Invisible Until It Burst: Unexpected Subarachnoid Hemorrhage From a Rapid-Onset Infectious Aneurysm in a Patient With Endocarditis.

Infective endocarditis (IE) can lead to serious neurological complications, including septic embolism and infectious intracranial aneurysms (IIAs). Although IIAs are rare, their rupture often results in catastrophic outcomes. Predicting their formation, especially within a short period, remains a clinical challenge. We present the case of a man in his 70s who was newly diagnosed with colon cancer. During preoperative evaluation, transthoracic echocardiography revealed vegetations on the aortic and mitral valves, leading to a diagnosis of IE caused by Streptococcus sanguinis. On the third day of hospitalization, the initial brain magnetic resonance imaging (MRI) revealed asymptomatic cerebral infarction, but magnetic resonance angiography (MRA) did not show any aneurysms. Despite appropriate antibiotic therapy, the patient developed sudden left hemiparesis and impaired consciousness on day 6. Emergent computed tomography (CT) and computed tomography angiography (CTA) revealed a subarachnoid hemorrhage and a newly formed ruptured aneurysm in the M1 segment of the middle cerebral artery. Given the patient's overall prognosis, neurosurgical intervention was deemed inappropriate, and best supportive care was initiated. The patient passed away shortly thereafter. This case highlights the unpredictable nature of IIAs in IE. Although imaging performed just three days prior showed no aneurysms, a rapidly formed and ruptured IIA resulted in fatal subarachnoid hemorrhage. It underscores the challenge of predicting the rupture of infectious aneurysms in IE and emphasizes the importance of frequent imaging follow-up, even when initial imaging findings are normal.