Bartter Syndrome Presenting as Arginine-Vasopressin Resistance: A Report of 2 Cases

Introduction

Bartter syndrome (BS) is an autosomal recessive, inherited salt-losing tubulopathy with a prevalence of 1: 1 000 000 [1]. It is a life-threatening condition that requires an expedite diagnosis and treatment. Five subtypes of BS have been identified so far, with different phenotypes and severity depending on the gene mutated [2].

The primary molecular defect results in impaired salt reabsorption in the thick ascending limb of Henle’s loop, leading to polyuria, dehydration, metabolic alkalosis, hypokalemia, and, depending on the affected gene, hypercalciuria and nephrocalcinosis [3]. A history of polyhydramnios, prematurity, failure to thrive, and dysmorphic features (triangularly shaped face, prominent forehead, protruding ears, and drooping mouth) are the most common clinical features [3]. Several previously published reports of BS have been uncovered after investigation of apparent arginine-vasopressin (AVP) resistance [2] due to polyuria, hypernatremia, and failure to respond in water deprivation test; however, distinctively, patients with BS usually do not have urinary osmolalities <100 mosmol/kg H2O. We report 2 challenging, additional cases of BS type 1 (OMIM # 601678) and type 2 (OMIM # 241200) presenting with apparent AVP insufficiency/resistance, one of them with urinary osmolalities <100 mosmol/kg H2O.

Case Reports

CASE 1:

A 9-month-old (corrected age: 7-month-old) male infant was admitted to the pediatric ward due to incoercible vomiting, without diarrhea or fever. He had dehydration, metabolic alkalosis, and hypernatremia. Despite dehydration, there was a great number of urine-soaked diapers changed during the patient’s stay in the Emergency Department. He had a past history of prematurity (gestation: 28 weeks) due to polyhydramnios, with several associated morbidities: very low birth weight (1020 g), hyaline membrane disease type II, transient renal failure, intraperiventricular hemorrhage, hemorrhagic stroke, hydrocephaly with implantation of a ventriculoperito-neal shunt, and severe retinopathy with blindness. Failure to thrive and global development delay were noted since birth. Physical examination was remarkable for protuberant forehead, triangular-shaped face (Figure 1, for which informed consent was obtained from the legal representatives), severe short stature (height: −6.56 SD; Figure 2A), and emaciation (body mass index [BMI]: −3.4 SD; Figure 2B). Polyuria was confirmed with a 24-h urine volume of 2.4 L/m2. After initial stabilization with intravenous fluids, the patient resumed oral hydration for further surveys. Biochemical analysis revealed high plasma osmolality and hyposthenuria. AVP insufficiency/resistance was suspected, and a short desmopressin test was performed (overnight fast; desmopressin 20 μg given intranasal; plasma/urine collected at baseline, and urine collected hourly after desmopressin, for 4 h) [4] and showed a urinary concentration of more than 50% from baseline, but a urinary osmolarity to plasma osmolarity ratio <1.5. A test of oral desmopressin was attempted for 7 days, unsuccessfully (urine volume: 2.34 L/m2). A broad revision of the case showed, additionally, hypokalemia, persistent metabolic alkalosis, high PTH, hyper-calcemia, and hyperphosphatemia. Further surveys revealed secondary hyperaldosteronism, hypercalciuria, high chloride fractional excretion (Table 1) [5], and type 2 nephrocalcinosis on renal ultrasound (Figure 3). Genetic screening by a panel of next-generation sequencing including BS-associated genes (BSND, CLCNKA, CLCNKB, KCNJ1, SLC12A1, SLC12A3) revealed a previously published [6] homozygotic pathogenic variant in SLC12A1, c.1432G>A, p.(Gly478Arg) (Figure 4A), diagnostic of BS type 1. His parents were heterozygous carriers of this pathogenic variant (Figure 4B), and after genetic counseling were referred to the fertility clinic for medical-assisted reproduction. Indomethacin was initiated (1 mg/kg/day, divided in 3 doses) and titrated to 5 mg/kg/day, which led to resolution of all presenting symptoms (urinary volume: 1.71 L/m2) and a significant improvement in most of electrolytic imbalances (Table 1) [5]. There was also an improvement in his auxologic parameters (height: −4.61 SD; BMI −2.65 SD; Figure 2A, 2B), although they remained in the severe range, despite titration of indomethacin above the recommended dose.

CASE 2:

A 6-year-old boy was referred to a pediatric endocrinology appointment due to suspected short stature. His mother reported a salt craving pattern, polydipsia, polyuria, and enuresis, leading to suspected AVP insufficiency/resistance. He had a history of polyhydramnios since the gestational age of 22 weeks, and delivery was carried out at 34 weeks of gestation, due to placental abruption. He was admitted to the Neonatal Intensive Care Unit due to perinatal asphyxia, transient acute renal failure, hyponatremia, hyperkalemia, symptomatic hypoglycemia, and neonatal apnea. A presumptive diagnosis of pseudohypoaldosteronism type 1 was made, and supportive therapy consisting of a salt-rich diet and water ad libitum was recommended. Physical examination revealed a triangular face, abducted ears, and a drooping mouth. He had a stature of 114.7 cm (−1.01 SD; target height: 179.3 cm, +0.5 SD; Figure 5A) and a BMI of 14.1 kg/m2 (−1.36 SD; Figure 5B). He was admitted to the ward for reliable 24-h urinary samples and further investigations of AVP insufficiency/resistance. Marked polyuria (4.7 L/m2) was shown, as was low urinary with high plasma osmolalities (Table 1) [5]. Blood tests showed hypokalemia, metabolic alkalosis, hyperreninemic hyperaldosteronism, hypercalciuria, chloride renal wasting, and isosthenuria (Table 1) [5]. Renal ultrasound ruled out nephrocalcinosis. Upon a broader clinical review of all findings, the likelihood of BS was deemed high, and further investigations of AVP insufficiency/resistance were not performed. Genetic screening by next-generation sequencing of BS-associated genes (BSND, CLCNKA, CLCNKB, KCNJ1, SLC12A1, SLC12A3) revealed compound heterozygosity for pathogenic variants c.139G>A, p.(Gly47Arg) and c.389C>T, p.(Ser130Leu) of KCNJ1, confirming BS type 2 (Figure 4C). The variant c.389C>T, p.(Ser130Leu) is novel (gnomAD allele frequency: not reported) and in silico analysis (REVEL: “deleterious”; metaLR: “deleterious”; MutationAssessor: “disease”; MutationTaster: “medium deleterious probability”; Franklin: “deleterious”) predicted pathogenicity, whereas the variant c.139G>A, p.(Gly47Arg) was previously reported [7]. His parents denied genetic screening as they did not desire further pregnancies. He started on indomethacin (1 mg/kg/day), which led to resolution of all symptoms and a catch-up growth toward his target height (−0.27 SD; Figure 5A). A parallel improvement was seen in biochemical results (Table 1) [5], except for hypokalemia and hyperparathyroidism, which occurred in the follow-up.

Discussion

We presented 2 cases resembling AVP insufficiency/resistance that uncovered BS types 1 and 2 upon a broader clinical revision.

The furosemide-sensitive sodium-potassium-2-chloride co-transporter (NKCC2) located in the thick ascending limb is responsible for the reabsorption of sodium, chloride, and potassium. Intracellularly, sodium is then actively transported to the interstitial medulla by the basolateral sodium-potassium ATPase, while chloride leaves the cell in the basolateral membrane through chloride channels (ClC-Ka and/or ClC-Kb). Potassium is secreted in the tubular lumen of the thick ascending limb through the medullary external potassium channel (ROMK), which maintains the positive voltage that drives the absorption of ions through NKCC2. For this reason, ROMK deficiency (as in BS type 2) leads to similar effects that are seen with NKCC2 deficiency (as in BS type 1). The loss of transepithelial voltage gradient in the thick ascending limb, which drives paracellular reabsorption of calcium and magnesium, also leads to hypercalciuria and eventually nephrocalcinosis. Without the transport of sodium to the renal interstitium, there is a loss of corticomedullary osmotic gradient and the ability to concentrate urine in the distal convoluted tubule, leading to a state that can clinically overlap with AVP insufficiency/resistance. The entrance of chloride to the macula densa, which signals normal tubuloglomerular filtration, is compromised because this transport is also driven by NKCC2 channels. Consequently, the macula densa is permanently activated and stimulates the juxtaglomerular apparatus, which produces large amounts of prostaglandin E2 (PGE2) and renin. Thus, there is vasodilatation of afferent arterioles and renal hyper-filtration, permanent activation of renin-angiotensin-aldosterone axis, and distal renal secretion of potassium and hydrogen, leading to further dehydration and hypokalemic alkalosis [3]. PGE2 also enhances vascular perfusion in renal interstitium, inducing washout of corticomedullary osmotic gradient and aggravating renal wasting of water [3,8].

Although all 5 subtypes of BS share the same primary defect of impaired salt reabsorption in the thick ascending limb leading to the abovementioned biochemical alterations, genetic screening to differentiate these subtypes is paramount, as it can inform age of clinical manifestation (eg, BS type 3 usually manifests later in life), therapeutic needs (eg, marked hypokalemia in BS types 3 and 4, which need aggressive supplementation and close monitoring), and prognosis (eg, BS type 5 remits spontaneously) [9].

The severe morbidity that can develop in patients with BS related to preterm labor and extreme prematurity renders prenatal diagnosis and treatment of paramount importance.

Polyhydramnios due to fetal polyuria is almost always caused by BS, and this diagnosis should always be considered in such circumstances. Prenatal genetic diagnosis is the recommended step, with calculation of the “Bartter index” (total protein multiplied by alpha-fetoprotein; measurements on the amniotic fluid) as an alternative [9]. Treatment with non-steroidal anti-inflammatory drugs (NSAIDs) and serial amniocentesis have been used with satisfactory success in case reports [10,11], although risks of therapy – especially with NSAIDs – should be carefully monitored, preferably in a multidisciplinary setting [9]. In addition, uncovering a genetic etiology allows genetic counseling and reproductive assisted techniques to parents that desire further healthy pregnancies, as shown in our first case.

Due to the marked polydipsia and polyuria, and sometimes hypernatremia, that patients with BS types 1 and 2 patients often present, AVP insufficiency/resistance is frequently considered before the former diseases. Indeed, negative genetic screening for causative mutations of AVP resistance are usually followed by genetic testing of KCNJ1 and SLC12A1 mutations, as per protocol in some institutions, as their prevalence in this setting is approximately 5% [12]. The distinction between BS and AVP insufficiency/resistance is essential, as treatment and prognosis differ between the 2 entities. Endocrine tests performed to survey apparent AVP insufficiency/resistance are consistent with a nephrogenic form (AVP resistance) but, with rare exceptions [13,14], patients with BS typically do not have urinary osmolalities below 100 mosmol/kg H2O. This feature reflects the ability adaptation of the distal convoluted tubule to increase salt reabsorption in patients with BS type 1 and 2 [3]. However, urinary osmolalities persistently <100 mosmol/ kg H2O in our first patient led to diagnostic confusion and delay of proper treatment [12]. Thus, a broad approach that considers all pertinent data from history, physical features, and biochemical findings is paramount to a correct diagnosis and expedited treatment of BS.

Transient hyperkalemia, and sometimes concurrent hyponatremia, is often seen in patients with BS type 2 during the newborn period. This biochemical finding can suggest and lead to a misdiagnosis of pseudohypoaldosteronism type 1 [3,9,15], delaying the correct diagnosis and treatment, decreasing quality of life and auxologic development for many years, as seen in our second case. ROMK deficiency can lead to transient hyperkalemia due to the role of this channel in the cortical collecting duct, where it participates in potassium secretion [12].

Hyperparathyroidism is frequently seen in patients with BS types 1 and 2, but the mechanisms underlying this finding remain unclear [14]. Direct stimulation of parathyroid glands by PGE2 [16] and increased PGE2-mediated activation of 1,25 (OH)2 D [17] contribute to high serum PTH and calcium, respectively, but reversal of hyperparathyroidism in BS patients with indomethacin therapy is reported in some but not all studies [18,19]. Hyperreninemic hyperaldosteronism occurring in BS patients can also play a role, as both angiotensin II and aldosterone increase PTH secretion [20]. Along with chronic high PTH, excess aldosterone, as seen in primary hyperaldosteronism, can also have direct and detrimental effects in bone of BS patients, as osteoclasts, osteoblasts, and osteocytes express mineralocorticoid receptors [21–23]. Indeed, low bone mineral density has been described in patients with BS types 1 and 2, and life-long bone mineral density monitoring is being recommended [18]. Cinacalcet has been successfully used in reversing hyperparathyroidism in some but not all patients with BS type 1 [2,24], and indomethacin may indirectly decrease aldosterone and renin levels through normalization of volume and electrolytes imbalances. Prospective studies to inform benefits of these therapeutic interventions on bone health are lacking for patients with BS.

Salt supplementation should be considered as a supportive treatment that could aid in restoring extracellular volume and normal electrolyte levels. Sodium chloride supplements are recommended, although they should be avoided in BS patients with apparent AVP resistance with osmolalities in urine lower than plasma, as they can aggravate polyuria and precipitate severe hypernatremia. Chloride potassium should also be considered when hypokalemia supervenes, with a target of 3 mmol/L; above this range, careful surveillance is advised. Potassium salts should be avoided, as they aggravate metabolic alkalosis [9].

Indomethacin, ibuprofen (non-selective NSAID), and celecoxib (selective COX-2 inhibitor) are recommended pharmacologic options to suppress excess PGE2 [9]. As shown in our cases, they induced a considerable improvement in debilitating symptoms, polydipsia/polyuria, growth, serum electrolytes, renal wasting of calcium, normalization of renin/aldosterone, and attenuation of hyperparathyroidism [22,23]. However, there are no studies showing superiority of one NSAID over the others, although cumulative experience is higher with indomethacin [19,25]. The recommended maximum dose of this NSAID is 4 mg/kg/day, divided in 3 to 4 doses [9]. Nevertheless, as shown in our first case, some patients can need higher doses (eg, 7 to 9 mg/kg/day) to reach clinical and biochemical meaningful benefits [25,26]. Titrating the dose to normal renin levels has also been shown to be successful [19]. It should be kept in mind that chronic NSAID use should always be individualized, and that tapering to the lowest dose or even discontinuation in stable patients should be maintained as an option during follow-up [9].

Adverse effects related to chronic NSAID use have been reported, especially gastrointestinal and renal, although their prevalence does not seem to be high [19,27,28]. In the first months of life, celecoxib may be preferable, due to the increased susceptibility to necrotizing enterocolitis. If a non-selective NSAID is used, proton-pump inhibitors should be prescribed at all ages to prevent gastrointestinal adverse effects [9]. Chronic kidney disease is common in patients with BS, although its etiology is judged to be multifactorial (eg, prematurity, nephrocalcinosis, proteinuria related to hyperfiltration, chronic hypovolemic states) and not only a consequence of long-term use of NSAIDs. Indeed, studies of BS patients treated with indomethacin reported stable biochemical and histologic renal parameters after 9 to 14 years of follow-up [19,29]. Except for BS type 3, renal wasting of calcium and nephrocalcinosis is frequently seen in the other types, although progression to symptomatic nephrolithiasis and obstructive uropathy seems to be rare, possibly related to the benefits of indomethacin in reducing renal calcium wasting [19,29]. Nevertheless, renal ultrasound is recommended each 12 to 24 months to monitor for the occurrence of urologic complications [9].

Growth retardation is a common complication in BS, with short statures as severe as – 4.9 SD [29]. In experimental studies, chronic hypokalemic states showed decreased pituitary growth hormone (GH) secretion, decreased IGF-1 mRNA expression in epiphysis, impaired chondrocyte hypertrophy in growth plate, and resistance to GH therapy [29]. Salt supplements and indomethacin have shown excellent outcomes in terms of longitudinal growth in some but not all cases, as described in our patients [27,29]. In addition, some individuals can have synchronous GH deficiency, and this possibility should be surveyed if there is poor growth despite adequate titration of standard therapy of BS. GH treatment in BS patients with GH deficiency has been reported to increase height velocity by 4 to 6.5 cm/ year [30]. However, in BS patients with severe short stature without proved GH deficiency, GH treatment did not significantly reduce the prevalence of severe short stature in the majority of cases reported in one survey, although data are scarce [27].

Conclusions

We described 2 patients with antenatal BS types 1 and 2 that were diagnosed after a broad clinical revision of apparent AVP insufficiency/resistance. Standard medical therapy of BS led to meaningful benefits in terms of clinical and biochemical parameters, although variable success was seen in terms of auxology, and data are scarce on the pertinence of GH therapy for BS patients without proved GH deficiency. Severe and life-long incapacitating morbidity related to extreme prematurity demands a low suspicion threshold for BS in all cases of polyhydramnios, and requires expedited antenatal intervention to prolong gestation as long as possible in cases of confirmed genetic BS. This diagnosis also allows a personalized treatment and follow-up to patients and genetic counseling to their parents if future pregnancies are desired, avoiding further cases of BS and associated complications in their offspring.