Midline Structures in Neonatal Brain Ultrasound

Cerebral ultrasound (CUS) is a cornerstone in the diagnosis of cystic brain lesions in neonates and infants. Using the open anterior fontanelle, posterior fontanelle, temporal bone and mastoid bone as acoustic windows, ultrasound provides a clear view of the brain's structure in patients typically under 12–18 months of age, before the fontanelles close. Its non-invasive nature, real-time imaging, and absence of radiation make it ideal for early evaluation of conditions such as subependymal cysts, choroid plexus cysts, cystic encephalomalacia and other cystic structures.

However, before exploring various cystic lesions, it is essential to first understand the normal anatomical structures visible on neonatal brain ultrasound. Recognizing these midline structures is crucial for distinguishing between normal variants and pathological findings, thereby preventing unnecessary follow-ups or additional imaging. Here are some examples of non-pathological structures that should not be mistaken for pathological cysts.

Question: 

Look at the midline structures on the brain ultrasound of this 2-month-old infant. The loop has been taken in the coronal plane; below you see still images from the coronal (at the level of the interventricular foramen of Monro) and sagittal planes.

What do you notice? Describe and name the structures! 

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Cavum Septi Pellucidi and Cavum Vergae

During fetal development, the septum pellucidum contains a single continuous midline cystic structure. This structure comprises the cavum septi pellucidi (located anterior to the foramen of Monro) and the cavum Vergae (located posteriorly). Both components are typically present early in gestation but undergo a closure process that progresses from back to front, beginning around 6 months of gestation.

By full-term birth, the posterior portion (cavum Vergae) has closed in approximately 97% of infants, leaving only the cavum septi pellucidi. Within 3 to 6 months after birth, the cavum septi pellucidi closes in 85% of infants. However, in some cases, the septum remains partially or fully open into adulthood.

Choroid Plexus Cysts in the Neonate

A choroid plexus cyst appears as a round-shaped anechoic structure with well-defined walls within the choroid plexus. Choroid plexus cysts vary in size from less than 4 mm to over 1 cm. They are usually unilateral, more frequently on the left than on the right, and situated in the dorsal aspect of the choroid plexus. Color Doppler is helpful to distinguish vessels in the choroid from cysts. No follow-up is required in most cases unless the size of the cyst exceeds 1 cm or abnormal neurological signs or other associated anomalies are present. A study from 2002 found isolated choroid plexus cysts in 8.8% of neonates but reported no delay in early childhood development [1].

Recently, a systematic review [2] has corroborated the view that choroid plexus cysts are generally not linked to adverse health or developmental outcomes, even when following patients until adolescence. Rare cases of symptomatic choroid cysts associated with obstructive hydrocephalus have been reported but are probably related to some specific cause rather than to the choroid plexus cyst itself [3].

In summary, no follow-up is required in most cases of unilateral choroid plexus cysts, unless the size of the cyst exceeds 1 cm or abnormal neurological signs or other associated anomalies are present. The latter aspect is also corroborated by the 2023 review that shows a potential association with chromosomal abnormalities. However, genetic testing is not recommended for most cases (Singal, Adamczyk et al., 2023). Most isolated choroid plexus cysts resolve spontaneously.

Choroid plexus cysts are best visualized through the anterior fontanelle, in coronal and parasagittal views. Color Doppler is helpful to distinguish vessels in the choroid from cysts.  

Here’s an interactive case for you:

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Stay tuned to learn more! Thank you to Andrei Satsuk, MD, for providing us with these interesting cases and amazing loops and images! 

And don't forget to check out our Neonatal & Pediatric Ultrasound BachelorClass which also has a chapter on brain ultrasound in neonates!

References:

1. Hung KL, Liao HT. Neonatal choroid plexus cysts and early childhood developmental outcome. J Formos Med Assoc. 2002 Jan;101(1):43-7. PMID: 11911036.
2. Singal K, Adamczyk K, Hurt L, Woolner A, Paranjothy S. Isolated choroid plexus cysts and 3. health and developmental outcomes in childhood and adolescence: A systematic review. Eur J Obstet Gynecol Reprod Biol. 2023 Nov;290:115-122. doi: 10.1016/j.ejogrb.2023.09.013. Epub 2023 Sep 16. PMID: 37778251.
3. Naeini RM, Yoo JH, Hunter JV. Spectrum of choroid plexus lesions in children. Am J Roentgenol. 2009;192(1):32-40.