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Renal Tubular Acidosis

Renal Tubular Acidosis
Renal tubular acidosis is characterized by non-gap metabolic acidosis caused by defects in acid excretion or bicarbonate reabsorption.
Distal Renal Tubular Acidosis (Type 1)
Associated with decreased hydrogen secretion in the distal nephron.
As a result of decreased hydrogen secretion, no new bicarbonate can be generated, which leads to acidosis.
Causes Include:
    • Amphotericin B toxicity
    • Analgesic nephropathy
    • Autoimmune diseases (SLE and Sjogren's)
    • Urinary tract obstructions
Signs and Symptoms:
    • Polydipsia and polyuria
    • Muscle weakness
    • Nephrolithiasis (due to hypercalciuria and hypocitraturia)
    • In children: growth retardation and rickets.
Urine and Serum Values:
    • Urine pH is relatively basic > 5.5
    • Serum bicarbonate is reduced, between 10-15 mmol/L
    • Serum potassium is reduced (hypokalemia).
Diagnosis:
Perform an ammonium load test and look for a positive urinary anion gap.
Treatment:
Administrate alkaline substances, such as sodium bicarbonate or potassium citrate, and thiazide diuretics.
Proximal renal tubular acidosis (Type 2 RTA)
Decreased bicarbonate reabsorption in the proximal tubule.
Causes Include:
    • Fanconi syndrome, in which proximal tubule malfunction leads to excess excretion of several solutes (glucose, bicarbonate, phosphate, uric acid, potassium, and some amino acids).
    • Carbonic anhydrase inhibitors, which are used to treat glaucoma, epilepsy, congestive heart failure, seizures, and other disorders.
    • Complication of multiple myelom, in which the excreted light chains are toxic to the proximal nephron cells.
Signs and Symptoms:
    • Muscle weakness
    • Growth retardation
    • Rickets
Urine and Serum Values:
    • Urine pH is variable; it is > 7 as long as plasma bicarbonate remains normal, but falls to < 5.5 when plasma bicarbonate is depleted.
    • Serum bicarbonate is somewhat low, 16-20 mmol/L.
    • Potassium is reduced.
Diagnosis:
Proximal RTA is indicated when a bicarbonate loading test produces Fractional Excretion of bicarbonate of greater than 15% and a urine pH of > 7.5.
Treatment:
Alkaline solutions and thiazide diuretics.
Hyperkalemic renal tubular acidosis (Type 4)
Characterized by aldosterone deficiencies or resistance in the distal and collecting tubules; this leads to hyperkalemia and reduced acid excretion in the form of NH4+ (ammonium).
Causes Include:
    • Diabetic and obstructive nephropathies, chronic interstitial nephritis, adrenal insufficiency
    • Several medications, including ACE inhibitors, angiotensin II receptor blockers (ARBs), potassium-sparing diuretics, and trimethoprim-sulfamethoxazole.
Signs and Symptoms:
    • Most patients are asymptomatic, but if hyperkalemia is severe, they may experience muscle weakness or cardiac arrythmias.
Urine and Serum Values:
    • Urine pH is low, < 5.5
    • Acidosis is milder with serum bicarbonate levels > 17 mmol/L.
    • Elevated serum potassium levels are key to differentiating this type of renal tubular acidosis from the proximal and distal forms.
Diagnosis:
Urinary potassium is less than 40 mmol/L and Fractional Excretion of potassium is less than 20%, both the result of potassium retention in this disorder.
Treatments:
Volume expansion and dietary potassium restriction with potassium-wasting diuretics.

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