Infant Neck Swelling Differential Diagnosis

The question describes an infant presenting with a unilateral neck swelling. Key characteristics provided are: soft, cystic, partially compressible, and brilliantly transilluminant.

Clinical Feature Analysis

• Unilateral Neck Swelling: Indicates a localized process on one side of the neck.
• Soft, Cystic, Partially Compressible: Suggests a fluid-filled lesion that can be manually compressed.
• Brilliantly Transilluminant: This is a crucial sign, indicating the lesion is filled with clear fluid and allows light to pass through easily. It strongly points towards a lymphatic or fluid-filled origin.

Evaluating Diagnostic Options

• Branchial Fistula: Typically presents as an external opening or sinus tract, not a distinct swelling. It is not usually cystic or transilluminant.
• Branchial Cyst: While cystic and located in the neck, branchial cysts are often firmer, less compressible, and not typically described as "brilliantly transilluminant". They usually arise from remnants of the second branchial arch.
• Cystic Hygroma: This is a congenital malformation of the lymphatic system (lymphatic malformation). It characteristically presents as a soft, compressible, often large cystic mass. The fluid-filled nature makes it brilliantly transilluminant, matching the description perfectly. Common locations include the posterior triangle of the neck.
• Thyroglossal Cyst: This cyst is typically found in the midline of the neck and characteristically moves upward when the patient swallows or protrudes their tongue. It is less likely to be unilaterally located or brilliantly transilluminant.

Conclusion

Based on the specific combination of clinical findings – particularly the unilateral location, soft cystic nature, partial compressibility, and especially the brilliant transillumination – Cystic Hygroma is the most probable diagnosis.