Multiloculated cervical thymic cyst.

In this study, we report a rare case of cervical thymic cyst in an 8-year-old child.

INTRODUCTION

The thymus is derived mostly from the third pharyngeal pouch. During embryogenesis, the primordial thymic tissue descends down into the mediastinum, initially connected to the pharynx by hollow structures named “thymopharyngeal ducts”.[1] During its path of descent, remnants of thymic tissue may be left behind, leading to formation of a cyst. Cervical thymic cysts are rare,[2] and most are unilocular.[3] Thymic cysts account for 0.3% of pediatric congenital cervical cysts.[3] A variant which communicates with the pharynx is designated “thymopharyngeal duct cyst,”[4]

CASE REPORT

An 8-year-old child presented with a swelling involving the upper cervical region on the left side of the neck for 3 months. The swelling was soft to cystic in consistency and was situated in the carotid triangle. The lesion had diffuse borders, smooth external surface, and appeared fluctuant. On clinical examination, the lesion was diagnosed as “branchial cyst”. A fine needle aspiration cytology performed elsewhere had been reported as “benign retention cyst”.

The ultrasound examination showed a cystic mass that measured 3.7 × 2.5 × 2.4 cm in its greatest dimension. The lesion was surgically excised. During surgery, the lesion was found connected to the pharynx by a firm cord. The excised specimen was a multiloculated pale brown, cystic tissue with a knobby surface, 5 × 4 × 2 cm. On sectioning, the surface displayed multiple locules between 0.3 and 1 cm in diameter with pale brown serous fluid; separated by fine translucent septae with tiny grey-white nodules [Figure 1]. On microscopic examination, thymic tissue with lobules of lymphoid follicles with germinal centers and large keratinous to amorphous appearing Hassal's corpuscles were present. Many of the Hassal's corpuscles had a distended appearance with flat epithelial lining and luminal debris, suggesting early cyst formation [Figure 2]. The stroma had cholesterol clefts with foreign body reaction.

Figure 1

Cut section of the specimen revealing brownish multiloculated cyst with thin septae

Figure 2

Microphotograph showing degenerating Hassal's corpuscle within thymic lymphoid tissue (H and E, ×400)

DISCUSSION

Cervical thymic cysts are uncommon lesions, more often affecting the left side of the neck, with a slight male preponderance.[4] They usually manifest during the first decade of life, as a slow, painless mass in the region between the angle of mandible to sternum, in the vicinity of the carotid sheath, located along the path of descent of embryonic tissue.[1] Thymic cysts may be connected to the pyriform fossa through the thyrohyoid membrane.[5]

Contrast enhanced computed tomogram (CT) scans can differentiate thymic cysts from other pediatric neck swellings, such as branchial cleft cysts and lymphangiomas; the second branchial cysts are located superficial and lateral to internal jugular vein and common carotid artery, and lymphangiomas are found in the posterior triangle of the neck while thymic cysts are situated in close association with the carotid sheath, between internal jugular vein and carotid vessels.[67] Also, thymic cysts tend to be longer, extending toward the anterosuperior mediastinum.[7]

The cysts measure up to 15 cm in diameter, most are unilocular.[4] The cyst contents are either clear, serous or brownish fluid. Microscopic diagnosis requires that the cyst wall contains thymic tissue.[1] Microscopy is characterized by typical thymic tissue with large, degenerating Hassal's corpuscles and cysts lined by single or multilayered epithelium. The parathyroid gland may be found in the periphery of the cyst, indicative of the common embryologic origin from the third branchial pouch.[8]

The origin of cervical thymic cysts has been attributed to two differing mechanisms. The unilocular cysts are thought to be due to the persistence of thymopharyngeal duct, while the multilocular cysts result from cystic degeneration of thymic Hassal's corpuscles.[49]

Surgical removal is the treatment of choice. Thymectomy during childhood has been documented to produce impairment of immune status in later life.[10] Hence, it is imperative that the existence of mediastinal thymus is confirmed before proceeding with the excision of the cervical thymic tissue. Magnetic Resonance Imaging (MRI) and Fine Needle Aspiration Cytology (FNAC) may be performed to confirm the presence of a mediastinal thymus; and in the absence of which the surgical procedure should be deferred.[6] Cervical thymic cysts are not known to recur or undergo malignant transformation.