Thyroglossal Duct Cyst

Etiology

Knowledge of neck embryology and anatomy aids in understanding how thyroglossal duct cysts develop. A thyroglossal duct cyst is an embryological remnant that forms due to the failure of the thyroglossal duct to close, which extends from the foramen cecum in the tongue to the thyroid's position in the neck. The thyroid begins to develop in the third week of gestation as a median outgrowth from the primitive pharynx. The thyroid primordium originates at the foramen cecum, located at the junction of the anterior two-thirds and posterior one-third of the tongue.

The thyroid subsequently descends to the neck, passing anteriorly and near the developing hyoid bone. The thyroid reaches its final position in the inferior pretracheal neck by the seventh week of gestation. The thyroglossal duct is a narrow, tubular structure that serves as a remnant of the thyroid gland's descent, connecting the thyroid gland to the foramen cecum. In approximately 50% of individuals, the distal part of the duct develops into the pyramidal lobe of the thyroid gland. Typically, the thyroglossal duct involutes by the tenth week of gestation. However, if any portion of the duct remains, secretion from its epithelial lining can cause inflammation and lead to the formation of a thyroglossal duct cyst.[2][3]

Epidemiology

Thyroglossal duct cysts affect approximately 7% of the global population and occur equally in males and females. While these cysts are the most common mass in children, they can also develop in adults with varying frequency. Thyroglossal duct cysts are closely associated with the hyoid bone, with their locations distributed as follows: 20% to 25% are found at the suprahyoid level, 15% to 20% at the level of the hyoid, and 25% to 65% at the infrahyoid level.[4][5] Furthermore, thyroglossal duct cysts can appear in all age groups.[6]

Histopathology

Thyroglossal duct cysts are fluid-filled structures lined with either respiratory epithelium, squamous epithelium, or a combination of both types of tissue. Due to their high susceptibility to infection, these cysts often show signs of inflammation, characterized by the presence of granulation tissue or giant cells. In approximately 70% of cases, small areas of ectopic thyroid gland tissue can be found, typically located within the wall of the cyst.

History and Physical

Thyroglossal duct cysts typically present as mobile masses in the midline of the neck, positioned near the hyoid bone. Although they are often asymptomatic, they can occasionally manifest as an abscess or a sinus that drains intermittently. The thyroglossal mass moves upward when the tongue is protruded or during swallowing. Additionally, thyroglossal duct cysts are closely associated with the hyoid bone and are most commonly located at or below the level of the hyoid.

Evaluation

Diagnostic Imaging Studies

Imaging is essential for both diagnosing a thyroglossal duct cyst and assessing the presence of healthy thyroid tissue. If normal thyroid tissue is absent in the lower neck, the patient and their caregivers should be informed about the potential need for lifelong thyroid replacement therapy following surgery.

Ultrasound is the preferred initial imaging method to evaluate patients with thyroglossal masses. Ultrasound imaging is widely available, cost-effective, and noninvasive, making this modality ideal, especially in children, as it does not involve ionizing radiation or sedation. While computed tomography (CT) scans and magnetic resonance imaging (MRI) can also be used to evaluate thyroglossal duct cysts and check for normal thyroid tissue, ultrasound is typically sufficient.

Laboratory Studies

Some surgeons recommend routine thyroid function tests as a precaution before surgical treatment is performed. Presurgical laboratory studies may be beneficial if ectopic thyroid tissue is suspected; however, current literature does not support routine laboratory work for uncomplicated thyroglossal duct cysts. An ectopic thyroid gland cannot be ruled out even if TSH levels are within normal limits.[7][3][8] Additionally, thyroglossal cysts and thyroid disease may coexist in adults.[9]

Treatment / Management

Antibiotic therapy is recommended before surgical excision if the cyst is infected, as this promotes safer dissection and removal. The antibiotic agent utilized should be guided by culture and sensitivity results. However, oral amoxicillin, augmentin, or clindamycin provides sufficient bacterial coverage in most cases. If preoperative evaluation reveals that no other functional thyroid tissue is present, removal can still be performed with the understanding that hormone replacement therapy may be necessary postoperatively.

The primary treatment for thyroglossal duct cysts is surgical excision, as this intervention not only prevents recurrent infections but also addresses the small risk of malignancy. However, simple excision of a thyroglossal duct cyst is linked to high recurrence rates, ranging from 45% to 55%. Therefore, the Sistrunk operation is the standardized surgical approach due to its significantly lowered recurrence rates. Moreover, this procedure involves more extensive surgical resection, including the removal of the central third of the hyoid bone and a core of tissue from the base of the tongue.[10][11] Ultrasound-guided ethanol ablation for thyroglossal duct cyst has shown some potential as a new treatment strategy.[12][13]

Differential Diagnosis

The differential diagnoses of thyroglossal duct cysts include a wide array of midline and cystic neck masses that may present with similar clinical features. Cystic metastatic lymph nodes, dermoid or epidermoid cysts, and second branchial cleft cysts should also be considered. Cystic metastatic lymph nodes typically originate from papillary thyroid carcinomas or squamous cell carcinomas of the upper aerodigestive tract. Additionally, dermoid or epidermoid cysts may present as midline cystic neck masses.

A key feature that helps differentiate thyroglossal duct cysts from similar conditions is their close association with the hyoid bone. However, the final diagnosis is usually confirmed through pathology. In contrast, second-branchial cleft cysts appear as cystic masses in the anterior neck but are located laterally and are not associated with the hyoid bone. Additionally, obtaining an accurate preoperative diagnosis is crucial, as the most common misdiagnosis is often a presumed dermoid cyst.[14]

Surgical Oncology

Less than 1% of thyroglossal duct cysts develop into carcinoma. The most common type of malignancy identified is papillary carcinoma, accounting for 92.1% of cases, followed by squamous cell carcinoma at 4.3%. Thyroglossal duct cyst carcinoma typically presents as an asymptomatic midline neck mass. 73.3% of these carcinomas are diagnosed incidentally during final pathological analysis. Patients with thyroglossal duct cyst carcinoma are usually adults and tend to be older on average than those with typical thyroglossal duct cysts.[11][12] The treatment for papillary carcinoma of the thyroglossal duct cyst typically involves a Sistrunk procedure. Following this, the lateral neck lymph nodes and the thyroid gland are assessed. Depending on the extent of the disease, additional treatments may include a total thyroidectomy, lateral neck dissection, and/or radioactive iodine therapy. The prognosis for this condition is excellent, with a 99.4% survival rate and a 4.3% recurrence rate.

Prognosis

The prognosis after the Sistrunk procedure is generally excellent. About 10% of thyroglossal duct cysts may recur following surgery. The surgical treatment technique utilized is the most significant factor affecting recurrence, as recurrence is around 5% after the Sistrunk procedure.[10] The recurrence rate is notably higher, at 55.6%, when a simple excision is performed without removing the middle third of the hyoid bone. Notably, approximately 1% of thyroglossal duct cysts are malignant and are only diagnosed after surgical removal.

Complications

The most common complication of the Sistrunk procedure is the recurrence of the thyroglossal duct cyst, which occurs in approximately 10% of cases. Factors contributing to recurrence include incomplete excision, intraoperative rupture, the surgeon’s skill and experience, and the presence of infection.[15] Acknowledging that recurrence can still occur even after a technically proficient procedure is essential.

A rare but potentially serious complication of the Sistrunk procedure is laryngotracheal injury, which can lead to airway, swallowing, and voice difficulties. This injury may occur if the surgeon mistakenly resects the thyroid cartilage instead of the hyoid bone. Correctly identifying the hyoid bone, thyroid cartilage, and thyrohyoid membrane is crucial during surgery to prevent this complication.

Additionally, hypoglossal nerve injury, although rare, has been reported following the Sistrunk procedure. This injury can result in paralysis on 1 side of the tongue. The hypoglossal nerve runs lateral to the hyoglossus muscle and medial to the stylohyoid muscle and lingual nerve, near the lateral aspect of the hyoid bone. To avoid injury to the hypoglossal nerve, the resection of the hyoid bone must be performed medial to the lesser cornu of the hyoid.