Cervical ectopic thymus is a rare form of congenital neck mass in children.1 It usually occurs in the age-group of 2–13 years children with slightly higher incidence in boys. The masses are mostly located on the left side of the neck, which may cause the change in vocal pitch, difficulty breathing, or difficulty swallowing.2–4 Pediatric cervical ectopic thymus generally presents as a cervical thymic cyst or ectopic thymic tissue, the latter of which was more likely found by the age of 3 years or younger.5 Despite the low prevalence, it is necessary to take the ectopic thymus into consideration during the differentiation diagnosis of cervical masses.

Theoretically, an ectopic thymus could occur in any other location outside the normal descend pathways, such as pharynx, trachea, posterior neck or mediastinum, or esophagus,6 where subglottic ectopic thymus is a rare condition. A total of 20 children with cervical thymic remnant were summarized from 1975 to 2006 at Cincinnati Children’s Hospital Medical Center, none of which were in the subglottic lesion.7 Up till now, only six cases of subglottic ectopic thymus have been reported separately in English literature. Basic information and characteristics of these cases were summarized in Table 1 with adding our case in this study.5,6,8–11 Noteworthily, it is the first time to report a highly suspected subglottic ectopic thymus patient before surgical intervention.

Normally, thymus tissue originates from the ventral development of bilateral third pharyngeal sacs during the sixth week of embryonic development.12,13 Then it splits into a dorsal wing and produces the thymus.14,15 The dorsal wing extends into the thymopharyngeal tube.12,14,15 At the eighth week, the thymopharyngeal tube moves down from the posterior sternum to the mediastinum, and fuses in the midline. This descent depends on the association with mesodermal components of the medial and posterior thymus. At the third month, the thymus is mature with cortex and medulla, and Hassall corpuscles are seen.16 The relative size of the thymus reaches its maximum at the age of four and begins to degenerate after puberty, but Hassall corpuscles remain.17

Nowadays, mainstream view of the occurrence of subglottic thymus is that could be a result of the failure descent of thymic in the embryo. If thymic precursors are pulled medially before cricoid cartilage formation at seventh week, it may be trapped within the annulus.18 In addition, subglottic thymus may derive from metaplasia of the subglottic pluripotent endodermal cells.10 However, this theory has not yet been clarified.

If a mass in the neck or anterior mediastinum is suspected of an abnormal thymus tissue, ultrasound is the first choice for the evaluation. In the published cases,1,19 ectopic thymus presents as a clear hypoechoic lesion with punctate and linear echogenic lesions giving the characteristic “starry sky” appearance similar to the normal anterior mediastinal thymus. Ultrasonic diagnosis is based on mediastinal thymic homogeneity and similarity.20,21

CT scan as the criterion standard for radiological diagnosis could be suggested when ultrasound results cannot provide a hint to make a definitive diagnosis.6 The feature of uniform low-density mass with regular edges is shown in CT scan.20,22 A normal thymus may be barely visible on Positron Emission Tomography (PET) scans. However, it may show striking Fluorodeoxyglucose (FDG) avidity in rebound hyperplasia, causing false alarm for recurrent lymphoma.

Some authors believe that MRI scanning is the best non-invasive method to identify abnormal solid thymic problems. MRI of the normal thymus shows uniform signal intensity or slightly higher intensity compared with the muscles on T1-weighted images, and the signal intensity close to that of fat on T2-weighted images.10,23,24 Ectopic cervical thymus tissue is similar to those of mediastinal thymus. However, the cystic components of the abnormal thymus appear bright on T1-weighted images, unlike the mediastinal thymus, which appears dark on T1-weighted images.

The present preoperative examination could not accurately identify the subglottic mass as ectopic thymus tissue. Owing to these limitations and rarity, six cases of ectopic subglottic thymus previously reported in the literatures were misdiagnosed preoperatively. Indeed, a correct diagnosis can be made only by pathohistological examination of the excised specimen. Hassall corpuscles, which are restricted to the medulla and characterized by a concentric pattern of keratinization, are the most typical feature of thymus.25 Since thymus is mainly composed of epithelial cells and thymic lymphocytes, the immunohistochemistry shows positivity characters of these two types of cells.25

The broad differential diagnosis of pediatric neck masses typically fits into three main categories: inflammation, tumors, or structural abnormalities. There is cervical lymphadenitis, branchial arch cysts, lymphatic malformations, infantile hemangioma, squamous papilloma, parathyroid adenomas, dermoid or epidermoid cysts, carcinoids, and mucoepidermoid carcinomas in common causes.6,7 From the perspective of the subglottic lesion, infantile hemangioma as the most common disease could be easy to confuse. Most uncomplicated cases are treated conservatively and usually without pathohistological diagnosis. Review of the previous reports revealed that three out of the six reported cases (Cases 1, 3, 6) were misdiagnosed as hemangiomas. In our case, hemangioma was successfully ruled out before the surgery without tentative treatment for hemangioma.

For differential diagnosis, imaging examination and pathohistological evaluation of the biopsy are required. Clinic radiological approach based on morphologic features is useful for evaluation of a suspected thymic mass. DLB examination should be performed to visualize the mass for proper differential diagnosis.

The heterotopic subglottic thymus may cause recurrent respiratory distress and stridor. Although these symptoms can be partially alleviated by drug therapy, surgery is the gold standard for the purpose of both diagnosis and treatment, especially in cases of any symptomatic subglottic mass. A complete excision of the ectopic thymus is also important, as cases of malignant degeneration have been reported.26

In previous reports, several surgical approaches were used for the excision of ectopic thymus. In Case 1, under the condition of tracheotomy, a relatively unvascularized mass was found between the left superior pole of the thyroid gland and the trachea, extending backwards to the tracheoesophageal sulcus, which was completely removed. In Cases 2 and 4, thymic cysts were removed using a microlaryngoscopic surgery with a combination of cold instruments and microdebrider. No clear boundary was found between normal mucosa and the subglottic mass, and therefore marsupialization was deemed sufficient. But Case 4 repeated endoscopic surgery for the recurrence. In Cases 3 and 6, transverse neck incisions were performed to fully expose the trachea, cricoid cartilage and thyroid cartilage. Vertical midline incision was made through the cricoid cartilage to fully expose the subglottic lesions. Temporary tracheostomy or ETT inserted in the lower airways was performed for adequate exposure of the subglottic lesion in Case 3. In our case, we conducted a standard transverse neck incision, opening the cervical fascia vertically along the midline, preserving the strap muscles, and allowing a good exposure of the anterior thyroid and cricoid cartilage and the first two or three tracheal rings. The cricoid cartilage was cut vertically along the midline up to 1 cm below the vocal fold plane. An obvious mass between the perichondrium of the cricoid cartilage and the mucosa of the subglottic trachea was fully resected without tracheostomy. The intact subglottic mucosa was reconstructed to avoid the risk of postoperative stenosis.

The presence of mediastinal thymus should be confirmed before surgical resection of subglottic mass, because theoretically the removal of the thymus tissue may cause mental abnormalities, affect the development of normal immune function or induce autoimmune diseases.20,27

In the six published cases, no recurrences were found during the follow-up. In our case, a planned DLB was performed two months later, and no recurrent airway lesions were found.