Background: Renal hypouricemia (RHUC) is an inherited heterogenous disorder caused by faulty urate reabsorption transporters in the renal proximal tubular cells. Anaerobic exercise may induce acute kidney injury in individuals with RHUC that is not caused by exertional rhabdomyolysis; it is called acute renal failure with severe loin pain and patchy renal ischemia after anaerobic exercise (ALPE). RHUC is the most important risk factor for ALPE. However, the mechanism of onset of ALPE in patients with RHUC has not been elucidated. The currently known genes responsible for RHUC are SLC22A12 and SLC2A9.
Case presentation: A 37-year-old man presented with loin pain after exercising. Despite having a healthy constitution from birth, biochemical examination revealed hypouricemia, with a uric acid (UA) level of < 1 mg/dL consistently at every health check. We detected acute kidney injury, with a creatinine (Cr) level of 4.1 mg/dL, and elevated bilirubin; hence, the patient was hospitalized. Computed tomography revealed no renal calculi, but bilateral renal swelling was noted. Magnetic resonance imaging detected cuneiform lesions, indicating bilateral renal ischemia. Fractional excretion values of sodium and UA were 0.61 and 50.5%, respectively. Urinary microscopy showed lack of tubular injury. The patient's older sister had hypouricemia. The patient was diagnosed with ALPE. Treatment with bed rest, fluid replacement, and nutrition therapy improved renal function and bilirubin levels, and the patient was discharged on day 5. Approximately 1 month after onset of ALPE, his Cr, UA, and TB levels were 0.98, 0.8, and 0.9 mg/dL, respectively. We suspected familial RHUC due to the hypouricemia and family history and performed genetic testing but did not find the typical genes responsible for RHUC. A full genetic analysis was opposed by the family.
Conclusions: To the best of our knowledge, this is the first report of ALPE with hyperbilirubinemia. Bilirubin levels may become elevated as a result of heme oxygenase-1 activation, occurring in exercise-induced acute kidney injury in patients with RHUC; this phenomenon suggests renal ischemia-reperfusion injury. A new causative gene coding for a urate transporter may exist, and its identification would be useful to clarify the urate transport mechanism.
Keywords: Acute kidney injury; Acute renal failure with severe loin pain and patchy renal ischemia after anaerobic exercise; Hyperbilirubinemia; Renal hypouricemia.