Renal tubular acidosis is a
metabolic acidosis due to impaired
acid excretion by the kidney. Hyperchloraemic
acidosis with a normal anion gap and normal (or near normal) glomerular filtration rate, and in the absence of diarrhoea, defines this disorder. However, systemic
acidosis is not always evident and
renal tubular acidosis can present with hypokalaemia, medullary
nephrocalcinosis and recurrent
calcium phosphate stone disease, as well as growth retardation and
rickets in children, or short stature and
osteomalacia in adults. Renal dysfunction in
renal tubular acidosis is not always confined to
acid excretion and can be part of a more generalised renal tubule defect, as in the
renal Fanconi syndrome. Isolated
renal tubular acidosis is more usually acquired, due to drugs,
autoimmune disease, post-obstructive uropathy or any cause of medullary
nephrocalcinosis. Less commonly, it is inherited and may be associated with
deafness,
osteopetrosis or ocular abnormalities. The clinical classification of
renal tubular acidosis has been correlated with our current physiological model of how the nephron excretes
acid, and this has facilitated genetic studies that have identified mutations in several genes encoding
acid and base ion transporters. In vitro functional studies of these
mutant proteins in cell expression systems have helped to elucidate the molecular mechanisms underlying
renal tubular acidosis, which ultimately may lead to new therapeutic options in what is still treatment only by giving an oral
alkali.