Disorders of sodium + water balance Flashcards
What is the most common electrolyte abnormality?
hyponatraemia
What is the classification of hyponatraemia?
according to severity
130-135 = mild
125-129 = moderate
<125 = severe
Clinical features of acute hyponatraemia (<48h)
seizures
coma
respiratory distress
Clinical features of chronic hyponatraemia (>48h)
frequently mild or no symptoms
headache
restlessness
muscle cramps
nausea
vomiting
lethargy
confusion
disorientation
What can acute hyponatraemia do to the brain?
brain oedema
What can happen if you rapidly correct chronic hyponatraemia?
osmotic demyelination syndrome
(central pontine myelinolysis)
What can cause isoosmolar or hyperosmolar hyponatraemia?
TURP and hysteroscopy (associated with absorption of irrigation solutions)
What is pseudohyponatraemia?
an uncommonly encountered laboratory abnormality defined by a serum sodium concentration of less than 135 mEq/L in the setting of a normal serum osmolality (280 to 300 mOsm/kg)
What is TURP syndrome?
This occurs when too much of the fluid used to wash the area around the prostate during the procedure is absorbed into the bloodstream
intravascular absorption of hypotonic irrigating fluids
causes acute volume overload –>pulmonary oedema
How is TURP syndrome treated?
supportive care of cardiac and pulmonary functions
airway protection if indicated
fluid restriction
furosemide
hypertonic saline for severe hypernatraemia (<120)
What test should be done if you suspect pseudohyponatraemia?
measure serum sodium concentration using direct potentiometry
What drugs commonly cause hyponatraemia?
thiazide and thiazide-like diuretics
amiloride
carbamazepine
sulphonylureas (but not gliclazide)
PPIs
antidepressants, particularly SSRIs
ACE-is and ARBs
opiates
In a hyponatraemic patient, a high urine osmolality is indicative of what?
SIADH
Diagnostic criteria for SIADH
effective serum osmolality <275mOsm/kg
urine osmolality >100mOsm/kg
urine Na+ >30mmol/L with normal dietary salt and water intake
absence of adrenal, thyroid, pituitary or renal insufficiency
no recent use of diuretic agents
What defines over correction of sodium?
> 8mEq/L in first 24h
18mEq/L in first 48h
How do you treat excessive sodium correction?
IV desmopressin with dextrose
Why do you assess hydration status in a hyponatraemic patient?
hyponatraemia is:
- hypovolaemic (dehydrated)
- hypervolaemic (overloaded)
- euvolaemic
What are the 2 main overarching causes of hypovolaemic hyponatraemia, and how can you differentiate between them?
due to renal sodium loss
due to sodium loss elsewhere (extra-renal)
urine sodium >20mmol/L suggests renal cause - kidneys are not conserving sodium
urine sodium <20mmol/L suggests non-renal cause - kidney is conserving sodium but sodium is being lost from elsewhere
Renal causes of hypovolaemic hyponatraemia
Renal failure (high creatinine, low eGFR)
Addison’s disease - associated hyperkalaemia
Excess diuretic medications
Osmotic diuresis - severe hyperglycaemia
Non-renal causes of hypovolaemic hyponatraemia
GI losses - vomiting, diarrhoea, small bowel obstruction, fistulae
skin loss - excess sweating, burns
Describe what hypervolaemic hyponatraemia is
in a fluid overloaded patient, fluid accumulates in the extracellular (3rd) space
extra fluid has dilutional effect on serum sodium
more water than sodium, so relative sodium deficiency
Causes of hypervolaemic hyponatraemia
anything causing fluid overload
cardiac failure
liver cirrhosis (hypoalbuminaemia reduces oncotic pressure in bloodstream, causing fluid loss into 3rd space, portal hypertension also causes ascites)
end-stage renal failure
nephrotic syndrome - inappropriate loss of albumin through the glomerulus leads to hypoalbuminaemia, which decreases plasma oncotic pressure, and fluid accumulates in the extracellular space)
Euvolaemic hyponatraemia causes
SIADH
primary polydipsia
severe endocrine disturbances (severe hypothyroidism/cortisol deficiency)
What serum and urine osmolalities would be present in SIADH?
raised urine osmolality
low serum osmolality
kidney inappropriately producing concentrated urine despite low serum osmolality
What should you test in euvolaemic hyponatraemia?
urine osmolality
If urine osmolality is low in euvolaemic hyponatraemia, what could the causes be?
primary polydipsia (water intoxication)
severe hypothyroidism
glucocorticoid deficiency
Signs of hypovolaemia
tachycardia
hypotension
dry mucous membranes
reduced skin turgor
Signs of hypervolaemia
peripheral oedema
raised JVP
bibasal lung field crepitations
Neurological signs of severe hyponatraemia
cognitive impairment
drowsiness
signs of seizure activity/brainstem herniation (indicate cerebral oedema)
Serum osmolality calculation
(2 x Na) + Glucose + Urea (all in mmol/L)
What is pseudohyponatraemia and what can cause it?
Hyperlipidaemia, hyperproteinaemia or hyperbilirubinaemia (secondary to obstructive jaundice) can interrupt some laboratory analysis methods of measuring serum sodium, leading to falsely low readings
What can you test to show it could be pseudohyponatraemia?
Serum osmolarity is classically normal if measured. Pseudohyponatraemia is rare but important to exclude before initiating treatment for hyponatraemia
Describe hyperglycaemia associated hyponatraemia
In patients with significant hyperglycaemia (e.g. DKA / HHS), the increase in serum glucose raises serum tonicity, pulling water out of cells and expanding the extracellular space, causing a dilutional effect on serum sodium concentrations.
In this case, total body sodium remains unchanged, and sodium concentration will generally correct with the correction of the hyperglycaemia. Calculated serum osmolality is classically raised.
Acute severe hyponatraemia treatment
IV hypertonic saline bolus (100ml 3% NaCl) with close monitoring of serum sodium
How is hyponatraemia without severe neurological symptoms treated?
In the absence of acute severe hyponatraemia, a cautious approach to management is taken. If the sodium is corrected too quickly, patients are at risk of severe complications (osmotic demyelination syndrome)
correct by no more than 6mmol/L in the first 6 hours and no more than 10mmol/L in the first 24 hours
Hypovolaemic hyponatraemia
Rehydration with intravenous 0.9% normal saline, with regular monitoring of serum sodium.
Hypervolaemic hyponatraemia
Fluid restriction (<1.5L/24h), with regular monitoring of serum sodium.
Euvolaemic hyponatraemia
Fluid restriction (1.5L/24h), with regular monitoring of serum sodium. Oral salt tablets may be required if fluid restriction alone is ineffective.
Hypovolaemic hyponatraemia with no neurological symptoms treatment
Rehydration with intravenous 0.9% normal saline, with regular monitoring of serum sodium.
Hypervolaemic hyponatraemia with no neurological symptoms treatment
Fluid restriction (<1.5L/24h), with regular monitoring of serum sodium
Euvolaemic hyponatraemia with no neurological symptoms treatment
Fluid restriction (1.5L/24h), with regular monitoring of serum sodium. Oral salt tablets may be required if fluid restriction alone is ineffective
Complications of hyponatraemia
gait disturbance and falls
cerebral oedema - can lead to brainstem herniation and death
osmotic demyelination syndrome - typically presents with quadriplegia and pseudobulbar palsy
How does osmotic demyelination syndrome typically present?
2-4 days after hyponatraemia treatment
quadriplegia and pseudobulbar palsy
