By Dr Wayne Smit, General Pathologist
Perth Pathology
Dr Wayne Smit is a General Pathologist. He completed his Fellowship in Melbourne in 1996 and has worked in Perth since 1999, including a term as CEO at Clinipath Pathology (Sonic). Wayne co-founded Perth Pathology with like-minded ‘corporate refugees’ in 2006 and remains Managing Partner.
(Perth Medical Laboratories Pty Ltd APA) 152 High Street Fremantle WA 6160 Ph 9433 5696 Fax 9433 5472
www.perthpathology.com.au Collection centres throughout the Perth metropolitan area including: Fremantle (Main Lab); Perth CBD, Atwell, Bedford, Belmont, Bentley, East Perth, Ellenbrook, Hilton, Joondalup, Kardinya, Kinross, Maddington, Malaga, Palmyra, South Lake, South Perth, Southern River, Subiaco, West Leederville
Hyponatraemia By Dr Wayne Smit, General Pathologist
H
yponatraemia (low serum sodium) is the most common electrolyte disorder. Maintenance of sodium is important to sustain blood pressure and normal nerve and muscle function. In its most severe form, hyponatraemia is associated with significant morbidity and mortality. While the list of causes is long (see Table 1), unexpected cases can usually be resolved with clinical assessment and some basic laboratory investigations.
Clinical presentation Most patients with hyponatraemia are asymptomatic, especially if the fall in sodium has been slow. Symptoms may not develop until the [Na+] falls to 120mmol/L or less, and even then may be nonspecific (e.g. lethargy, headache, and nausea). However when reduction in sodium has been rapid, patients may be symptomatic well before a level of 120mmol/L is reached. In severe hyponatraemia neurological symptoms predominate. As the level of sodium outside the cells falls, there is movement of water from the extracellular fluids into the cells in an attempt to maintain the normal sodium concentration. Most tissues can
accommodate this cellular swelling but the brain is more sensitive because of the confinement by the skull bones. The risk of seizures and coma increases as the sodium level decreases.
Assessment & Investigation The identification of hyponatraemia should be followed by clinical assessment of the patient for symptoms but particularly to exclude important causes including CCF, liver or renal failure, malignancy, hypothyroidism, Addison’s, and IV or irrigation fluid administration. The patient’s hydration status should also be assessed and hydration status is important to help distinguish causes of hypotonic hyponatraemia.
In many cases the underlying cause will be evident following clinical assessment but where the aetiology is less clear, further investigation may be indicated. Applicable laboratory tests may vary with the clinical situation but serum osmolality, urine osmolality and urine sodium concentration are frequently useful. LFTs, TFTs, cortisol, glucose and lipids may be helpful to narrow the differential diagnoses. Assay of ADH (antidiuretic hormone) is rarely helpful.
Management Treatment, if indicated, is generally directed at the underlying cause of the hyponatraemia. Water restriction may be useful. In euvolaemic hyponatraemic patients, hypertonic saline may also be used but the risks of osmotic pontine demyelination caused by rapid restoration of sodium levels should be considered. References are available on request.
Table 1: Common Causes of Hyponatraemia: (after Walmsley) Pseudohyponatraemia (normal plasma osmolality)
Hyperlipidaemia Hyperproteinaemia
Hypertonic hyponatraemia (high plasma osmolality)
Hyperglycaemia (Diabetes)
Hypotonic hyponatraemia (low plasma osmolality) Hypovolaemia
- Extrarenal (urine Na <20mmol/L) GI loss: diarrhea, vomiting Skin loss: excessive sweating - Renal (urine Na >20mmol/L) Diuretic Rx Salt losing nephritis Addision’s disease
Euvolaemia (urine Na <20mmol/L) Acute water overload
Increased intake PLUS hypovolaemia: haemorrage, burns, drugs including IV hypotonic fluids Stress: post surgery, psychogenic Renal insufficiency
Euvolaemia (urine Na>20mmol/L) Chronic water overload
SIADH Drugs Chronic renal failure Endocrine: hypothyroidism, cortisol deficiency
Hypervolaemia (urine Na <20mmol/L): Oedematous states
Cardiac failure Hepatic cirrhosis Nephrotic syndrome
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