You are called to review a 54-year-old female who is obtunded, 5 days post total knee replacement. She has a history of hypertension and mild depression and is on regular medication for both conditions. She has no other known co-morbidities.

Her biochemistry profile is as follows:

Parameter Patient Value Normal Adult Range
Sodium 114 mmol/L* 135 – 145
Potassium 4.6 mmol/L 3.5 – 5.0
Chloride 87 mmol/L* 95 – 105
Bicarbonate 18 mmol/L* 24 – 32
Urea 6.6 mmol/L 2.9 – 8.2
Creatinine 72 µmol/L 70 – 120

a) What are the likely causes for these results in this patient?

b) Briefly outline how you will determine the underlying cause.

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College Answer

a)
Inappropriate fluid therapy post op
SIADH (possible SSRI therapy)
Thiazide diuretic
Vomiting and/or diarrhoea
Salt-wasting (cerebral or renal)
Less likely as no other co-morbidity CCF, cirrhosis, hypoadrenalism, hypothyroidism (kidney failure
excluded from results)

b)
History of medications and fluid input/output
Clinical assessment of fluid status, presence of heart/liver failure
Serum osmolality
Urine osmolality and sodium
Random cortisol
TFTs

Discussion

a)

Causes of hyponatremia in this patient could include:

  • SIADH:
    • recent surgery
    • possible SSRI therapy
  • True hypovolemia
    • Underfilled post op
    • Vomiting post anaesthetic
    • Unable to drink (due to decreased level of consciousness)
  • Excess of sodium-poor fluid
    • Inappropriate fluid resuscitation choices, eg. 5% dextrose
  • Increased sodium excretion
    • Thiazide diuretic or spironolactone (has history of hypertension)
  • Unlikely:
    • Renal tubular acidosis
    • Hypothyroidism
    • Hypoadrenalism
    • Cerebral salt wasting

In general:

Causes of Hyponatremia

Spurious result

Isotonic

  • High triglycerides
  • High serum protein
  • Glycine (TURP syndrome)

Hypertonic

  • Hyperglycaemia
  • Mannitol
  • Sorbitol
  • Maltose
  • Radiocontrast dye

Water retention

High urine sodium

  • Renal failure
  • Cirrhosis
  • Congestive cardiac failure
  • Diuretics (but not enough!)
  • SIADH

Low urine sodium

  • Psychogenic polydipsia
  • True hypovolemia

Sodium excretion

  • Post-ATN diuresis
  • Hypoaldosteronism
  • Diuretic excess
  • Cerebral salt wasting
  • Inappropriate fluid replacement (5% dex)

b)

An approach to the diagnosis of a hyponatremia should involve the following structured steps:

  • History (including an audit of the medication chart and fluid orders)
  • Examination (focusing on volume status)
  • Investigations (most importantly, serum osmolality and urine soidum)

History: The following bits of historical information are important:

  • Medication history (diuretics, steroids, drugs which cause SIADH eg. SSRIs)
  • Fluid chart (has somebody been mindlessly charting dextrose)
  • Psychosocial history (is psychogenic polydipsia even a possibility; are they on a weird diet)
  • Alcohol history (liver disease, cirrhosis, beer potomania)
  • Oedema history (Ascites worse recently? Sleep on twenty pillows?)
  • Trauma history (cerebral salt wasting, pituitary injury)
  • Urine output (massive diuresis of HONK or ATN recovery phase, or oliguria or chronic renal failure)
  • Recent procedures: TURP, contrast CT, recent surgery, etc.

The following standard battery of tests can be launched; particularly if history is unhelpful, or one cannot bring oneself to interview the patient or their family.

Essential tests:

  • Serum osmolality (to classify the disorder)
  • Urine osmolality
  • Urinary sodium

Optional tests:

  • Serum triglycerides
  • Serum protein level
  • TFTs
  • LFTs
  • Urea and creatinine
  • Random cortisol
  • Short synacthen test

Potential causes:

This is essentially the content of Box 93.1 from Anthony Delaney and Simon Finfer's chapter for Oh's Manual.

Causes of Hyponatremia

Spurious result

Isotonic

  • High triglycerides
  • High serum protein

Hypertonic

  • Hyperglycaemia
  • Mannitol
  • Sorbitol
  • Maltose
  • Radiocontrast dye
  • Glycine (TURP syndrome)

Water retention

High urine sodium

  • Renal failure
  • Cirrhosis
  • Congestive cardiac failure
  • Diuretics (but not enough!)
  • SIADH

Low urine sodium

  • Psychogenic polydipsia
  • True hypovolemia

Sodium excretion

  • Post-ATN diuresis
  • Hypoaldosteronism
  • Diuretic excess
  • Cerebral salt wasting
  • Inappropriate fluid replacement (5% dex)

Diagnosis on the basis of the above lab tests and historical findings:

  • Serum osmolality testing:
    • Hyperosmolar hyponatremia:
      • Hyperglycaemia
      • Mannitol therapy
      • Other unmeasured solutes, eg. glycine
      • Glycine (TURP syndrome)
    • Isoosmolar hyponatremia
      • High triglycerides
      • High serum protein
    • Hypoosmolar hyponatremia
      • Further investigations will be required to distinguish between water retention and sodium excretion.
  • Urinary sodium and urinary osmolality
    • Low urinary sodium: water retention disorders;
      • Polydipsia, beer potomania - low urine osmolality
      • true hypovolemia, heart failure, cirrhosis, nephrotic syndrome - high urine osmolality
    • High urinary sodium: sodium wasting disorders;
      • Acute renal failure, post-obstructive diuresis, polyuric phase of ATN - low urine osmolality
      • Thiazides, SIADH, cerebral salt wasting, hypoadrenalism, hypothyrodism - high urine osmolality

References