Bartter Syndrome: Description, Causes and Risk Factors:
Bartter syndrome is an inherited renal tubular disorder with hypokalemia
, hypochloremic metabolic alkalosis, normal blood pressure with hyperreninemia and increased urinary loss of sodium, potassium and chloride.
The cause is unknown. Currently, the disorder is best explained as a primary defect in chloride reabsorption in the ascending limb of the loop of Henle. The resultant decrease in sodium chloride reabsorption in this portion of the loop reduces medullary hypertonicity, perhaps explaining the concentrating defect. The defect in chloride reabsorption presents extra sodium chloride to the distal tubule, where sodium is reabsorbed in exchange for potassium; the result is urinary potassium wasting. The induced hypokalemia stimulates the synthesis of prostaglandins (which may account for the vascular insensitivity to pressor agents and the defect in platelet aggregation); these, in turn, activate the renin-angiotensin-aldosterone system by increasing renin release and by stimulating aldosterone synthesis. The latter exacerbates renal potassium wasting.
The inheritance of Bartter syndrome is autosomal recessive with 3 subtypes. The neonatal type is characterized by fever, diarrhea, vomiting, osteopenia and elevated urinary excretion of prostaglandin E. In neonatal Bartter syndrome type 1, frame shift, nonsense and missense mutations occur in the gene located on chromosome 15q15-q21 encoding for bumetanide sensitive Na-K-2Cl cotransporter (NKCC2) of thick ascending loop of Henle. Mutations in ROMK gene located on chromosome 11q24-25 encoding adenosine triphosphate (ATP) sensitive K+ channels, that recycles reabsorbed K back into tubular lumen, results in neonatal form of Bartter syndrome type 2. In the third type of neonatal Bartter syndrome, there is associated sensorineural deafness and early chronic renal insufficiency due to mutations in gene BSND localised to chromosome 1p31, encoding for Barttin which is expressed in the ascending limb of Henle and inner ear.
The neonatal form differs from the classic Bartter syndrome by the age of onset, presence of nephrocalcinosis and very high urinary loss of sodium, calcium and chloride. Other differential diagnoses are Gitelman's syndrome (characterized by hypomagnesemia, hypocalciuria), pseudohyperaldosteronism (hypertension with no evidence of increased secretion of mineralocorticoids) and pseudoBartter syndrome due to administration of high doses of prostaglandin E1(6).
Estimates of prevalence of Bartter Syndrome vary from country to country. In Costa Rica, the frequency of neonatal Bartter syndrome is approximately 1.2 cases per 100,000 live births and is higher if all preterm births are considered. No evidence of consanguinity was found in the Costa Rican cohort. In Kuwait, the prevalence of consanguineous marriages or related families in patients with Bartter syndrome is higher than 50%, and prevalence in the general population is 1.7 cases per 100,000 persons. In Sweden, the frequency has been calculated as 1.2 cases per 1 million persons. Of the 28 patients Rudin reported, 7 came from 3 families; the others were unrelated.
The long-term prognosis of Bartter syndrome is uncertain. Many patients remain well, but some cases (especially those with glomerular or interstitial abnormalities) progress to renal insufficiency. Despite severe growth retardation in infancy, normal stature is ultimately obtained. The suggestion that mental retardation occurs in patients who have severe disease in the 1st yr of life remains to be confirmed.
This disease usually occurs in childhood. Symptoms may include:
- Growth failure.
- Increased frequency of urination.
- Low blood pressure.
- Kidney stone.
- Muscle cramping and weakness.
The diagnosis is suggested by the finding of hypokalemia; the serum potassium level is usually less than 2.5 mEq/L. Supportive findings include normal blood pressure; defective platelet aggregation; hypochloremia; metabolic alkalosis; elevated plasma levels of renin, aldosterone, and prostaglandin E2; and high urinary levels of potassium and chloride. Some patients may also have hypercalciuria, hyperuricemia, hypomagnesemia, and urinary sodium wasting. The diagnosis may be confirmed by the histologic demonstration of hyperplasia of the juxtaglomerular apparatus, but this abnormality is not found in all patients and is frequently absent in young children.
Prenatal diagnosis can be made by the highchloride content of the amniotic fluid andmutational analysis of genomic DNAextracted from cultured amniocytes obtainedby amniocentesis.
Bartter syndrome is treated by keeping the blood potassium level above 3.5 mEq/L. This is done by following a diet rich in potassium or taking potassium supplements. Many patients also need salt and magnesium supplements, as well as medicine that blocks the kidney's ability to get rid of potassium. High doses of nonsteroidal anti-inflammatory drugs (NSAIDs) may also be used.
The goals of therapy are to supply adequate nutrition and to maintain the serum potassium level above 3.5 mEq/L. Therapy is initiated with oral potassium chloride supplementation, increasing the dose until the serum potassium level reaches 3.5 mEq/L or the dosage reaches 250 mEq/24 hr. Reasonably well tolerated potassium preparations include K-Lyte/Cl (Mead Johnson Company, Evansville, IN), flavored effervescent tablets containing 25 or 50 mEq of potassium chloride, and Micro-K 10 Extencaps (A.H. Robins Company, Richmond, VA). Sodium chloride supplementation may also be required in small children. If the serum potassium level remains below 3.5 mEq/L (mmol/L) after reaching a dose of 250 mEq/24 hr of potassium chloride, then triamterene, 5-10 mg/kg/24 hr in divided doses, should be added. If this fails to resolve the hypokalemia, then indomethacin, 3-5 mg/kg/24 hr divided into three doses, should be given. Patients receiving indomethacin should be monitored for signs of gastrointestinal irritation.
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DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.