Renal tubular acidosis (RTA) is a pathological condition in which a increased quantity of acid can be seen in plasma due to the failure of kidneys to acidify urine in a proper manner .
In general RTA is asymptomatic, but it might involve some bone dysfunctions such as bone pain, osteomalacia in adults and rickets in children due to the marked levels of demineralization, which is particularly frequent in Type 1 and Type 2 RTA. In these types bone involvement is frequently combined with nephrolithiasis and nephrocalcinosis.
The signs and symptoms of hypokalemia, such as muscle weakening, hyporeflexia, and paralysis, are evident, especially in the already mentioned Type 1 and Type 2 RTA. These can sometime be combined with severe electrolyte disturbances, although these are very rare. Type 4 RTA too is usually asymptomatic, apart from a mild acidosis, but if hyperkalemia is severe cardiac arrhythmias or paralysis might become clearly pronounced. Type 2 RTA might also present with signs of extracellular fluid volume depletion due to urinary water loss and marked electrolyte excretion.
In adults the diagnosis of RTA is largely based on the observation of unexplained and incidental laboratory findings regarding blood, electrolyte concentrations and acid-base parameters, while in children it is mainly based on growth retardation, rickets, and failure to thrive.
Studying the clinical history might be useful, as many metabolic diseases, such as diabetes mellitus and adrenal insufficiency, can substantially increase the predisposition to RTA. The study of the patient’s clinical history should also include past exposure to certain drugs and toxins, which as previously seen might damage the nephron tubule both in the distal and the proximal region. The harmful drugs include carbonic anhydrase inhibitors, amphotericin-B, and ibuprofen, while RTA-inducing toxins include heavy metals and cis-platinum.
Laboratory evaluation of blood, urine acid-base parameters and electrolyte concentrations can be performed with routine procedures regularly used in clinical settings. Usually, low plasma bicarbonate concentrations is observed in both acidosis and respiratory alkalosis. Therefore, differentiation between these two disorders in paramount. In this regard, observing elevated pH levels might be very useful, as it can cause low plasma bicarbonate levels in the cases of respiratory alkalosis. The plasma anion gap is normal in all type of RTA, therefore increased levels of this parameter should suggest the presence of another type of acidosis  . In general, decreased plasma bicarbonate levels, hyperchloremia, acidemia, and a normal serum anion gap should support the diagnosis of RTA.
Urine analysis should show impaired urinary acidification and failed bicarbonate reabsorption, while plasma bicarbonate should be low and coupled with hyperchloremia. Aldosterone deficiency or resistance can be indicated by hyperkalemia in the cases of distal RTA.
Furthermore, radiologic findings can be useful for the diagnosis of RTA. Abdominal X-ray or abdominal CT scan can detect nephrocalcinosis and the presence of stones in the kidneys in the cases of classic distal RTA. Evaluation of urinary tract obstruction, instead, can be performed with ultrasound, nuclear renal scan, or spiral CT scan in the case of hyperkalemic distal RTA.
The first goal of the treatment of RTA is to restore blood pH and bicarbonate levels, which can be done by giving the patient daily doses of sodium-bicarbonate or a solution of sodium citrate and citric acid (Shohl solution).
In Type 1 (distal) RTA alkali replacement therapy is performed to correct metabolic acidosis and maintain plasma potassium levels, so than consumption of skeleton and muscle mass can be prevented. Potassium deficits might be significant, but its administration should be controlled to avoid adverse side effects. If necessary, potassium supplements can be provided.
In Type 2 (proximal) RTA patients usually need higher doses of alkali which can be provided by adjusting the Shohl solution concentration accordingly. Also in this case, potassium supplements might be provided if necessary. It should be noted that as more alkali is given and bicarbonaturia increases, urinary potassium loss is exacerbated. In this case, to generate a mild volume depletion and increase proximal reabsorption, hydrochlorothiazide can be given to patients. However, this drug might worsen potassium loss, thus potassium supplementation is highly advised also in this case.
Replacement of bicarbonate loss is paramount in Fanconi syndrome, which can be done by following the same treatment for Type 2 RTA. Phosphate supplementation and vitamin D are required, while amino acids loss can be addressed by following an appropriate diet. The description of the treatments for the other metabolic disorders usually associated with Fanconi syndrome goes beyond the purposes of this discussion.
The major issues in Type IV RTA are acidosis and hyperkalemia, which in this condition might be severe. Correction of potassium overload might cause acidosis to increase significantly, due to increased ammonia production and excretion. Therefore, the doctor and patient should pay particular attention to avoid drugs associated with hyperkalemia and salt substitutes containing potassium. When Type 4 RTA is caused by aldosterone deficiency, fludrocortisones might be provided as replacement, while dietary potassium restriction is effective when hyperkalemic distal RTA is the result of inappropriate response to aldosterone. In these cases patients do not respond to fludrocortisone, which should be replaced by furosemide or other loop diuretics. Patients can received Shohl solution, while K-Shohl solution and other forms of citric acid preparations should be avoided.
The prognosis is good, especially in terms of prevention of renal stone formation and amelioration of nephrocalcinosis in the cases of distal RTA. If treatment is started early, it can even restore growth in children with severe proximal RTA, serum potassium can be controlled and acidosis ameliorated in the cases of hyperkalemic distal RTA, even though the long-term outcomes of treatment are still unknown.
The prognosis for Fanconi syndrome depends on the clinical case considered. If the disorder is due to inherited metabolic diseases, prognosis might not be good, and patients might fail to recover. The situation is different when it comes to cases of RTA due to drugs or toxin exposure, where the Fanconi syndrome resolves once the kidneys heal from the injury and recovery can occur once the toxin is removed and further exposure is avoided.
By contrast, in the cases of proteinuria-associated diseases the prognosis largely depends on the treatment of the primary disorder, which is really responsible for the kidney dysfunction, and recovery is only possible when this primary disorder is under control.
The etiology of RTA includes a variety of many factors which can be either genetic and acquired in nature. The genetic etiological factors involve mutations in the genes of those proteins with key role in the renal function. For instance, one of the most important of these genetic factors is represented by the mutations that occur in the gene decoding proton pump and anion exchanger, responsible for removing bicarbonates from the tubule     , or on the gene decoding the sodium bicarbonate cotransporter   . The acquired factors include episodes of interstitial inflammation and injury  , responsible for tubule cell injury and loss of proton pumps  , or aldosterone deficiency  . RTA might also be triggered by the use of many drugs, such as amphotericin B, toluene, and nonsteroidal anti-inflammatory drugs (NSAIDs).
Particularly frequent as etiological factor is also Fanconi syndrome, a disease of the proximal renal tubules in which glucose, amino acids, uric acid, phosphate and bicarbonate are lost in great quantity into the urine instead of being reabsorbed. The syndrome affects the proximal tubule and might have genetic origins, even though it is frequently associated to the use of certain drugs or the ingestion of heavy metal.
The prevalence and incidence of RTA are not exactly known as the disorder is often not recognized and its frequency underestimated. However, inherited forms of RTA appear to be much rarer then acquired ones   such as those due to renal failure. In the US, the most common forms of RTA are hyperkalemic distal RTA in urinary tract obstruction and hyperkalemic distal RTA secondary to aldosterone deficiency in diabetes.
Fanconi syndrome is very rare as primary cause of RTA, while metal exposure varies significantly according to the location, residence, occupation and in general the social status of the people affected. The incidence of RTA induced by drugs has been constantly increasing over the last years, perhaps due to the unclear side effects or the not always well-defined pharmacological interaction among these medications.
The pathophysiology of RTA varies according to the type and its etiological factor. There are four type of RTA: Type 1 (distal) RTA, Type 2 (proximal) RTA, Type 3 RTA, and Type 4 RTA.
Patients are recommended to take alkali on a daily basis. Mixed solution of cold water or lemonade with sweeteners might be a very effective prevention measure, together with a strict avoidance of potassium foods and drugs causing hyperkalemia. As RTA is a lifelong issue, prevention measures should always be followed by patients through prevention programs previously planed with the doctor.
The cause of renal tubular acidosis (RTA) lays in a dysfunction of the kidneys. In normal conditions, blood is filtered in the kidneys through the action of their functional unit known as nephron. The filtrate produced by this system passes through a long tubule where it is progressively processed, allowing the exchange of salts and other important biological compounds which then drain into the bladder to be expelled as urine. Such system is pivotal for maintaining the biochemical balance of blood plasma, that has to be continuously kept within the appropriate physiological ranges.
RTA occurs when this system fails to recover a sufficient quantity of bicarbonates from the filtrate and to secrete the appropriate quantity of hydrogen ions into the urine, in other words an insufficient urinary acidification without urea and anion retention. This condition is an example of metabolic acidosis, a term which indicates a decrease of the plasma pH. Episodes of metabolic acidosis might also be triggered by renal insufficiency, but it is only in RTA that can develop in physiologically-functioning kidneys.
Renal tubular acidosis (RTA) is a pathological condition in which a increased quantity of acid can be seen in plasma due to the failure of kidneys to acidify urine in a proper manner. The consequence of this condition is an impaired balance of major plasma physiological parameters, such as pH, bicarbonate levels and amino acids concentration, which result in the main features of RTA, like acidemia, hypokalemia or bone demineralization. There are four types of renal tubular acidosis.
Patients are recommended to take alkali on a daily basis. Mixed solution of cold water or lemonade with sweeteners might be a very effective prevention measure, together with a strict avoidance of potassium foods and drugs causing hyperkalemia. As RTA is a lifelong issue, prevention measures should always be followed.