The cervical thymus (plural: cervical thymi) refers to an ectopic location of the thymus in the neck above the level of the brachiocephalic veins.
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Clinical presentation
A cervical thymus usually presents before adolescence as a painless unilateral midline or lateral neck mass.
Pathology
Etiology
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.
Location
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 veins.
Radiographic features
Ultrasound
Sonographic characteristics of the parenchyma are the same as normally positioned thymic tissue:
- multiple linear hyperechoic septa
- discrete homogeneously distributed hyperechoic foci giving a "speckled" or "starry sky" 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 an anatomic variant.
Differential diagnosis
The clinical differentials for a cervical thymus may include:
However, if the characteristic normal thymic echo pattern is demonstrated, differential diagnoses are usually easy to exclude.