Acute Hyponatraemia

Key facts:

Authors: Stephanie Horne and Andrew Stein
Top Tips: Hyper-, eu- or hypo-volaemia is possible; Rx is quite different; beware Central Pontine Myelinosis; get senior help

Key Differential Diagnosis

• Reduced conscious level, due to another cause + low sodium

Key Investigations

• Urinary sodium
• U+E, LFT, Bone, Glucose, TFTs
• FBC, ESR, CRP
• CXR (cause ?SIADH(

Key Treatment

• Look for (and Rx) underlying cause
• Remove drug causes (especially diuretics and SSRIs)
• ± PO/IV FUROSEMIDE 40-80 mg od (hypervolaemic)
• ± IV FLUIDS (hypovolaemic)

Key Management Decisions

• (Hypervolaemic) fluid restriction
• (Euvolaemic) mild fluid restriction
• (Hypovolaemic) fluid push or IV fluids

Background

Severe hyponatraemia is a factor in 5% of all acute medical admissions

Introduction

• Hyponatremia is a decrease in plasma Na concentration to
• Common community-acquired causes include drugs (diuretics, SSRIs), heart failure, SIADH and renal disease. In-hospital causes include excessive use of IV 5% Dextrose (effectively water) especially post-operatively, and drugs
• Whatever the cause, bring the Na up slowly;
• Clinical manifestations are primarily neurological (due to an osmotic shift of water into cerebral cells, causing cerebral oedema), especially in acute hyponatremia. They include headache and confusion. Eventually seizures and coma may occur
• The patient may be hypervolaemic, euvolaemic or hypovolaemic. Deciding on the fluid state is vital, as it alters Rx fundamentally. Treatments are very different, depending on the cause and fluid state; eg diuretics can be the cause or Rx of the problem
• In all patients, search for and Rx the underlying cause, as well
• It is common (5% of admissions) and has a high mortality (5% if cut-off

Definition

• = Plasma sodium < 135 mmol/L

Epidemiology

• 5% hospital admissions (20% require ITU); the commonest electrolyte abnormality in hospitalised patients
  [Ref]
• 70% have pre-existing hyponatraemia: [Ref]

Types

• Hypervolaemic
• Euvolaemic
• Hypovolaemic

Severity

• Mild = 125-134 mmol/L
• Moderate = 116-124
• Severe = < 116

Causes

Big Three

• Drugs (diuretics/SSRIs)
• Heart failure
• SIADH
  Note: often combination of above

Other

• Hypervolaemic (poor prognosis)
  CCF
  Hepatic/Renal failure
  Nephrotic syndrome
  Iatrogenic (xs IV DEXTROSE post-op = commonest hospital-acquired cause)
  Note: many of these patients have more than one cause, eg CCF, mild CRF, on diuretics (or given IV fluids incorrectly)
• Euvolaemic
  Artefact (taken IV from arm)
  Psychogenic polydipsia
  Hypothyroidism
  SIADH (many causes, especially lung/cerebral pathology and drugs eg SSRIs)
• Hypovolaemic
  Renal: Diuretics (commonest community acquired cause), diuresis (eg osmotic diuresis (urea, glucose (DKA), mannitol), hypoadrenalism, Na-losing nephropathies
  GI: diarrhoea, vomiting
  Other: burns, pancreatitis

Symptoms

• Nil, if mild
• Depends on fluid state (ie could be thirsty and dizzy if hypovolaemic, or c/o SOB/oedema if hypervolaemic)
• Graduation of symptoms: from nil (mild; Na⁺ 125-134), to lethargy + anorexia (moderate; Na⁺ < 116-124) to agitation, confusion, fits and coma (severe; Na⁺ <116) 

Key questions

• "Has anyone changed your tablets in last 4-6 weeks?"
• "Have you had any infections, diarrhoea or vomiting recently?"

Signs

• Nil, if mild
• Depends on fluid state (JVP estimation very important)
• May be confused, drowsy, or fitting

Investigation

Even though it is traditional to measure plasma and urine osmolality, they rarely affect initial decision making. This is based on how unwell the patient is, the Na level and the patient's fluid state

Blood

• U+E, LFT, Bone, Glucose
• FBC, CRP, ESR

Other

• CXR (cause of ?SIADH)

Key investigations

• Sodium
• CXR

Specialist investigations

Blood

• Serum osmolality (275-285 mosmol/L)

Urine

• Urinary sodium (normal 10-20 mmol/L)
• Urine osmolality (70-1200; low <300; high >800 mosmol/L)
  Note: these tests are all difficult to interpret if the patient is on diuretics; serum osmo decreased (<260); urine osmo increased (>500) + urine Na⁺ increased (>20); ie the 'inappropriate' thing is that despite dilute blood, the patient is excreting concentrated urine
• SIADH
• Excess renal losses (renal failure, salt-losing nephropathy, hypoadrenalism)
• Note:Serum osmo decreased (<260); urine osmo decreased (<300) + urine Na⁺ decreased (<10); ie the 'appropriate' response to dilute blood, is to excrete dilute urine
• Extra-renal losses and causes of hypervolaemia

Other

• CT head

Treatment

Treatments are very different, depending on the cause eg diuretics can be either cause or treatment. In all patients, search for and Rx the underlying cause

Treatment

• Hypervolaemia (moderate fluid restriction (<1L/d) ± PO/IV FUROSEMIDE 40-80 mg od)
• Euvolaemia (mild fluid restriction, <1.5L/d)
  Note: in SIADH, treat the cause, mild fluid restriction, occasionally PO DEMECLOCYCLINE 300 mg bd
• Hypovolaemia (stop diuretcs, IV fluids, consider half-normal saline)

Prescribing issues

• DEMECLOCYCLINE, like most tetracyclines, are catabolic in renal failure; hence, use DOXYCYCLINE (or according to local guideline or protocol)

Key management decisions

• (Hypervolaemic) fluid restriction
• (Euvolaemic) mild fluid restriction
• (Hypovolaemic) fluid push or IV fluids

Stop

• Diuretics, if hypovolaemic, and thought to be cause
  Note: though in other cases giving/inc diuretics can be treatments
• Other drugs that may cause hyponatraemia (eg SSRIs)
• IV fluids (esp 5% dextrose, post op)

Admit

• Usually (unless mild, and cause obvious)

Bed plan

• Any medical ward
• ± Endocrine/DM
• ± ITU, if Na⁺

Referrals

Medical    

• Endocrine (renal?)

Other

• Pharmacist (drug causes)

The Rest

It is a medical emergency, but Na should not be raised too quickly (or Central Pontine Myelinosis can occur)

Prognosis

• Poor; 5% mortality if Na cut-off
• But 50% if Na cut-off is
• It is unclear whether hyponatraemia is the cause of this mortality, or is a marker of being a 'sick' patient: [Ref]

2° Prevention + Health Promotion

• Monitor Na⁺ in patients with diuretics, SSRIs

Don't forget

• Usually caused by an excess of water relative to solute
• 70% have pre-existing hyponatraemia
• Common causes include drugs (diuretics, SSRIs), heart failure, SIADH and renal disease
• In-hospital causes also include excessive use of IV 5% Dextrose (effectively water) especially post-operatively
• May be hypervolaemic, euvolaemic or hypovolaemic. Deciding on the fluid state is vital, as it alters Rx fundamentally
• Hypoadrenalism (especially if associated with K↑); rare but treatable
• Whatever the cause, bring Na up slowly;
• Faster risks central pontine myelinolysis

Red flags

• Na+ < 120 mmol/L
• Drowsy 
• Fitting

References