A. What is an arachnoid cyst 1. An arachnoid cyst is an accumulation of cerebrospinal fluid within the arachnoid layer, and the cyst may be connected to the subarachnoid space. Primary arachnoid cysts are caused by abnormal development of the soft meninges, while secondary or acquired arachnoid cysts are caused by adhesions of the arachnoid membrane and contain cerebrospinal fluid within it. 2, Arachnoid cysts are rare, accounting for about 1% of intracranial occupying lesions, and are more common in male than female fetuses, and in the left side of the brain than in the right side of the brain. They are mostly located on the surface of the brain, usually near the cerebral fissure in the pterygoid saddle area, and can be located in the anterior cranial fossa, middle cranial fossa and posterior cranial fossa. Cysts near the corpus callosum pressure are usually called CVI cysts. 3. Arachnoid cysts may be isolated or may be combined with other brain malformations, such as agenesis of the corpus callosum, hyaline septal cavity defects, cerebellar lobar defects, and type I Arnold-Chiari malformations. Many of these associated malformations are not apparent by ultrasound until late in mid-trimester. Ultrasound manifestations of fetal arachnoid cysts 1. Echo-free cystic masses with thin and smooth walls and no blood flow signal within the capsule appear in the brain. The adjacent cerebral hemispheres, cerebellum or brainstem may be compressed or displaced. Arachnoid cysts do not communicate with the lateral ventricles. The occupying effect of large cysts may obstruct the flow of cerebrospinal fluid and lead to hydrocephalus. 2, Arachnoid cysts can be diagnosed early in midterm pregnancy, but most are diagnosed relatively late. 3, The role of ultrasound is to rule out associated abnormalities and to monitor the size of the cyst and the presence of signs of compression, such as ventricular enlargement and giant skull. Clinical management of fetal arachnoid cysts 1. The natural prenatal course of arachnoid cysts is unclear; some may continue to enlarge, leading to obstructive hydrocephalus, and some may disappear spontaneously. MRI is most helpful after 20 weeks of gestation to confirm the extra-axial nature of arachnoid cysts and to determine whether the cortex beneath them shows normal, allowing dynamic monitoring of cyst changes and the presence or absence of hydrocephalus. 2. The presence of clinical symptoms in newborns after birth depends on the size of the cyst. For the vast majority of asymptomatic lesions, serial imaging and neurological examinations are sufficient. If there are symptoms of increased intracranial pressure, seizures, focal neurological deficits or cognitive impairment, surgery is indicated. Surgical options include: craniotomy for partial or complete cystectomy, subarachnoid opening, or cyst-abdominal shunt. IV. Prognosis of fetal arachnoid cysts 1. An analysis included 47 fetuses with intracranial extra-axial cysts and 24 fetuses with arachnoid cysts, of which 73% had CNS-related abnormalities and 14% had abnormalities other than CNS. The most common were ventricular enlargement and corpus callosum abnormalities. 6% of fetuses had chromosomal abnormalities, but all fetuses with simple cysts were haploinsufficient. 2. A study reviewed neurological signs and symptoms in 45 pediatric patients (2-17 years of age) at the time of presentation and found that headache was the predominant symptom (61%), while 31% had epilepsy. Of those patients with seizures, 91% had cysts located in the left temporal region. 3. Cysts in other sites also have characteristic presentations. Suprasellar cysts usually result in obstructive hydrocephalus; cysts in this region can occasionally cause visual and/or endocrine dysfunction. Cysts in the tetrahymena and posterior cranial fossa may cause brainstem symptoms and hydrocephalus.