Renal tubular acidosis is a rare kind of kidney disorder that occurs due to the inability of the kidneys to excrete acids from the blood into the urine normally. This leads to 

disruption of the normal acid-base balance, increased acid levels in the blood and a condition called metabolic acidosis. 

Renal tubular acidosis is of four types— type 1 or distal renal tubular acidosis, type 2 or proximal renal tubular acidosis, type 3 or mixed renal tubular acidosis, and type 4 or hyperkalemic renal tubular acidosis. Type 1 or distal renal tubular acidosis leads to nephrolithiasis or the formation of kidney stones, apart from causing other symptoms. 

Nephrolithiasis in type 1 renal tubular acidosis

• Untreated distal or type 1 renal tubular acidosis often results in hypocitraturia, nephrolithiasis, increased urinary pH, hypercalciuria, and bone abnormalities.

• There are several factors that contribute to nephrolithiasis or stone formation in distal renal tubular acidosis. The main reason is the release of increased calcium phosphate from the bone when the body tries to buffer the retained acid. The higher the degree of acidemia, the higher the calcium levels and the higher the chances of stone formation. 

• Distal RTA is characterised by extremely low citrate levels and alkaline urine, both of which increase the risk of developing kidney stones. The patients should ideally be under the treatment of a nephrologist or a urologist. Distal RTA should be excluded in any patient who presents with calcium phosphate kidney stones, increased urinary pH ( early morning pH>5.5), and decreased urinary citrate. 

• In distal or type 1 RTA, H+ ions are not excreted effectively along the distal part of the nephron, including the distal tubule and collecting duct. This leads to the formation of alkaline urine and calcium phosphate precipitation and stones. 

• Other factors include increased intestinal calcium absorption, persistently increased urinary pH, decreased citrate excretion, and reduced reabsorption of certain ions from the tubules. Increased urinary pH for a long time leads to precipitation of calcium phosphate. 

• The histopathological features of kidney stones formed due to distal renal tubular acidosis show calcium phosphate deposition on the Bellini ducts and inner medullary layer, which leads to fibrosis. Even areas that are not plugged with calcium phosphate show interstitial fibrosis. 

• Proximal renal tubular acidosis does not lead to kidney stones.