Cystic hygroma is a benign congenital vascular anomaly of the lymphatic system characterized by the progressive expansion of fluid-filled lymphatic spaces arising from the sequestration or obstruction of developing lymphatic pathways.[1] Almost 80% to 90% of cases are diagnosed in infants and children under two years of age, its manifestation is uncommon in adult population.[2] In adults, the exact aetiology remains idiopathic; however, proposed theories suggest that external triggers such as previous upper respiratory tract infections or localized head and neck traumamay stimulate this condition.[3] Adult onset cervical cystic hygromas most commonly present as painless, gradually enlarging,soft, fluctuant masses, predominantly located in the posterior cervical triangle.[4] Establishing a definitive preoperative diagnosis of an adult cervical cystic hygroma can bequite challenging, as these lesions are frequently misdiagnosed as more common benign anomalies like branchial cleft cysts, thyroglossal duct cysts, dermoid cysts, or cold abscesses.[5] Preoperative evaluation relies on high-resolution ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI) to delineate anatomical boundaries and map relationships with surrounding vital neurovascular structures.[6] Complete surgical excision remains by far the definitive gold-standard therapeutic option with aim to prevent airway compromise and minimize the risk of recurrence.[7]. CASE REPORT A 35-year-old malepresented to the ENT OPD with 3-year history of left lateral neck swelling with mild discomfort.Physical examination revealedapproximately 8x8cm soft,cystic, gradually enlarging, fluctuant,non-tender mass. MRI of neck showed well-defined multiloculated cystic lesion in the left posterior triangle, extending into the left supraclavicular region, measuring approximately 7.0 x 6.6 x 3.9 cm (TR x CC x AP). The lesion appeared T1 hypointense and T2hyperintensewith multiple thin internal septation showing mild post-contrast enhancement without any enhancement of solid component. Superiorly, the lesion abuts the sternocleidomastoid muscle with maintained intervening fat planes, inferiorly abuts the left subclavian artery and medially, it is seen abutting the left internal carotid artery, without evidence of vascular enhancement or luminal compression. The lesion demonstrated insinuation along facial planes.The imaging features were suggestive of a cystic lymphatic malformation likely cystic hygroma. 8 x 8 cm multiloculated cystic mass extending inferiorly into the supraclavicular region was identified intraoperatively. The lesion extended anteriorly to the posterior border of the sternocleidomastoid muscle and posteriorly to the anterior border of the trapezius muscle. Careful blunt dissection was performed over the very thin-walled cyst wall to separate it from the surrounding cervical tissues. A small amount of milky-white lymphatic fluid was noted intraoperatively. The spinal accessory nerve was identified and preserved. Complete excisionof the mass was achieved. Histopathological examination showed multiple variably sized cystic spaces separated by fibrous septa and lined by flattened endothelium without cytological atypia. Lumen contains focal eosinophilic proteinaceous fluid. Intervening septa consisted of loose fibro collagenous to fibro-adipose tissue with lymphocytic aggregates and lymphoid follicles. Many thin-walled blood vessels with smooth muscle bundles were also noted. Hence,histopathological features were consistentwithcystic lymphangioma (Cystic Hygroma).

Cystic hygroma is predominantly a congenital lymphatic malformation of childhood, with 80% to 90% of cases presenting before 2 years of life.[1]Presentation in adulthood is uncommon, making this 35-year-old man's case unusual. These benign lesions may progressively enlarge to considerable dimensions remaining without any symptoms. Despite their massive dimensions, compressive symptoms such as dysphagia, dyspnea, or airway compromise are uncommon due to their slow, insinuating growth along fascial planes. [8] Consequently, adult cervical cystic hygromas present a formidable diagnostic dilemma, frequently mimicking other neck masses such as branchial cleft cysts, thyroglossal duct cysts, lipomas, or lymphomas.[7] MRI is the gold standard for preoperative surgical planning of adult cervical cystic hygromas.[9] In this case, the 7.0 x 6.6 x 3.9 cm multiloculated lesion in left posterior triangle, exhibiting T1 isointense to T2 hyperintense, with thin, mildly enhancing internal septations and no solid enhancement component.[10] Insinuating growth pattern along fascial planes is its hallmark radiological feature. These imaging features were consistent with benign macrocytic lymphatic malformation and were instrumental in delineating its relationship with adjacent neurovascular structure. [9,11]. Complete surgical excision remains the gold standard treatment for adult cervical cystic hygroma [6]. In this case, the 8 x 8 cm mass occupied the posterior triangle of the neckextended deeply into the supraclavicular fossa. Approaching this mass via a transverse cervical incision placed in a natural skin crease optimizes cosmetic healing and reduces tension across the elevated subplatysmal flaps [12]. During the surgical exploration of adult-onset macrocystic lesions, the cyst wall is typically thin, making intraoperative leakage often characterized by the outflow of milky white lymphatic fluid a common event [1]. Although cyst decompression can facilitate deeper dissection, surgeons must ensure meticulous, complete removal of the entire cyst wall, as partial resection dramatically increases recurrence rates [13]. Most importantly, operations in the posterior cervical triangle demand active identification and preservation of the spinal accessory nerve, protecting it from transient or permanent injury [14]. Finally, placing a negative-pressure active suction drain, such as a 14 Fr Romovac, is essential to collapse dead space and prevent postoperative seroma [15].

Adult cervical cystic hygromas is rare and may pose a significant diagnostic challenge due to its resemblance with other neck mass.MRI is important for confirming diagnosis, defining the extent of mass, assessing it relationshipswith adjacent neurovascular structures in the posterior cervical triangle. Complete surgical excision with meticulous dissection and preservation of spinal accessory nerve remains an effective treatment.

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