Hyponatraemia

Hyponatraemia

Key Points

·        Normal value of Na: 135 – 145 mmol/L

·        Hyponatraemia is not a diagnosis – it is found in diverse conditions. Body Na may be low, normal or high. Relative water retention is a common factor

·        Condition and treatment can be hazardous.  If correct too fast then pontine demyelination

·        Treatment must be slow and monitored closely. Treatment can range from water restriction or diuresis to sodium restriction or normal saline. Need to know underlying cause

·        Don‟t use hypotonic fluids post-op unless Na is high. Eg dextrose saline – glucose absorbed very quickly post surgery ® hypotonic

Symptoms

·        The big boogie is underlying cerebral oedema. Bigger problem if abrupt onset. Rapid correction can cause central pontine melanosis

·        Symptoms don‟t correlate well with [Na]

·        Early: anorexia, headache, nausea, vomiting, muscle cramps, weakness

·        Advanced: mutism, dysarthria, impaired response to verbal or painful stimuli, bizarre behaviour, hallucinations, asterixis, incontinence, respiratory insufficiency, spastic quadriparesis in 90%

·        Far advanced: (too late to do much) decorticate or decerebrate posturing, bradycardia, hypo or hypertension, dilated pupils, seizures, respiratory arrest, coma, polyuria (central diabetes insipidous)

·        Should always be a differential in post-operative coma

Aetiology

·        Either Na depletion or water gain (usually water gain)

·        Inappropriate water retention: eg drugs (most common – eg antiepileptics), ­ADH, kidney or thyroid problems

·        May be borderline hyponatraemic before (eg long term use of diuretics)

·        Normal ADH will ­ if ­osmolality or ¯blood volume

·        Operative stress or serious illness ® syndrome of inappropriate ADH (in most people) ® water retention (especially in women, smaller starting fluid volume). NB it‟s not really inappropriate – the body is making a justifiable physiological response: I‟m stressed so conserve water rather than throwing it out.

·        Ageing impairs fluid homeostasis ® wider swings happen easily

Assessment

·        History: fluid losses, diuretics, other medications

·        Clinical findings: pulse, blood pressure, volume assessment, oedema, thirst, skin, input/output

·        Laboratory:

o   Creatinine, urea, glucose, HCO₃, K, plasma osmolarity, urine Na and Osmolarity

o   Severe hyponatraemia is < 125 mmol/l: nausea, malaise, headache

o   < 115 mmol/l: convulsions

·        Look for:

o   Low Na and ¯ serum osmolality

o   Urine osmolality higher than expected (>200 and usually > serum osmolality)

o   Urinary sodium higher than expected (> 30)

o   Normal pituitary, adrenal, cardiac, and renal function

·        Clinically useful grouping (Þ volume assessment critical):

o   Hyponatraemia with oedema: heart failure + diuretic, cirrhosis, nephrosis (impairment of water loss via increased ADH +/- Na loss)

o   Hyponatraemia with dehydration:

§  Urine [Na] > 20 mmol/l: Diuretics, Addison‟s Disease, Salt losing nephritis

§  Urine [Na] < 20 mmol/l: Vomiting, Diarrhoea, Skin loss

§  Usually rehydrate slowly with normal saline

o   Hyponatraemia with euvolaemia and reduced plasma osmolality:

§  Urine [Na] > 20 mmol/l: Chronic water overload (eg primary polydypsia, chronic SIADH – central or malignancy, etc)

§  Urine [Na] < 20 mmol/l: Acute water overload (eg acute SIADH, oxytocin for induced labour, etc)

·        Treat with fluid restriction < 1000 ml/day, and treat underlying cause

·        Complicating factors:

o  If plasma osmolarity is high then measure glucose. Hyperglycaemia ® shift of water out of muscle cells: Na ¯ 1 mmol/L for every 4 mmol/L ­ in glucose

o  If osmolarity is normal then pseudo-hyponatraemia (eg hyperlipidaemia, hyperprotinaemia). An artefact: Na has been incorrectly measured in plasma volume rather than plasma water

Syndrome of Inappropriate ADH secretion

·        = SIADH

·        See Diabetes Insipidous

·        Ectopic ADH Production (relatively rare): malignancies of lung, bronchus, brain, kidney, duodenum, pancreas

·        Central production:

o  Cerebral infections, trauma, tumours, haemorrhage

o  Lung disease, eg pneumonia

o  Drugs, eg morphine, carbamazepine (anti-epileptic)

o  Can be seen in AIDS patients (?combination of above factors)

Common Scenarios

·        Prolonged vomiting and rehydration with Gastrolyte – only contains 60 mmol/L Na

·        If dehydrated (eg vomiting) and on diuretic, ADH still conserves water, but ¯Na retention so ¯[Na]. We preserve volume at the expense of osmolarity

·        Serious post-operative problem. Especially women after elective surgery (eg gynaecology wards). Hypothesis: surgery ® ­ADH (eg due to pain), dextrose also given in belief that it slows catabolism and promotes healing – but together they lead to ¯[Na]

·        Sample cases:

·        

Treatment

·        Principles:

o  Raise the sodium at a safe rate

o  Treat the cause

·        Basic regimes:

o  If volume depleted (Renal/GI losses, diuretics, adrenal insufficiency): saline isotonic to the patient or normal saline. Extra Na will have a small effect but ­volume ® ¯ADH ® excess water excreted 

o  Normovolaemic or oedematous (SIADH, renal failure, polydypsia, oedema): Water restriction

o  If severe symptoms or if sodium < 110 then ?hypertonic saline. ­Na by no more than 12 mmol per 24 hours: keep rate smooth. Key judgement is speed of infusion. No front loading. Animal studies show correction by > 14/mmol/24 hours ® lesions in 71% of dogs. If no symptoms – maybe go slower

·        Monitor 2 hourly.  Manage in high dependency unit.  Detect and treat hypoxia

·        Adverse neurological consequences of rapid correction: myelin breakdown in the pons, patchy symmetrical lesions elsewhere in the brain. But risk of not treating acute cerebral oedema far exceeds the small risk of osmotic demyelination

·        Maybe frusemide to ­free water excretion

Dehydration or Volume Depletion

·        Dehydration:

o   Often used loosely to describe a volume depleted patient

o   Correctly it refers to ¯intracellular water, following fluid shifts from ICF to ECF

o   Water is lost (either as pure water or as hypotonic fluid) ® ­osmolality and thirst

o   Treatment is water replacement (dextrose)

·        Volume depletion:

o   Losses from the ECF (isotonic sodium) ® ¯circulating volume

o   ¯BP, ­tachycardia, ¯tissue turgor

o   Treatment is replacement of NaCl

·        Dehydration and volume depletion can co-exist