What is the most common cause of a midline hypoechoic (anechoic) structure inferior to the thalami on fetal brain ultrasound?

Cavum Septi Pellucidi (CSP)

The most common cause of a midline hypoechoic (anechoic) structure inferior to the thalami on fetal brain ultrasound is the normal cavum septi pellucidi (CSP), a fluid-filled space between the leaflets of the septum pellucidum that is a normal anatomical landmark present in all fetuses during the second and third trimesters.

Normal Anatomy vs. Pathology

The cavum septi pellucidi is:

• A normal midline cerebrospinal fluid-filled cavity
• Located between the frontal horns of the lateral ventricles
• Positioned inferior and anterior to the corpus callosum
• Present in 100% of fetuses between 18-37 weeks gestation
• Appears as an anechoic (hypoechoic) rectangular or box-like structure on axial views

When evaluating a midline hypoechoic structure in this location, the key is determining whether it represents normal CSP or a pathological finding.

Pathological Differential Diagnosis

If the structure is not the normal CSP, consider these pathological entities:

Holoprosencephaly (Most Important Pathological Cause)

• Characterized by fusion of the thalami and absence of the midline falx 1
• Alobar form shows a single monoventricle with a dorsal cyst in many cases (5/10 cases) 1
• Semilobar form demonstrates incomplete thalamic fusion 1
• Associated facial anomalies include proboscis, midline cleft, and hypotelorism 1, 2
• Reliably distinguished from other causes of hydrocephalus by demonstrating absent interhemispheric fissure and fused thalami 1

Arachnoid Cysts

• Hypoechoic lesions external to brain matter 3
• Doppler ultrasound distinguishes these from vascular malformations 3
• Generally associated with good outcomes when isolated (4/5 cases) 3

Porencephalic Cysts

• Fluid-filled lesions within brain matter that communicate with lateral ventricles (9/10 cases) 3
• Always predict poor outcome 3

Clinical Approach

When you identify a midline hypoechoic structure inferior to the thalami:

1. First, confirm normal anatomy:

   • Verify presence of normal midline falx cerebri
   • Confirm thalami are separate (not fused)
   • Assess normal ventricular configuration
   • Identify the structure as normal CSP

2. If abnormal features present:

   • Perform detailed fetal anatomic scan 4
   • Consider fetal MRI at ≥22 weeks for CNS anomalies, particularly for isolated ventriculomegaly, agenesis of corpus callosum, or absent CSP 4
   • MRI is especially helpful for CNS anomalies and provides complementary information 4

3. Look for associated anomalies:

   • Facial dysmorphism (proboscis, cleft, hypotelorism)
   • Extracranial abnormalities (polydactyly, renal dysplasia, omphalocele)
   • These associations significantly impact prognosis

Key Pitfalls to Avoid

• Do not mistake normal CSP for pathology - it is universally present in mid-gestation fetuses
• Do not overlook thalamic fusion - this is the critical distinguishing feature of holoprosencephaly 1
• Do not assume isolated findings are benign - absent CSP warrants MRI evaluation even when isolated 4
• Do not delay MRI - optimal timing is ≥22 weeks, though 18-22 weeks may have value in certain cases 4

The normal CSP should close postnatally, with absence in adults being a normal variant. However, prenatal absence of CSP is pathological and requires thorough evaluation for associated midline brain malformations 5.