Adult-Onset Cervical Lymphatic Malformation: A Case Report.

Lymphatic malformations (LMs) are low-flow vascular malformations of lymphatic origin that are typically diagnosed in childhood, with cervicofacial LMs posing particular risks for functional and cosmetic morbidity. Fewer than 10% of LMs present after adolescence, and adult-onset cervical cases are exceedingly rare, with only isolated reports in the literature. Adult-onset cervical LMs often mimic other cystic neck lesions, complicating diagnosis and delaying management. We report the case of a 20-year-old male who presented with a painless, slowly enlarging left upper neck mass. Ultrasound demonstrated a 3.2 × 3.0 × 1.4 cm multiloculated cystic avascular lesion, while contrast-enhanced CT revealed a 6.7 × 2.7 × 2.2 cm multiloculated cystic mass located between the parotid tail and the sternocleidomastoid muscle. MRI confirmed a T2-hyperintense, multiseptated lesion with areas of macroscopic intralesional fat and minimal enhancement, findings most consistent with LM rather than teratoma. Follow-up MRI nine months later demonstrated stability (2.7 × 1.2 × 3.7 cm). Active surveillance was chosen over intervention due to the patient's asymptomatic status, stable morphology, and the potential morbidity associated with surgical or sclerotherapeutic management. Adult cervical LMs remain diagnostically challenging due to overlap with branchial cleft cysts, ranulas, venous malformations, and cystic nodal metastases. The presence of intralesional fat can further complicate diagnosis, as it is more typically associated with dermoid cysts or teratomas, yet has been documented in LMs. Multimodal imaging is essential, with MRI offering the greatest sensitivity for delineating lesion extent. Management should be individualized: surveillance is appropriate in stable, asymptomatic cases, whereas progressive or symptomatic lesions may require sclerotherapy, surgical excision, or systemic therapies such as sirolimus or PI3K inhibitors. This case underscores the importance of including LM in the differential diagnosis of adult cervical cystic lesions and highlights that conservative management is often safe in clinically stable, asymptomatic patients.