Cryptogenic brain abscess: Right-to-left shunt in pulmonary arteriovenous malformation and hepatopulmonary syndrome.

Background: Pulmonary arteriovenous fistula (PAVF) and hepatopulmonary syndrome (HPS) both cause right-to-left shunting, allowing pathogens to bypass the pulmonary filtration barrier and enter systemic circulation, leading to brain abscess. Despite distinct etiologies, both share the same pathological pathway-right-to-left shunt-resulting in the same devastating neurological complication. Case presentation: Two male patients with cryptogenic brain abscesses caused by PAVM (59 years old) and HPS (52 years old), respectively, are reported. The patient with PAVM presented with fever, dyspnea, and headache, and was diagnosed by contrast-enhanced chest computed tomography. Fusobacterium nucleatum was detected in cerebrospinal fluid (CSF). The patient recovered following anti-infective therapy combined with surgical resection of pulmonary lesions, achieving a favorable prognosis at the 3-month follow-up. The HPS patient had a history of hepatitis C complicated by liver cirrhosis. He presented with headache, fever, and orthostatic hypoxemia. Diagnosis was confirmed by a contrast-enhanced transcranial Doppler bubble test (c-TCD). Blood culture identified Streptococcus intermedius. The brain abscess recurred after antibiotic treatment due to refusal of liver transplantation. At the 3-year follow-up, the patient could maintain only a sedentary lifestyle. Conclusion: Both pulmonary arteriovenous fistula (PAVM) and hepatopulmonary syndrome (HPS) can cause cryptogenic brain abscess through right-to-left shunt. PAVM can be eradicated by intervention or surgery, while HPS fundamentally requires liver transplantation. Notably, Clinical and radiological improvement following antimicrobial therapy may not represent definitive cure if the underlying right-to-left shunt remains untreated, and recurrence remains possible.