Endocrinology - Hypernatraemia

Question 1

what is hypernatraemia?

Answer 1

Serum Na+ >145 mmol/L.

Question 2

name the possible causes of hypernatraemia

Answer 2

1. pure free water loss (dehydration)
   - inadequate water intake, inc. thirst impairment (e.g. dementia or hypothalamic lesions impairing osmoR function or thirst response)
   - diabetes insipidus (cranial or nephrogenic)
2. hypotonic fluid loss (dehydration + hypovolaemia)
   - dermal loses: burns or excessive sweating (e.g. endurance sports people under heat stress, pyrexia)
   - GI losses: non-secretory diarrhoes (inc. laxative abuse), vomiting, NG drains, fistulas
   - urinary losses: loop diuretics, osmotic diuresis (e.g. hyperglycaemic states), ATN (polyuric in early stage)
3. hypertonic Na+ gain (may cause hypervolaemia)
   - iatrogenic: use of hypertonic saline, etc.
   - excess salt ingestion: inadvertent, e.g. infant formula error, salt poisoning
   - hyperaldosteronism (usually only mild increase in Na+)
4. intracellular shift of water (rare)
   - v. strenuous exercise or electroshock-induced seizure causes transient increase in cell osmolality and thus water moves into cells

Question 3

describe the symptoms caused by hypernatraemia

Answer 3

Severe symptoms usually only found in acute (<48hrs) and large rises in serum Na+ (>160 mmol/L).

• lethargy and weakness
• altered mental status, irritability
• seizures and coma
• focal neurological findings, e.g. muscle weakness, loss of sensation

Depending on aetiology, Sx may also inc.

• polyuria and polydipsia (e.g. hypervolaemic causes like DI or osmotic diuresis)
• signs of dehydration, e.g. orthostatic hypotension, dry mucous membranes

Question 4

why can hypernatraemia cause focal neurological signs/symptoms?

Answer 4

Hypernatraemia from free water loss causes cell dehydration as intracellular water is drawn out of cells into ECF. Brain cell dehydration and brain shrinking may cause blood vessel rupture and thus intracerebral/subarachnoid haemorrhages.

Question 5

how can cause of hypernatraemia to differentiated between Na+ overload and dehydration?

Answer 5

Dehydration:
- hypovolaemic
- urinalysis:
1/ if caused by GI or skin losses, osmolality >600 mOsm/kg as kidneys retaining water and eliminating decreased volume of concentrated urine (<500 ml/day)
2/ if caused by renal water loss (DI or osmotic diuresis), osmolality <600 mOsm/kg as kidneys can’t retain water despite increased Na+

Na+ overload:

• hypervolaemic (odema)
• urinalysis: osmolality >600 mOsm/kg and increased Na+ (>100 mmol/L)

Question 6

how would you manage a pt with hypernatraemia?

Answer 6

1/ Estimate total water deficit: total body water (L) x [(serum Na+/140)-1].
Where TBW = weight (kg) x 0.6 in men or 0.5 in women.
2/ Administer IV fluids to correct water deficit and decrease serum Na+:
- if sig. hypovolaemia, use isotonic fluid (0.9% saline) to restore circulating volume- if hypervolaemic from hypertonic Na+ gain, give diuretics + 5% dextrose to offload fluid and provide free water
- otherwise give hypotonic fluids - 0.45% saline, 5% dextrose, oral water

Question 7

what is the max rate Na+ can be corrected by? why?

Answer 7

Max of 10 mmol/L/day or increased risk of cerebral oedema.

Question 8

suggest possible complications of hypernatraemia

Answer 8

Cerebral bleeding, subarchnoid haemorrhage, permanent brain damage and death secondary to brain shrinkage in acute hypernatraemia.