A cervical thymus usually presents before adolescence as a painless unilateral midline or lateral neck mass.
Around the 4th to 5th week of gestation, development of the thymus begins from the 3rd branchial pouch. Towards the end of the 6th week, the thymus traverses caudomedially into the inferoanterior mediastinum before fusing in the midline.
Ectopic tissue may occur anywhere along this path secondary to failure of or incomplete descent, implantation or persistence of remnant tissue or failure of involution; resulting in a cervically positioned thymus above the brachiocephalic vein.
Sonographic characteristics of the parenchyma are the same as normally positioned tissue:
- multiple linear hyperechoic septa
- discrete homogeneously distributed hyperechoic foci giving a "speckled" appearance
Contiguity with the normally positioned thymus may be seen with the thymopharyngeal duct.
Treatment and prognosis
No intervention is required as normal involution will usually occur with age. Like normal thymic tissue, residual focal thymic tissue may persist throughout life as a normal variant.
The clinical differentials for a cervical thymus can include:
- branchial cleft anomaly
However, if the characteristic normal thymic echo pattern is demonstrated, other differential diagnoses are easily excluded.
- 1. Bansal AG, Oudsema R, Masseaux JA, Rosenberg HK. US of Pediatric Superficial Masses of the Head and Neck. (2018) Radiographics : a review publication of the Radiological Society of North America, Inc. 38 (4): 1239-1263. doi:10.1148/rg.2018170165 - Pubmed
- 2. Scott KJ, Schroeder AA, Greinwald JH. Ectopic cervical thymus: an uncommon diagnosis in the evaluation of pediatric neck masses. (2002) Archives of otolaryngology--head & neck surgery. 128 (6): 714-7. Pubmed
- 3. Costa NS, Laor T, Donnelly LF. Superior cervical extension of the thymus: a normal finding that should not be mistaken for a mass. (2010) Radiology. 256 (1): 238-42. doi:10.1148/radiol.10091792 - Pubmed