Chapter outlines

PHYSIOLOGICAL PREDISPOSING FACTORS

1. Lower baseline plasma sodium. The normal range of serum sodium concentration is lower (130–140 mEq/L) in pregnancy compared to its value in non-pregnant adults (135–145 mEq/L) [4, 5]. In the late third trimester, about 6 to 8 liter of water is retained, which is responsible for the dilutional hyponatremia and the lower baseline plasma sodium value.
2. An impaired renal ability to excrete water [6, 7]. In healthy women, the normal capacity of kidneys to excrete a water load is 900 ml/h. While in late pregnancy, the maximum renal ability to excrete a water load reduces to 600 mL/h [8]. In the presence of the tendency of kidneys to retain water, hyponatremia is likely to occur if the excess fluid is consumed orally or administered intravenously.
3. Antidiuretic effect of endogenous oxytocin. As the structure of oxytocin is similar to vasopressin, oxytocin causes water retention and results in dilutional hyponatremia in laboring women [9]. Reasons for the higher concentration of oxytocin during labor are the greater release of endogenous secretion of oxytocin and IV administration of synthetic oxytocin to induce or augment labor.

CAUSES

1. Excessive oral water intake during prolonged labor [10, 11].
2. Hyponatremia is likely in women who choose home delivery but are transferred to the hospital after prolonged labor due to unsuccessful delivery or obstetric complications [3]. The mechanism for developing hyponatremia in such patients is the high endogenous secretion of oxytocin and excessive oral hypotonic fluid intake.
3. Liberal fluid administration causes iatrogenic maternal fluid overload.
4. Increased oral fluid intake and intravenous fluid administration are major causes of hyponatremia. Hyponatremia occurs when the intake of low sodium-containing fluid exceeds a woman’s ability to excrete it during labor. Water retention causes a decrease in the concentration of sodium by diluting the blood. As a result, the incidence of hyponatremia at delivery among women is 1% for those with a total fluid intake (oral + IV) of up to 1 liter during labor and 5% for those with a total fluid intake between 1 to 2.5 liters [2]. The risk of hyponatremia increases to as high as 26% for those with a total fluid intake above 2.5 liters.
5. Administration of a large volume of oxytocin in hypotonic solutions (i.e., 5% dextrose) for a prolonged period for induction and augmentation of labor can cause hyponatremia [2].
6. Other causes: Hyperemesis gravidarum, psychiatric disorders, and maternal medications like diuretics, antidepressants like selective serotonin reuptake inhibitors [SSRIs], and synthetic recreational drugs like ecstasy.

HARMFUL EFFECTS

PREVENTION

1. Strict fluid balance chart and early recognition: Record oral/intravenous fluid intake and urine output every four hours for close monitoring. When an electrolyte imbalance is suspected, it is important to measure the serum electrolytes. If a woman’s fluid balance exceeds positive 1500 ml, her risk of developing hyponatremia is high, and her blood sodium level should be checked.
2. Neutral fluid balance: Maternal dilutional hyponatremia can be prevented during labor by maintaining a neutral fluid balance. Therefore, excessive fluid intake during labor should not be encouraged for women.
3. Avoid the administration of hypotonic IV fluids like 5% dextrose during labor.
4. When oxytocin is to be administered in a high dose for a prolonged period, use a higher concentration of oxytocin to reduce the total volume of infused fluid, use electrolyte-containing solutions such as normal saline or Ringer’s lactate as a diluent, and avoid electrolyte free solution like 5% dextrose as a diluent.
5. During labor, it is important to monitor serum sodium levels in women who require oxytocin infusion, have a positive fluid balance exceeding 1500 ml, develop symptoms of hyponatremia, or have a serum sodium level less than 130 mEq/L.

TREATMENT

REFERENCES

1. GAIN Guideline for the prevention, diagnosis and management of hyponatraemia in labour and the immediate postpartum period march 2017.
2. Moen V, Burdin L, Rundgren M, et al. Hyponatremia complicating labour—rare or unrecognised? A prospective observational study. BJOG 2009;116(4):552‑61.
3. Lassey SC, Napoe GS, Carusi D, et al. Hyponatremia among parturients transferred to the hospital after prolonged labor during an attempted home birth. Obstet Gynecol. 2019;134(1):106–108.
4. Costantine MM. Physiologic and pharmacokinetic changes in pregnancy. Front Pharmacol. 2014;5:65.
5. Narelle H. Biochemical Changes in Pregnancy-What Should A Clinician Know? J Gynecol Women’s Health. 2017;4(1):555626.
6. Schrier RW. Body water homeostasis: clinical disorders of urinary dilution and concentration. J Am Soc Nephrol 2006;17(7):1820–1832.
7. Cheung KL, Lafayette RA. Renal physiology of pregnancy. Adv Chronic Kidney Dis. 2013;20(3):209–14.
8. Hytten FE, Klopper AI. Response to a water load in pregnancy. J Obstet Gynaecol Br Commonw 1963;70:811–6.
9. Chunling Li, Weidong W, Sandra NS, et al. Molecular Mechanisms of Antidiuretic Effect of Oxytocin. J Am Soc Nephrol. 2008;19(2):225–32.
10. Johansson S, Lindow S, Kapadia H, et al. Perinatal water intoxication due to excessive oral intake during labour. Acta Paediatr. 2002;91(7):811–4.
11. Solomon N, Many A, Orbach R, et al. Maternal and neonatal hyponatremia during labor: a case series. J Matern Fetal Neonatal Med. 2019;32(16):2711–2715.
12. West CR, Harding JE. Maternal water intoxication as a cause of neonatal seizures. J. Paediatric Child health. 2004;40(12):709–710.
13. Walter KN, Montgomery J, Amess P, et al. Hyponatraemia and brain oedema in newborns following oral water intoxication during prolonged labour. Klinische Paädiatrie. 2012;224(4):266–267.
14. Chapman TH, Hamilton M. Water intoxication presenting as maternal and neonatal seizures: a case report. Journal of medical Case Reports. 2008;2:366.
15. Ophir E, Solt I, Odeh M, et al. Water intoxication - a dangerous condition in labor and delivery rooms. Obstet Gynecol Surv 2007;62(11):731–8.
16. Valerio E, Fantinato M, Giovannini IA, et al. Severe asymptomatic maternal antepartum hyponatremia leading to neonatal seizures: prevention is better than cure. Matern Health Neonatol Perinatol. 2015;1:25.