An enlarged cerebellar cistern is a finding identified during routine prenatal screenings. It refers to the cisterna magna, a naturally occurring, fluid-filled space located behind the cerebellum, the part of the brain responsible for balance and coordination. While the term “enlarged” may cause concern, it is often a normal variation in fetal development. Understanding the context of this finding is helpful for interpreting its significance.
Anatomy and Purpose of the Cerebellar Cistern
The cerebellar cistern, or cisterna magna, is one of several spaces in the subarachnoid area surrounding the brain and spinal cord. It is located in the posterior fossa of the skull, a small space at the lower back of the head between the cerebellum and the occipital bone.
The primary purpose of the cisterna magna is to hold cerebrospinal fluid (CSF). CSF is a clear fluid that is continuously produced and absorbed by the body, circulating within the brain’s ventricles and around the spinal cord. This fluid serves several functions, including cushioning the brain from impact, delivering nutrients, and clearing metabolic waste, ensuring the brain remains protected.
Standard Measurement During Fetal Development
The size of the cerebellar cistern is a standard measurement taken during the second-trimester anatomy scan, performed between 18 and 22 weeks of gestation. During this ultrasound, a sonographer assesses various aspects of fetal growth and development, including brain structures. To measure the cisterna magna, the sonographer obtains a specific view of the fetal head to visualize the posterior fossa structures.
The measurement is taken as the distance in millimeters from the back of the cerebellar vermis, the central part of the cerebellum, to the inner edge of the occipital bone. A measurement of less than 10 millimeters is considered within the normal range. This data point is evaluated in conjunction with the size and structure of the cerebellum and other brain structures to build a comprehensive picture of the fetus’s health.
Understanding an Enlarged Cerebellar Cistern
An enlarged cerebellar cistern is diagnosed when its measurement is 10 millimeters or greater, a condition also referred to as a mega cisterna magna. It is important to distinguish between a finding that is “isolated” and one that is associated with other anatomical differences.
When an enlarged cisterna magna is an isolated finding, it means that all other aspects of the brain, including the cerebellar vermis and hemispheres, appear normal, and there are no other abnormalities detected in the fetus. In these cases, the enlarged space is considered a normal anatomical variant, and research indicates the vast majority of individuals with an isolated mega cisterna magna have a normal neurological outcome.
The presence of an enlarged cistern can sometimes be associated with other conditions. These can range from structural brain anomalies, such as issues with the development of the cerebellar vermis (Dandy-Walker malformation), to certain chromosomal abnormalities like trisomy 18. For this reason, its discovery prompts a thorough evaluation to look for any other findings.
Diagnostic Evaluation for Associated Abnormalities
If an enlarged cerebellar cistern is detected, healthcare providers will recommend further evaluation to determine if the finding is isolated. The goal is to obtain a complete view of the fetal anatomy, with a focus on the brain and cerebellum. The first step is a high-resolution, targeted ultrasound, which is more advanced than a routine screening ultrasound and is performed to meticulously examine the fetal brain.
In some cases, a fetal magnetic resonance imaging (MRI) scan may be recommended. A fetal MRI can provide more detailed images of the brain than an ultrasound and is particularly useful for visualizing complex structures. It does not use ionizing radiation and is considered safe during pregnancy. Additionally, a genetic counselor can discuss the option of diagnostic testing, such as amniocentesis, to screen for chromosomal conditions that are sometimes linked to posterior fossa abnormalities.