Makindo Medical Notes.com

Related Subjects: |Sodium Physiology |Hyponatraemia |TURP Hyponatraemia syndrome |Hypernatraemia |Diabetes Insipidus

A sudden drop in Na leads to progressive cerebral oedema with cell swelling and dysfunction. Progression can lead to brain herniation and neurological damage. A gradual drop is often better tolerated and needs less aggressive management. Ensure that Na rise is less than 12 mmol/L per day and correction may be over 2-3 days.

Physiology

• Normal response to low sodium is a reduced ADH and free water excretion.
• Inappropriate ADH retains free water with an inappropriately raised urine osmolality (> 100) and the rest should have low osmolalities.
• Normal response to hypovolaemia is ADH release + thirst + Aldosterone mediated Na retention with decreased Urinary Na.
• Hyponatraemia and low serum osmolality causes free water to enter cells causing swelling. This is most notable in the brain with progressive cerebral oedema

Clinical

• Examine drug chart and fluid balance and obs always and State of hydration. Context is important
• You need to decide if patient is
• Hypovolaemic: Dehydrated: reduced skin turgor, low BP, dehydrated, Tachycardia, postural drop, dry mucous membranes, concentrated urine. Post operative or Addison's.
• Euvolaemic: normal BP, HR, No oedema: Consider SIADH: See topic.
• Hypervolaemia: Renal/Cardiac/Liver failure. Normal or high BP, Ascites, Oedema

Clinical Types and causes

• Hypovolaemic (dry): Dehydrated, Poor capillary return. Low BP, Vomiting and diarrhoea (Low volume concentrated urine with low Na) or diuretic overuse or Cerebral salt wasting seen in SAH, drains, burns, Addison's disease, Salt losing nephropathy. Drugs: Diuretics impair salt retention (e.g. thiazide diuretics, amiloride, loop diuretics),
• Euvolaemic: SIADH, Pain, Stress, Medication e.g. opiates. (All of these occur with surgery), Alcohol excess and malnutrition, Excess IV Dextrose, Psychogenic polydipsia. NSAIDs, Neuroleptics SSRIs, Opiates, ecstasy
• Hypervolaemic (wet): CCF, Chronic liver disease, Chronic kidney disease, Liver failure or Free water overload - TURP syndrome or excessive hypotonic IV fluids or water intake e.g. potomania if kidneys cannot rapidly excrete free water,

Surgical causes of post op hyponatraemia

• Being given 5% Dextrose alone
• Post surgical rise in ADH causes free water retention
• Pain and Opiates induce ADH release
• Post Turp hyponatraemia syndrome - see the topic

Medical causes of hyponatraemia

• Diuretics causing Salt and Water loss
• Heart/Liver/Renal Failure with water retention
• SIADH with excess water reabsorption in kidneys
• Pseudohyponatraemia - high lipids or plasma proteins

Severity

• Symptoms come with rapid changes or levels below 120 mol/L. Coma or seizures or level below 110 mmol/l is considered an emergency
• Na⁺ 110-115 mmol/l : headache and irritability, delirium, weakness
• Na⁺ < 110 mmol/l : Coma, seizures, ataxia, tremor

Investigations

• U&E : Mild Na⁺ 135-130 mmol/L, Mod 125-129 mmol/L, Severe < 125 mmol/L or symptoms
• Serum osmolality < 275 mosm/kg in SIADH
• Urine osmolality > 100 mosm/kg H₂O suggests SIADH when there is hypotonic plasma
• Urine Na⁺: < 20 in Low in Hyperldosteronism, Na > 30 in SIADH
• TFT: High TSH in Hypothyroidism
• Exclude Addison's disease 9 am Low Cortisol +/- Short Synacthen test:
• CXR: tumour , infection
• CT head if any question of acute neurology - may be seen with SAH but should be clear from context.
• Lipids: rarely a very high lipids level can give pseudohyponatraemia

Central Pontine Myelinosis a consequence of rapid correction of low Na

Typical Algorithm

Differential

• Pseudohyponatraemia: Excessive lipids or proteins in plasma inflate the plasma volume. There is a Low Na but all else normal. Bloods show high protein or cholesterol. Plasma osmolality will be normal or even high

Management

• ABC and move to HDU setting as needed to closely monitor U&E, NEWS score and CNS status
• Full assessment of condition, drugs taken, recent issues. Appropriate Monitoring based on current status. The cause is usually apparent from context.
• Main issue is a clinical assessment of the state of hydration if the patient is euvolemic or dry (hypovolaemic and clinically dry with low BP, tachycardia and low urinary Na) or wet (hypervolaemic - ascites, heart or liver failure). The issue is always is this salt loss or water excess. These can be difficult to manage but there are some general principles. Always take more senior advice if the patient is symptomatic or Na < 120 mmol/L or unclear of cause.
• Hyponatraemia causes cellular swelling, Sudden correctional rises in Na⁺ from a period of low Na or hypo-osmolar state can cause devastating neurological damage - central pontine myelinosis. In the emergency setting the desire is not a normal Na but a Na > 120 mmol/L as this should reduce CNS complications. Therefore Na⁺ must be held such that it comes up no faster than 12 mmol/24 hours. If the Na has fallen quickly e.g. due to water dilution with prostate surgery then it can be corrected quickly. Those with prolonged hyponatraemia are at the highest risk of CNS damage from rapid correction.
• If dehydrated and salt deficient then simple consider N-Saline and slowly replace. The best advice is to start slow and monitor frequently. Take expert advice.
• If euvolemic and likely SIADH then fluid restrict 800-1000 mls/day. For those with coma/seizure consider small volumes e.g. 150 ml of 3% saline over 15 minutes for very severe symptomatic hyponatraemia to give a one-off change off change of 5 mmol/L. This may be repeated after 20 minutes. This can be followed up with fluid restriction and Tolvaptan 15 mg OD or Demeclocycline 150-300 mg. Look for drugs causes e.g. thiazides and SSRI but check all drugs taken.
• Suspected Addison's disease will need short synACTHEN test and Steroids - IV Hydrocortisone 100 mg acutely
• Post-TURP Syndrome or Sudden Excess hypotonic fluids e.g. IV Dextrose or huge intake of oral water excess can require IV diuretics. See Postoperative Hyponatraemia topic
• Hypervolaemic hyponatraemia: Fluid and salt restriction. Consider diuretics Treat the underlying cause.

References

• The diagnosis and management of inpatient hyponatraemia and SIADH
• Guidelines for the management of hyponatraemia in hospitalised patients