The embryology, presentation, imaging, and treatment of the thyroglossal duct cyst will be reviewed. Anatomic features and surgical technique to prevent complications and recurrence will be discussed. Included in the discussion will be the management of thyroglossal duct cyst malignancy and ectopic thyroid.
Key points
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Thyroglossal duct cyst carcinomas.
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Posterior Hyoid Space.
Introduction
The thyroglossal duct cyst (TGDC) is the most common congenital malformation in the neck, accounting for 70% of all congenital neck lesions. Although TGDCs may occur at any age, most are detected in the first 2 decades of life. A previous histopathologic study of adult larynges showed TGDCs to be present in 7% of the population. TGDCs occur as a result of failure of the thyroglossal duct to obliterate during embryonic development. During the fourth week of fetal development, epithelium located in the floor of the pharynx (an area that later becomes the foramen cecum of the tongue) invaginates to form the thyroglossal duct. This duct then extends in a fingerlike fashion to a terminus in the lower midline neck, where its distal end becomes bilobed and further differentiates into the thyroid gland. Thyroid development is completed at the eighth week of gestation, after which the thyroglossal duct involutes between the eighth and 10th week. A TGDC may occur if viable epithelium persists anywhere along the course of the thyroglossal duct. As a consequence, the cyst can form anywhere from the level of the foramen cecum to the thyroid. During its descent, the thyroglossal duct passes close to the hyoid bone, which is concomitantly undergoing anterior fusion. Consequently, the 2 structures assume a close anatomic relationship. In a previous histopathologic study of TGDCs, most cyst tracts were noted to be ventral and closely adherent to the hyoid bone. A more recent study reported that the tract may be found posterior to the hyoid in approximately 30% of cases, which has important implications in treatment. The tract has never been reported to lie within the bone itself.
TGDCs normally present as indolent, asymptomatic soft masses, which move upward on tongue protrusion and swallowing. However, restricted movement of the cyst does not rule out diagnosis. According to Shah and colleagues, locations of TGDCs are classified into 4 subdivisions: (1) intralingual, (2) suprahyoid or submental, (3) thyrohyoid, and (4) suprasternal. TGDCs usually appear as midline cervical masses located just above or below the hyoid bone, but approximately one-third may present submentally or in lower cervical levels. Less than 1% are located off the midline. Most TGDCs are asymptomatic; however, the lingual subtype may cause laryngeal stridor, respiratory obstruction, and dysphagia. TGDCs may show similarities to dermoid cysts, lymphadenopathy, lymphatic malformations, ectopic thyroid gland, branchial cleft cysts, hemangiomas, lipomas, and sebaceous cysts. Thyroglossal duct carcinomas occur in less than 1% of TGDCs.
The epithelial lining of a TGDC ranges from squamous epithelium to pseudostratified ciliated columnar epithelium. Salivary gland tissue or thyroid gland tissue may be seen in the wall of the cyst. The cyst fluid can be described as mucoid, gelatinous, or purulent yellowish-white to dark brown, and may contain cholesterol. Sonographic studies have shown that stratified squamous epithelium tends to line cysts located near the foramen cecum, whereas thyroidal acinar epithelium tends to line cysts located more proximal to the thyroid gland.
Radiologic criteria for diagnosing TGDCs are usually based on the presence of ductular or cystic structures.
Introduction
The thyroglossal duct cyst (TGDC) is the most common congenital malformation in the neck, accounting for 70% of all congenital neck lesions. Although TGDCs may occur at any age, most are detected in the first 2 decades of life. A previous histopathologic study of adult larynges showed TGDCs to be present in 7% of the population. TGDCs occur as a result of failure of the thyroglossal duct to obliterate during embryonic development. During the fourth week of fetal development, epithelium located in the floor of the pharynx (an area that later becomes the foramen cecum of the tongue) invaginates to form the thyroglossal duct. This duct then extends in a fingerlike fashion to a terminus in the lower midline neck, where its distal end becomes bilobed and further differentiates into the thyroid gland. Thyroid development is completed at the eighth week of gestation, after which the thyroglossal duct involutes between the eighth and 10th week. A TGDC may occur if viable epithelium persists anywhere along the course of the thyroglossal duct. As a consequence, the cyst can form anywhere from the level of the foramen cecum to the thyroid. During its descent, the thyroglossal duct passes close to the hyoid bone, which is concomitantly undergoing anterior fusion. Consequently, the 2 structures assume a close anatomic relationship. In a previous histopathologic study of TGDCs, most cyst tracts were noted to be ventral and closely adherent to the hyoid bone. A more recent study reported that the tract may be found posterior to the hyoid in approximately 30% of cases, which has important implications in treatment. The tract has never been reported to lie within the bone itself.
TGDCs normally present as indolent, asymptomatic soft masses, which move upward on tongue protrusion and swallowing. However, restricted movement of the cyst does not rule out diagnosis. According to Shah and colleagues, locations of TGDCs are classified into 4 subdivisions: (1) intralingual, (2) suprahyoid or submental, (3) thyrohyoid, and (4) suprasternal. TGDCs usually appear as midline cervical masses located just above or below the hyoid bone, but approximately one-third may present submentally or in lower cervical levels. Less than 1% are located off the midline. Most TGDCs are asymptomatic; however, the lingual subtype may cause laryngeal stridor, respiratory obstruction, and dysphagia. TGDCs may show similarities to dermoid cysts, lymphadenopathy, lymphatic malformations, ectopic thyroid gland, branchial cleft cysts, hemangiomas, lipomas, and sebaceous cysts. Thyroglossal duct carcinomas occur in less than 1% of TGDCs.
The epithelial lining of a TGDC ranges from squamous epithelium to pseudostratified ciliated columnar epithelium. Salivary gland tissue or thyroid gland tissue may be seen in the wall of the cyst. The cyst fluid can be described as mucoid, gelatinous, or purulent yellowish-white to dark brown, and may contain cholesterol. Sonographic studies have shown that stratified squamous epithelium tends to line cysts located near the foramen cecum, whereas thyroidal acinar epithelium tends to line cysts located more proximal to the thyroid gland.
Radiologic criteria for diagnosing TGDCs are usually based on the presence of ductular or cystic structures.
Treatment goals and planned outcomes
Surgical extirpation of TGDCs is recommended, because of the likelihood of recurrent infections and the rare possibility of malignancy. Surgical management requires a thorough understanding of the embryogenesis of the thyroid gland and of the hyoid bone. The main aim of surgical treatment is the complete removal of the cyst and the duct. Historically, TGDCs were treated by simple excision or incision and drainage, which resulted in recurrence rates of greater than 50%.
Surgical approaches to the TGDC have been developed primarily to avoid complications associated with chronic infection, usually resulting in a cutaneous fistula. Schlange in 1893 was the first to propose excision of the TGDC, along with the central portion of the hyoid bone based on embryologic procedures. This technique was later refined in 1920 by Sistrunk, who included not only resection of the central hyoid but also a cuff of lingual musculature oriented toward the foramen cecum. The Sistrunk procedure is now recognized as the most efficacious surgical treatment of TGDCs, because of its low rate of recurrence. Nevertheless, recurrence remains a frequent complication in the treatment of TGDCs, even when the Sistrunk procedure is adhered to. Several investigators have proposed variants of the Sistrunk procedure in an attempt to further reduce these recurrence rates. These variants consist of different types of wider dissections, which include more tongue base tissue, an extension to the infrahyoid space including strap muscles, or an en bloc central neck dissection. However, none of them has been proved to significantly improve the outcome. Several other investigators have focused on different factors related to TGDC recurrence. Perkins and colleagues reported results of the Sistrunk procedure for 231 patients, based on 23 years’ experience. These investigators reported that TGDC recurrence was related to incorrect initial diagnosis in 50% of cases, previous infection in 15% of cases, TGDC presentation in unusual locations (ie, lateral neck or base of tongue) in 15% of cases, and lack of removal of tongue base muscle tissue and relative inexperience of the surgeon, both in 2% of cases. These factors identified to be related to TGDC recurrence are in agreement with those found in previous studies. It is our opinion that performing the wrong operation secondary to an error in diagnosis does not represent recurrence. Preoperative infection is a common TGDC complication affecting surgical outcome, with rates ranging from 10% to 70%. Traditional management has been to avoid incision and drainage and to rely on antibiotics and possible needle aspiration for refractory cases. The reason for this approach has been that incision and drainage create scarring and obscure natural tissue planes, making dissection difficult and decreasing the likelihood of successful surgical extirpation. Previous studies have reported the rate of recurrence after Sistrunk procedure to be higher when a previous incision and drainage had been required than when they were not. However, a more recent study contradicts this finding, showing that incision and drainage of previous infections were not associated with an increased risk of recurrence when compared with treatment with antibiotics alone.
Marianowski and colleagues reported age to be a factor of importance; an age older than 2 years at time of surgery was in their study correlated to a significantly lower TGDC recurrence rate. However, this finding is contradicted by Shah and colleagues, who found TGDC recurrence to be independent of age at presentation.
In 2010, Maddalozzo and colleagues defined a previously undescribed anatomic area, the posterior hyoid space (PHS), which plays an important role in TGDC surgery and minimizes the risk of recurrence. The investigators ascribe the recurrence of TGDCs to incomplete resection of (1) the suprahyoid ductules, which are microscopic, or (2) infrahyoid and perihyoid tissue. The PHS is demarcated caudally by the inferior rim of the hyoid, cranially by the superior rim of the hyoid and thyrohyoid membrane, ventrally by the posterior surface of the hyoid, and dorsally by the thyrohyoid membrane ( Figs. 1–4 ). Identification of the PHS allows surgeons to accurately assess the dimensions of the hyoid in both its inferior to superior aspect, as well as the anterior to posterior. This assessment facilitates complete resection of the hyoid and unroofing of the PHS, allowing evacuation of abnormal tissue from this area. These investigators reported that understanding and applying the anatomic concept of the PHS can prevent inadequate cyst resection in the area of the hyoid, reporting a recurrence rate of less than 2%.
Preoperative planning and preparation
Although clinical examination is often sufficient, most surgeons request preoperative imaging in diagnosing a TGDC. The purpose of imaging is to primarily identify a normal thyroid gland and to rule out ectopic thyroid tissue, which may represent the patient’s only functioning thyroid. Secondarily, it is used to appreciate the characteristics of the cyst. The role of computed tomography (CT) and MRI have been previously well established. However, high-resolution ultrasonography (US) remains the ideal initial investigation of choice in diagnosing TGDCs. This option is particularly applicable to children, because US does not involve ionizing radiation, does not usually require sedation, is readily available, and in most cases, provides the necessary preoperative information. Echographic patterns for TGDCs in children range from anechoic to heterogeneous. Uncomplicated TGDCs tend to be anechoic, whereas a pseudosolid echo pattern most likely represents cholesterol crystals, proteinaceous fluid content, or keratin. Evidence of thick walls and internal septae indicates inflammation. Although US is the most common preoperative diagnostic technique used for pediatric TGDCs, the literature suggests that CT is used more often in adults. Typically, TGDCs are visualized on CT as well-circumscribed lesions, with homogeneous fluid attenuation surrounded by a thin enhancing rim. However, an increased density may be observed, as in US, caused by cholesterol crystals, fluid-containing proteins, and keratin. The use of MRI for TGDCs is largely reserved for recurrences. Although MRI has the benefit of not involving any radiation, it often does require sedation in children. Lingual TGDCs ( Fig. 5 ), which present superior to the hyoid bone, require the aid of other diagnostic modalities, such as flexible fiber-optic laryngoscopy ( Fig. 6 ), direct laryngoscopy ( Fig. 7 ), or three-dimensional CT imaging for diagnosis. Fine-needle aspiration biopsy has also been used to confirm TGDC diagnosis as well as to determine the composition of the cyst fluid. Fine-needle aspiration biopsy can also, to a limited degree, confirm malignancy, although the rate of true-positive results is only 53%. A recent study by Lee and colleagues concluded that fine-needle aspiration is not routinely necessary for diagnosing TGDC in children, especially given the concerns about possible injury, low sensitivity, and low positive predictive value.
