De novo aneurysm formation on choroidal anastomosis during pregnancy in moyamoya disease: illustrative case.

backgroundHemorrhagic stroke during pregnancy can cause devastating outcomes in women with moyamoya disease (MMD). The bleeding risk associated with choroidal anastomosis, a unique collateral manifestation of MMD, remains unknown in the context of pregnancy. OBSERVATIONS: A 29-year-old woman with asymptomatic MMD became pregnant. She had not undergone bypass surgery, and choroidal anastomosis had developed in the right hemisphere. After an uneventful pregnancy for 34 weeks, she suddenly manifested HELLP (hemolysis, elevated liver enzymes, and low platelets) syndrome and underwent an emergency cesarean section. Although both she and her neonate remained healthy, MR angiography after delivery revealed on the choroidal anastomosis a small aneurysm, which had been absent before conception, with the possible finding of minor bleeding. After recovering from HELLP syndrome, she underwent direct bypass surgery, which resulted in the disappearance of the aneurysm and marked reduction of the choroidal anastomosis. LESSONS: Choroidal anastomosis may carry a potential risk of aneurysm formation causing bleeding and should be carefully monitored with noninvasive imaging modalities during the antenatal and postpartum periods. https://thejns.org/doi/10.3171/CASE25112.