A filar cyst is an incidental finding on neonatal lumbar sonography located in the filum terminale of the spinal cord. It is considered a normal variant and is often confused for a ventriculus terminalis, a smooth dilated cavity of the central canal, located within the conus medullaris.
The incidence of filar cysts is inversely related to age and they have not been reported in adults, suggesting a developmental variant which regresses with age. However, it also remains a question whether they truly exist or are simply artifactual, as no correlate is typically identifiable on MRI nor has been verified on autopsy.
Typically, no correlate to the filar cysts on ultrasound are seen on MRI, which could be due to insufficient resolution of spinal MRI in small neonates