A 16-week obstetric ultrasound is performed on a 28-year-old multiparous woman. A cystic mass is noted in the fetal neck. Which single ultrasound feature best distinguishes a cystic hygroma from a branchial cleft cyst?

## Cystic Hygroma vs Branchial Cleft Cyst: Internal Septations as the Key Ultrasound Discriminator **Key Point:** The **presence of internal septations and multiloculated appearance** is the single best *ultrasound* feature that distinguishes cystic hygroma from branchial cleft cyst. Both lesions can occur in the posterior or lateral neck, but their internal architecture on sonography is the defining discriminator. ### Comparative Table | Feature | Cystic Hygroma | Branchial Cleft Cyst | | --- | --- | --- | | **Internal architecture** | **Multiloculated, septated ("honeycomb")** | **Unilocular, simple, anechoic** | | Location | Posterior triangle (predominant) | Anterior/lateral to SCM | | Embryological origin | Lymphatic channels (jugular lymph sac) | Branchial apparatus remnant | | Wall thickness | Variable, may be thick | Thin | | Hydrops association | 50–80% | <5% | | Chromosomal anomalies | 50–70% (trisomy 13, 18, 21) | <5% | | Prognosis | Guarded | Good | **High-Yield:** On ultrasound, cystic hygroma characteristically shows **multiple internal septations creating a multiloculated or "honeycomb" pattern**, reflecting its origin from a network of lymphatic spaces. Branchial cleft cysts (typically 2nd cleft) are **unilocular, anechoic, and thin-walled** — indistinguishable from a simple cyst on imaging. This internal architecture difference is the most reliable *sonographic* discriminator. ### Why Not Location (Option B)? Location alone is unreliable as a single discriminating ultrasound feature because: - Cystic hygromas can extend anterolaterally and may not be confined to the posterior triangle. - Branchial cleft cysts can occasionally present posteriorly. - Both lesions may appear in overlapping cervical regions, especially in the fetus. ### Why Not Hydrops/Chromosomal Association (Option C)? While hydrops fetalis and chromosomal anomalies are strongly associated with cystic hygroma, these are **clinical/prognostic** features — not direct ultrasound morphological features. The question asks for the best *ultrasound* discriminating feature. ### Why Not Anechoic Appearance (Option D)? An anechoic, thin-walled appearance is characteristic of **branchial cleft cyst**, not cystic hygroma. Moreover, this feature describes one lesion rather than distinguishing between the two. **Clinical Pearl:** When a fetal neck cyst shows internal septations on ultrasound, cystic hygroma must be the primary diagnosis — prompt karyotyping and detailed fetal anomaly survey are indicated. [cite: Callen Ultrasonography in Obstetrics and Gynecology 6e Ch 21; Rumack Clinical Diagnostic Ultrasound 4e]