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.
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