A 28-year-old primigravida from Delhi presents to the obstetric ultrasound clinic at 20 weeks of gestation for a routine anomaly scan. On transabdominal ultrasound, the fetus shows a large fluid-filled cavity in the posterior fossa measuring 12 mm in anteroposterior diameter, with normal cerebellar hemispheres bilaterally and an intact vermis. The lateral ventricles measure 10 mm. Nuchal fold thickness is 5 mm. What is the most likely diagnosis?

Explanation

## Diagnosis: Mega Cisterna Magna ### Key Imaging Findings **Key Point:** Mega cisterna magna is defined as enlargement of the cisterna magna (posterior fossa fluid space) to 10–15 mm in the second and third trimesters, with normal cerebellar anatomy and normal ventricular size. In this case: - Posterior fossa fluid collection: 12 mm (within the 10–15 mm range for mega cisterna magna) - Cerebellar hemispheres: normal and symmetrical - Vermis: intact - Lateral ventricles: 10 mm (normal; <10 mm at 20 weeks is reassuring) - Nuchal fold: 5 mm (borderline; >6 mm is concerning) ### Differential Diagnosis of Posterior Fossa Enlargement | Feature | Mega Cisterna Magna | Dandy-Walker Malformation | Arachnoid Cyst | Hydrocephalus | |---------|-------------------|--------------------------|----------------|---------------| | **Cisterna magna size** | 10–15 mm | >15 mm (often >20 mm) | Variable | Normal or enlarged | | **Cerebellar vermis** | Normal | Hypoplastic/absent | Normal | Normal | | **Cerebellar hemispheres** | Normal | Hypoplastic, splayed | Normal | Normal | | **Lateral ventricles** | Normal | Enlarged | Normal | Enlarged (>10 mm) | | **Associated anomalies** | Rare | Common (cardiac, skeletal, CNS) | Rare | Variable | | **Prognosis** | Excellent (benign) | Poor | Variable | Depends on cause | ### Clinical Pearl **Clinical Pearl:** Mega cisterna magna is a **benign anatomical variant** with an excellent prognosis. It does NOT represent pathology and requires no intervention. The key distinguishing feature is the **preservation of normal cerebellar anatomy and normal ventricular size**. ### High-Yield Distinguishing Points **High-Yield:** - **Dandy-Walker malformation** would show a hypoplastic or absent vermis, splayed cerebellar hemispheres, and enlarged lateral ventricles—none of which are present here. - **Arachnoid cyst** would typically show a mass effect or mass-like appearance, not a simple fluid collection in the expected location of the cisterna magna. - **Hydrocephalus** would manifest with enlarged lateral ventricles (>10 mm at 20 weeks), which are normal in this case. ### Management **Tip:** When mega cisterna magna is diagnosed: 1. Confirm normal cerebellar anatomy and vermis 2. Confirm normal ventricular size 3. Look for associated anomalies (cardiac, skeletal, other CNS) 4. Reassure parents: this is a benign finding with no clinical consequences 5. Standard prenatal care; no additional surveillance needed ### Borderline Nuchal Fold The nuchal fold of 5 mm is at the upper limit of normal (normal <6 mm at 20 weeks). While this may warrant follow-up imaging in the third trimester, it does not change the diagnosis of mega cisterna magna itself.