Inherited Tubular Disorders in Children

Common Features of Inherited Tubular Disorders

  • The primary function of renal tubules is urine concentration. When tubules malfunction, urine concentration is impaired, leading to excessive urine output, known as polyuria.
  • Polyuria: Daily urine output exceeding 2 liters per meter squared (2 L/m²) of body surface area.
  • Polydipsia: Excessive thirst resulting in increased water intake due to polyuria.
  • Failure to Thrive: Impaired weight and height gain, often due to impaired nutrient reabsorption (glucose, amino acids) in the tubules.

Overview of Specific Disorders

  • Bartter Syndrome: Defect in the ascending limb of the loop of Henle.
  • Gittelman Syndrome: Defect in the distal convoluted tubule (DCT).
  • Liddle Syndrome: Defect in the collecting duct.

Specific Defects in Each Syndrome

  • Bartter Syndrome: Defect in the sodium-potassium-two chloride (Na-K-2Cl) cotransporter. This mimics the action of loop diuretics like furosemide.
  • Gittelman Syndrome: Defect in the sodium-chloride (Na-Cl) cotransporter, mimicking the action of thiazide diuretics.
  • Liddle Syndrome: Gain-of-function mutation in the gene coding for the epithelial sodium channel (ENaC), leading to increased channel activity.

Inheritance Patterns

  • Bartter Syndrome: Autosomal recessive
  • Gittelman Syndrome: Autosomal recessive
  • Liddle Syndrome: Autosomal dominant

Bartter Syndrome in Detail

  • Autosomal recessive disorder with onset in infancy (less than one year).
  • Characterized by loss-of-function mutations in genes coding for:
    • Sodium-potassium-chloride cotransporter
    • Renal outer medullary potassium (ROMK) channel
    • Chloride channel (Barttin)
  • Due to loss of function, these channels become non-functional.

Consequences of Channel Dysfunction

  • Loss of sodium, potassium, and chloride in the urine.
  • Sodium loss leads to water loss, causing dehydration and decreased intravascular volume.
  • This stimulates renin release, activating the renin-angiotensin-aldosterone system.
  • Aldosterone acts on collecting ducts, causing sodium reabsorption but also potassium secretion, further increasing potassium loss in urine.
  • Results in hypokalemic, hypochloremic metabolic alkalosis.
  • Impaired calcium absorption leads to hypercalciuria, potentially causing kidney stones.
  • Some cases (Type IV Bartter syndrome) may involve chloride channel defects in the cochlea, leading to sensorineural hearing loss.

Summary of Bartter Syndrome

  • Hypokalemic, hypochloremic metabolic alkalosis
  • Predisposition to kidney stones due to hypercalciuria
  • Potential for sensorineural hearing loss (Type IV)
  • Onset in infancy

Gittelman Syndrome in Detail

  • Onset typically in late childhood or adolescence.
  • Characterized by loss-of-function mutations in:
    • Sodium-chloride cotransporter
    • TRPM6 (transient receptor potential cation channel, subfamily M, member 6), involved in magnesium absorption.
  • Occurs in the distal convoluted tubule.

Consequences of Channel Dysfunction

  • Loss of sodium and chloride in urine.
  • Loss of magnesium in urine due to TRPM6 dysfunction, leading to hypomagnesemia.
  • Sodium loss leads to decreased intravascular volume, activating the renin-angiotensin-aldosterone system.
  • Results in sodium absorption and potassium excretion, leading to hypokalemia.
  • Also results in hypokalemic, hypochloremic metabolic alkalosis.

Summary of Gittelman Syndrome

  • Hypokalemic, hypochloremic metabolic alkalosis
  • Hypomagnesemia
  • Onset in late childhood or adolescence.

Comparison of Bartter and Gittelman Syndromes

  • Common Feature: Hypokalemic, hypochloremic metabolic alkalosis and normal blood pressure.
  • Differentiating Features:
    • Bartter Syndrome: Hypercalciuria (predisposition to stones)
    • Gittelman Syndrome: Hypomagnesemia

Treatment

  • Correction of electrolyte abnormalities (potassium, chloride supplementation).
  • Gittelman Syndrome: Additional magnesium supplementation.
  • Bartter Syndrome: Indomethacin (prostaglandin inhibitor) may be effective in some cases due to increased renal prostaglandin levels.

Liddle Syndrome in Detail

  • Autosomal dominant disorder.
  • Caused by gain-of-function mutations in genes (SCNN1D and SCNN1G) coding for the epithelial sodium channel (ENaC).
  • Increased activity of ENaC in the collecting duct leads to excessive sodium absorption.
  • Excessive sodium absorption increases blood sodium levels, leading to hypertension.
  • Increased sodium in the blood suppresses aldosterone levels.
  • This condition is called pseudo-hyperaldosteronism because it mimics the effects of increased aldosterone (sodium retention, hypertension) but aldosterone levels are low.
  • Excessive sodium absorption also causes potassium secretion and loss in the urine, resulting in hypokalemic alkalosis.

Summary of Liddle Syndrome

  • Hypokalemic alkalosis
  • Hypertension
  • Low aldosterone levels (pseudo-hyperaldosteronism)

Treatment

  • Amiloride, an epithelial sodium channel blocker, is the treatment of choice.

Summary of Inherited Tubular Disorders

  • Common features: Polyuria and failure to thrive.
  • All conditions: Hypokalemic alkalosis.
  • Bartter Syndrome and Gittelman Syndrome: Normal blood pressure.
  • Liddle Syndrome: Increased blood pressure.
  • Differentiating features:
    • Bartter Syndrome: Hypercalciuria and stone formation.
    • Gittelman Syndrome: Hypomagnesemia.