A Painful Way to Get Hyponatremia: Acute Intermittent Porphyria

Introduction: Acute intermittent porphyria (AIP) is a rare metabolic disorder caused by abnormal enzyme activity in the heme biosynthesis pathway. Symptoms can be nonspecific which can make it challenging to diagnose and treat. Hyponatremia is seen in severe cases and thought to be due to syndrome of inappropriate antidiuretic hormone (SIADH). We present a case of AIP in a young woman presenting with SIADH and recurrent hospitalizations for abdominal pain. Case Description: A 22-year-old female with a history of endometriosis presented with acute abdominal pain. Vital signs revealed a blood pressure of 137/82 mm Hg. Abdominal examination was significant for diffuse tenderness to palpation without rebound or guarding. She reported experiencing similar episodes in the past year and underwent extensive evaluation and prior hospitalization. Initial laboratory results were significant for sodium (Na) level of 143 mmol/L that rapidly decreased to 122 mmol/L the next day. She was aggressively fluid resuscitated, and Na decreased to 118 mmol/L. Serum osmolality was 257 mosm/kg, urine sodium was 149 mosm/L, and urine osmolality was 642 mosm/kg, consistent with SIADH that was attributed to pain. Sodium did not improve despite medical management including 3% saline infusion. Abdominal imaging revealed an ileus. Due to the combination of her symptoms, AIP was considered. Urine porphobilinogen was checked and was found to be elevated 176.8 mg/L consistent with AIP. A Hemin infusion was started and the patient’s pain and hyponatremia improved. Genetic testing was done, which later confirmed AIP. Discussion: AIP is characterized by neurovisceral attacks associated with high accumulation of early porphyrin precursors. This presence of SIADH in AIP is thought to be multifactorial. Firstly, pain because of visceral spasms is a direct stimulator for ADH. Secondly, presence of paralytic ileus, as in our case, accompanying abdominal pain, leads to intestinal sequestration of water and electrolytes. This leads to activation of angiotensin II and subsequent stimulation of ADH. Treatment includes prompt administration of hemin. Delays in recognizing and treating attacks can lead to serious complications including motor axonal neuropathy, seizures, psychosis and severe hyponatremia. This case highlights the importance of considering the diagnosis of AIP in patients presenting with recurrent abdominal pain and hyponatremia.