A 75-year-old lady is brought in by ambulance after being found collapsed by neighbours. She is a chronic smoker and is recently under investigation for Ca Lung. On examination she is drowsy and becomes agitated when attempts are made to rouse her. Routine observations show that her blood pressure is 170/95 mmHg and heart rate is 72 bpm. Her biochemistry results come back as urea & electrolytes: Na+ 116 mmol/litre, K+ 3.0 mmol/litre, urea 6.5 mmol/litre, creatinine 92 ΅mol/litre; plasma osmolality: 251 mosmol/kg. The most probable diagnosis is:
a. Ectopic adrenocorticotropic hormone secretion
b. Ectopic antidiuretic hormone secretion
c. Ectopic parathyroid hormone secretion
d. LambertEaton syndrome
Correct answer: b Ectopic antidiuretic hormone secretion (SIADH)
There are many causes for SIADH of which the paraneoplastic, ie nonmetastatic, syndrome associated with small cell carcinoma of the lung is one.
Inappropriate levels of the hormone lead to hypervolaemic hyponatraemia with inability of the body to diurese in response to falling plasma osmolality.
Severe hyponatraemia with Na+ < 110 mmol/litre can lead to generalised fits and coma.