Pathogenesis and management of hyponatremia

The vasopressin-aquaporin-2 pathway syndromes

Vasopressin is the key hormone involved in water conservation and regulation of water balance, essential for life. In the renal collecting duct, vasopressin binds to the V2 receptor, increasing water permeability through activation of aquaporin-2 redistribution to the luminal membrane. This mechanism promotes rapid water reabsorption, important for immediate survival; however, only recently it has become clear that long-term adverse effects are associated with alterations of the vasopressin-aquaporin-2 pathway, leading to several syndromes associated with water balance disorders. The kidney resistance to the vasopressin action may cause severe dehydration for patients and, conversely, nonosmotic release of vasopressin is associated with water retention and increasing the circulatory blood volume. This chapter discusses the relevance of the altered vasopressin-aquaporin-2 pathway in some diseases associated with water balance disorders, including congenital nephrogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The emerging picture suggests that targeting the vasopressin-AQP2 axis can provide therapeutic benefits in those patients.

Sodium Abnormalities in Cardiac Surgery With Cardiopulmonary Bypass in Adults: A Narrative Review

Perioperative sodium abnormalities or dysnatremia is not uncommon in patients presenting for cardiac surgery and is associated with increased morbidity and mortality. Both the disease process of heart failure and its treatment may contribute to abnormalities in serum sodium concentration. Serum sodium is the main determinant of serum osmolality, which in turn affects cell volume. Brain cells are particularly vulnerable to changes in serum osmolality because of the nondistensible cranium. The potentially catastrophic neurologic sequelae of rapidly correcting chronic dysnatremia and the time-sensitive nature of cardiac surgery can make the management of these patients challenging. The use of cardiopulmonary bypass to facilitate surgery adds another layer of complexity in the intraoperative management of sodium and water balance. This narrative review examines the definition and classification of dysnatremia. It also covers the etiology and pathophysiology of dysnatremia, implications during cardiac surgery requiring cardiopulmonary bypass, and the perioperative management of dysnatremia.

Therapeutic effect of conivaptan bolus dosing in hyponatremic neurosurgical patients

STUDY OBJECTIVE

To determine the natremic response of a single 20-mg bolus dose of conivaptan, an arginine vasopressin antagonist, in hyponatremic neurosurgical patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH).

DESIGN

Retrospective medical record review.

SETTING

Neurosurgical intensive care unit of a tertiary care referral hospital.

PATIENTS

Thirty-two hyponatremic patients with SIADH who were admitted to the neurosurgical intensive care unit and received a single 20-mg bolus dose of conivaptan between January and December 2011.

MEASUREMENTS AND MAIN RESULTS

Each patient's natremic response over 48 hours was determined. The primary end point was an increase in serum sodium level of 4 mEq/L or greater over the first 24 hours. The mean ± SD baseline serum sodium level was 129.8 ± 3.4 mEq/L, which increased to 133.1 ± 3.2 mEq/L at 6 hours after administration of the bolus dose of conivaptan. The serum sodium level at 24 hours was 134.2 ± 3.2 mEq/L, indicating a 24-hour natremic response of 4.3 ± 2.6 mEq/L (range 1-13 mEq/L) from baseline (p<0.001). Eighteen patients (56%) met the primary end point. The mean ± SD fluid balance over the first 24 hours was -783 ± 440 ml. The mean ± SD change in serum sodium level from 24 to 48 hours was 0.5 ± 1.3 mEq/L. No adverse effects or injection-site reactions were noted. The patients who failed to reach the primary end point were treated with repeated doses of conivaptan plus other agents.

CONCLUSION

We recommend a single 20-mg dose of conivaptan as the preferred initial approach to treating patients with SIADH who are in the neurosurgical intensive care unit. The 24-hour natremic response should then dictate whether additional doses of conivaptan or other therapeutic interventions are required. We believe that such an approach is safe and will result in a controlled and predictable increase in the serum sodium concentration.

Syndrome of inappropriate secretion of antidiuretic hormone associated with imatinib

OBJECTIVE

To describe a patient with Bcr-abl(+) acute lymphoblastic leukemia who developed the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) while being treated with high-dose imatinib.

CASE SUMMARY

A 29-year-old woman was diagnosed with Bcr-abl(+) acute lymphoblastic leukemia, and treatment was initiated with chemotherapy and imatinib 800 mg daily. Following imatinib initiation, a gradual decrease in serum sodium level was noticed. Prolonged aplasia and neutropenic fever prompted discontinuation of therapy for 4 weeks. Following the patient's recovery, complete remission was achieved and monotherapy with imatinib 800 mg daily was restarted; however, hyponatremia recurred a few days later. The clinical findings and laboratory workup were compatible with the diagnosis of SIADH, which was attributed to high-dose imatinib. Fluid restriction and imatinib dosage reduction (to 600 mg/day) restored sodium levels. According to the Naranjo probability scale, this adverse reaction was probably associated with imatinib.

DISCUSSION

Imatinib emerged as the first tyrosine kinase inhibitor to enter everyday clinical practice for the treatment of Ph(+) leukemias. Due to its molecular specificity, imatinib lacks the broad cytotoxicity of conventional chemotherapy. Inhibition of kinases in normal tissues accounts for many of imatinib's adverse reactions. To our knowledge, this is the first reported case of imatinib-induced SIADH.

CONCLUSIONS

We recommend monitoring for SIADH if a patient receiving high-dose imatinib develops hyponatremia.

Plasma arginine vasopressin and atrial natriuretic peptide concentration in patients with hyponatremia at diagnosis and following treatment

BACKGROUND

Much evidence for arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) in the pathogenesis of hyponatremia in humans is based on single measurements. To study the roles of AVP and ANP in the pathogenesis and recovery of hyponatremia, sequential measurements of ANP and AVP were taken during treatment in a group of hyponatremic patients with different etiologies.

METHODS

Consecutive adult patients with hyponatremia (serum Na <130 mmol/l) and healthy controls were studied. Volume status was determined by clinical and laboratory criteria. Plasma AVP and ANP, fractional sodium excretion, and urine osmolality were determined daily until serum Na was above 135 mmol/l or for at most 7 days.

RESULTS

A total of 16 controls and 40 hyponatremic patients (12 normovolemic, 9 hypervolemic, and 19 hypovolemic) were studied. Patients' plasma AVP on the first day [1.0 (0.3-2.3) ng/l] and on the last day [1.1 (0.3-2.5) ng/l] of the study did not differ from that of controls [0.7 (0.5-1.0) ng/l]. Serum sodium concentration increased significantly in patients between the first and the last day. Patients had significantly lower ANP concentrations, both on the first day [25 (15-46) ng/l] and on the last day [29 (17-46) ng/l], than controls [41 (28-51) ng/l]. Plasma AVP was elevated relative to serum osmolality on the first day and to a lesser extent on the last day of the study.

CONCLUSIONS

AVP is inappropriately high in a majority of hyponatremic patients. Plasma AVP and ANP concentrations do not change during treatment in hyponatremic patients despite a significant increase in serum osmolality. A low ANP concentration in clinically normovolemic and hypovolemic patients indicates volume depletion, which may lead to baroreceptor-stimulated AVP secretion.

Thyroid transcription factor-1 facilitates cerebrospinal fluid formation by regulating aquaporin-1 synthesis in the brain

In the brain, aquaporin-1 (AQP-1), a water channel for high osmotic water permeability, is mainly expressed in the apical membrane of the ventricular choroid plexus and regulates formation of cerebrospinal fluid (CSF). Although the physiology of AQP-1 has been the subject of several publications, much less is known about the trans-acting factors involved in the control of AQP-1 gene expression. Here we report that TTF-1, a homeodomain-containing transcriptional regulator, is coexpressed with AQP-1 in the rat brain choroid plexus and enhances AQP-1 gene transcription by binding to conserved core TTF-1-binding motifs in the 5'-flanking region of the AQP-1 gene. Intracerebroventricular administration of an antisense TTF-1 oligodeoxynucleotide significantly decreased AQP-1 synthesis and reduced CSF formation. In addition, blockade of TTF-1 synthesis increased survival of the animals following acute water intoxication-induced brain edema. These results suggest that TTF-1 is physiologically involved in the transcriptional control of AQP-1, which is required for CSF formation.

Alterations in electrolyte equilibrium in patients with acute leukemia

BACKGROUND AND AIM

A wide array of disturbances in electrolyte equilibrium is commonly seen in patients with acute leukemia (AL). These abnormalities present a potential hazard in these patients, as that of enhancing the cardiotoxic effects of certain chemotherapeutic regimens. The literature dealing with AL-related electrolyte abnormalities and their interactions in leukemic patients was reviewed.

DATA SYNTHESIS

Sources included MEDLINE and EMBASE. The search strategy was based on the combination of 'acute leukemia', 'electrolyte abnormalities', 'acid-base disorders', 'potassium', 'sodium', 'magnesium', 'calcium', and 'phosphorus'. References of retrieved articles were also screened. A decrease in serum potassium, mainly owing to lysozyme-induced tubular damage, appears to be one of the most frequent and potentially hazardous abnormalities. Other clinically significant metabolic perturbations include hyponatremia and hypercalcemia.

CONCLUSION

A broad spectrum of electrolyte abnormalities is encountered in the clinical setting of AL, which are related to the disease process per se and/or to the therapeutic interventions. Clinicians should be vigilant for early detection and appropriate management of these disorders before the initiation of chemotherapy regimens as well as during treatment.

Treatment of electrolyte disorders in adult patients in the intensive care unit

PURPOSE

The treatment of electrolyte disorders in adult patients in the intensive care unit (ICU), including guidelines for correcting specific electrolyte disorders, is reviewed.

SUMMARY

Electrolytes are involved in many metabolic and homeostatic functions. Electrolyte disorders are common in adult patients in the ICU and have been associated with increased morbidity and mortality, as has the improper treatment of electrolyte disorders. A limited number of prospective, randomized, controlled studies have been conducted evaluating the optimal treatment of electrolyte disorders. Recommendations for treatment of electrolyte disorders in adult patients in the ICU are provided based on these studies, as well as case reports, expert opinion, and clinical experience. The etiologies of and treatments for hyponatremia hypotonic and hypernatremia (hypovolemic, isovolemic, and hypervolemic), hypokalemia and hyperkalemia, hypophosphatemia and hyperphosphatemia, hypocalcemia and hypercalcemia, and hypomagnesemia and hypermagnesemia are discussed, and equations for determining the proper dosages for adult patients in the ICU are provided. Treatment is often empirical, based on published literature, expert recommendations, and the patient's response to the initial treatment. Actual electrolyte correction requires individual adjustment based on the patient's clinical condition and response to therapy. Clinicians should be knowledgeable about electrolyte homeostasis and the underlying pathophysiology of electrolyte disorders in order to provide the optimal therapy to patients.

CONCLUSION

Treatment of electrolyte disorders is often empirical, based on published literature, expert opinion and recommendations, and patient's response to the initial treatment. Clinicians should be knowledgeable about electrolyte homeostasis and the underlying pathophysiology of electrolyte disorders to provide optimal therapy for patients.

Increased aquaporin-1 expression in choroid plexus epithelium after systemic hyponatremia

The expression of aquaporin-1 (AQP1) protein, a water channel, in the choroid plexus epithelium was examined in the rat to determine the role of AQP1 in the pathophysiology of systemic hyponatremia. Systemic hyponatremia was produced by coadministration of 30 ml hypotonic dextrose solution (140 mM) intraperitoneally and 3 microg 1-deamino-8-d-arginine vasopressin (dDAVP) subcutaneously. This hydration protocol produced severe systemic hyponatremia: 2h after drug administration, normonatremic serum osmolarity and Na(+) concentration (296+/-5mOsm/L and 140+/-5 mM) were reduced to 252+/-5mOsm/L and 117+/-2 mM, respectively. AQP1 expression in the third ventricle choroid plexus epithelium was determined by using immunohistochemistry and quantitative immunoblot analysis. After 2h of systemic hyponatremia, AQP1 expression was significantly increased by 28.2+/-2.7% (n=6, P<0.05), whereas it was attenuated to 15.7+/-2.2% (n=6, P<0.05) after 6h of systemic hyponatremia. In addition, TUNEL staining revealed apoptotic granule cells in the dentate gyrus (DG) adjacent to the third ventricle with a peak measured 2h after the start of the systemic hyponatremia protocol, while virtually no apoptotic granule cells left 6h after the start of the hyponatremic treatment. These findings suggest that AQP1-mediated excessive water influx into the brain through the blood-cerebrospinal fluid barrier during systemic hyponatremia may elevate the intracranial pressure and result in the apoptotic brain cell death. Thus, AQP1 in the choroid plexus may play an important role in the pathogenesis of systemic hyponatremia.

Alpha syntrophin deletion removes the perivascular but not the endothelial pool of aquaporin‐4 at the blood‐brain barrier and delays the development of brain edema in an experimental model of acute hyponatremia

The formation of brain edema, commonly occurring as a potentially lethal complication of acute hyponatremia, is delayed following knockout of the water channel aquaporin-4 (AQP4). Here we show by high-resolution immunogold analysis of the blood-brain-barrier that AQP4 is expressed in brain endothelial cells as well as in the perivascular membranes of astrocyte endfeet. A selective removal of perivascular AQP4 by alpha-syntrophin deletion delays the buildup of brain edema (assessed by Diffusion-weighted MRI) following water intoxication, despite the presence of a normal complement of endothelial AQP4. This indicates that the perivascular membrane domain, which is peripheral to the endothelial blood-brain barrier, may control the rate of osmotically driven water entry. This study is also the first to demonstrate that the time course of edema development differs among brain regions, probably reflecting differences in aquaporin-4 distribution. The resolution of the molecular basis and subcellular site of osmotically driven brain water uptake should help design new therapies for acute brain edema.

The molecular basis of water transport in the brain

Brain function is inextricably coupled to water homeostasis. The fact that most of the volume between neurons is occupied by glial cells, leaving only a narrow extracellular space, represents an important challenge, as even small extracellular volume changes will affect ion concentrations and therefore neuronal excitability. Further, the ionic transmembrane shifts that are required to maintain ion homeostasis during neuronal activity must be accompanied by water. It follows that the mechanisms for water transport across plasma membranes must have a central part in brain physiology. These mechanisms are also likely to be of pathophysiological importance in brain oedema, which represents a net accumulation of water in brain tissue. Recent studies have shed light on the molecular basis for brain water transport and have identified a class of specialized water channels in the brain that might be crucial to the physiological and pathophysiological handling of water.

Increased expression of tyrosine hydroxylase immunoreactivity in paraventricular and supraoptic neurons in illnesses with prolonged osmotic or nonosmotic stimulation of vasopressin release

Our previous studies indicated that in the human paraventricular (PVN) and supraoptic (SON) nuclei, tyrosine hydroxylase (TH)--the first and rate-limiting enzyme in catecholamine synthesis--is localized mainly in magnocellular neurons and that antemortem factors regulate its expression. The purpose of the present study was to investigate the distribution of TH-immunoreactive (TH-IR) perikarya of the hypothalami of a large sample of well-documented adult subjects without neurological, psychiatric or endocrinological disease in order to identify factors that could regulate the expression of TH in the human neurosecretory neurons. Our material consisted of the hypothalami of 38 subjects studied immunohistochemically for TH using the peroxidase-antiperoxidase method. Striking individual differences were observed among the subjects studied concerning the number and distribution of TH-IR perikarya within the PVN and SON. These differences were evident throughout the entire rostrocaudal length of the hypothalamus and appeared to be related neither to the age or sex of the subjects nor to the postmortem interval or staining procedures. In the sample studied, a large number of TH-IR perikarya were observed specifically in all subjects that had suffered from right-sided heart failure due to pulmonary hypertension, liver cirrhosis or dehydration. In all the above illnesses, increased production and secretion of vasopressin (VP) are reported to occur due to a decrease in 'effective' blood volume or to osmotic stimulation. We conclude that somatic illnesses leading to prolonged osmotic or nonosmotic stimulation of VP release may induce increased expression of TH immunoreactivity in the human neurosecretory neurons related to neuronal activation.

Abnormal regulation of ENaC: syndromes of salt retention and salt wasting by the collecting duct

Although the aldosterone-responsive segments of the nephron together reabsorb <10% of the filtered Na+, certain single-gene defects that affect the epithelial Na+ channel (ENaC) in the luminal membrane of the collecting duct (CD) or its regulation by aldosterone cause severe hypertension, whereas others cause salt wasting and hypotension. These rare defects illustrate the key role of the distal nephron in maintaining normal extracellular volume and blood pressure. Genetic defects that increase the Cl- conductance of the junctional complexes may also lead to salt retention and hypertension. Less dramatic alterations in regulatory actions of other hormones such as vasopressin (VP), either alone or with other genetic variations, diet, or environmental factors, may also produce Na+ retention or loss. Although VP acts primarily to regulate water balance, it is also an antinatriuretic hormone. Elevated basal plasma VP levels, and/or augmented VP release with increased Na+ intake, have been linked to essential hypertension in humans and in animal models of congestive heart failure and cirrhosis. Norepinephrine, dopamine, and prostaglandin E2 can inhibit the antinatriuretic effects of VP, and changes in the actions of these autocrine and paracrine regulators may also be involved in abnormal regulation of Na+ reabsorption.