PAROTID GLAND DEVELOPMENTAL ANOMALIES
- Imaging is central to a proper diagnosis of branchial apparatus and other parotid-region anomalies.
- Computed tomography and magnetic resonance imaging are the prime diagnostic imaging tools since a sinus tract cannot usually be identified or, if found, injected.
- Ultrasound is cost additive, not definitive, and should be avoided in almost all cases for these reasons.
- A cystic parotid mass in an adult should be presumed to be neoplastic or infectious rather than of branchial origin unless characteristic associated findings are identified.
Developmental abnormalities involving the parotid region are mainly those due to first branchial apparatus anomalies and venolymphatic malformations. Syndromic craniofacial anomalies may also involve the first branchial apparatus as part of a more complex developmental anomaly. Epidermoid and dermoid cysts and teratomas rarely present as strictly parotid-region masses (Fig. 176.1).
Congenital anomalies of first branchial apparatus origin affecting the parotid region come to imaging in both the pediatric and adult population. Their embryologic origin explains the diverse appearance of these anomalies on imaging studies as well as their various clinical presentations (Figs. 153.7, 153.8, and 176.2–176.4). In the parotid region, anomalous branchial apparatus development results in cysts and fistulas with occasionally associated epidermoid cysts and temporal bone anomalies (Figs. 153.7, 153.8, and 176.3). Their anatomic relationships and appearance on imaging studies predicts their embryologic origin and raises related treatment implications.
Proliferative hemangioma and venolymphatic malformations may originate in the parotid gland and spread transcompartmentally, be isolated to the parotid gland, or involve the gland secondarily (Figs. 176.5–176.8).
ANATOMIC AND DEVELOPMENTAL CONSIDERATIONS
Embryology and Mechanisms of First Branchial Apparatus Anomalies
The general development of the branchial apparatus is summarized in Chapter 153. The branchial apparatus has six mesodermal arches separated externally by ectodermal-lined branchial clefts or grooves and internally by endodermal-lined pharyngeal pouches1,2 (Figs. 153.1). The mesoderm of the branchial arches contributes to specific bone, cartilage, and vascular structures, and each arch is related to a specific cranial nerve (Fig. 153.4). The facial and trigeminal nerve are both related to the first arch.1
The mechanism of first branchial apparatus developmental errors is incomplete obliteration of one or more components leaving buried cell rests trapped where they do not belong during the embryologic stage, where they later produce branchial apparatus–related problems.1,2 This may produce aberrant migration and ectopias of glandular tissue and/or cysts, sinuses, and fistulas related to migrational pathways.1–3 The first branchial cleft persists as an adult structure providing the epithelium of the external auditory canal (Figs. 153.1–153.5). The first arch contributes to facial development. The first pharyngeal pouch forms part of the eustachian tube and tubotympanic recess. The parotid gland, a first pouch derivative, is an epithelial growth starting at the oral cavity and migrating toward the ear.
First branchial cleft cysts and fistulas are the most common and well-known anomalies in the parotid region. Incomplete migration of endoderm can result in a rare absence of the parotid gland. Rests of cells left in the parapharyngeal space may account for prestyloid parapharyngeal space–origin benign and malignant salivary gland tumors.
The developmental background related to proliferative hemangiomas and vascular malformations and their classifications is discussed in Chapter 8.