A 1-year-old boy is brought to the clinic for a painful mass in the neck. The parents noticed a small area of nontender swelling in the neck a month ago. Over the last few days, it has enlarged and become painful, and the patient has a fever. Examination shows a mobile, tender, fluctuant, and warm mass approximately 2 cm in diameter that is located at the right mandibular angle. There is purulent drainage from the skin surface just anterior to the right sternocleidomastoid muscle. This patient's clinical findings are most likely due to which of the following embryologic processes?
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This patient's neck mass anterior to the sternocleidomastoid muscle is likely an infected branchial cleft cyst. This congenital anomaly is related to abnormal in utero development of the pharyngeal apparatus, an early embryologic structure comprising the following:
Pharyngeal (or branchial) arches: Six pairs of arches are precursors to the bones, muscles, and vasculature of the face, neck, and pharynx.
Pharyngeal pouches: The inner surface of the arches is lined by endoderm, and the pockets created by each arch are pouches, which differentiate into distinct fetal structures (eg, third pouch forms the thymus).
Pharyngeal clefts (or grooves): The outer surface of the arches is lined by ectoderm, and the clefts are the recesses between the arches. Most clefts obliterate; for example, the second, third, and fourth clefts merge to form the cervical sinus, which then involutes.
Failed obliteration of a pharyngeal cleft leaves a pathologic remnant in a predictable anatomic location. This patient likely has the most common type, a second pharyngeal groove anomaly (branchial cleft cyst), which is characteristically located anterior to the sternocleidomastoid muscle. The cyst is often unrecognized until secondary infection leads to a tender, fluctuant mass with or without purulent drainage (ie, sinus, fistula).
(Choice A) The parathyroid glands are derivatives of the third and fourth pharyngeal pouches; failure to descend to their anatomic position leads to ectopic glands. An associated adenoma can lead to symptoms of persistent hyperparathyroidism (eg, bone disease, nephrolithiasis), not a fluctuant mass and drainage.
(Choice B) Failed neural crest cell migration to the pharyngeal arches would affect development of skeletal structures of the face and neck (eg, bone, cartilage) and would not explain a cystic mass.
(Choice C) Inadequate fusion of the mandibular prominences (derivatives of the first pharyngeal arch) is rare and affects development of the lower lip, lower jaw, and tongue, findings not seen in this patient.
(Choice E) In utero, the thyroid gland descends from the base of the tongue to its final position down the thyroglossal duct, which then obliterates. A persistent thyroglossal duct can lead to a cyst along the path of descent that can become secondarily infected, but the cyst would be midline.
Educational objective:
Branchial cleft cysts are most often located anterior to the sternocleidomastoid muscle and result from incomplete obliteration of a pharyngeal cleft (or groove). Patients typically have a tender, fluctuant mass (due to secondary infection) with or without purulent drainage.