Thyroglossal Cyst
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Direct answer
Midline neck swellings around the hyoid gravitate to one diagnosis in the viva: the thyroglossal cyst, a congenital remnant of the tract along which the thyroid anlage descends from the foramen caecum of the tongue base to its pretracheal position by about the seventh week of intrauterine life. The cyst presents as a smooth, fluctuant, painless swelling in the midline — most often just below the hyoid — that rises on tongue protrusion and swallowing because the tract remains tethered to the foramen caecum through or around the hyoid. Cure is surgical, and the operation is Sistrunk's: excision of the cyst with the entire tract, the body of the central hyoid, and a core of tongue-base muscle up to the foramen caecum. Simple enucleation recurs at several times the Sistrunk rate.
What you must remember
- Embryology in one line: thyroid anlage at the foramen caecum (in the posterior third of tongue, at the sulcus terminalis) descends in front of, through or behind the hyoid body to the pretracheal position; the duct normally obliterates, and retained epithelium anywhere along it secretes a cyst.
- Site frequency: subhyoid is the commonest position, followed by suprahyoid, lingual and suprasternal; the cyst is midline or just off midline, usually left of the midline when paramedian.
- The two movement signs: ascent with tongue protrusion (attachment to the foramen caecum via the hyoid) and movement with swallowing (attachment to the larynx, which rises); otherwise the cyst is freely mobile in the subcutaneous plane.
- The differential that decides surgery: dermoid cyst does not move on tongue protrusion; ectopic thyroid is a swelling that may be the patient's only thyroid tissue — scan before excision; subhyoid bursitis, lymph node and branchial cyst (lateral) complete the list.
- Mandatory preoperative workup: ultrasound confirms the cystic lesion and, critically, a normally located thyroid gland; scintigraphy if an ectopic thyroid is suspected; thyroid function tests.
- Sistrunk operation (1920): excise cyst, entire tract, central 1-2 cm of hyoid body, and a cuff of tongue-base musculature tracked to the foramen caecum, usually through a transverse cervical incision.
- Recurrence arithmetic: simple cyst excision recurs in a substantial minority (figures of up to half in older series), while Sistrunk's operation reduces recurrence to a few percent; infection before surgery also raises recurrence.
A clinical case worked through
A 16-year-old notices a painless lump below the chin, present for a year, which recently became tender after a sore throat. Examination shows a 2 cm fluctuant swelling in the midline just inferior to the hyoid; when asked to protrude the tongue, the swelling rises — the sign that anchors the diagnosis. Ultrasound reports a cystic midline lesion and, equally important, a normal thyroid in the normal position. The acute inflammation is settled first, because operating through infected tissue both obscures planes and multiplies recurrence. At surgery, a transverse incision encircles any sinus tract; the cyst is mobilised, and the dissection is carried up with the thyrohyoid tract to the hyoid body, which is freed of muscle attachments and divided on either side to remove its central portion with the specimen — this is the step amateur operations omit. Above the hyoid, a core of genioglossus and tongue-base muscle around the tract is cored out towards the foramen caecum, felt as a dimple at tongue base, and ligated. Histology confirms a thyroglossal cyst. Had the ultrasound found no cervical thyroid, the plan would have changed completely — the swelling may have been a functioning lingual thyroid whose removal would render the patient hypothyroid.
Where students slip
The recurring mistake is treating this as "a midline cyst — excise it": examiners specifically ask what makes the thyroglossal cyst move on tongue protrusion, and the answer must invoke the tract's persistent attachment to the foramen caecum through the hyoid, which is also precisely why the hyoid body must be resected. The second slip is the ectopic thyroid trap — excising a midline swelling that is in fact the only thyroid tissue the patient owns; hence the scanning rule before every Sistrunk. Third, the differential question "midline swelling in a child" expects an ordered answer: thyroglossal cyst, dermoid, ectopic thyroid, suprasternal notch structures, lymphadenopathy (submental), and bony swellings. In Indian viva convention, Sistrunk's name, the year, and the components of the operation are asked as identifiers, and the movement signs as the clinical pearl.
Frequently asked questions
Why does a thyroglossal cyst move on tongue protrusion?
The persistent tract attaches to the foramen caecum at the tongue base and passes through or around the hyoid, so protrusion pulls the cyst upwards.
What does the Sistrunk operation include?
Excision of the cyst with the entire tract, the central body of the hyoid, and a core of tongue-base musculature up to the foramen caecum.
Which is the commonest site of a thyroglossal cyst?
Just below the hyoid bone (subhyoid), followed by suprahyoid, lingual and suprasternal positions along the descent tract.
Why must the thyroid gland be imaged before excision?
To confirm a normally located thyroid and exclude the swelling being an ectopic thyroid — the only functioning tissue in some patients, whose removal causes hypothyroidism.
Why does simple cyst excision fail?
Rests of duct epithelium left along the tract, in the hyoid and at the tongue base recur as new cysts; recurrence after enucleation is many-fold higher than after Sistrunk's operation.