Renal Tubular Acidosis

• Renal tubular acidosis (RTA) involves metabolic acidosis due to pathology in the tubules of the kidneys.
• The tubules balance hydrogen (H+) and bicarbonate ions (HCO3–) between the blood and urine to maintain a normal pH.
• There are four types of renal tubular acidosis, with different pathophysiology.
• Renal tubular acidosis type 1 involves a pathology in the distal tubule that prevents it from excreting hydrogen ions, resulting in high urinary pH and low serum potassium.
• Renal tubular acidosis type 2 involves a pathology in the proximal tubule that prevents it from reabsorbing bicarbonate, resulting in high urinary pH and high serum potassium.
• Renal tubular acidosis type 3 involves low aldosterone or impaired aldosterone function, resulting in high urinary pH and high serum potassium.
• Renal tubular acidosis type 4, the most common type, involves inadequate hydrogen excretion in the distal tubules, with both type 1 and 4 involving inadequate hydrogen excretion.
• In type 1 renal tubular acidosis, there is hypokalaemia.
• In type 4 renal tubular acidosis, there is hyperkalaemia.
• Type 2 Renal Tubular Acidosis (or proximal RTA) occurs when the proximal tubule cannot reabsorb bicarbonate from the urine to the blood, with excessive bicarbonate excreted in the urine.
• Treatment for renal tubular acidosis includes oral bicarbonate, which corrects the acidosis and electrolyte imbalances.
• Low aldosterone or impaired aldosterone function leads to insufficient potassium and hydrogen ion excretion.
• Low aldosterone or low aldosterone activity can be due to adrenal insufficiency, diabetic nephropathy, or medications.
• Normally, ammonia is produced in the distal tubules to balance the excretion of hydrogen ions.
• Hypokalaemia, due to failure of the hydrogen and potassium exchange (H+/K+ ATPase), is a symptom of renal tubular acidosis.
• Type 3 Renal Tubular Acidosis (or mixed RTA) is a combination of type 1 and 2, with pathology in the proximal and distal tubules.
• Many conditions can cause renal tubular acidosis, including genetic, systemic lupus erythematosus, Sjögren’s syndrome, primary biliary cholangitis, hyperthyroidism, sickle cell anaemia, Marfan’s syndrome.
• Type 4 Renal Tubular Acidosis (or hyperkalaemic RTA) is caused by reduced aldosterone.
• High urinary pH (above 6) due to the absence of hydrogen ions is a symptom of metabolic acidosis.
• Renal tubular acidosis presents with failure to thrive in children, recurrent UTIs, bone disease (rickets or osteomalacia), muscle weakness, arrhythmias, and hypokalaemia.
• Management of hyperkalemia includes fludrocortisone (a mineralocorticoid steroid) and oral bicarbonate.
• Hyperkalemia suppresses ammonia production, so the urine becomes acidotic.
• Aldosterone stimulates sodium reabsorption and potassium and hydrogen ion excretion in the distal tubules.