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Electrolytes
 

Hyponatraemia

 
 

Overview

  • Hyponatraemia is an imbalance between sodium and water balance in the body, producing a reduced serum sodium concentration of <135 mmol/L.
      • Normal Range

      • Serum: 135 - 145 mmol/L
      • Urine: 40 - 220 mmol/L

Physiology of Sodium

  • Sodium is the principle cation in extracellular fluid. Its extracellular concentration is normally 12 mmol/L, while its extracellular concentration is approximately 140 mmol/L.
    • Intake

    • Dietary sodium comes from many different sources. Average dietary salt intake is much higher than recommended levels for most western people.
    • Excretion

    • Sodium is primarily excreted by the kidneys; it is freely filtered through the glomerulus and reabsorbed within the proximal convoluted tubule. Sodium is also excreted through sweat, tears, and faeces.
    • Roles of Sodium

    • Maintenance of blood volume
    • Maintenance of cellular osmotic pressure gradients
    • Maintance of membrane potentials, and activation of action potentials
    • Nerve conduction
    • Sodium Homeostasis

    • Pressure natriuresis - increased blood volume and/or pressure will stimulate increased sodium excretion through increased glomerular filtration.
    • Renin-angiotensin-aldosterone system (RAAS) - reduced perfusion within the juxtaglomerular apparatus stimulates renin release, which in turn stimulates the production of angiotensin I. Angiotensin I is converted to angiotensin II by ACE. Angiotensin II modulates pressure natriuresis and increases tubular sodium resorption, both directly and indirectly by stimulating aldosterone.
    • Aldosterone - once secreted by the adrenal cortex will stimulate increased active reabsorption of sodium.
    • Antidiuretic hormone (ADH) - the pituitary gland secretes ADH in response to reduced serum water concentration, which then acts on the distal convoluted tubule to salvage water and concentrate urine.

Aetiology of Hyponatraemia

  • Hyponatraemia may occur due to loss of salt through the kidneys or elsewhere, through dilution or through incorrect sampling.
    • Causes of Hyponatraemia

    • Salt Depletion

    • Extrarenal - vomiting, diarrhoea, third spacing
    • Renal - loop or thiazide diuretic abuse, Addison’s, renal tubular acidosis
    • Dilution

    • Osmotic shift - hyperglycaemia, mannitol
    • Excess water intake - primary polydipsia, beer potomania
    • Antidiuresis - SIADH, hypothyroidism, ACTH deficiency
    • Fluid overload - heart failure, cirrhosis, nephrotic syndrome, renal failure
    • Factitious

    • Pseudohyponatraemia - lab error, hyperproteinaemia, hyperlipidaemia
    • Causes of SIADH

    • Malignancy - lung cancers, pancreatic cancer, colorectal cancer, bladder cancer, lymphoma, sarcoma
    • CNS - tumours, demyelination, ischaemic stroke, haemorrhage, infection, vasculitis, traumatic brain injury
    • Pulmonary - pneumonia, tuberculosis, abscess, vasculitis
    • Drugs - desmopressin, SSRIs, TCAs, carbemazepine, levetiracetam, haloperidol, cyclophosphamide, vincristine
    • Postoperative state

Clinical Features

  • Mild or gradual onset hyponatraemia generally presents with few symptoms, while severe or rapid onset hyponatraemia can be life-threatening.
  • Acute hyponatraemia may present with nausea, vomiting, headaches or confusion; as the sodium level falls patients may present with seizures (including status epilepticus) or coma.
  • Chronic hyponatraemia may also present with nausea, vomiting or headaches. Patients may also report polydipsia, lethargy or muscle cramps.

Approach to Diagnosis

  • In order to further investigate the cause of hyponatraemia, first check the serum and urine sodium and osmolality at the same time
  • Normal serum osmolality (280 - 285 mOsm/kg) indicates either a spurious result (pseudohyponatraemia) or excessive administration of isotonic solutes like isotonic mannitol.  Pseudohyponatraemia is the false laboratory measurement of sodium due to an increased solid phase of plasma, such as with hyperproteinaemia or hyperlipidaemia.
  • Elevated serum osmolality (>285mOsm/kg) indicates movement of water from cells to the interstitium and intravascularly, driven by extracellular solutes such as sugars. This can occur with hyperglycaemia and hypertonic mannitol. A sodium correction should be performed to account for serum glucose - see the Calculators section for this tool.
  • Low serum osmolality (<280 mOsm/kg) suggests excess total body water in relation to total body sodium, due to sodium loss or water retention. Examine the patient's fluid status to further delineate a cause.
  • Low serum osmolality with dehydration suggests salt and water loss from the kidneys or elsewhere. Low urinary sodium (<20 mEq/L) in these patients is suggestive of GI or sequestrational loss, such as due to vomiting, diarrhoea or third spacing. Normal urinary sodium (>20 mEq/L) is suggestive of renal loss, such as due to diuretics, mineralocorticoid deficiency, renal tubular acidosis, cerebral salt wasting or salt wasting nephropathy.
  • Low serum osmolality with euvolaemia is suggestive of redistribution. Urine osmolality <100 mOsm/kg may be due to primary polydipsia or beer potomania syndrome; urine osmolality >100mOsm/kg suggest SIADH, hypothyroidism or glucocorticoid deficiency.
  • Low serum osmolality with hypervolaemia is suggestive of water retention. Low urinary sodium (<20 mEq/L) in this case is suggestive of renal failure as a cause, while normal urinary sodium (>20 mEq/L) suggests other causes of fluid overload such as heart failure, liver cirrhosis, nephrotic syndrome or hypoalbuminaemia.
    • Approach to Diagnosis
Last updated on January 5th, 2019
 
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Associated Diseases
BETA

Endocrinology
Neurological
Oncology
CNS Cancers
Renal & Urinary
Renal Tubular Acidosis
Other
Cardiovascular
Gastrointestinal
Immunology
Infectious Diseases
Tuberculosis
Nutrition & Metabolic
Psychiatry
Respiratory
Trauma

Associated Drugs
BETA

Cardiovascular
Loop Diuretics
Drug & Alcohol
50% Dextrose
Neurologic
Levetiracetam
Oncology
Vincristine
Psychiatric
Selective Serotonin Reuptake Inhibitors
Haloperidol
 

Read More...

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