Thyroglossal Duct Cyst

Synonyms: Thyroglossal cyst, Thyrolingual cyst, Thyroglossal duct remnant, TGDC, TDC, TGDR

Definition: Thyroglossal duct cyst is a congenital malformation that occurs due to incomplete closure of the thyroglossal duct which presents as a cystic midline neck swelling at birth. Thyroglossal fistulas are usually recognized by their external opening (typically located at the thyroid notch) and the associated discharge. Thyroglosaal fistula is never congenital and forms after draining incision of infected cyst.

Embryological origin of thyroglossal cyst:

1. The thyroid gland originates as a ventral midline diverticulum in the floor of pharynx between the tuberculum impar and copula during the 4th week of development. In the fully developed fetus, this site is marked by the foramen cecum at the junction of anterior 2/3 and posterior 1/3 of the tongue.
2. The gland then descends on the thyroglossal duct anterior to, an rarely through the hyoid body, and often has a diverticulum that hooks below and behind the hyoid, before it courses towards a thyroglossal duct cyst or the thyroid gland.
3. The gland reaches its final pretracheal position by the end of 7th week.
4. The obliteration of the duct usually occurs by the 10th week of gestation and persistence of any portion of this duct gives rise to thyroglossal cyst.
5. The hyoid bone develops latera and has a variable relationship to the thyroglossal duct.

The foramen cecum in the midline of the tongue and the pyramidal lobe of the thyroid gland are viewed as the remnants of the thyroglossal duct.

Theories for formation of cyst:

1. Cystic degeneration
2. Retention phenomenon

Location of thyroglossal cyst: It can occur anywhere in the course of thyroid.

1. Lingual (Base of tongue): 2%
2. Suprahyoid: 25%
3. Thyrohyoid: 60%
4. Suprasternal: 13%
5. Mediastinal: Rarely

The commonest position is subhyoid.

Epidemiology:

1. 90% of the cases present before 10 years of age or may remain asymptomatic until infected (can present at any age)
2. Most common congenital neck swelling
3. 2nd  most common benign neck mass after lymphadenopathy
4. Most common anomalies of thyroid development

Pathology:

1. Lined by columnar or cuboidal epithelium
2. Surrounded by a rim of lymphoid tissue
3. Contains thick jelly like fluid which may contain cholesterol crystals

Characteristics of swelling:

1. Located in midline of neck, anywhere along the course of embryological thyroid (Inferior lesions tend to be more off midline due to the presence of thyroid gland)
2. Cystic and rounded (often fluctuant but rarely transilluminate)
3. Usually 2-4 cm in diameter
4. Invariably painless and pain suggests infection
5. Size increases with Upper respiratory tract infection (URTI)     
6. Mobile horizontally but not vertically
7. Moves up on swallowing and protrusion of tongue (because of close anatomical relation to hyoid)

If the position is lingual (base of tongue), it may cause swallowing and respiratory difficulties.

Associations: Ectopic thyroid (40%)

Differential diagnoses:

The differentials for Thyroglossal cyst includes other causes of midline neck swelling. One diagnostic sign of thyroglossal cyst is its movement upwards during swallowing or protrusion of tongue.

1. Dermoid cyst (Cheesy secretion)
2. Infected lymph node (Purulent secretion)
3. Lipoma (Slip sign positive)
4. Sebaceous cyst (Doughy feel)
5. Hypertrophic pyramidal lobe of thyroid

Complications:

1. Recurrent infection
2. Thyroglossal fistula
3. Malignancy (<1%): Usually papillary carcinoma of thyroid

Investigations:

The main aim of pre-operative evaluation is to confirm the diagnosis and to detect if the cyst contains ectopic thyroid tissue, which may be the only functioning thyroid and excision would result in profound hypothyroidism. Hence, along with baseline investigations, these are commonly performed:

1. Ultrasonography
2. Radioisotope scan (131 I scan)
3. Thyroid function test (TFT)

Note:

• Ultrasonography may also detect papillary carcinoma as a solid component or calcification.
• Diagnostic needle aspiration is contraindicated.
• Thyroglossal fistula can be confirmed by radiographic contrast examination.

Treatment:

1. Preoperative antibiotics: To control infection if present, before the surgery.

2. Sistrunk operation/procedure:

Principle: Complete excision of cyst and tract up to the foramen cecum at base of tongue along with excision of anterior portion of body of hyoid. The excision of body of hyoid helps in excision of any retrohyoid part of the tract and also complete excision of the thyroglossal tract upto the foramen cecum. This reduces the incidence of recurrence.

Anesthesia: General anesthesia

Position: Supine with neck extended with sandbag between the shoulder blades

Procedure:

3. Percutaneous ethanol sclerotherapy (Minimally invasive modality): Newer modality which can be performed in OPD setting but is not effective in all cases.

Reccurence:

• With local excision: As high as 40%
• With sistrunk operation: 1-5%