The incidence of sternoclavicular sinus may vary around the world. This malformation was described for the first time in 1994 in 18 cases of Japanese patients. [3] Since then, a few of cases were reported in literatures. [4–12] In our study, we collected 88 patients in two years. It was more common than the second branchial anomaly and pyriform sinus in our hospital. The large number of the cases may be explained by two reasons. First, our institution is the largest tertiary hospital in west of China, which covers four provinces and more than 171 million of population. Second, though the actual incidence was unclear, most of the published reports were from Japan and China. The genetic factors may play an important role the in the pathogenesis of this disease.
The embryological origin of the sternoclavicular sinus was unclear. Thus, the names used by the authors to refer to the anomaly were considerable inconsistency. In early published study, it was suggested that the sternoclavicular sinus was derived from ectoderm and designated as a dermoid fistula. [9] The other similar names included congenital sternoclavicular dermoid sinus and congenital dermoid fistulas of the anterior chest region. [1] Dermoid cysts are resulted from entrapment of epithelial elements in the deeper tissues along embryonic lines of fusion (inclusion type of dermoid). [13] The sternum arises from two bands of somatopleural mesenchyme near the median plane of the thorax about 6 weeks after fertilization, when the blastemal clavicles are appearing. [14] The proximal portion of the clavicle fuses to the upper part of the sternum. Histologically, dermoid cyst/sinus is lined by squamous epithelium of ectodermal origin with adnexial structures of endodermal origin (hair follicles, sebaceous glands and sweat glands. [15] It was observed in most of sternoclavicular sinus.
However, there were two characteristics of the sternoclavicular sinus cannot be explained by the hypothesis of dermoid sinus. Firstly, in accordance with previous report, most of the sinuses are in the left side. [2, 3, 6] The frequently involved area of dermoid cyst is the periorbital area, followed by the nasal dorsum, submental area, and the suprasternal area. [16] The nasal, submental and suprasternal dermoid are in the midline. As to the periorbital dermoid, we did not find it occur in one side more likely than the other side. [17] Another similar malformation is preauricular sinuses. [18] They are not of true branchial cleft origin and represent related to embryonic ectodermal mounds (auditory hillocks) that essentially form the auricles of the ear. The preauricular sinuses also have race/ethnicity difference in the occurrence and are more commonly on the right side. But, the lining cells of preauricular sinuses are stratified squamous epithelium. The ciliated columnar epithelium or salivary gland are never seen in it. The second characteristic of sternoclavicular sinus not conforming to dermoid sinus was ciliated columnar epithelium or salivary gland being detected histologically in the specimen. The salivary gland and ciliated epithelium are endodermal origin and the characteristics of branchial remnant fistula. [19]
All these findings suggested that the congenital sternoclavicular sinus was related to the fourth branchial remnant fistula. Both third and fourth branchial anomalies are rare congenital malformation of the neck. These fistulas enter the pyriform sinus and are difficult to differentiate them in clinic. [20] The fourth arch anomaly usually presents as cyst in newborn and suppurative thyroiditis outside the neonatal period. [21] Most of the symptomatic pyriform sinuses (more than 90%) located on the left side. [22] Theoretically, the entire course of the fourth branchial fistula originates at the apex of the pyriform sinus, descends beneath the aortic arch, and then ascends anterior to the carotid artery to end in the vestigial cervical sinus of His. [23] Actually, the fistula of complete remnant of the fourth branchial apparatus has never been reported. The external opening site of the fourth branchial fistula is near the sternoclavicular joint area. Therefore, the sternoclavicular sinus may be the skin side of the remnant of the fourth branchial remnant fistula. This can also explain the findings of ciliated epithelium and salivary gland in the histological examination of the sinuses. The hypothesis was also supported by study performed by Ohno et al. [5]
The main complication of the sternoclavicular sinus was infection and abscess formation. The infection can be occurred as early as 2 months old. I&D should be performed in most patients once the abscess formation. And the infection tended to recur in a short period even after I&D. Surgical resection the sinus was indicated in all patients diagnosed with sternoclavicular sinus. Complete resection was simple in patient without previous infection for the short course of sinus. However, it would be difficult to identify the sinus in patient with recurrent infection and prior I&D. The en bloc excision of the involved tissue was sufficient to prevent the recurrence.
The observational and retrospective nature of this study was an important limitation. As an academic institution and tertiary medical center, there may be more patients with rare diseases referred to our hospital. The prevalence may be overestimated.