Objective

To explore the imaging manifestations，genetic characteristics，and pathological features of fetal arachnoid cysts（AC）.

Methods

A case of a 32-week-gestational-age fetus with a prenatal ultrasound showing a large AC treated at Dongguan People's Hospital in July 2023 was selected in the study.A retrospective analysis was conducted on the fetal ultrasound examination and fetal head MRI examination results before termination of pregnancy，as well as the genetic testing and pathological autopsy results after termination of pregnancy.The literature on fetal AC was retrieved from the China National Knowledge Infrastructure，Wanfang Data Knowledge Service Platform，and Pub Med，Web of Science databases using the Chinese and English keywords"arachnoid cyst""fetus""fetal""intracranial".The literature search time was set from the establishment of the above databases to January 2024.This study followed the procedures in accordance with the 2013 revision of the Declaration of Helsinki by the World Medical Association and adhered to the ethical standards set by the Ethics Committee of the Tenth Affiliated Hospital of Southern Medical University ^（Dongguan People's Hospital） （Approval No.IRB2023-WZ-156）.The pregnant woman gave informed consent for the diagnosis and treatment.

Results

①The fetus's ultrasound examination at 32 weeks of gestation indicated that the fetal development was equivalent to 32 weeks，with a 9.7 cm×3.3 cm cystic occupancy in the right arachnoid，clear boundaries，good transmission，no obvious blood flow signals，asymmetrical bilateral cerebral hemispheres，the left side larger than that of the right，the midline of the brain shifted to the left，and the maximum measurement of the left ventricle was 1.4 cm，tentatively diagnosed as right AC，and dilatation of the left ventricle.②The fetus's head MRI at 32 weeks of gestation showed：the arachnoid space of the right cerebral hemisphere of the fetus was overall widened in a crescent shape，with the maximum diameter measured in the axial position as 10.6 mm×2.3 mm，the cerebrum and the falx cerebri were shifted to the left as a whole，no abnormal signal was seen in the cerebral substance，the bilateral cerebral hemispheres were asymmetrical，the right hemisphere was smaller than that of the opposite side，the cerebral sulci were asymmetrical bilaterally，some of the right cerebral sulci were shallower，and the maximum diameter of the left ventricle was 1.6 cm，indicating right AC，delayed development of the right cerebrum compared to the opposite side，and dilatation of the left ventricle.③The pathological autopsy of the fetus showed：the falx cerebri was shifted to the left，and there was a small amount of fluid between the right outer arachnoid and the surrounding tissue.The bilateral cerebral hemispheres were slightly asymmetrical，with the right cerebral volume slightly smaller than that of the left；the meningeal tissue section was stained with HE，and under high magnification，it was shown that the dura mater and arachnoid were separated，forming a cavity.④Whole-exome sequencing（WES）results of the family of this case showed no pathogenic/likely pathogenic variations related to the clinical phenotype of this family.⑤The literature review showed that a total of 10 related documents were retrieved，including 465 fetuses diagnosed with AC prenatally.The most common location of AC was the interhemispheric space of the supratentorial region，followed by the suprasellar region.And 50%to 70%of the cysts showed no significant change in volume during pregnancy，about 20%of the cysts continued to enlarge during pregnancy or after birth，and 3%to 28%of the children needed surgical treatment after birth，and the prognosis of isolated cysts was good，with 70%to 91%of the fetuses followed up normally after birth.

Conclusions

The results of prenatal ultrasound and fetal MRI were consistent with the pathological autopsy results，and imaging examination can provide an effective basis for the diagnosis of fetal AC.