Definition: Thyroglossal cyst arises from the persistant thyroglossal duct.
Embryology: The thyroid gland originates from the foramen cecum present in the floor of the pharyngeal gut on the 17th day of gestation. The gland then descends in front of the pharynx as a bilobed diverticulum which is initially patent. It reaches its final position in the neck by the 7th week of gestation. The duct usually disappears by the 10th week of gestation. Persistence of any portion of this duct could give rise to thyroglossal cyst.
Clinical Photograph of a patient with Thyroglossal cyst
Diagram showing the embryology of thyroglossal cyst along with its possible locations
Since the hyoid bone develops later and joins from lateral to medial, the thyroglossal duct may get trapped in the substance of the body of hyoid bone, resulting in the tract running inside the body of the bone. The hyoid bone rotates to reach its adult position dragging the duct posteriorly and superiorly at the inferior edge of its body.
No natural internal opening of thyroglossal duct has been demonstrated at the level of foramen cecum so far. This has been attributed to the fact that the tongue and foramen cecum forms after the complete descent of the thyroglossal duct. Rarely a tract could be found at the level of foramen cecum. This tract has been attributed to the persistence of lingual duct, which represents the point of union between the anterior and posterior components of the tongue.
The lowermost portion of the thyroglossal duct could remain in some as the pyramidal lobe of the thyroid gland.
Theories of thyroglossal cyst:
Cystic degeneration: This theory suggests that recurrent throat infections could possibly stimulate the epithelial remnants of the tract causing it to undergo cystic degeneration.
Retention phenomenon: This theory suggests a block in the thyroglossal duct could cause the cyst to expand because of retained secretions. Foramen cecum has been postulated as the possible site of obstruction. The thyroglossal duct epithelium is supposed to contain mucinous and serous glands. The continued secretions from these glands could cause enlargement of the cyst.
Clinical features: Thyroglossal cyst is the second commonest benign neck lesion next only to lymphadenopathy. Commonly it manifests as a cystic midline neck mass before the age of 5. The mass moves with deglutition and on protrusion of the tongue. Theoretically speaking the cyst could lie anywhere within the thyroglossal tract. These cysts may also be located laterally as well.
Site of occurrence:
1. Hyoidal - 61%
2. Suprahyoidal - 24%
3. Infrahyoid - 13%
4. Intralingual - 2%
The masses are invariably painless. When these cysts are associated with pain then infection of the cyst should be considered. Enlargement of intralingual cysts may cause respiratory obstruction.
Differential diagnosis: These cysts should be differentiated from
1. Dermoid cyst - cheesy secretion
2. Infected lymph node - purulent secretion
3. Lipoma - slippery edges
4. Sebaceous cyst - doughy feel
5. Hypertrophic pyramidal lobe of thyroid
Commonly these cyst could contain thyroid tissue, hence I 131 study should be considered in all patients with suprahyoid and infrahyoid masses. Rarely this could be the only functioning thyroid gland tissue.
CT scan neck showing cystic lesion in the anterior neck (thyroglossal cyst)
Is mainly surgery. The procedure commonly performed is Sistrunk operation which involves exposing the whole cyst along with its tract. The anterior portion of the body of the hyoid bone should be included in the dissection to prevent recurrence. It was Wenglowski who suggested that along with the body of hyoid bone a core of tissue between the hyoid bone and the tongue should also be removed to reduce the incidence of recurrence.
Causes of failure of Sistrunk operation:
1. Missing a dumb-bell cyst deep in the back of hyoid
2. A cyst which has already ruptured with formation of a number of pseudocysts
3. Leaving part of a cyst wall
4. Presence of multiple tracts
In the case of infected thyroglossal cyst a fistula may also form. If fistula occurs then it must be removed in toto. The mouth of the fistulous tract must be included in the incision.
Suprahyoid thyroglossal cyst