Type 1 (Distal) RTA
Type 1 or distal RTA is referred to as the classic RTA and is a disorder of acid excretion involving the collecting tubules. The disorder is characterized by a hypokalemic, hyperchloremic metabolic acidosis.
The disorder is due to defective H+ ion secretion in the distal tubule. Impairment in H+ ions secretion result in an inability to acidify the pH beyond 5.5 which retards the excretion of titratable acids (H2PO4) and NH4+ ions, thus resulting in a reduction in net acid excretion. The plasma bicarbonate is significantly reduced and may fall below 10 meq/L.
The impairment in H+ ions secretion is most commonly thought to be due to a defect in the luminal H+ -ATPase pump located in the intercalating cells of the collecting tubule. The H+ -ATPase pump is primarily responsible for the hydrogen secretion in the distal nephrons.
These patients tend to have urinary K+ wasting and hypokalemia. The etiology of the hypokalemia is unclear but is thought to be due to increased potassium secretion by distal tubular cells in the setting of diminished H+ ion secretion.
Hypercalciuria, hyperphosphatemia, nephrolithiasis (calcium phosphate stones) and nephrocalcinosis are frequently associated with untreated type 1 RTA. The hypercalciuria is thought to be due to 1) increased calcium phosphate release from bone as a result of bone buffering of excess acid and 2) reduction in tubular calcium reabsorption secondary to chronic acidosis. The hypercalciuria, alkaline urine, and reduced excretion of citrate in the urine (which normally prevents calcium crystallization) promote the precipitation of calcium phosphate and stone formation. The hypocitraturia is thought to be due to the effects of acidosis and hypokalemia on proximal tubule reabsorption.
Many different conditions have been associated with type I RTA. See table below. The most common identifiable causes in adults are autoimmune disorders, such as Sjogren’s syndrome . In children, RTA is most often a primary hereditary condition.
The loss of calcium salts from bones in these patients can result in failure to thrive, rickets and stunting of growth in children and osteomalacia or osteopenia in adults. Patients may otherwise be asymptomatic or may present with symptoms of severe acidosis or hypokalemia (polyuria, polydypsia, weakness and fatigue)
Major causes of distal RTA
Idiopathic or sporadic in children