Sun28November0333PM 34

A 35-year-old man is admitted to the high dependency unit following the de-bulking of a pituitary tumour. Twenty hours following surgery, he is noted to be polyuric and clinically dehydrated.

The following biochemistry results are available:

Urinary specific gravity 1.004

Urinary sodium 40 mmol/L

Urinary osmolality 185 mOsm/kg

Plasma sodium 153 mmol/L

Plasma osmolality 309 mOsmol/kg

What is the most likely diagnosis?

(Please select 1 option)

Mannitol-induced diuresis

Furosemide-induced diuresis

Cerebral salt wasting syndrome

Inappropriate secretion of antidiuretic hormone (SIADH)

Central diabetes insipidus Correct

Explanation

Central diabetes insipidus (DI) results from the failure of antidiuretic hormone (ADH) release from the pituitary, resulting in a diuresis of dilute urine. The inappropriate water loss can lead to a high serum sodium concentration and a state of clinical dehydration. DI is associated with subarachnoid haemorrhage, traumatic brain injury, and pituitary surgery.

Biochemically, DI can be diagnosed by:

urine osmolality <200 mOsm/kg

urinary [Na] 20-60 mmol/L

plasma osmolality >305 mOsmol/kg

serum [Na] >145 mmol/L, and

urinary specific gravity <1.005.

Treatment includes increasing oral water intake. In the unconscious patient, nasogastric water and/or intravenous 5% dextrose can be administered. Synthetic ADH can be given intranasally or intravenously if the urine output continues to be greater than 250 ml/hr.

Cerebral salt wasting syndrome causes polyuria and dehydration secondary to urinary sodium losses, but it is also characterised by hyponatraemia and a serum osmolality < 280 mOsm/kg, which is not present in this patient.

Furosemide-induced diuresis is associated with a serum [sodium] <135 mmol/L and a serum osmolality <280 mOsmol/kg.

Inappropriate secretion of antidiuretic hormone (SIADH) can follow brain injury and results from an inappropriately high ADH, but patients are not polyuric and the biochemical picture is one of hyponatraemia (<135 mmol/L) and low serum osmolality (<280 mOsmol/L).

Mannitol-induced diuresis should not be selected as although mannitol causes polyuria and a high serum osmolality (>305 mOsmol/L), it is associated with a low serum sodium (<135 mmol/L) that is not seen in this patient's plasma biochemistry.

Reference:

Bradshaw K, Smith M. Disorders of sodium balance after brain injury. Contin Educ Anaesth Crit Care Pain. 2008;8:129-133.

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