Hyponatraemia is the most common electrolyte disorder encountered in clinical practice and is estimated to occur in up to 35% of hospital inpatients.[1]Adrogué HJ, Tucker BM, Madias NE. Diagnosis and Management of Hyponatremia: A Review. JAMA. 2022 Jul 19;328(3):280-91.
http://www.ncbi.nlm.nih.gov/pubmed/35852524?tool=bestpractice.com
It is defined as a serum sodium <135 mmol/L (<135 mEq/L) (normal serum concentration is in the range of 135 to 145 mmol/L (135 to 145 mEq/L); severe hyponatraemia is defined as a serum sodium <125 mmol/L (<125 mEq/L).[2]Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.
https://academic.oup.com/ejendo/article/170/3/G1/6668028
http://www.ncbi.nlm.nih.gov/pubmed/24569125?tool=bestpractice.com
Patients with hyponatraemia have increased morbidity and mortality compared with patients without hyponatraemia.[3]Lien YH, Shapiro JI. Hyponatremia: clinical diagnosis and management. Am J Med. 2007 Aug;120(8):653-8.
http://www.ncbi.nlm.nih.gov/pubmed/17679119?tool=bestpractice.com
Mild hyponatraemia is an independent risk factor of adverse outcome and mortality even in the general population.[4]Sajadieh A, Binici Z, Mouridsen MR, et al. Mild hyponatremia carries a poor prognosis in community subjects. Am J Med. 2009 Jul;122(7):679-86.
http://www.ncbi.nlm.nih.gov/pubmed/19559171?tool=bestpractice.com
Hyponatraemia is primarily a disorder of water balance. A low serum sodium concentration indicates dilute body fluids or an excess of water. The clinical manifestations of hyponatraemia depend on the rate of decline of serum sodium. An acute fall in sodium over 24 to 48 hours produces severe cerebral oedema, which can be fatal.[5]Wakil A, Ng JM, Atkin SL. Investigating hyponatraemia. BMJ.2011 Mar 7;342:d1118.
http://www.ncbi.nlm.nih.gov/pubmed/21382929?tool=bestpractice.com
A gradual fall in sodium over several days or weeks can be compensated for by the brain, produces relatively modest morbidity and may be asymptomatic.[5]Wakil A, Ng JM, Atkin SL. Investigating hyponatraemia. BMJ.2011 Mar 7;342:d1118.
http://www.ncbi.nlm.nih.gov/pubmed/21382929?tool=bestpractice.com
In most cases, hyponatraemia reflects hypotonicity or low effective osmolality. It is often iatrogenic and avoidable. Common causes of hyponatraemia include true volume depletion, effective arterial volume depletion (e.g., congestive heart failure, cirrhosis), and medicine-induced hyponatraemia due to thiazide diuretics or antidepressants.[5]Wakil A, Ng JM, Atkin SL. Investigating hyponatraemia. BMJ.2011 Mar 7;342:d1118.
http://www.ncbi.nlm.nih.gov/pubmed/21382929?tool=bestpractice.com
[6]Montanana PA, Modesto i Alapont V, Ocon AP, et al. The use of isotonic fluid as maintenance therapy prevents iatrogenic hyponatremia in pediatrics: a randomized, controlled open study. Pediatr Crit Care Med. 2008 Nov;9(6):589-97.
http://www.ncbi.nlm.nih.gov/pubmed/18838929?tool=bestpractice.com
[7]Grant P, Ayuk J, Bouloux PM, et al. The diagnosis and management of inpatient hyponatraemia and SIADH. Eur J Clin Invest. 2015 Aug;45(8):888-94.
https://onlinelibrary.wiley.com/doi/10.1111/eci.12465
http://www.ncbi.nlm.nih.gov/pubmed/25995119?tool=bestpractice.com
Types of hyponatraemia
Hyponatraemia can result from a variety of conditions, based on different mechanisms, and can have multiple drivers. It may result from an inappropriate hypotonic fluid intake, inappropriate fluid retention by excessive antidiuretic hormone (ADH) which increases renal water re-absorption, or inadequate renal re-absorption of sodium.
Hyponatraemia can be classified into 5 main types. The first three represent types of hypotonic, or true, hyponatraemia and are further classified according to fluid volume status:
Hypovolaemic hyponatraemia (hypotonic): total body water decreases, but total body sodium decreases to a greater extent. The extracellular fluid volume is also decreased.
Euvolaemic hyponatraemia (hypotonic): total body water increases, but total body sodium remains unchanged. There is a modest increase in extracellular fluid volume, but not enough to cause oedema.
Hypervolaemic hyponatraemia (hypotonic): total body water and sodium both increase, but total body water increases to a greater extent. The extracellular fluid volume is markedly increased, causing oedema.
Hypertonic (redistributive) hyponatraemia: increased osmotic pressure in the extracellular compartment causes water to shift from the intracellular to the extracellular compartment diluting extracellular sodium. However, total body sodium and water are unchanged. This is commonly seen with hyperglycaemia and mannitol administration. This simple formula can be used to correct sodium level in the presence of hyperglycaemia: serum sodium is decreased by 2.4 mmol/L (2.4 mEq/dL) for every 5.6 mmol/L (100 mg/dL) elevation of serum glucose over 5.6 mmol/L (100 mg/dL).
Pseudohyponatraemia: excessive lipids or proteins dilute the aqueous phase of the extracellular compartment and the measured sodium levels are low. However, this decrease is an artifact and should be excluded before proceeding with further investigations. Total body sodium and water are unchanged, and there has not been a shift of fluid between compartments. The use of ion-specific electrodes has helped reduce the incidence of this artifact.[8]Lippi G, Aloe R. Hyponatremia and pseudohyponatremia: first, do no harm [letter]. Am J Med. 2010 Sep;123(9):e17.
http://www.amjmed.com/article/S0002-9343(10)00390-6/fulltext
http://www.ncbi.nlm.nih.gov/pubmed/20800135?tool=bestpractice.com
Hyponatraemia can also be classified according to its rate of onset.[2]Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.
https://academic.oup.com/ejendo/article/170/3/G1/6668028
http://www.ncbi.nlm.nih.gov/pubmed/24569125?tool=bestpractice.com
Acute hyponatraemia is defined as hyponatraemia with a duration of <48 hours.
Chronic hyponatraemia is defined as hyponatraemia with a duration of at least 48 hours. Chronic hyponatraemia is much more common than acute, and cases where the duration of hyponatraemia is unclear should be considered to be chronic unless there is clinical evidence suggesting otherwise.
Central nervous system effects of hyponatraemia
Hyponatraemia is significant when it is associated with a decline in extracellular osmolality, as it causes cellular oedema. Most tissues can tolerate cellular oedema, apart from the bony calvarium due to space limitations. Brain cells have long-term adaptive mechanisms that can compensate for low serum sodium and osmolality by giving up ions like potassium and synthesising organic osmolytes to preserve cell volume. If the sodium concentration falls slowly over several days or weeks, the brain is able to use such mechanisms to adapt. For this reason, patients with chronic hyponatraemia have relatively modest cerebral oedema and do not develop brainstem herniation. However, if the sodium concentration falls rapidly over 24 to 48 hours, the compensatory mechanisms of the brain are overwhelmed and severe cerebral oedema occurs, leading to brainstem herniation, respiratory arrest, and death. This can occur even with a modest fall in sodium (125-130 mmol/L [125-130 mEq/L]).
Failure to correct hyponatraemia can lead to permanent neurological damage, as can correcting sodium levels too rapidly.[2]Spasovski G, Vanholder R, Allolio B, et al; Hyponatraemia Guideline Development Group. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014 Feb 25;170(3):G1-47.
https://academic.oup.com/ejendo/article/170/3/G1/6668028
http://www.ncbi.nlm.nih.gov/pubmed/24569125?tool=bestpractice.com
When hyponatraemia is chronic and the serum sodium concentration increases too rapidly, osmotic demyelination syndrome (ODS; also known as central pontine myelinolysis) may develop. ODS is characterised by altered mental status, reduced motor functioning, and/or abnormalities of balance.[9]Lindner G, Schwarz C, Haidinger M, et al. Hyponatremia in the emergency department. Am J Emerg Med. 2022 Oct;60:1-8.
https://www.sciencedirect.com/science/article/pii/S0735675722004600
http://www.ncbi.nlm.nih.gov/pubmed/35870366?tool=bestpractice.com
[10]Wang P, Li T. Osmotic demyelination syndrome: clinical and neuroimaging characteristics in a series of 8 cases. Quant Imaging Med Surg. 2023 Jul 1;13(7):4785-91.
https://qims.amegroups.org/article/view/113564/html
http://www.ncbi.nlm.nih.gov/pubmed/37456327?tool=bestpractice.com
