Subarachnoid Hemorrhage From Ruptured Aneurysms at the Internal Carotid Artery-Posterior Communicating Artery Bifurcation Not Detectable on Preoperative Imaging Studies.

Since subarachnoid hemorrhage (SAH) due to the re-rupture of cerebral aneurysms severely worsens the prognosis, an accurate initial diagnosis is essential. Computed tomography (CT) and magnetic resonance imaging (MRI) usually detect aneurysmal subarachnoid hemorrhage (aSAH). However, in rare cases, its identification on CT- and MRI scans is difficult, and a cerebrospinal fluid (CSF) examination is required. We present preoperative imaging and intraoperative findings in patients whose aSAH detection necessitated a CSF examination. Of 225 aSAH patients who underwent preoperative imaging studies at our institution between April 2010 and August 2019, 3 females (1.3%, mean age 57.3 years) harbored undetectable aSAH due to the rupture of an internal carotid artery-posterior communicating artery (ICA-PcomA) aneurysm. The aneurysmal orientation was inferolateral. Intraoperatively, the anterior petroclinoid ligament hampered the detection of the aneurysms that firmly adhered to the surrounding arachnoid membrane. Sustained arterial pulsation and successive minor hemorrhage can lead to the gradual adhesion of an ICA-PcomA aneurysm to the surrounding arachnoid membrane and explain their atypical rupture undetectable on imaging studies and the development of acute subdural hematoma without SAH.