**4.1 Bartter's syndrome**

92 Basic Nephrology and Acute Kidney Injury

from potassium-sparing ones, cause increased urinary losses of potassium. Magnesium depletion may also lead to renal potassium wasting. A 24 h urine collection can be used to assess renal potassium excretion in a hypokalemia patient. This should be < 15 mmol/24 h if there is extra renal potassium wasting. In a similar way, a spot urine for a potassium/creatinine ratio should be less than 1 in the presence of extra renal potassium wasting. Calculations of transtubular potassium gradient will give similar information. Urinary chloride will also be low in cases of significant GI volume losses (vomiting, diarrhoea, laxative abuse). If renal K wasting is suspected and confirmed then further thought regarding the blood pressure and acid base status of the patient aids the diagnosis

of hypokalemia**.** 

Hereditary

Table 1. Causes of extra-renal hypokalemia

distal renal tubular acidosis.

Extra-renal Hypokalemia

Spurious Extra-renal Loss Redistribution

spherocytosis GI fistulas Insulin excess

Dietary insufficiency

High WBC count Diarrhoea Acid-base disturbance

Cutaneous losses

Reasons to suspect an inherited tubulopathy may include evidence for a persistent electrolyte disturbance, the presence of renal impairment, nephrocalcinosis and renal stone formation (Sayer &Pearce, 2001). A detailed and extensive family history is necessary. In paediatric cases, faltering growth, abnormal growth patterns, developmental delay and deafness may all be clues to an inherited tubulopathy. Neonates may present with a saltwasting crisis and are particularly sensitive to severe hypovolemia and electrolyte disturbance due to immature tubular physiology and low-salt intakes in standard feeds. With nephron maturation, the propensity to present with salt-wasting crises decreases. Individual tubular diseases can be differentiated by their serum and urine biochemical profiles (discussed below). Hypokalemia may be the only presenting feature of a tubulopathy and it is important to follow the diagnostic 'lead' to differentiate the many causes. Nephrocalcinosis should be thoroughly investigated and differential diagnosis such as hyperparathyroidism, vitamin D intoxication and sarcoidosis first excluded. Renal tubular disorders associated with nephrocalcinosis include Bartter's syndrome, Dent's disease, hypomagnesemic hypercalciuric nephrolithiasis, idiopathic hypercalciuria and

Renal tubulopathies are best investigated by urine pH and 24-hour urine collection for potassium, calcium, magnesium, citrate and phosphate along with serum biochemistry. Nevertheless, renal tubulopathies are rare and they should be considered only after

Laxative abuse Beta-adrenergic agonists

catecholamines

thyrotoxic)

Drugs / toxins e.g. Barium,

Hypokalemic periodic paralysis (familial and Bartter's syndrome is an autosomal recessive renal disorder described by Bartter et al. in 1962 (Bartter, et al., 1962). It has estimated incidence of 1.2 per million (Rudin, 1988). Impaired salt (sodium chloride) reabsorption in the thick ascending loop of Henle (TAL) leads to renal salt wasting and a hypokalemic metabolic alkalosis. The majority of cases present in the early neonatal period with salt-losing crises. An antenatal diagnosis can also be suspected if a pregnancy is complicated by polyhydramnios or premature birth (Sieck &Ohlsson, 1984). Bartter's syndrome is also associated with short stature, growth retardation in infancy, muscle weakness, polyuria and polydipsia. Some children have characteristic facies that are triangular-shaped with a prominent forehead, large eyes, protruding ears and a drooping mouth. Blood pressure is normal. Bartter's syndrome should be suspected in patient presenting with the above symptoms and signs and the following laboratory findings: hypokalemic alkalosis, high urinary chloride and urinary potassium levels and normal or raised urinary calcium level. Serum magnesium levels are typically normal or mildly low.

The biochemical abnormalities are a consequence of renal salt wasting in the TAL. This stimulates the renin-angiotensin II-aldosterone system (RAAS) and causes hyperplasia of juxtaglomerular apparatus, a feature originally noted by Bartter (Bartter, et al., 1962). Raised RAAS increases sodium reabsorption in the distal nephron (via ENaC) in exchange for K+ and H+, which leads to hypokalemic alkalosis. Increases in prostaglandin E2 synthesis aggravate the salt wasting by its effect on ROMK1 and NKCC2. Despite raised RAAS, leading to hyperreninemia and hyperaldosteronism, patients with Bartter's syndrome remain normotensive. The various phenotypes of Bartter's syndrome are now more simply classified by their underlying genetic mutation.

Antenatal Bartter's syndrome (Type 1) is caused by homozygous or compound heterozygous mutation in the *SLC12A1* gene, encoding NKCC2 (Simon, et al., 1996a). NKCC2 is kidney specific, electroneutral transporter protein, located at the apical membrane of the TAL (Simon, et al., 1996a).

Type 1 is a severe form of Bartter's syndrome which may present in utero with marked polyhydramnios and premature birth. Amniocentesis shows high chloride (and aldosterone) levels. Analysis of a pregnant mother's urine may also suggest the diagnosis, demonstrating low Na+, Cl- and Ca2+ (Matsushita, et al., 1999). A definitive diagnosis may be made using mutational analysis of DNA from amniocytes (Konrad, et al., 1999).

Prenatal diagnosis is important as indomethacin may be a useful treatment for polyhydramnios (Smith et al., 1990). and early neonatal treatment may be life saving. Neonates typically present with severe salt-wasting crises, hypokalemic alkalosis, vomiting and diarrhoea. The latter two symptoms are due to renal activation of prostaglandin synthesis, as a consequence of hypokalemia. A feature that distinguishes this type from others is marked hypercalciuria, which causes nephrocalcinosis and osteopenia in infancy. Treatment is with potassium supplements, often combined with potassium-sparing diuretics (such as spironolactone and amiloride) and inhibitors of

Renal Potassium Handling and Associated Inherited Tubulopathies Leading to Hypokalemia 95

2002,Watanabe, et al., 2002). The mechanism leading to this phenotype is thought to be constitutive activation of the mutant CASR, located on the basolateral membrane of the TAL, by normal serum calcium levels, leading to a secondary inhibition of sodium chloride transport in the TAL, thus mimicking Bartter's syndrome. Of note, the degree of metabolic

Pseudo antenatal Bartter's syndrome has been reported in a preterm child with cyanotic heart disease treated with high dose prostaglandins (Langhendries, et al., 1989). The biochemical phenotype of Bartter's may also be mimicked by loop diuretic abuse. Hypokalemic metabolic alkalosis may also be seen in patients with cystic fibrosis, bulimia and laxative abuse. In such cases, urinary chloride levels are low, given the salt wasting is

Gitelman's syndrome refers to an autosomal recessive congenital condition, which is characterized by a hypokalemic alkalosis with hypocalciuria and often hypomagnesaemia (Gitelman, et al., 1966). Gitelman's syndrome has incidence of 1:40,000, which makes it one of the commonest inherited tubulopathies. The electrolyte disturbance mimics that of chronic thiazide diuretic use. Patients with this syndrome have genetic defect in the *SLC12A3* gene encoding the thiazide-sensitive sodium-chloride cotransporter (NCCT) (Simon, et al., 1996a). This cotransporter is located in the apical membrane of distal convoluted tubular (DCT) cells. Defects in NCCT result in reduced sodium reabsorption in the DCT, leading to increased delivery of sodium to the CCD. This leads to increased absorption of sodium via ENaC, coupled with K excretion, leading to hypokalemia. Subsequent stimulation of K reabsorption via H+K+ATPase in intercalated cells results in a metabolic alkalosis. Within the DCT, transcellular absorption of magnesium (via the apical magnesium channel TRPM6 and a putative basolateral Na+/Mg2+ exchanger) is also reduced. The exact mechanism of hypocalciuria has been the subject of speculation for some time. Evidence from studying thiazide treatment in murine models now suggests that as result of volume contraction enhanced proximal tubular sodium reabsorption occurs, and with it, an increase in proximal tubular paracellular absorption of calcium (Nijenhuis,

Gitelman's syndrome is often asymptomatic into adult life, presenting with weakness, paraesthesia, fatigue and tetany. Salt craving may be a feature. Typically the patients are normotensive and may have polyarthritis and chondrocalcinosis secondary to severe and longstanding hypomagnesaemia (Cobeta-Garcia, et al., 1998). Clinical diagnostic features for Gitelman's syndrome include a low urinary calcium:creatinine ratio (typically <0.2), low serum magnesium (<0.65 mmol/L) and a hypokalemic metabolic alkalosis, once thiazide use is ruled out. Surreptitious ingestion of thiazide may be ruled out by screening the urine. Cisplastin nephrotoxicity may resemble Gitelman's syndrome as may some of the magnesium wasting tubulopathies (Knoers et al., 2003) discussed below. Treatment is with lifelong potassium and magnesium supplements and diet rich in sodium and potassium. Amiloride and spironolactone may also be useful treatments to ensure maintained serum potassium levels. Occasionally, a severe childhood onset phenotype

alkalosis in these patients was mild (Sayer &Pearce, 2003).

**4.2 Pseudo Bartter's syndrome** 

not secondary to a tubular defect.

**4.3 Gitelman's syndrome** 

et al., 2005).

prostaglandin-stimulated renin release (such as indomethacin and specific cyclooxygensae (COX-2) inhibitors).

Antenatal Bartter's syndrome (Type 2) is clinically indistinguishable from Type 1. It is caused by loss-of-function mutations in the *KCNJ1* gene (alias *ROMK1*) encoding the ATPsensitive potassium channel ROMK1, also located apically in the TAL (Simon, et al., 1996b). ROMK1 allows potassium recycling from the TAL cells back to the luminal filtrate, and is electrogenic driving paracellular reabsorption of sodium, calcium and magnesium. ROMK1 is a regulator of the NKCC2 co-transporter and functional coupling of ROMK1 with NKCC2 is essential for effective NaCl reabsorption. Therefore, functional defects in the ROMK1 protein severely disrupt electrogenic chloride reabsorption in TAL, resulting in a similar antenatal phenotype, although the hypokalemia may be less severe (Kurtz, 1998). The treatment of Bartter's syndrome Type 2 is the same as for Bartter's syndrome Type 1.

Classic Bartter's syndrome (Type 3) is caused by mutations in *CLCNKB* encoding the CLC chloride channel member CLC-KB. This channel protein is located on the basolateral membrane of TAL cells, where it co-localizes with Barttin. The presentation of Type 3 / Classic Bartter's syndrome tends to be with weakness and hypovolemia during early childhood, with a milder defect of urinary concentrating ability and normal urinary calcium levels. Hence, nephrocalcinosis or nephrolithiasis is rarely a feature. Severe and chronic hypokalemia may lead to medullary cyst formation (Ariceta &Rodriguez-Soriano, 2006). Hyperuricemia may occur due to volume contraction. Renal function is usually preserved initially but may decrease as a result of chronic hypokalemia and tubulointerstitial damage. The primary defect alters chloride reabsorption, with defective basolateral exit of chloride via CLC-KB in the TAL. Clinical features in Type 3 Bartter's syndrome include short stature and salt wasting. However, *CLCNKB* mutations may present with variable phenotype, ranging from neonatal salt losing crises to asymptomatic patients detected in adulthood by routine electrolyte testing (Konrad, et al., 2000). A milder phenotype may be confused with Gitelman's syndrome. Indeed, some patients with *CLCKNB* mutations have profound hypomagnesaemia and hypocalciuria, closely mimicking Gitelman's syndrome (Jeck, et al., 2000,Konrad, et al., 2000).

Bartter's syndrome Type 4A is a form of infantile Bartter's syndrome caused by a mutation in the gene *BSND* encoding Barttin (Estevez, et al., 2001). Barttin is two-transmembrane protein and an essential subunit of the CLC chloride channels CLC-KB and CLC-KA. Barttin modulates both membrane insertion and function of both CLC-KA and CLC-KB (Scholl, et al., 2006,Waldegger, et al., 2002). In the kidney Barttin is expressed in the thin limb and the TAL of Henle. This type of Bartter's syndrome is associated with congenital sensorineural deafness (termed BSND) which may explained by the localisation of Barttin protein as a subunit for CLC-KA in inner ear cells. Again, the phenotype can be severe neonatal salt wasting or a more mild adult presentation (Miyamura, et al., 2003).

Type 4B Bartter's syndrome is associated with simultaneous mutations in both chloride channel genes, *CLCNKA* and *CLCNKB* resulting in sensorineural deafness and renal salt wasting. This form of Bartter's syndrome is rare but should be considered when BSND mutations are not detected. Children from consanguineous parents with homozygous mutations in both genes (Schlingmann, et al., 2004) and non consanguineous parents with digenic compound heterozygous mutations have been described (Nozu, et al., 2008).

Bartter's syndrome Type 5 is better known as autosomal dominant hypocalcaemia with Bartter'syndrome. Mutations in the *CASR* gene encoding the Calcium-sensing receptor can occasionally be associated with a Bartter's like phenotype (Vargas-Poussou, et al., 2002,Watanabe, et al., 2002). The mechanism leading to this phenotype is thought to be constitutive activation of the mutant CASR, located on the basolateral membrane of the TAL, by normal serum calcium levels, leading to a secondary inhibition of sodium chloride transport in the TAL, thus mimicking Bartter's syndrome. Of note, the degree of metabolic alkalosis in these patients was mild (Sayer &Pearce, 2003).
