Torcular Herophili Thrombosis Initially Suspected to Represent an Arachnoid Cyst: A Prenatal Case Report.

Torcular Herophili thrombosis is a rare fetal dural venous condition with a heterogeneous course, ranging from spontaneous regression to parenchymal injury, neurological impairment, or perinatal death. Prenatal ultrasound is generally the first-line imaging modality, whereas fetal magnetic resonance imaging (MRI) provides complementary information regarding lesion origin and associated intracranial abnormalities. After diagnosis, serial ultrasonographic follow-up is recommended to monitor lesion evolution and fetal well-being. We report a fetus referred at 23 weeks' gestation with a presumed arachnoid cyst. Serial prenatal ultrasonography demonstrated antenatal regression, and postnatal ultrasonography, brain MRI, and MRV confirmed complete resolution with normal patency of the major dural venous sinuses. This case illustrates a recognized diagnostic pitfall in the evaluation of midline posterior intracranial cystic-appearing lesions and highlights the complementary roles of Doppler ultrasound, fetal MRI, and postnatal venous follow-up.