A thyroglossal duct cyst (TDC) is a fluid-filled sac or lump appearing in the midline of the neck. It is the most frequently encountered congenital anomaly of the neck, meaning it is present from birth, though it may not be visible for many years. TDC is a common cause of a benign midline neck mass, typically located near the hyoid bone. While often diagnosed in children, these cysts can become apparent later in life, presenting to adults as a noticeable swelling or painful lump.
The Embryological Origin of the Cyst
The formation of a thyroglossal duct cyst is linked to the developmental journey of the thyroid gland during the first few weeks of gestation. The thyroid gland begins forming at the base of the tongue, at the foramen cecum. Around the third week of embryonic development, the thyroid tissue migrates downward to its final position in the lower front of the neck.
This migration path creates a temporary connection between the tongue base and the final thyroid location, called the thyroglossal duct or tract. By the tenth week of gestation, this duct is meant to degenerate and completely disappear, a process called involution.
The underlying cause of the cyst is the failure of this embryonic duct to fully involute and close off. When the duct does not disappear, it leaves behind remnants of epithelial tissue along the migratory path. These remnants secrete fluid and mucus, leading to gradual accumulation within the closed-off segment.
This fluid accumulation results in the formation of the thyroglossal duct cyst. The cyst can be located anywhere from the base of the tongue down to the thyroid gland, but it is most commonly found near or below the hyoid bone. The cyst is a persistent embryological remnant.
Why It Becomes Noticeable in Adulthood
Although the thyroglossal duct remnant is present from birth, it often remains small, dormant, and asymptomatic for many years. Up to half of all thyroglossal duct cysts are not diagnosed until adult life, sometimes presenting between the ages of 33 and 55 years.
The transition from a silent remnant to a symptomatic cyst requires a trigger causing rapid cystic dilation. The most common trigger is an upper respiratory tract infection, such as a cold or flu. The inflammation associated with the infection can travel to the epithelial lining of the dormant duct remnant.
This inflammation leads to increased secretion of fluid and mucus inside the remnant, causing it to swell and become palpable. The resulting mass may suddenly appear or enlarge rapidly, becoming tender or painful. Trauma to the neck can also stimulate the latent cyst, making it inflamed and visible.
In adults, the most common presentation is an infected or painful mass, unlike the asymptomatic swelling often seen in children. This inflammatory reaction draws attention to the underlying anomaly. If infected, the cyst may rupture, leading to spontaneous drainage through the skin and creating a sinus tract.
Identifying and Managing the Condition
Identifying a thyroglossal duct cyst begins with a physical examination of the neck mass. The mass is usually found in the center of the neck. A distinctive feature is its upward movement with swallowing or when the patient protrudes the tongue, due to the cyst’s attachment to the hyoid bone.
Initial diagnostic investigation involves an ultrasound to confirm the cystic nature of the mass and determine its size and location. Imaging also helps ensure a healthy, functioning thyroid gland is present in its normal position. This is important because, in rare cases, the cyst itself may contain the only functional thyroid tissue.
A computed tomography (CT) scan may be used to further delineate the extent of the cyst and rule out other potential neck masses, such as enlarged lymph nodes. If an infection is present, antibiotics are administered first to reduce inflammation and swelling before definitive treatment is considered.
The standard treatment is surgical removal, known as the Sistrunk procedure. This operation prevents recurrence by excising the cyst, the entire thyroglossal tract up to the tongue base, and the central portion of the hyoid bone. Removing the segment of the hyoid bone is necessary because the duct often passes directly through it, and failure to remove it results in a high rate of recurrence.