2. Hyponatremia Induced by AVP Analogs
AVP analogs include desmopressin and oxytocin and can induce hyponatremia by acting as V2R agonists. Desmopressin selectively binds the V2R in the kidney and stimulates adenylyl cyclase activity and cAMP production in collecting duct epithelial cells
[7][6]. This enhances osmotic water reabsorption through the upregulation of the aquaporin-2 (AQP2) water channel. Currently, desmopressin not only is used for treating diabetes insipidus but also is prescribed for relieving nocturnal polyuria in older adults. Even low doses of desmopressin can induce hyponatremia in susceptible patients with nocturnal polyuria because an advanced age is an important risk factor for hyponatremia
[8][7]. Compared with AVP, desmopressin has a greater antidiuretic effect because of its longer half-life and selective binding to the V2R
[9][8]. A meta-analysis reported that the incidence of desmopressin-induced hyponatremia was 7.6% in adults with nocturia
[10][9].
Oxytocin may also induce hyponatremia when it is used in obstetrics to induce abortion and to induce or augment labor. Its antidiuretic activity is presumed by the fact that oxytocin and AVP are closely related peptides secreted from the posterior pituitary and that both are nine amino-acid peptide hormones, of which seven are identical
[11][10]. Furthermore, the action of oxytocin as an antidiuretic hormone has been demonstrated in previous studies. Oxytocin increased osmotic water permeability in perfused inner medullary collecting ducts isolated from Sprague Dawley rats
[12][11], and its hydro-osmotic action was mediated by V2R
[13][12]. In Sprague Dawley rats, oxytocin treatment induced apical and basolateral translocation of the AQP2 protein along the collecting duct. This response was blocked by pretreatment with a V2R antagonist
[14][13]. The antidiuretic action of oxytocin was also demonstrated in humans in association with AQP2 upregulation
[15][14]. In brief, pharmacological doses of oxytocin can induce antidiuretic effects as a result of V2R stimulation and subsequent AQP2 upregulation
[16][15].
3. Hyponatremia Induced by Anticancer Chemotherapeutic Agents
Hyponatremia is a common complication in cancer patients because SIAD is potentially caused by malignancies and it can be related to anticancer medical therapy as well. Vincristine, vinblastine, cisplatin, carboplatin, cyclophosphamide, and ifosfamide are the chemotherapeutic agents that are most frequently associated with hyponatremia
[17][16]. Traditionally, these were believed to stimulate AVP release from the pituitary gland or to increase the production of AVP in the hypothalamus. Chemotherapy-induced nausea may be a potential stimulus to AVP secretion
[18][17]. However, evidence supporting AVP hypersecretion induced by chemotherapeutic agents is limited.
Previous studies have shown that SIADH underlies the mechanism of vincristine-associated hyponatremia. A 3-year-old girl who was inadvertently administered an overdose of vincristine developed clinical features compatible with SIADH. Her blood AVP level was more than four times the normal value
[19][18]. In addition, urinary AVP excretion was markedly elevated in a child with acute lymphatic leukemia following the administration of vincristine
[20][19]. Furthermore, animal studies have suggested that SIADH may result from a direct toxic effect of vincristine on the neurohypophysis and the hypothalamic system
[21,22][20][21].
Cisplatin is a platinum-based chemotherapeutic agent that potentially causes nephrotoxicity. It rarely induces hyponatremia via increasing plasma AVP levels
[23][22]. Moreover, cisplatin nephrotoxicity may produce renal salt wasting causing hypovolemic hyponatremia
[24][23]. Carboplatin may have lesser nephrotoxicity than cisplatin but is rarely associated with hyponatremia
[25][24]. Whether plasma AVP level is increased by carboplatin administration is unclear.
Cyclophosphamide and ifosfamide are representative alkylating agents that may be associated with hyponatremia. Hyponatremia can be induced by various doses of cyclophosphamide during the treatment of malignancy and rheumatologic disease
[26][25]. However, plasma AVP concentrations are not elevated in patients following the administration of intravenous cyclophosphamide
[27,28,29][26][27][28]. Furthermore, antidiuresis was reported to occur in response to intravenous cyclophosphamide in patients with central diabetes insipidus
[30[29][30],
31], excluding the possibility of SIADH. This was confirmed by in vitro experiments using primary cultured rat inner medullary collecting duct (IMCD) cells, in which the active metabolite of cyclophosphamide (4-hydroperoxycyclophosphamide) increased cAMP production, AQP2 protein and mRNA expression, and V2R mRNA expression in the absence of vasopressin stimulation
[32][31].
4. Hyponatremia Induced by Psychotropic Agents
Psychotropic agents are a broad category of drugs including antipsychotics and antidepressants used for psychiatric patients, and anticonvulsants are a category of central nervous system-acting drugs for neurologic patients. These three drug classes are the major contributors to drug-induced hyponatremia in current practice. Although they were previously described as inducing SIADH in many case reports
[37][32], a diagnosis of SIAD is more appropriate because plasma AVP levels were undetermined
[3]. More specifically, psychotropic agents were recently found to act as V2R agonists and to induce nephrogenic antidiuresis, i.e., NSIAD. In primary cultured rat IMCD cells, they stimulated V2R, increased cAMP production, and led to AQP2 upregulation in the absence of vasopressin
[38][33]. This intrarenal mechanism is reminiscent of chlorpropamide-induced hyponatremia. Chlorpropamide is a long-acting first-generation sulfonylurea that is no longer used. It was shown to bind to the V2R within the rat renal tubular basolateral membrane in a competitive manner
[39][34] and to increase the V2R density in rat renal papillary membranes
[40][35].
5. Thiazide-Induced Hyponatremia (TIH)
5.1. Clinical Presentation of TIH
Thiazide and thiazide-like diuretics are the common cause of hyponatremia that is usually induced within a few weeks of starting medication but can occur at any time and rapidly in susceptible patients. They are frequently used for the treatment of hypertension and edematous disorders. According to a retrospective cohort study, approximately 3 in 10 patients who exposed to steady use of thiazides develop hyponatremia
[67][36]. Unlike hypokalemia, hyponatremia is dose-independent
[68][37]. Hypertensive old women are particularly at risk of hyponatremia; the major risk factors for TIH are old age, female gender, low body mass, hypokalemia, and concurrent use of other medications that impair free water excretion
[69][38]. Hyponatremia and inability to excrete a water load resolve within 10 to 14 days of drug withdrawal
[70][39].
Serum sodium levels are variable at presentation. Mild hyponatremia, ranging from 125 to 132 mmol/L, is usually asymptomatic, although vague symptoms such as fatigue or nausea are possible
[71][40]. More severe hyponatremia can be asymptomatic or associated with symptoms including headache, vomiting, confusion, dizziness, lethargy, seizures, and even coma. These symptoms of TIH primarily reflect osmotic water shift into brain cells rather than ECF volume depletion
[72][41].
5.2. Pathogenesis of TIH
The mechanisms of TIH are complicated and not fully understood at present.
Table 21 summarizes how thiazides cause hyponatremia from renal and extrarenal mechanisms. Renal mechanisms are primary and derived from the action of thiazides on renal tubules. Extrarenal mechanisms are subsidiary and include insufficient solute intake, polydipsia, and transcellular cation exchange. Low protein intake reduces urea generation and diminishes urine concentration. Patients with TIH may have a higher fluid intake at baseline and during thiazide use than normonatremic individuals
[73][42]. Hypokalemia concurrently induced by thiazide diuretics can also promote hyponatremia. Extracellular Na
+ will enter cells when K
+ exits because of transcellular ion exchange. The renal mechanisms are detailed in the following paragraphs.
Table 21.
Mechanisms of thiazide-induced hyponatremia.
Renal (Primary) |
NCC inhibition-related |
Sodium loss leading to GFR reduction and enhanced proximal tubular fluid reabsorption |
Impaired urinary dilution |
Independent of NCC inhibition |
AQP2 upregulation in the collecting duct |
Direct effect |
Prostaglandin E2-mediated |
Extrarenal (subsidiary) |
Insufficient solute intake |
Excessive water intake |
Coexistent hypokalemia leading to transcellular cation exchange |