In comparison to adults, parotid lesions in children are more likely to result from infectious or inflammatory conditions than neoplastic processes. The etiology of the condition may often be determined through a combination of imaging and fine needle aspiration biopsy. Pediatric parotid surgery may be performed safely and effectively, provided that the surgeon is cognizant of important anatomical differences in the position of the facial nerve in children compared to adults.
8 Pediatric Parotidectomy
In comparison to adults, lesions of the parotid gland are significantly less common in pediatric patients. Nonetheless, there are a multitude of conditions that may involve the parotid gland in children, many of which ultimately require surgical intervention. Pediatric parotid surgery is safe and effective, provided that important anatomical differences between children and adults are recognized. The purpose of this chapter is to review the anatomy and development of the parotid gland in children; provide an overview of common pathology affecting the parotid gland in the pediatric population; and to provide a guide for performing safe and effective parotid surgery in children.
8.2 Anatomy and Embryology
A thorough understanding of the embryology and anatomy of the parotid gland, facial nerve, and mastoid bone is essential to performing safe and effective parotid surgery in the pediatric population. Importantly, there are several anatomic differences between children and adults that must be considered as part of the surgical planning and approach.
Like all salivary glands, the parotid gland is derived from rests of oral ectoderm, which first appear between the fourth and sixth weeks of gestation. These ectodermal rests pierce the surrounding mesoderm, arborize, and subsequently form the acini of the gland. The developing facial nerve reaches the parotid gland around the seventh week of gestation, and by the eleventh to twelfth week of gestation, the nerve has been completely enveloped by the gland parenchyma. Because expanding parotid parenchyma envelops the facial nerve, there is no true “plane” dividing the superficial from the deep lobe of the gland. Head and neck lymphatic development occurs between the twelfth and fourteenth weeks of gestation, which is prior to encapsulation of the parotid gland by the surrounding mesenchyme and the deep cervical fascia. 1 Although the parotid gland is the first major salivary gland to develop embryologically, it is the last major salivary gland to be encapsulated by fascia. Therefore, the parotid gland is the only salivary gland to contain lymph nodes within the gland parenchyma. Typically, 2–20 nodes are found within the superficial portion of the gland and 1–4 nodes are located in the deep portion of the gland. 1
Drainage of saliva from the parotid gland is accomplished by Stensen’s duct, which arises from the anterolateral surface of the gland. The duct courses anteriorly superficial to the masseter muscle, before turning medially at the anterior border of the masseter to pierce the buccinator and enter the oral cavity. The Stensen’s duct os is typically located across from the second maxillary molar. Detached accessory parotid glands may be identified along Stensen’s duct in up to 21% of individuals. 2
The saliva produced by the parotid gland is predominately serous in nature and is responsible for nearly 50% of stimulated saliva production. By contrast, only 30 to 40% of resting saliva production originates from the parotid. Salivary flow from the parotid gland is regulated by parasympathetic and sympathetic innervation. Parasympathetic innervation of the parotid gland originates in the inferior salivatory nucleus in the medulla. These fibers travel with cranial nerve IX to exit the skull at the jugular foramen. The fibers leave the glossopharyngeal nerve as Jacobsen nerve before re-entering the skull through the inferior tympanic canniliculus and passing through the middle ear space as the tympanic plexus. These nerves coalesce into the lesser petrosal nerve, which exits the skull base through the foramen ovale. The presynaptic fibers synapse in the otic ganglion, with the postsynaptic fibers traveling with the auriculotemporal nerve to innervate the parotid parenchyma. Postganglionic sympathetic innervation of the parotid gland occurs through the superior cervical plexus. Parasympathetic stimulation produces large quantities of low-protein, serous saliva while sympathetic stimulation produces a small and variable quantity of thick saliva.
As noted above, the facial nerve is intimately associated with the parotid gland. In adults, well-defined anatomic landmarks are used to identify the main trunk of the facial nerve during parotid surgery (▶ Table 8.1 ). However, in pediatric patients the facial nerve is far more superficial, often lying just deep to the subcutaneous tissue. In addition, in young children for whom the mastoid is poorly pneumatized, the facial nerve takes a more abrupt and horizontal course from the stylomastoid foramen to the parotid gland. Based on cadaveric dissections of three stillborn infants, Farroir and Santini 3 recommend searching for the facial nerve as it exits the stylomastoid foramen in a triangle bordered by the cartilaginous ear canal, the anterior border of the sternocleidomastoid muscle, and the digastric muscle (▶ Table 8.1). Although the length of the facial nerve trunk distal to the stylomastoid foramen in healthy newborns remains poorly defined, studies on human fetuses have indicated that the main trunk ranges in length between 9 and 26 mm. 4 Branching patterns of the facial nerve were initially described by Davis et al in 1956 based on cadaveric dissections in 350 craniofacial halves. 5 Most commonly, the facial nerve bifurcates (~80%) into superior and inferior divisions with a variable degree of anastomosis, 6 but trifurcation may also be identified (~20%) (▶ Fig. 8.1). 4 It is important to note that the marginal mandibular branch of the facial nerve also takes a more superior course over the mandible in children compared to adults.
Differences in the extracranial course of the facial nerve between children and adults result largely from the relative absence of the mastoid process in young children. The mastoid antrum is the first air cell to develop, and can be first recognized at 21 to 22 weeks of gestation with full development by 34 weeks. At birth, the antrum is typically the only fully developed air cell, representing an area of 2.9 cm 2 . 7 Mastoid growth occurs at a rate of ~1 cm 2 per year up to age 6, at which point there is a more gradual increase in area until the mastoid reaches the adult size of approximately 12 cm 2 . 8 As the mastoid air cell system increases in size and volume, the facial nerve subsequently takes a more medial and protected course behind the mastoid tip and tragal cartilage. Therefore, it is critical to consider patient age and the extent of mastoid development when dissecting towards the expected course of the facial nerve in pediatric patients.
8.3 Patient Evaluation
8.3.1 History and Physical Examination
Assessment of pediatric parotid gland disorders requires a detailed history regarding the course, onset, duration, severity, and frequency of symptoms. Parotid enlargement presenting in the perinatal period typically represents congenital lesions, while slowly enlarging, painless masses, especially those presenting in older children, may be more concerning for malignancy. By contrast, acute-onset painful swelling, particularly if associated with fever, suggests an infectious or inflammatory etiology. The quality and quantity of secretions should also be sought. Parents may attribute sialorrhea to the overproduction of saliva; however, this typically results from an inability to clear oral secretions and may be associated with underlying neuromuscular disorders. On the other hand, complaints of decreased salivary production, particularly in association with dry eye or dental caries, may suggest autoimmune conditions. Unilateral symptoms are generally suggestive of congenital, infectious, neoplastic, or traumatic disease processes, while bilateral or multiglandular involvement suggests systemic autoimmune or inflammatory disease. A history of trauma may be indicative of ductal disruption.
Physical examination for parotid lesions should always begin with a complete head-and-neck exam, including bimanual palpation of the parotid glands. The examiner should note the size, symmetry, consistency, and mobility of the parotid glands along with any nodularity, tenderness, and any overlying skin changes. The buccal mucosa should be carefully examined to identify the Stensen’s duct orifice and scars, lesions, or bite marks that could potentially contribute to ductal obstruction. Saliva should be expressed by massaging the gland in a superior–anterior direction beginning at the angle of the mandible, noting the quantity, clarity, and viscosity of saliva. An absence of salivary flow may indicate ductal obstruction, while purulent fluid typically indicates bacterial parotitis. Erythema of the Stensen’s duct orifice with clear saliva production is suggestive of viral sialadenitis. Although systematic evaluation of facial nerve function is sometimes challenging in pediatric patients, particularly young children who are unable or unwilling to follow commands, analysis and documentation of facial nerve function is essential to the evaluation of parotid lesions. Solid parotid gland lesions are more likely to be malignant in pediatric patients, and a history of a slowly growing mass combined with facial nerve weakness should immediately raise the concern for a malignant process.
8.3.2 Laboratory Evaluation
A thorough history and physical examination will guide the need for laboratory studies. Concern for an infectious etiology may indicate the need to evaluate white blood cell count, erythrocyte sedimentation rate, and/or C-reactive protein levels. By contrast, concern for an autoimmune etiology may necessitate serologic tests to evaluate for Sjögren’s syndrome or sarcoidosis. Cystic multiglandular enlargement is suggestive of human immunodeficiency virus (HIV)–associated salivary gland disease and should prompt HIV testing. Serologic testing for Mumps may also be useful, particularly in unvaccinated children with parotid swelling.
8.3.3 Imaging Studies
Ultrasound remains the workhorse imaging modality for parotid lesions in children because it is inexpensive, widely available, and avoids radiation exposure. 9 In addition, ultrasound examinations can be performed rapidly on awake patients, potentially avoiding the need for sedation. It should be noted that ultrasound evaluation is highly user-dependent and therefore studies may vary significantly in quality. In experienced hands ultrasound can frequently identify sialoliths >2 mm and may help distinguish between benign and malignant lesions. 10 An important caveat regarding the use of ultrasound is that this modality has low specificity and accuracy regarding the diagnosis of solid parotid tumors, 11 although semi-quantitative ultrasound algorithms have been developed to more readily distinguish between benign and malignant solid masses. 12
One particular advantage of ultrasound is that image-guided fine needle aspiration biopsy (FNAB) may be performed at the time of imaging, although sedation may be required, particularly for young children. A recent meta-analysis indicated that FNAB has a sensitivity and specificity of 88% and 99.5% with 19% probability of non-diagnostic or indeterminate cytology. 13 When applied specifically to pediatrics, a small study by Lee et al indicated that FNAB has a sensitivity of 100%, positive predictive value of 85%, and accuracy of 85.7% for the diagnosis of benign parotid lesions. 14 For superficial lesions that can be completely visualized on ultrasound and are confirmed to be benign on FNAB, additional imaging is often not required prior to surgical extirpation. 15 It is important to note that FNAB is contraindicated for vascular lesions due to the risk of bleeding.
Despite the advantages of ultrasound and its utility as an initial diagnostic modality, additional imaging studies are often required to evaluate for deep lobe lesions, involvement of the parapharyngeal space, cervical lymphadenopathy, and skull base extension. Computed tomography (CT) scanning with and without contrast is often the preferred modality of the evaluation of suspected inflammatory or obstructive conditions including sialolithiasis, sialadenitis, ranulas, and abscesses. Advantages of CT include the high degree of anatomic detail provided and operator-independent image acquisition permitting an unbiased analysis of the lesion and surrounding structures. Additionally, modern CT scanners have extremely rapid image acquisition times and therefore sedation is often not required. An important concern regarding the use of CT scanning, particularly among pediatric patients, is the development of secondary malignancies as a result of exposure to ionizing radiation. Several large studies have examined the risk of malignancy associated with CT scanning in pediatric patients and have indicated that CT scanning results in approximately one excess malignancy per 10,000 patients under 10 years of age exposed to CT scanning with a 10-year follow-up. 16 – 18 Despite the low risk of secondary malignancy, the risks and benefits of performing a CT scan should be carefully considered and dose-reduction protocols should be utilized to reduce the associated risk. 19
In the case of solid parotid masses, where malignancy is a concern, magnetic resonance imaging (MRI) is the imaging modality of choice. MRI provides excellent soft tissue resolution, which can be used to identify tumor margins, depth of invasion, facial nerve involvement, and/or perineural spread. The use of gadolinium contrast combined with additional imaging protocols such as fat suppression, diffusion-weighted imaging, and dynamic contrast may be used to help distinguish between benign and malignant lesions. Importantly, many solid parotid masses have characteristic imaging characteristics on MRI, which may suggest diagnosis (▶ Table 8.2). Generally, malignant lesions display reduced signal intensity on T2-weighted imaging associated with central necrosis, soft tissue invasion, perineural invasion, and variable contrast enhancement. 20 , 21 In pediatric populations a particular advantage of MRI is the lack of ionizing radiation. However, despite improvements in image acquisition time MRI remains a lengthy study that typically requires sedation or general anesthesia to minimize motion artifact. In addition, MRI is significantly more expensive than other imaging modalities and is frequently less readily available.
Imaging of the salivary ductal system has traditionally required sialography, which involves cannulation of Stensen’s duct and injection of radio-opaque contrast material. However, the invasive nature of this study generally limits its applicability in pediatric patients. MR sialography can provide accurate mapping of first-, second-, and occasionally third-order branches using heavily T2-weighted imaging. Moreover, this modality may provide information regarding strictures, sialolithiasis, and acute or chronic sialadenitis. 22 , 23 Performing MR sialography adds minimal additional time to a traditional MRI studies, and therefore may be considered if the child is already undergoing general anesthesia. Advantages over traditional sialography include the lack of ionizing radiation and the non-invasive nature of the procedure.
Nuclear imaging can also play an important role in quantifying parotid gland function and in detecting aspiration in at-risk children. Tecnetium-99m pertechnetate (99mTc) concentrated in all major salivary glands after intravenous injection and is secreted in saliva. Therefore, 99mTc studies can be used to measure saliva secretion in response to sialogogues or to measure residual gland function following ductal ligation or in the cases of autoimmune glad destruction. As a sensitive test for aspiration, particularly in children who are unable to cooperate with videofluoroscopic swallow studies, 99mTc-labeled sulfur colloid is placed sublingually and the radiotracer is followed by dynamic imaging and chest images at the conclusion of the study. Aspiration is confirmed by the presence of the radiotracer in the bronchi or lungs.
Sialendoscopy was first introduced in the early 1990s as a method to directly examine salivary ductal anatomy and remove or repair obstructive stones or strictures. 24 , 25 In general, sialendoscopes are either diagnostic, which contain only fiberoptic, light, and irrigation channels, or interventional, which include an additional working channel through which various instruments may be passed. Recent studies have indicated that sialendoscopy is safe and effective for the treatment of sialolithiasis and juvenile recurrent parotitis (JRP) in pediatric populations, although it should be noted that endoscopic treatment of JRP may not be more efficacious than conservative measures. 26 , 27 Some pediatric patients >8 years of age may also tolerate sialendoscopy under local anesthesia, 28 thereby obviating the need for sedation and general anesthesia. Accordingly, sialendoscopy is a potentially valuable and/or therapeutic tool for pediatric parotid disorders. The details of performing sialendoscopy are beyond the scope of this chapter, and the reader is referred to several excellent resources on the topic. 29 , 30
8.3.5 Pediatric Parotid Gland Disorders
In pediatric populations, the parotid gland may be affected by a variety of congenital or acquired disorders (▶ Table 8.3). Generally speaking, infectious or inflammatory etiologies are more common in children compared to adults; however, solid parotid lesions in children are more commonly malignant and therefore full evaluation of these lesions should not be delayed.
8.3.6 Congenital Lesions
Congenital lesions of the parotid gland include first branchial cleft cysts, dermoid cysts, ductal cysts, and congenital venolymphatic malformations (▶ Table 8.3). First branchial cleft cysts are divided into two types based on embryology, histology, and location. Type I cysts represent an ectodermal duplication of the membranous external auditory canal, pass lateral to the facial nerve, and end in a bony cul-de-sac near the mesotympanum. By contrast, type II cysts are composed of both ectoderm and mesoderm, and represent duplications of the external auditory canal and pinna. These cysts track medial to the facial nerve and end near the angle of the mandible. Because of their embryologic origin, Type II cysts may contain cartilage. First branchial cleft cysts may present with swelling near the angle of the mandible, parotid mass, or even otorrhea due to the presence of a fistula between the cyst and the external auditory canal. Treatment consists of surgical excision with facial nerve dissection and preservation.
Dermoid cysts of the parotid are rare, with only 18 cases reported in the literature. 31 Imaging by US, CT, or MRI typically reveals a “sack of marbles” appearance, due to fat globules within the cyst, which is considered nearly pathognomonic. Treatment consists of a superficial parotidectomy to avoid recurrence. Ductal cysts result from congenital ductal dilations manifesting as swelling of the parotid gland in infancy. These dilations can be visualized on sialography or US, and in the absence of repeated infections management consists of observation.