Revisiting Imaging Features and the Embryologic Basis of Third and Fourth Branchial Anomalies

Discussion

Anomalies that are thought to arise from third or fourth branchial remnants are typically found in the neck as a sinus tract communicating with the piriform sinus of the hypopharynx. In children, they commonly present with recurrent neck infections or abscesses, most often occurring on the left. There is considerable discrepancy noted between the usual clinical and radiologic presentation and the theoretic embryologic course of these remnants.^4,5

These embryonal pouch remnants often present with recurrent neck infections, mainly on the left side, with associated thyroiditis or abscess following bouts of upper respiratory infection.^3–15 The sinus opening acts as a conduit for the spread of infection from the pharynx. Eighty percent of the lesions in the current study were on the left side, in agreement with the previous reports.^3–7 This is perhaps due to the asymmetric vascular development of these branchial arches.¹⁶ Many of these patients had recurrent neck infections with a history of multiple occurrences of surgical abscess drainage and antibiotic therapy. Infected lesions were often associated with significant inflammatory adenopathy in the neck (90%).

Only 2 patients in our series presented without infection, and both anomalies were on the right side, remaining adjacent to but not traversing the thyroid gland. Direct pharyngoscopic examination showed a pit or sinus opening in the piriform fossa in all except the 2 noninfected patients. It has been claimed that the third pouch remnant will leave a rostral pit and the fourth pouch will have an apical opening in the piriform fossa and that this information can potentially differentiate them.¹⁶ However, more recent reports negate this claim and show that considerable overlap exists.⁴ All suspected cases of neck and thyroid infection in children, especially on the left side, should undergo a direct pharyngoscopic examination, which could potentially demonstrate the branchial pouch origin of these lesions and hence aid in planning for early definitive surgery. This is especially important because several published reports note the presence of significant delay between the initial clinical presentation and definitive surgery.^3–6,17

Evaluation of branchial embryology assists understanding of the nature of these anomalies. The primitive pharynx has ectodermal branchial clefts and endodermal branchial pouches separated by mesenchymal branchial arches.^4–6 The thymus develops from the ventral component, and the inferior parathyroid gland originates from the dorsal component of the third pouch, whereas the superior parathyroid sprouts from the dorsal expansion of the caudal pharyngeal complex of the fourth pouch and the rudimentary fifth pouch.^2,3 The fourth branchial pouch may contribute to thyroid development (the ventral portion forms the ultimobranchial body) and also the thymus gland.⁷ The lumen of the thymus primordia from the third pouch is initially patent and usually closes after the fusion of the thymic tissues from either side. Failure of this lumen to close may result in persistence of the thymopharyngeal duct (Fig 4), thus leading to a branchial sinus in communication with the pharynx.⁵ The close proximity of the duct remnants to the thyroid gland can also be explained by descent of the thyroid and the thymic glands in the neck at around the same time (the seventh and eighth weeks of gestation).⁶

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Fig 4.

Noninfected third branchial cleft cyst on the right side of the neck following the course of the embryonal thymopharyngeal duct (arrows). A, Coronal T2 fat-saturated image. B, Schematic representation of the course of the thymopharyngeal duct.

Although terminology is often used interchangeably in the radiology literature,^18,19 as emphasized by Rea et al,⁴ it is important to use standardized nomenclature in the description of the branchial apparatus anomalies. A “sinus” is a blind-ending tract. A “branchial cleft sinus” connects to the skin, whereas a “branchial pouch sinus” connects to the pharynx, neither of them arising from the mesenchyme of the branchial arch.⁴ A “true fistula” is a communication between 2 epithelialized surfaces; a “congenital branchial fistula” should be present at birth and should communicate between a persistent pouch and a cleft. Most of the branchial fistulas reported in the literature are actually pseudofistulas, formed between a pouch remnant and a skin opening produced following an infection or a surgical incision, or are just sinuses. There was no true congenital fistula in our series, as also noted in previous studies.^4,5 One should adhere to strict definitions of these terms in reporting these lesions to avoid further confusion in the literature.

None of the cases in the present series followed the classic description of the course of the third and fourth branchial remnants found in the literature.⁴ This was also noted by previous investigators, and a third or fourth branchial pouch remnant following the complete classic course passing above the hypoglossal nerve or winding down around the aortic arch or subclavian artery is yet to be demonstrated.^3–5,17 We suggest that the explanation for the discrepancy between the theoretic and observed courses of the branchial anomalies arising from an opening in the piriform fossa is that they do not arise from true third or fourth arch fistulas but that they are sinuses arising from persistence of a patent thymopharyngeal duct. A true fistula from endodermal mucosa to ectodermal skin would be expected to follow the more convoluted courses around structures of the third and fourth arches, but it appears that such true fistulas across these arches are exceptionally rare, or possibly nonexistent. In our surgical experience, all lesions arising from the piriform sinus passed caudal to the superior laryngeal nerve. This is taken as demonstrating the route of descent of the thymus from the third pouch down through the fourth arch into the mediastinum, so it does not indicate that the anomaly is of fourth-pouch origin as has been suggested previously.⁴

A contrast-enhanced CT study has been considered as the investigation of choice in demonstrating the site and extent of the lesion. The presence of air within a left-sided infected lesion in relation to the thyroid gland is thought to be characteristic of a branchial pouch sinus as the underlying cause; however, this is seen in only a very few cases. MR imaging can also show the inflammatory lesion but may not be adequate to demonstrate the presence of air in the lesion. Sonography with a “trumpet maneuver” may also demonstrate the sinus.¹⁹

A barium study of the pharynx is considered essential by some authors, who suggest that it should be performed after the acute inflammation is over to reduce the false-negative results.^20,21 However, another study has reported only a 50% sensitivity for barium examination compared with pharyngoscopy.¹⁸ In our experience, a barium study is not required to diagnose these lesions, and all the suspected cases should undergo a rigid pharyngoscopy with a straight blade laryngoscope, in an attempt to improve the sensitivity of detecting a piriform sinus opening. The only case in our series in which a barium study was available failed to demonstrate the piriform sinus pit seen on pharyngoscopy.

There were 2 cysts in our series (10%) that failed to show a pyriform sinus opening and presumably did not have any sinuses passing from the pyriform sinus through the thyroid gland. We postulate that these could be cysts in which the sinus opening has closed over spontaneously with time, obliterated by recurrent inflammation, or that the cysts were presumed to be different from the other lesions that were associated with sinuses. The lining of these cysts indicates that they arise from the endoderm of a branchial pouch, but we cannot be certain whether they arise from the third or fourth pouch. One case showed bilateral pits, but the inflammatory neck lesion was unilateral.

Histopathologic demonstration of the sinus tract lined with squamous or respiratory epithelium in the resected specimen is important for the diagnosis of branchial cleft lesions and for ensuring complete surgical excision. However due to chronic inflammation and recurrent scarring, it may not always be possible to demonstrate it. The presence of inflammation and fibrosis of the adjacent thyroid tissue can also be seen quite often.