Hyponatremia is the commonest electrolyte abnormality presenting to Medical Admissions and when acute, severe and symptomatic, is associated with high mortality. The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most common underlying disorder in hospitalised patients with euvolaemic hyponatraemia and is a complication of many clinical conditions and drug therapies.
We discuss the cases of two patients presenting with acute severe hyponatraemia secondary to SIADH, admitted to the same hospital within a 2-week period. Patient 1 presented with severe abdominal pain and hypertension and was initially investigated for a pelvic mass. Serum sodium was normal on admission but dropped 11 mmol/L in 8 hours, precipitating a seizure. She was admitted to the intensive treatment unit (ITU) for close monitoring. Patient 2 presented with increasing confusion, lethargy and abdominal pain. Her admission sodium was 108 mmol/L. She had a seizure in the emergency department and was intubated and transferred to ITU.
Both patients received several boluses of 2.7% hypertonic saline and, showing biochemistry consistent with SIADH, were fluid restricted to 750 mls daily. Extensive imaging in each case failed to identify an underlying cause. The combination of unexplained SIADH, hypertension and abdominal pain raised the possibility of porphyria as a unifying diagnosis. This was confirmed by the presence of elevated urinary porphobilinogen and total porphyrin concentration in both patients. They were commenced on IV haem arginate leading to normalisation of sodium levels.
Acute intermittent porphyria (AIP) results from partial deficiency of porphobilinogen deaminase activity. Clinical manifestations include recurrent abdominal pain, peripheral neuropathy and neuropsychiatric symptoms. Hyponatraemia occurs in approximately 20% of cases of symptomatic AIP.
The number of possible causes of SIADH is extensive and maybe difficult to determine despite thorough investigation. We discuss the importance of considering rarer causes for unexplained hyponatraemia in those with additional symptoms suggestive of underlying pathology.