During a routine second-trimester anatomy scan at 20 weeks, a fetal intracranial mass is identified. The lesion is located in the midline, appears cystic with internal echoes, and compresses the lateral ventricles. What is the most common intracranial mass detected on prenatal ultrasound?

Explanation

## Fetal Intracranial Masses on Prenatal Ultrasound **Key Point:** Choroid plexus cysts (CPCs) are the most common intracranial mass detected on second-trimester ultrasound, present in 0.3–3.6% of fetuses. Most are benign and resolve spontaneously by the third trimester. ### Ultrasound Characteristics of Choroid Plexus Cysts | Feature | Details | |---------|----------| | **Location** | Within choroid plexus (lateral ventricles) | | **Appearance** | Anechoic or hypoechoic cystic lesion | | **Size** | Usually <1.5 cm | | **Bilateral** | Can be unilateral or bilateral | | **Associated findings** | May be associated with trisomy 18 (Edwards syndrome) | | **Prognosis** | >90% resolve by 24 weeks | **High-Yield:** Choroid plexus cysts are considered a **soft marker** for trisomy 18 when isolated, but isolated CPCs have a low risk of aneuploidy (<1%) in the absence of other anomalies. Detailed anatomy scan and maternal serum screening are recommended. ### Differential Diagnosis of Fetal Intracranial Cysts | Lesion | Location | Appearance | Clinical Significance | |--------|----------|-----------|----------------------| | **Choroid plexus cyst** | Lateral ventricles | Anechoic, small | Soft marker; usually benign | | **Arachnoid cyst** | Subarachnoid space | Cystic, mass effect | Rare; may cause hydrocephalus | | **Porencephalic cyst** | Cerebral parenchyma | Irregular, often unilateral | Associated with stroke/infection | | **Vein of Galen aneurysm** | Midline, deep | Pulsatile flow on Doppler | High-output cardiac failure | **Clinical Pearl:** When a choroid plexus cyst is identified, the key management principle is to perform a detailed anatomy scan to exclude other markers of aneuploidy (cardiac defects, renal anomalies, growth restriction). If isolated and maternal age is <35 years, reassurance is appropriate; if maternal age >35 or other markers present, consider amniocentesis or cell-free fetal DNA testing. **Mnemonic: CPC BENIGN** - **C**horoid plexus — location - **P**erfectly — usually normal - **C**ystic — appearance - **B**enign — >90% resolve - **E**arly — detected at 18–22 weeks - **N**ormally — no mass effect - **I**solated — soft marker only - **G**enerally — reassuring - **N**o — intervention needed **Warning:** Do not confuse a choroid plexus cyst with hydrocephalus. Hydrocephalus involves enlargement of the ventricles (>10 mm at lateral atrium); a choroid plexus cyst is a small lesion within the plexus itself and does not cause ventricular dilatation.