Introduction

24.1.1 Development of the Pharyngeal Arches

In a developing embryo, the most typical features in the head and neck region are the external clefts that are part of the “branchial” or “pharyngeal” arches. The word branchial is derived from early observations of embryologists studying the development of the gills of fishes, and while “branchial” is still used to describe head and neck structures in humans, the terms pharyngeal arches, pouches, and clefts are preferred.

These rudimentary structures appear in the fourth and fifth weeks of development. Each pharyngeal arch consists of a core of mesenchyme, which is covered externally by ectoderm and internally by epithelium of endoderm origin (the primitive foregut). Each arch has its own muscular component and each of these components has their own cranial nerve supply. ¹ They consist of bars of mesenchyme tissue separated by deep external clefts and internal pouches.

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  The first pharyngeal arch includes the maxillary and mandibular processes and Meckel’s cartilage, which will later form the incus and malleus. Musculature of the first arch includes the muscles of mastication, and the nerve supply is provided by the mandibular branch of the trigeminal nerve.

  
  
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  The cartilage of the second or hyoid arch gives rise to the stapes, thyrohyoid process, and parts of the hyoid bone. The nerve of the second arch is the facial nerve.

  
  
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  The fourth and sixth pharyngeal arches (the fifth is rudimentary) contribute to the formation of the laryngeal cartilages. The fourth arch musculature is supplied by the superior laryngeal branch, and the sixth arch by the recurrent laryngeal branch of the vagus.

  
  
  
  

  
  

  The ongoing development of the first pharyngeal pouch and cleft contributes much to the formation of the ear ( ▶ Fig. 24.1).

The first cleft penetrates the underlying mesenchyme and gives rise to the external auditory meatus with the lining at the bottom end of the meatus participating in the formation of the eardrum.

The mesenchyme of the second arch migrates inferiorly to overlap the third and fourth clefts. This forms a cavity lined with ectoderm epithelium, that is, the cervical sinus, which ultimately involutes.

The thyroid gland develops at the level of the foramen cecum in the floor of the pharynx and subsequently descends in front of the primitive foregut as a bilobed diverticulum.

The gland reaches its final position in front of the trachea at the seventh week.

24.2 Congenital Neck Masses

Congenital neck masses in children may be apparent at birth (e.g., lymphangioma, fibromatosis colli) or may declare themselves later, for example, thyroglossal duct cysts.

24.2.1 Dermoid Cysts in the Neck

A dermoid cyst is thought to result from the inclusion of epithelial cells along the lines of embryonic closure. The resulting cyst contains ectodermal and mesodermal elements and can result in the presence of hair follicles and sebaceous glands within the lesion.

Presentation

A dermoid cyst often presents in a similar fashion to a thyroglossal duct cyst with a small midline mobile smooth swelling. Infection is rare. It is often found in an inferior position in the neck to that of a thyroglossal duct cyst ( ▶ Fig. 24.2); it is usually more superficial and mobile and it does not move on protrusion of the tongue. In reality, it can be difficult to distinguish from a thyroglossal duct cyst.

An ultrasound scan is of limited benefit but will show the extent of the cyst and its position in relation to deeper structures ( ▶ Fig. 24.3). It is important to ask the radiologist to look for evidence of a connection to adjacent structures, for example, the presence of a track extending to the tongue base, as this would not be in keeping with a dermoid cyst but point to a thyroglossal duct cyst instead.

Management

Surgery for a clinically obvious dermoid cyst is performed through a skin crease incision and then a simple cystectomy is carried out. If there is any doubt about the diagnosis, then it is best to assume that the lesion is a thyroglossal duct cyst and proceed accordingly (discussed next). A dermoid cyst typically contains a white cheesy material, whereas a thyroglossal duct cyst will usually yield a viscous gelatinous fluid.

24.2.2 Thyroglossal Duct Cyst

This is the most common congenital abnormality that presents in the head and neck region, making up approximately 70% of cases. Thyroglossal duct cysts occur because the duct fails to involute between the 8th week and the 10th week of gestation and a cyst can form anywhere along the duct’s natural course from the foramen cecum to the thyroid gland. The majority of cysts are found in close proximity to the middle portion of the hyoid bone, and in approximately 30% of cases, a tract is found posterior to or adherent to the hyoid. Hence, resecting a segment of hyoid bone to ensure there are no cell rests left behind is important.

Presentation

A thyroglossal cyst most commonly presents in childhood with a midline lump in close proximity to the hyoid bone ( ▶ Fig. 24.4). It can occasionally become infected resulting in a red, painful swelling, which can discharge. Typically, the lump moves on swallowing and on protruding the tongue, but this is not always reliable.

Investigations

It is used to establish the presence of a cystic structure in the neck, ² but also and more importantly to confirm that there is a separate and normal-looking thyroid gland present. This is important to prevent the rare possibility of inadvertent removal of an ectopic thyroid gland. In most cases, no further investigations are needed, but other imaging modalities may be helpful, depending on the ultrasound findings or in cases of recurrence.

Treatment

The operation is classically described by Sistrunk who appreciated the importance of excising the middle portion of the hyoid bone, the cyst, and a core of tongue base muscle up to the foramen cecum ( ▶ Fig. 24.5). Histological examination of the resected specimens has confirmed that the thyroglossal duct can have small branches extending laterally.

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  The incision should be carried out through a horizontal skin crease. If the skin has been involved because of previous infection, then an ellipse should be taken and the edges undermined to permit a good aesthetic repair.

  
  
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  The cyst is identified and the tissues dissected over a wide plane superiorly up toward the hyoid bone. It is important to identify the thyrohyoid membrane that will prevent inadvertent entry into the airway and will also guide the surgeon to the undersurface of the hyoid bone.

  
  
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  The hyoid bone should be transected at the level of the lesser cornu bilaterally. In an older child, it may be well ossified and very hard, so a sharp bone-cutting forceps is needed.

  
  
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  Cutting diathermy is useful to extend the dissection up to the tongue base taking a triangular core.

It is advisable to insert a drain for the first few hours postoperatively. There is often some oozing from the hyoid bone and there is also a significant dead space. Hematoma is therefore a distinct possibility.

Recurrence rates are very low if a wide local excision as described is performed, but simple cystectomy results in a 50% recurrence rate.

24.3 Pharyngeal Arch Disorders

24.3.1 First Pharyngeal Arch Anomalies

There are a number of classifications of this anomaly and the most frequently reported is that of Work, who described duplication anomalies of the first branchial groove, type I and type II. ³ Although these classifications may indicate the underlying embryological abnormality, they are not able to predict accurately the relationship of any abnormality to the facial nerve, which is, of course, of great importance in the surgical management of these patients.

Presentation

First pharyngeal arch anomalies usually present as a cyst, sinus, or mass in the periauricular area ( ▶ Fig. 24.6) or they can present with a discharge from the ear in the presence of a normal tympanic membrane (due to a sinus in the external auditory canal). These abnormalities are rare and the presentations vary. Unfortunately, this often means that the diagnosis is made at a late stage following several episodes of infection in the same area. This can make excision more challenging.

It is important to distinguish a first pharyngeal arch anomaly from a simple preauricular sinus (see ▶ 6). Openings above the tragus are unlikely to be complex arch anomalies; they are more likely to arise from aberrant fusion of the developing aural tubercles.

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