Hyponatraemia 💧

Clinical guide for evaluating and managing hyponatraemia. Step-by-step algorithm, volume status assessment, SIADH, and correction rates for on-call doctors.

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🚨 Life-Threatening Hyponatraemia

⚠️ ESCALATE IMMEDIATELY if life-threatening features present
Life-threatening features: Severe neurological symptoms (seizures, coma, severe confusion), signs of cerebral oedema, very low Na (<120 mmol/L) with severe symptoms, or rapid onset with severe symptoms
Management: Proceed to life-threatening hyponatraemia algorithm per IAEM guideline
Note: This topic covers NON-LIFE-THREATENING hyponatraemia only

🧠 Definition & Classification

Hyponatraemia = Serum Na⁺ < 135 mmol/L
Most common electrolyte disorder in hospitalised patients
Most common form: Hypotonic hyponatraemia (serum osmolality <275 mOsmol/kg)
Use the interactive algorithm below for systematic evaluation (based on IAEM guideline Figure 1)

📋 Clinical History & Examination

History: Focus on symptoms of hypothyroidism, glucocorticoid deficiency, CCF, renal/liver failure
Note recent medication changes or extra fluid intake
Symptoms (Na <130): Fatigue, lethargy, somnolence, nausea, vomiting, anorexia
Signs of cerebral oedema: Seizures, confusion, lethargy
Examination: Assess vital signs, examine for liver/renal/heart failure, hypothyroidism, Addison's disease
Fluid status assessment: Pulse, BP, capillary refill, mucous membranes, skin turgor, pitting oedema, respiratory exam, JVP, urine output, weight trend

🩺 Volume Status Assessment

Volume Status	Clinical Features
Hypovolaemic	Dry mucous membranes, decreased skin turgor, postural hypotension, tachycardia, low JVP, reduced urine output, poor capillary refill
Euvolaemic	Normal volume status, no signs of dehydration or overload. Most common cause: SIADH
Hypervolaemic	Peripheral oedema, raised JVP, ascites, pulmonary oedema. Associated with heart failure, cirrhosis, nephrotic syndrome, renal failure

🧪 Core Investigations

Serum sodium
Serum osmolality (or calculate: 2 × Na + glucose + urea)
Venous blood gas (VBG)
Urinary sodium
Urinary osmolality
Urea and electrolytes
Full blood count
Liver function tests
Thyroid function tests
CXR (to assess for pulmonary oedema)

🔍 Diagnosis of Hypotonic Hyponatraemia (IAEM Table)

Volume Status	Urinary Na	Likely Diagnoses
Hypervolaemic	<30 mmol/L	Liver failure, Cardiac failure
Hypervolaemic	>30 mmol/L	Renal failure, Diuretic treatment in cardiac failure
Euvolaemic	<30 mmol/L	Hypothyroidism, Iatrogenic (IV fluids), Water intoxication
Euvolaemic	>30 mmol/L	SIADH, Adrenocortical deficiency
Hypovolaemic	<30 mmol/L	Diarrhoea and vomiting, Reduced PO intake, Burns
Hypovolaemic	>30 mmol/L	Salt wasting (renal/cerebral), Diuretic medication, Addison's disease

🔍 Other Forms of Hyponatraemia

Type	Serum Osmolality	Causes
Isotonic	275-295 mOsmol/kg	Renal failure, High serum lipids/protein/glucose
Hyperosmolar	>295 mOsmol/kg	Infusion of hypertonic solutions (e.g. mannitol)

💊 Treatment by Volume Status (Non-Life-Threatening)

Volume Status	Management
Hypervolaemic	Treat the cause, Fluid restrict if appropriate
Euvolaemic	Treat the cause, Stop potential contributing medications, Fluid restrict if appropriate
Hypovolaemic	Treat the cause, Administer IV 0.9% NaCl or Hartmann's at 0.5-1ml/kg/hr, Reassess Na every 8 hours, Target Na rise <10mmol/L in 24h (healthy) or <8mmol/L (malnourished)

⚠️ Osmotic Demyelination Syndrome (ODS)

Risk: If hyponatraemia present >48 hours, brain cell osmolality falls and brain becomes dehydrated
Overcorrection can cause ODS: paresis, paralysis, seizures, coma
Prevention: Target Na rise <10mmol/L in 24h (healthy) or <8mmol/L (malnourished)
Never correct rapidly in chronic hyponatraemia
Monitor closely during correction

📈 Monitoring During Correction

Repeat clinical assessment and sodium monitoring every 8 hours (early in treatment)
Frequency depends on patient stability and rate of sodium change
Monitor urine output during treatment while fluid balance is being adjusted
Watch for complications: Cerebral oedema (acute), ODS (chronic, overcorrection), fluid overload
If Na rises too quickly: Stop correction, consider DDAVP or hypotonic fluids, consult endocrinology/nephrology

🔗 External Resources

IAEM - Assessment and Management of Hyponatraemia in the Emergency Department Hypernatraemia Fluid Replacement Acute Kidney Injury Consult Endocrinology

Non-Life-Threatening Hyponatraemia Algorithm

Step-by-step assessment and diagnosis

Step 1: Serum Sodium (mmol/L) *

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Volume Status

Clinical Features

Hypovolaemic

Dry mucous membranes, decreased skin turgor, postural hypotension, tachycardia, low JVP, reduced urine output, poor capillary refill

Euvolaemic

Normal volume status, no signs of dehydration or overload. Most common cause: SIADH

Hypervolaemic

Peripheral oedema, raised JVP, ascites, pulmonary oedema. Associated with heart failure, cirrhosis, nephrotic syndrome, renal failure

Volume Status

Urinary Na

Likely Diagnoses

Hypervolaemic

<30 mmol/L

Liver failure, Cardiac failure

Hypervolaemic

>30 mmol/L

Renal failure, Diuretic treatment in cardiac failure

Euvolaemic

<30 mmol/L

Hypothyroidism, Iatrogenic (IV fluids), Water intoxication

Euvolaemic

>30 mmol/L

SIADH, Adrenocortical deficiency

Hypovolaemic

<30 mmol/L

Diarrhoea and vomiting, Reduced PO intake, Burns

Hypovolaemic

>30 mmol/L

Salt wasting (renal/cerebral), Diuretic medication, Addison's disease

Type

Serum Osmolality

Causes

Isotonic

275-295 mOsmol/kg

Renal failure, High serum lipids/protein/glucose

Hyperosmolar

>295 mOsmol/kg

Infusion of hypertonic solutions (e.g. mannitol)

Volume Status

Management

Hypervolaemic

Treat the cause, Fluid restrict if appropriate

Euvolaemic

Treat the cause, Stop potential contributing medications, Fluid restrict if appropriate

Hypovolaemic

Treat the cause, Administer IV 0.9% NaCl or Hartmann's at 0.5-1ml/kg/hr, Reassess Na every 8 hours, Target Na rise <10mmol/L in 24h (healthy) or <8mmol/L (malnourished)