A midline neck lump
1. What is this midline neck lump? 2. Describe it's embryological origin? 3. What are it's characteristic physical signs?
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This midline neck lump is a thyroglossal cyst. The thyroid gland appears as an epithelial proliferation in the floor of the pharynx between the tuberculum impar and copula during the 4th week of development. In the fully developed foetus this site is marked by the foramen caecum at the junction of the anterior 2/3 and the posterior 1/3 of the tongue. From this site the thyroid gland descend anterior to the pharynx closely related to the hyoid bone. It reaches it's final position in front of the tracheal cartilages during the 7th week of development. During descent the gland remains connected to the base of the tongue by the thyroglossal duct that later disappears in most individuals.
Several aberrations can occur in the development of the thyroid gland. A cystic remnant of the thyroglossal duct can persist causing a thyroglossal cyst. These can be found at any site in the path of descent of the thyroid gland but are usually found in close relationship to the hyoid bone. Rupture of thyroglossal cyst can result in a thyroglossal sinus. These are often erroneously termed thyroglossal fistulae. A thyroglossal fistula between the base of the tongue and neck skin due to persistence of the entire thyroglossal duct is very uncommon. Failure of decent of all or part of the thyroid gland results in aberrant thyroid tissue most commonly in the base of the tongue as a lingual thyroid. 75% of such patients have the entire gland in the aberrant site. 25% have some thyroid tissue at the normal location. Over 40% of thyroglossal cysts appear in childhood. The remainder can occur at any stage in adult life including old age. Despite their congenital origin they are rarely present at birth. Most are found close to, if not always exactly in, the midline. Over 50% are closely associated with the hyoid bone. They usually present as a painless midline neck lump. They may first become apparent when they become infected in which case sudden onset of a painful, tender lump can be the presenting feature. They are often fluctuant but rarely transilluminate. They can be easily moved from side to side but not up and down. The characteristic physical sign that they display is elevation on protrusion of the tongue. This is not a sign seen with other thyroid nodules. Histologically the cyst is lined by stratified squamous or ciliated pseudostratified columnar epithelium. Thyroid or lymphoid tissue is frequently found in the cyst wall. The thyroid tissue is frequently dysplastic with an increased risk of malignancy. The classic operation described for a thyroglossal cyst is Sistrunk's Operation. With the neck extended a transverse skin incision is made and platysma flaps elevated. These are retracted with a Joll's retractor. The cyst and any associated tract are mobilised. The tract is dissected between the infrahyoid muscles up to the hyoid bone. The central portion of the bone is excised. If necessary the tract is traced upwards in to the base of the tongue. Failure to excise the central portion of the hyoid bone results in an increased risk of recurrence. Recent papersBrewis C, Mahadevan M, Bailey C M, Drake D P. Investigation and treatment of thyroglossal cysts in children. J R Soc Med 2000; 93: 18-21. A not so recent paperSistrunk W E. The surgical treatment of cysts of the thyroglossal tract. Arch Surg 1920; 71:121-122.
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