Clinical History
A 3-year-old girl presented with repeated infections of the left neck involving the left lobe of thyroid. Each episode was managed with antibiotics and abscess drainage. At the time of this episode, she presented with acute pain, neck swelling, and fever, to the emergency department.
Imaging Findings
Contrast CT of the neck with multiplanar reconstructions reveals an expansile, fluid-containing process in the thyroid lobe. Smaller locules of fluid extend from the superior aspect of this collection to track upward in close proximity to the hypopharynx. A small locule of gas is also seen within the collection. On ultrasound, a large fluid containing an abnormality was seen through the left thyroid lobe.
Discussion
The branchial apparatus comprises of six mesodermal arches which are separated externally by ectoderm-lined "clefts" and internally by endoderm lined "pouches". Orderly embryologic differentiation of this apparatus results in formation of ectodermal, mesodermal and endodermal derivatives of the head and neck. Disordered differentiation of these structures can result in persistent embryonic communications leading to fistulas or sinuses.
Embryologically, the fourth branchial pouch connects to the pharynx via the pharyngobranchial duct which normally obliterates in utero. Failure of this process can result in persistence of embryonic communication between the pyriform sinus and thyroid gland (which is partially derived from the fourth branchial pouch). This is described as a pyriform sinus-thyroid fistula [1]. Alternatively, these abnormalities can manifest as intra-or perithyroidal cysts. These rare anomalies are more commonly seen on the left side [1].
Clinically, the affected patients present with recurrent upper respiratory infections, neck masses, and neck pain or tenderness [1].
Multiple imaging modalities contribute to the diagnosis. Ultrasound is usually the initial screening tool in these instances, and intra- or perithyroidal neck cysts or abscesses visualised on ultrasound should raise suspicion for an underlying fourth branchial remnant anomaly [1]. Gas within the fluid collection should be considered pathognomonic for this abnormality. CT, particularly when performed with “trumpet” manoeuvre (ask patient to blow his/her cheeks as if they were blowing a balloon or playing a trumpet), can complement ultrasound [1, 2]. A contrast swallow can outline the sinus tract once the acute infection has resolved [1].
Treatment includes management of the acute infection with antibiotics and surgical drainage of the abscess as necessary. Ultimately, the entire tract should be ligated and surgically excised. This includes excision of the cyst/inflammatory mass with en bloc removal of the involved thyroid tissue to prevent recurrence [1].
Teaching point:
Since thyroid infections are rare due to abundant thyroid vascularity and intrathyroidal iodine, suppurative thyroiditis, especially when recurrent, should raise strong concern for an underlying branchial remnant anomaly. The radiologist’s role in diagnosing this abnormality is crucial and appropriate imaging should be performed expeditiously.
Differential Diagnosis List
Fourth Branchial remnant anomaly-Pyriform sinus-thyroid fistula.
Thyroglossal duct cyst
Second Branchial Remnant Anomaly
Final Diagnosis
Fourth Branchial remnant anomaly-Pyriform sinus-thyroid fistula.
