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Imaging of the Head and Neck in the Pediatric Patient
Introduction
The old adage that “children are not young adults” is certainly true when evaluating pediatric head and neck lesions. The differential diagnosis of neck masses differs compared to masses that arise in adults. The most common pediatric head and neck lesions are congenital or developmental in origin, followed by inflammatory processes and tumors. This chapter will focus on the congenital/developmental and neoplastic processes; inflammatory lesions are covered in other chapters.
Benign Congenital Masses of the Neck
Thyroglossal duct cysts (TDCs) and anomalies of the branchial apparatus are the most common benign masses of the neck in children. Less common congenital benign masses of the head and neck in the pediatric age group include dermoid/epidermoid cysts and teratomas.
Thyroglossal Duct Cyst
TDC is the most common congenital neck mass and accounts for nearly 90% of nonodontogenic congenital cysts. TDC is the second most common benign cervical mass in children after reactive adenopathy and almost three times more common than branchial cleft cysts.
Embryogenesis of Thyroglossal Duct.
At about 3 weeks' gestation, the thyroid gland begins to develop as a midline endodermal diverticulum from the floor of the pharynx, and over the following 4 weeks of development, the primitive thyroid gland enlarges to form a bilobed diverticulum. For a short period of time, the developing thyroid gland is connected to the tongue by a narrow tube, the thyroglossal duct. This duct courses from the region of the junction of the anterior two thirds and posterior two thirds of the tongue (foramen cecum) to the hyoid bone and farther caudally to the region of the future location of the pyramidal lobe of the thyroid gland in the inferior neck. A TDC or fistula forms when a portion of this duct fails to involute and leaves behind rests of secretory epithelial cells that when stimulated by an inflammatory process cause a cyst to develop.
Presentation.
The TDC is usually a midline or near-midline lesion and can occur anywhere along the path of the duct. About 20% to 25% occur in the suprahyoid neck, 15% to 50% at the level of the hyoid bone, and 25% to 65% in the infrahyoid neck. They commonly occur near the hyoid bone and can be superior, anterior, inferior, or posterior to this bone.
TDCs usually present in the first 5 years of life, with about 66% noted before age 7 and 90% before age 10. A small percentage are diagnosed in patients older than 50 years. Patients typically present with a history of a gradually enlarging mass in the midline or paramedian neck and demonstrate a nontender, mobile, 2- to 4-cm subcutaneous midline or paramedian neck mass of variable firmness. Other presentations include complications of cyst infection, fistula formation, or rupture.
The lining of the cyst may be stratified squamous, pseudostratified ciliated, simple cuboidal, or columnar epithelium, and there may be residual thyroid tissue in the enlarging mass in the midline of the neck. Coexisting carcinoma in the wall of the cyst is reported in less than 1% of patients with TDC and typically occurs in adults, with papillary carcinoma being most common followed by papillary-follicular carcinoma.
Imaging Findings.
Suprahyoid neck cysts are almost always in the midline. When the cysts occur just caudal to the hyoid bone at the level of the thyrohyoid membrane of the larynx, they can stretch this membrane and bow it posteriorly, appearing to lie in the preepiglottic space of the larynx. In the infrahyoid neck, the cyst lies just off the midline, adjacent to the outer surface of the thyroid cartilage and deep to the infrahyoid strap muscles.
In the pediatric population, initial imaging evaluation is frequently performed with ultrasound, which demonstrates a well-defined, thin-walled, midline or paramedian mass with increased through transmission and variable internal echogenicity and no internal blood flow. Increased internal echogenicity can be due to multiple factors including inflammatory debris, hemorrhage, or proteinaceous content.
On computed tomography (CT), the TDC appears as a mass of relatively decreased attenuation to muscle. When the cyst is not infected, it will also demonstrate a thin, smooth rim, but when infected the cyst wall thickens and enhances ( Fig. 26-1 ). The attenuation of the cyst will vary based on its contents; usually they will have mucoid attenuation (10-25 Hounsfield units [HU]), but if there has been previous infection or hemorrhage, attenuation of the cyst contents can approach that of muscle. On magnetic resonance imaging (MRI) the T1 signal intensity can vary from low to high, while T2 signal intensity remains high. The variations in signal intensity are based on the variable protein content of the cyst. With infection or hemorrhage, septations can also arise in the cyst.
A TDC with an internal eccentric solid mass containing calcifications or infiltrative soft tissue characteristics should raise concern for an associated carcinoma.
The primary differential considerations of TDC by imaging include dermoid/epidermoid cyst, branchial cleft cyst (if paramedian in location), or rarely a sebaceous cyst.
Treatment.
Treatment is surgical removal of all tissue along the course of the duct as far as the foramen cecum (Sistrunk procedure). The hyoid bone rotates during maturation before assuming its final adult position. During this rotation, the thyroglossal duct is adherent to the hyoid along its anterior inferior edge and may be drawn posteriorly and cranially to lie behind the body of the hyoid bone; rarely the duct can be incorporated into the hyoid bone, trapped between the second and third arch components of the hyoid's body. Owing to this close relationship between the body and the TDC, Sistrunk proposed that the body of the hyoid bone should be removed during surgical resection of a thyroglossal duct, leading to a significant decrease in recurrences, reduced from nearly 50% to less than 4%.
Anomalies of the Branchial Apparatus
Embryogenesis of the Branchial Apparatus.
The anatomic structures of the face and neck predominantly derive from the branchial apparatus, which is a complex structure derived from neural crest cells that develops between the second and seventh weeks of gestation. The branchial apparatus consists of six paired arches separated on their outer surface by five paired ectodermal clefts and on their inner surface by five paired endodermally derived pharyngeal pouches. By the end of the fourth week of life, four well-defined pairs of arches are visible and the fifth and sixth arches are rudimentary. Each arch is composed of a central core of mesoderm and is lined externally by ectoderm and internally by endoderm.
The cervical sinus of His is formed after the branchial arches appear by accelerated mesodermal growth cranially of the first arch and a portion of the second arch, and caudally by growth of the epipericardial ridge, which develops from the mesoderm lateral to the fifth-sixth arch. The branchial apparatus typically disappears between the fourth and sixth weeks of life.
Etiology of Branchial Anomalies.
The etiology of branchial anomalies is controversial, and the most accepted theories to explain the development of branchial abnormalities propose that they are either vestigial remnants from incomplete obliteration of the branchial apparatus or the result of buried epithelial cell rests.
Classification.
Anomalies of the branchial apparatus are best understood by being classified into a spectrum of developmental anomalies that includes fistulas, sinuses, and cysts.
First Branchial Anomalies.
First branchial cleft anomalies account for up to 5% to 8% of all branchial anomalies: 68% are cysts, 16% are sinuses, and 16% are fistulas. Although Arnot and Work tried to subclassify first branchial anomalies into two distinct subtypes in 1971 and 1972, Olsen—after reviewing multiple cases—concluded that it is difficult to subclassify all these anomalies into one of the subtypes and proposed a simplified classification (cysts, sinuses, and fistulas).
Embryology.
The first branchial apparatus gives rise to portions of the middle ear, external auditory canal, eustachian tube, mandible, and maxilla, and the formation of these structures is completed by the sixth and seventh weeks of gestation. The parotid gland and facial nerve form and migrate between the sixth and eighth weeks of gestation. Because the facial nerve and parotid gland have a somewhat later embryologic development, vestigial branchial anomaly can be located in variable relationship to the parotid gland and the facial nerve. First branchial apparatus anomalies arise from incomplete closure of the ectodermal portion of the first branchial cleft. They can start at the junction of the bony and cartilaginous external auditory canal or in the cartilaginous portion of the external canal, the plane between the mandibular and hyoid arches, and end in the submandibular area. As a result, first branchial apparatus anomalies can arise in the middle ear cavity, external auditory canal, superficial or deep lobes of the parotid gland, superficial to the parotid gland, at the angle of the mandible, anterior or posterior to the pinna, or along the nasopharynx.
Presentation.
Although the majority of first branchial apparatus anomalies occur in the first decade of life, they can also be encountered in middle-aged patients, especially women. Patients typically present with recurrent abscesses or inflammatory processes with tenderness and swelling at the angle of the mandible or in the ear. If the process is related to the parotid gland, patients tend to present with recurrent parotid abscesses not responding to therapy. If there is drainage of the cyst into the external auditory canal, the initial presentation is typically otorrhea. Simple cysts can also present as a painless soft tissue mass. Sinus tracts are rare with first branchial anomalies. When present, they are usually found in the first decade of life.
Imaging findings.
First branchial cysts are usually related to the parotid gland, external canal of the ear, and/or the lower margin of the pinna or at the angle of the mandible. If a tract can be identified, directed toward the external auditory canal, then the diagnosis can be made with certainty. If a tract is not seen and the cyst lies within the parotid gland, possibilities would include a first branchial cyst, parotid suppurative lymph node and abscess, inflamed ectatic salivary gland, a lymphoepithelial cyst (human immunodeficiency virus [HIV]), Sjögren's syndrome, or a rare localized obstructive mucocele.
Uncomplicated cysts demonstrate mild rim enhancement or no enhancement. Infected cysts have variable wall thickness and enhancement characteristics ( Fig. 26-2 ). If the anomaly is located within the parotid gland, the imaging findings will be nonspecific. For example, the cyst contents of a first branchial cyst, a lymphoepithelial cyst, or an obstructive mucocele or sialocele are usually of mucoid attenuation on CT, and on MRI usually have low to intermediate T1-weighted and high T2-weighted signal intensity. T2-weighted or short tau inversion recovery (STIR) MRI sequences may be helpful in identification of small fluid-filled tracts leading to the external auditory canal. If the cyst is infected and has a tract, coronal T2-weighted and postcontrast T1-weighted fat-suppressed MRIs would best demonstrate the tract.
Treatment.
Treatment is complete surgical resection and carries an excellent prognosis. The major complication of surgery is related to facial nerve injury. Residual wall remnants can lead to recurrence.
Second Branchial Anomalies.
About 95% of all branchial anomalies are related to the second branchial apparatus, and these anomalies can occur anywhere from the tonsillar fossa to the supraclavicular region of the neck. The majority of these anomalies, about three fourths, are cysts. Cysts are more common between 10 and 40 years of age, and fistulas or sinuses usually present before age 10.
Classification.
Second branchial cysts are classified into four subtypes based on location per Bailey's classification. Type 1 cyst lies deep to the platysma muscle and anterior to the sternocleidomastoid (SCM) muscle ( Fig. 26-3 ). This type of cyst is felt to represent a remnant of the tract between the sinus of His and the skin. Type II cyst is the most common type and is thought to be the result of the persistence of the sinus of His. This cyst is located posterior and lateral to the submandibular gland, anterior and medial to the SCM muscle, and anterior and lateral to the carotid space ( Fig. 26-4 ). Type III cyst/fistula is thought to arise from a dilated pharyngeal pouch and courses medially between the internal and external carotid arteries; it can extend up to the lateral wall of the pharynx or skull base ( Fig. 26-5 ). Type IV cyst lies in the mucosal space of the pharynx adjacent to the pharyngeal wall and is thought to arise from a remnant of the pharyngeal pouch ( Fig. 26-6 ).
Presentation.
The most common presentation of a second branchial cleft cyst is as a fluctuant nontender mass at the lateral aspect of the mandibular angle. When infected the patient will present with a history of a slowly enlarging painful mass. Because the cysts contain lymphoid tissue, stimulus such as an upper respiratory tract infection can lead to an increase in size of the cyst. With a fistula, an ostium is often noted at birth at the anterior border of the junction of the middle and inferior thirds of the SCM muscle. The tract then courses deep to the platysma muscle, ascends laterally along the carotid sheath, lateral to the hypoglossal and glossopharyngeal nerves, and then passes between the internal and external carotid arteries before it terminates in the region of the palatine tonsillar fossa.
Imaging findings.
Ultrasound is usually the initial imaging study because the cysts often present as a neck mass in a young child and can range in size from 1 to 10 cm. If the anomaly is uncomplicated it will demonstrate sonographic characteristics of a simple cyst (thin wall, increased through transmission, anechoic, and compressible). On CT the anomaly will demonstrate a thin wall with decreased attenuation. On MRI it will appear as hypointense to slightly hyperintense to muscle on T1-weighted images and hyperintense to muscle on T2-weighted images. When it is infected it can demonstrate a thicker wall with increased internal echogenicity on ultrasound, increased attenuation on CT, and hyperintense T1-weighted signal on MRI, with variable enhancement. If the cysts are chronically complicated, they can demonstrate septations.
Sometimes there may be a “beak sign” present, which is pathognomonic of a Bailey type III second branchial cyst, where the medial aspect of the cyst is compressed and forms a beak as it extends between the internal and external carotid arteries on axial CT or MRI.
In children, differential considerations include suppurative adenopathy/deep neck infection, lymphatic malformation, dermoid/epidermoid cyst, and thymic cyst. Other rare considerations in this age group include cystic schwannoma of the vagal nerve, minor salivary gland tumor, and necrotic malignant adenopathy. Because the jugulodigastric node is seen in the same location as the most common form of second branchial cleft cyst, clinicians may mistake it for an enlarged, suppurative, reactive, or tumor-infiltrated jugulodigastric node.
Treatment.
Treatment is complete surgical resection, which carries an excellent prognosis.
Third and Fourth Branchial Anomalies.
These are rare anomalies, and third branchial anomalies account for 3% and fourth branchial anomalies account for about 1% to 2%. Third branchial anomalies are centered in the posterior cervical space, and despite their overall rarity they are the second most common congenital lesions of the posterior cervical space after lymphatic malformation. These must sometimes be distinguished from the large second branchial cleft cyst, which can protrude posteriorly in the posterior cervical space.
Embryology.
Third branchial anomalies most likely arise secondary to failure of involution of the third branchial apparatus, and fourth branchial anomalies are likely due to failure of regression of the fourth branchial pouch or distal sinus of His.
Third branchial apparatus cysts are located posterior to the common carotid or internal carotid artery, between the hypoglossal nerve (below) and glossopharyngeal nerve (above). If it is in the form of a fistula, there will be a cutaneous opening similar to a second branchial fistula that is anterior to the lower SCM muscle. It then courses posterior to the common or internal carotid artery, anterior to the vagus nerve and between the hypoglossal and glossopharyngeal nerves, and then pierces the thyrohyoid membrane and enters the pyriform sinus anterior to the superior laryngeal nerve.
Presentation and imaging findings.
Third branchial cleft cysts usually present as painless, fluctuant, 2- to 5-cm masses in the posterior triangle of the neck, often after a viral upper respiratory infection ( Fig. 26-7 ). The presentation is at an earlier age with a sinus or a fistula. On CT and MRI, uncomplicated cysts usually present as a unilocular cystic mass on imaging, and complicated cysts demonstrate increased wall thickness with variable enhancement characteristics and internal proteinaceous fluid.
Anomalies related to the fourth branchial pouch are usually in the form of a sinus tract that arises from the pyriform sinus, pierces the thyrohyoid membrane, descends along the tracheoesophageal groove, and continues into the mediastinum. These anomalies have variant presentations such as a nontender fluctuant mass anterior to the inferior SCM muscle for uncomplicated cases, and suppurative thyroiditis or recurrent neck abscesses for complicated cases ( Fig. 26-8 ). For diagnosing a fistulous tract by imaging, a contrast-enhanced CT following ingestion of barium or water-soluble contrast or direct injection of a fistulous tract is the imaging study of choice.
Although it is difficult to differentiate third from fourth branchial anomalies by imaging alone, the key anatomic markers for correctly diagnosing them include the carotid artery and the superior laryngeal nerve. Third branchial anomalies lie posterior to the internal carotid artery and above the superior laryngeal nerve, whereas fourth branchial anomalies are anterior to the internal carotid artery and lie below the superior laryngeal nerve. Third and fourth branchial anomalies can be related to the pyriform sinus and can appear similar to external laryngoceles on imaging.
Other differential considerations for third and fourth branchial anomalies include other branchial anomalies, TDC, lymphatic malformation, thyroid abscess, thyroid, parathyroid, or thymic cyst, and suppurative adenopathy.
Treatment.
For both third and fourth branchial anomalies, complete surgical resection is the treatment of choice. Persistence of the tract to the pyriform sinus can result in recurrent episodes of thyroiditis and can necessitate a partial thyroidectomy. If fourth branchial anomalies are complicated by infection and secondary thyroiditis, medical treatment should precede surgical treatment.
Cervical Thymic Remnant
Embryology and etiology.
Cervical thymic remnants are very rare lesions, and two main theories, congenital theory and acquired theory, have been reported to explain their etiology. The congenital theory states that the cysts are due to persistence of thymopharyngeal duct remnants and can be located anywhere from the level of the pyriform sinus to the superior mediastinum and located either beneath or medial to the SCM muscle. The acquired theory for thymic cysts proposes progressive cystic degeneration of thymic (Hassall's) corpuscles, primitive endodermal cells, lymphocytes, and epithelial reticulum of the thymus as sources for thymic remnants.
Presentation.
The thymus reaches its greater relative size at age 2 to 4 years and its greatest absolute size at puberty. Cervical thymic remnants are very rare lesions and can be found anywhere along the path of the thymopharyngeal duct.
Most are detected in childhood, and two thirds are found in the first decade of life, often between 3 and 8 years of age, and the remainder in the second and third decades of life.
Most patients are asymptomatic, and about 80% to 90% present with a painless slowly enlarging neck mass near the thoracic inlet, anterior or deep to the SCM muscle. In 50% of cases, a connection between the cervical thymic anomaly and the mediastinal thymic gland occurs in the form of direct extension or a remnant of the thymopharyngeal duct. Mediastinal thymic cysts can occur without a cervical component. There is a frequent association between cervical thymic cysts and thyroid and parathyroid inclusion cysts, and a fibrous strand is sometimes present between the cervical thymic cyst and the thyroid gland.
Imaging findings.
Because most ectopically located thymic masses are often cystic on ultrasound, CT and MRI will demonstrate imaging characteristics consistent with an uncomplicated cyst. A cystic lesion will course parallel to the SCM muscle and along the carotid sheath or lateral aspect of the visceral space on CT ( Fig. 26-9 ). Imaging characteristics will vary if the cysts are complicated by hemorrhage, cholesterol, proteinaceous fluid, or infection. Aberrant thymic tissue, parathyroid tissue, or lymphoid tissue can be present as soft tissue components related to the wall of the cyst.
Differential diagnosis.
A second branchial apparatus fistula is the main differential consideration of thymic remnants. A second branchial fistula will pass between the internal and external carotid arteries and end in the superior tonsillar pillar, whereas a thymic cyst will pass posterior to the carotid bifurcation and terminate in the pyriform sinus. Additionally, about 50% of cervical thymic cysts extend into the superior mediastinum, and some can be seen in the inferior pole of the thyroid gland. Other differential considerations for a thymic cyst include a fourth branchial anomaly, TDC, thyroid/parathyroid cyst, lymphatic malformation, cyst neuroblastoma, lymphadenopathy, external laryngocele, and vallecular cyst.
Treatment.
Treatment is surgical, and the prognosis is excellent if the resection is complete. There is a high recurrence rate with incomplete resection.
Dermoid/Epidermoid Cyst
Because of overlapping features, the terminology and classification of dermoid, epidermoid, and teratoid cysts in the head and neck can lead to confusion.
Embryology.
These masses are thought to arise from trapped epithelial cell rests at the site of midline closure. Dermoids and epidermoids are ectodermal-lined inclusion cysts, and teratoids can contain tissues of all three germ-cell layers. Dermoids contain epithelial and dermal elements, whereas epidermoids contain only epithelial elements.
Presentation.
There is no gender predilection with dermoid and epidermoid cysts. Dermoid cysts of the neck commonly occur near the midline, with the most common location being the floor of the mouth, (11.5% of all dermoids), followed by other locations including the anterior neck, tongue, palate, and orbit. Epidermoid cysts are rarely found in the head and neck, and when they occur they usually present in infancy.
Dermoid/epidermoid cysts become clinically apparent in the second or third decades of life, when they present as a painless, soft, slow-growing mass in the suprahyoid midline neck. The cyst can be variable in size, and rapid increase in size can be seen secondary to association with a sinus tract, pregnancy, or increasing desquamation of skin appendage products. Masses lying in the sublingual space are often clinically inapparent and demonstrate minimal mass effect, but masses in the submandibular space present with a more obvious swelling or mass effect. Additionally, a distinguishing factor between TDCs and dermoid/epidermoid cysts are that whereas TDCs move with protrusion of the tongue, owing to their association with the hyoid bone, dermoid/epidermoid cysts are nonmobile with this maneuver.
Dermoid cysts are well-defined encapsulated masses lined with squamous epithelium and may contain skin appendages in the form of sweat or sebaceous glands or hair follicles, which can lead to the formation of keratin and sebaceous material, including occasional hair. Epidermoid cysts lack skin appendages but have a histologic appearance similar to dermoids. The incidence of malignant degeneration into squamous cell carcinoma is about 5% for dermoids.
Imaging Findings.
On CT a dermoid cyst can appear as a thin-walled unilocular mass with decreased attenuation and at times with CT attenuation values approximating that of fat ( Fig. 26-10 ). Sometimes a virtually pathognomonic “sack-of-marbles” appearance, representing multiple small fatty nodules within the fluid, is seen within the cyst. These cysts can also present with a more heterogeneous appearance owing to differences in composition of the various skin appendage components, including calcification. Thin peripheral enhancement can be seen following contrast administration.
On MRI, signal changes vary depending on the composition of the various cyst components. T1 signal can be hyperintense due to lipid components or isointense to muscle, and T2 signal is usually hyperintense to muscle. If calcification is present, this will cause low T2 signal. If a dermoid contains minimal complex elements, it can appear similar to an epidermoid. Postcontrast images may or may not demonstrate rim enhancement. T1 postcontrast fat-saturation images can be helpful to diagnose dermoids by assessing for signal dropout.
Epidermoid cysts usually follow water signal, demonstrating homogenous low attenuation on CT and hypointense T1-weighted and hyperintense T2-weighted signal to muscle on MRI ( Fig. 26-11 ). The signal characteristics of this lesion can be confused with other cystic lesions such as an uncomplicated lymphatic malformation or ranula.
It is important to accurately localize the lesions when they are located in the floor of the mouth. If the lesion is located above the mylohyoid muscle, it lies in the sublingual space and is amenable to an intraoral surgical approach. If it lies in the submandibular space, a submandibular approach would be used. The optimal imaging plane for localizing these masses into the appropriate space would be coronal CT or MRI.
Teratoma
Approximately 9% of all pediatric head and neck neoplasms are teratomas, and less than 5% of all teratomas occur in the head and neck. In the pediatric population, most teratomas are extragonadal in location, with 82% in the sacrococcygeal area; locations in the head and neck include neck, nasal cavity, paranasal sinus, oral cavity, oropharynx, nasopharynx, temporal bone, ear, and orbit.
Teratomas are thought to arise from misplaced pluripotential primordial germ cells and are classified as true neoplasms because of their biological behavior with progressive and invasive growth patterns.
Cervical Teratomas.
Cervical teratomas are large and can present with extensive unilateral or more diffuse cervical swelling that can lead to obstetric complications such as difficult labor with malpresentation, premature labor, stillbirth, acute neonatal respiratory distress, and maternal polyhydramnios. The tumors that present later in life are smaller but have a higher incidence of malignancy.
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