Human Mutation. 24 (3): 272. doi: 10.1002/humu.9266 . PMID   15300855.
  17. ↑ Pushkin A, Abuladze N, Gross E, Newman D, Tatishchev S, Lee I, et al. (August 2004). "Molecular mechanism of kNBC1-carbonic anhydrase II interaction in proximal tubule cells". The Journal of Physiology. 559 (Pt 1): 55–65. doi:10.1113/jphysiol.2004.065110. PMC   1665076 . PMID   15218065.
  18. ↑ Fathallah DM, Bejaoui M, Lepaslier D, Chater K, Sly WS, Dellagi K (May 1997). "Carbonic anhydrase II (CA II) deficiency in Maghrebian patients: evidence for founder effect and genomic recombination at the CA II locus". Human Genetics. 99 (5): 634–7. doi:10.1007/s004390050419. PMID   9150731. S2CID   24978813.
  19. 1 2 Hyporeninemic Hypoaldosteronism at eMedicine
  20. ↑ Lightwood R. (1935). "Communication no. 1". Arch Dis Child. 10 (57): 205–6. doi:10.1136/adc.10.57.205. PMC   1975385 .
  21. ↑ Butler AM, Wilson JL, Farber S (1936). "Dehydration and acidosis with calcification at renal tubules". The Journal of Pediatrics . 8 (4): 489–99. doi:10.1016/s0022-3476(36)80111-5.
  22. ↑ Baines AM, Barelay JA, Cooke WT (1945). "Nephrocalcinosis associated with hyperchloremia and low plasma-bicarbonate". Q J Med. 14: 113–23.
  23. ↑ Lewis DW (December 1992). "What was wrong with Tiny Tim?". American Journal of Diseases of Children. 146 (12): 1403–7. doi:10.1001/archpedi.1992.02160240013002. PMID   1340779.
  24. ↑ Alvarez G, Ceballos FC, Quinteiro C (15 April 2009). "The role of inbreeding in the extinction of a European royal dynasty". PLOS ONE. 4 (4): e5174. Bibcode:2009PLoSO...4.5174A. doi: 10.1371/journal.pone.0005174 . PMC   2664480 . PMID   19367331.
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