Hyponatraemia

Definition

Serum sodium <135 mmol/L

Numbers

20% of all patient

Classification of hyponatremia

Biochemical

Mild 130-135mmol/l
Moderate 125-129mmol/l
Severe <125mmol/l

Symptoms

Severity	Common Symptoms
Mild	Headache ; Nausea ; Mild confusion or decreased ability to think ; Poor balance or dizziness ; Fatigue
Moderate	Vomiting ; Marked confusion or altered mental status ; Muscle cramps/twitching ; Irritability
Severe	Seizures ; Coma ; Profound confusion ; Hallucinations ; Brain swelling/cerebral edema

Symptoms do not always correlate with sodium level:

Profound biochemical hyponatraemia may be asymptomatic
Moderate cases can present with severe symptoms.

Symptomatic Hyponatraemia Moderate Symptoms No or Mild Symptoms Headache • Irritability Nausea I vomiting Mental slowing • Unstable gait / falls Confusion / delirium • Disorientation Severe Symptoms • Stupor I coma • Convulsions • Respiratory arrest — Symptomatic hyponatraemia

Volume status/aetiology

Volume Status	Major Causes	Examples
Hypovolemic	GI/renal losses, diuretics, mineralocorticoid deficiency, CSW	Vomiting, diarrhea, thiazides, Addison’s
Euvolemic	SIADH, drugs, endocrine disorders, polydipsia	SSRIs, hypothyroidism, beer potomania, MDMA
Hypervolemic	Edema-forming illnesses, excessive water intake	Heart failure, cirrhosis, nephrotic syndrome, AKI/CKD, EAH

Acuity of onset

Acute < 48 hours
Chronic > 48 hours

Pathophysiology

Hyponatremia arises from a relative excess of water compared to sodium in the body, often due to impaired water excretion rather than absolute sodium loss.

Acute hyponatremia → water influx into brain cells → cerebral edema.

The brain adapts over time by losing electrolytes and organic osmolytes to minimize swelling.

Early/acute hyponatremia causes brain swelling, leading to neurological symptoms and, in severe cases, brain herniation or death.

In chronic cases, slow adaptation reduces symptoms but increases the risk of osmotic demyelination syndrome (ODS) if sodium is corrected too quickly.

The brain undergoes volume adaptation in response to gradual-onset hyponatraemia of hypotonic state adaptation Water pin Loss sodium. potassiu and chloride LOSS of orgaNc osnWVtes Adrogue HO, Madias NE, NEJM. 2000; 342; 21: 1581-1589. — The brain undergoes volume adaptation in response to gradual-onset hyponatraemia

Assessment

General

Stop hypotonic fluids
Review drug card - long, long list of drugs that cause hyponatremia: PPI etc.

Specific

Plasma and Urine Osmolality
Urinary Na⁺
Glucose
TFT's
+/- Assessment of Cortisol
Assessment of underlying causes

Management

Principes

Treatment depends on symptom severity and duration of hyponatremia.
Use both Clinical symptoms and Sodium value to drive treatment

Acute serious symptoms (e.g., seizures, stupor) require prompt but controlled correction with hypertonic saline.

iv 150ml of 3% saline or equivalent over 20mins

best administered as intermittent boluses rather than continuous infusion for safety and efficacy.

Check serum Na⁺
Repeat twice until 5mmol/l increase Na⁺
After 5mmol/l increase, stop hypertonic saline, establish diagnosis, Na⁺ 6 hourly for 1ˢᵗ 24 hours, limit increase to 10mmol/l first 24 hour

In chronic or asymptomatic cases, correction must be slow to avoid ODS;

Strategies include

Fluid restriction
Addressing underlying causes
Careful monitoring of serum sodium rise.

Overcorrection should be avoided
Correction should not exceed 10 mmol/L in the first 24 hours (or 8 mmol/L if high risk), and 8 mmol/L in each subsequent 24-hour period.

Management algorithm of chronic hyponatraemia



graph TD
    A[Hyponatraemia] --> B{fluid overloaded}
    A --> C{Normovolaemic}
    A --> D{Dehydrated}

    B --> B_Group[
        Cirrhosis of liver/liver failure
        <br>CCF
        <br>inappropriate iv fluids
    ] --> C9

    C --> C_Group1[
        Serum + Urine Osmolalities
        <br>Spot Urine Sodium
        <br>TSH normal
        <br>Cortisol > 430 nmol/L
    ]
    C_Group1 --> C_Group2[
        SIADH
        <br>P Osm <275mOsmol/kg
        <br>U Osm >100mOsmol/kg
        <br>U Na >40mmol/l
    ]
    C_Group2 --> C9[Fluid Restrict 500 - 1000ml / 24hrs]

    D --> D_Group1[
        Low Urine Na
        <br>Vomiting and diarrhoea
        <br>Burns
        <br>Pancreatitis
        <br>Sodium depletion<br>after diuretics
    ]
    D_Group1 --> D6[Saline replacement]

    D --> D_Group2["
        Urine Na>40mmol/l
        <br>Diuretics
        <br>Addison's (or occasionally<br>pituitary failure)
        <br>Cerebral salt wasting
        <br>Salt wasting nephropathy"
    ]
   D_Group2 --> D6