Renal tubular acidosis

Patients with renal tubular acidosis (RTA) are often asymptomatic but may present with complaints of muscular weakness related to associated hypokalaemia.

Patients with severe acidaemia can show hyperventilation or Kussmaul's breathing due to respiratory compensation.

Patients with RTA have a low arterial pH and serum bicarbonate with hyperchloraemia and a normal serum anion gap.

The urine pH exceeds 5.5 in classic distal RTA, but is lower than 5.0 in patients with untreated proximal RTA and is low also in hyperkalaemic distal RTA.

Alkali therapy is the mainstay of treatment.

Potassium supplementation may be required for hypokalaemia, and low-potassium diets are used if hyperkalaemia is present.

If hyperkalaemic distal RTA is due to mineralocorticoid deficiency, fludrocortisone can be given unless it is contraindicated due to the presence of fluid overload or uncontrolled hypertension.

Proximal RTA occurs most often as a component of Fanconi's syndrome, which is characterised by generalised dysfunction of the proximal tubule, with the resultant urinary loss of bicarbonate, calcium, phosphate, urate, amino acids, glucose, and other organic acids and bases.

In children, Fanconi's syndrome causes growth retardation, renal rickets, and severe metabolic acidosis. Adult cases exhibit similar urinary losses, but the clinical impact is largely restricted to metabolic acidosis.

Fanconi's syndrome is marked by the appearance in the urine of all amino acids. Specific amino aciduria as seen in isolated cystinuria, glucose loss in isolated glycosuria, and isolated phosphaturia do not constitute Fanconi's syndrome.

The term renal tubular acidosis (RTA) describes any one of a number of disorders, in which the excretion of fixed acid (distal RTA) or the reabsorption of filtered bicarbonate (proximal RTA) is impaired to a degree that is disproportionate to any existing impairment of the glomerular filtration rate.[1]Batlle D, Moorthi KM, Schluter W, et al. Distal renal tubular acidosis and the potassium enigma. Semin Nephrol. 2006 Nov;26(6):471-8. http://www.ncbi.nlm.nih.gov/pubmed/17275585?tool=bestpractice.com The acid retention or bicarbonate loss results in the development of hyperchloraemic metabolic acidosis marked by hypobicarbonataemia and depressed arterial blood pH. In the absence of other acid-base disorders the serum anion gap is normal. Either hypokalaemia or hyperkalaemia may be present, depending on the nature of the acidification defect.

Fanconi's syndrome is characterised by a generalised dysfunction of the renal proximal tubule that results in the urinary loss of substances normally reabsorbed by the kidney at this site. The substrates lost include bicarbonate, glucose, amino acids, phosphate, small proteins and peptides, and organic acids and bases. Salt wasting, volume depletion, and potassium wasting often develop as secondary effects. The pathophysiological basis of these abnormalities depends upon the specific cause of the individual patient’s Fanconi's syndrome.