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Fig. 9.2. Branchial cleft cyst. Wall lined with stratified squamous epithelium

chial cleft cysts constitute approximately 75-80% of all branchial anomalies, and fistulae and sinuses together account for 15-20% of all such malformations [1]. In some series, external fistulae, sinuses and skin tags are more common than cysts [57].

Of all branchial anomalies, 92-99% are associated with the second branchial clefts apparatus, probably because it is deeper and longer than the others [8]. Use of the name "branchial cyst" without further qualification generally refers to a cyst of second branchial origin. Cysts are three times more common than sinuses and fistulae in this apparatus. They typically occur along the anterior border of the sternocleidomastoid muscle from the hy-oid bone to the suprasternal notch, but have infrequently been described in the midline, just as a thyroglossal duct cyst may occur laterally, as bilateral branchial cleft cysts, or even in the lateral wall of the nasopharynx [8, 85].

Branchial cleft anomalies have no gender preference. Most patients (75%) are aged 20-40 years at the time of diagnosis. Since fewer than 3% of cysts are found in patients older than 50 years, the pathologist must be careful in making this diagnosis in this age group; a metastatic cystic squamous cell carcinoma in a cervical lymph node may masquerade as a branchial cleft cyst.

On pathologic examination the cysts are unilocular, usually between 2 and 6 cm in diameter, and lined with stratified squamous epithelium (90%), respiratory epithelium (8%), or both (2%; Fig. 9.2). Lymphoid aggregates with or without reactive germinal centres beneath the lining epithelium are found in the majority of cysts (75-80%). Acute and chronic inflammation, foreign body giant-cell reaction and fibrosis are the secondary microscopic changes in the wall of the cyst. In exceptional cases, heterotopic salivary tissue may even be found in the wall of the cyst [111]. Carcinoma in situ has seldom been described in the lining of the cysts [132]. Regauer et al. have postulated that the cysts are initially lined with the endodermally derived pouch type of respiratory epithelium, which is replaced by squamous ep ithelium through an intermediate stage of pseudostrati-fied transitional-type epithelium [95].

Fistulae and sinuses are more often found at birth or in early childhood than cysts. The external opening, when present, is usually located along the anterior border of the sternocleidomastoid muscle at the junction of its middle and lower thirds. The tract, if there is one, follows the carotid sheath; it crosses over the hypoglossal nerve, runs between the internal and external carotid arteries and ends at the tonsillar fossa [122].

Thymic cyst and cystic low-grade mucoepidermoid carcinoma with prominent lymphoid stroma are considerations in the differential diagnosis. The cyst's benign lining distinguishes it from metastatic cystic squamous carcinoma.

Complete surgical excision of the cyst, sinus, or fistula is indicated. In a review of 274 patients with branchial remnants treated at the Mayo Clinic, the recurrence rate was only 2.7% for patients with no history of surgery or infection, 14% in those with a history of infection and 21.2% in those who had undergone prior attempts at surgical removal [28].

Anomalies from the first branchial arch accounted for only 8% of all branchial cleft anomalies at the Mayo Clinic [82]. Of these, 68% were cysts, 16% sinuses and 16% fistulae. These anomalies occur predominantly in females and are found in all age groups. In general, sinuses and fistulae tend to develop in infants and children, whereas cysts are more common in older groups. Clinically, they may masquerade as parotid tumours or as otitis with ear drainage [82].

Disorders of the first branchial cleft are classified into two types [119]. Type I are those that embryologically duplicate the membrane (cutaneous) external auditory canal. Accordingly, only ectodermal components are observed under the microscope. On histologic examination they are often confused with epidermoid cysts, for they are lined solely by keratinised, stratified squamous epithelium, with no adnexal structures or cartilage. Characteristically, they are located medial, inferior or posterior to the concha and pinna. Drainage from cysts or fistulae may occur in any of these sites. The fistula tract or sinus may parallel the external auditory canal and ends in a blind cul de sac at the level of the mesotympanum.

Type II deformities are composed of both ectodermal and mesodermal elements and therefore contain, in addition to skin, cutaneous appendages and cartilage (Fig. 9.3). Patients with this defect usually present with an abscess or fistula at a point just below the angle of the mandible, through the parotid gland, toward the external auditory canal. Type II defects are therefore more intimately associated with the parotid gland than are type I defects. Sometimes an anomaly cannot be distinguished as type I or type II. In those instances, Olsen et al. suggested that the abnormality be classified only as to whether it is a cyst, sinus, or fistula [82]. Complete excision is

Fig. 9.3. First branchial cleft cyst, Type II. Squamous epithelium Fig. 9.5. Bronchogenic carcinoma. Inset: malignant squamous lining the cystic cavity. Note the presence of skin appendage struc- and respiratory epithelium lining the cystic wall tures and cartilage in the stroma

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Fig. 9.4. Lymphoepithelial cyst. Note the absence of lymphoid hyperplasia the only effective treatment. In some cases, this may necessitate a superficial parotidectomy. First branchial cleft abnormalities must be differentiated pathologically from epidermal cysts (especially type I), dermoids (especially type II), and cystic sebaceous lymphadenoma.

Anomalies from the third and fourth branchial apparatuses are rare and together account for fewer than 5% of all branchial cysts, sinuses and fistulae [80]. A fistula in the pyriform sinus is one of the more common manifestations of a third branchial anomaly [32]. Recurrent infections of the lower neck, including suppurative thyroiditis, and a fistulous tract into the pyriform sinus are the features of a fourth branchial cleft or pouch anomaly [80].

Third and fourth branchial sinus anomalies can be distinguished only by detailed surgical exploration. A third branchial sinus always extends from the pyriform sinus through the thyroid membrane cranial to the superior laryngeal nerve. In contrast, a fourth branchial sinus extends from the pyriform sinus caudal to the superior laryngeal nerve and exits the larynx near the cri-cothyroid joint [127].

Neither the fifth nor the sixth branchial arch forms clefts or pouches in humans [126]. Branchial cleft cysts have been reported infrequently in the parotid, thyroid and parathyroid glands, floor of the mouth, tonsil, pharynx and mediastinum [17]. Many of these cysts have the microscopic features of lymphoepithelial cysts (Fig. 9.4).

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