3.9: Nasopharynx

Saugata Sen, Anisha Gehani, Jeevitesh Khoda, Dayananda Lingegowda

Introduction

Physical and naked eye examination of the nasopharyngeal region is not possible. Endoscopy and cross-sectional imaging are the only tools available for evaluating lesions in this region. Proximity to the skull base makes imaging an interesting and important proposition in the management protocol of nasopharyngeal lesions. Direct and indirect involvement of the nervous system and skull base demands diagnostic precision. Management varies according to the various diseases that occur in this region, and the skills of a radiologist are important for a planning treatment and prognostication.

In this chapter, we shall discuss the radiological anatomy of the nasopharynx and the pathologies that occur in this region. We will also go through the radiology of the common pathologies and discuss staging of malignant lesions. Imaging of treatment response and recurrence will be emphasized. We will also devote a small section for special consideration of imaging on diseases of paediatric and young adults.

Anatomy of the nasopharynx

The cranial most part of the pharynx is the nasopharynx. It lies just below the middle cranial fossa. The nasopharynx is a musculofascial columnar tube with an inner mucosal layer lining the airway.

The nasopharyngeal epithelium is complex. Stratified squamous epithelium is present at the anterolateral walls and inferior anterior and inferior posterior walls. Ciliated columnar epithelium is present around the nasal choanae and the posterosuperior wall or roof. The remaining areas have a mixture of the aforementioned two types. An intermediate epithelium is also present in the nasopharynx at the junction of nasopharynx and oropharynx. Muscular and fascial layers are present deep to the mucosa.

Boundaries of the nasopharynx

a. Superiorly, the nasopharynx is bordered from anterior to posterior by the basisphenoid and the basiocciput. As the roof is posteriorly sloping, the atlas (C1) and axis (C2) vertebrae are also part of the roof (Fig. 3.9.1).
b. The posterior choana forms the anterior boundary, through which the nasopharynx communicates with the nasal cavities and the exterior.
c. The inferior border is the level of the hard palate and the palatopharyngeus muscle, which forms the so-called “Passavant’s” muscle. The Passavant’s ridge is formed by contraction of this muscle and elevates to meet the soft palate to obliterate the communication between the oropharynx and the nasopharynx to prevent regurgitation of food and fluids during swallowing (Fig. 3.9.2).
d. The posterior and lateral walls of the nasopharynx are formed from superficial to deep by the pharyngobasilar fascia, the superior constrictor muscle and the buccopharyngeal fascia. A brief description is given in the following.
1. 1. The pharyngobasilar fascia (pharyngeal aponeurosis): This fascia is a part of the deep cervical fascia (middle layer) and lies between the mucosal layer and the muscular layer of the pharynx. It is strong and covers the lateral walls of the nasopharynx to maintain its contour and patency. It literally anchors the pharynx to the skull base.
2. 2. The superior constrictor: Along with the middle and inferior constrictor, the superior constrictor forms the key muscular component of the entire pharynx. The superior constrictor surrounds the nasopharyngeal column and assists in the swallowing reflex. The muscle originates from the skull base (pterygoid hamulus, pterygomandibular raphe and mandible) and inserts onto the median pharyngeal raphe.
3. 3. The buccopharyngeal fascia: This fascia is a part of the deep cervical fascia (middle layer) and covers the buccinator and the constrictor muscles of the pharynx.

Fig. 3.9.1 Lateral reconstructed CT in bone window algorithm showing the bony landmarks of the nasopharynx.

Fig. 3.9.2 Lateral radiograph of the neck soft tissues demonstrating various soft tissue relations of the nasopharynx.

The buccopharyngeal fascia separates the pharynx from the retropharyngeal space and parapharyngeal space (Fig. 3.9.3). The longus capitis muscles lie posterior to the nasopharynx and retropharyngeal space.

Fig. 3.9.3 Axial T1W image shows the relations of the nasopharynx to other cervical spaces.

The strong musculofascial covering of the mucosa and epithelium serves several important functions. The strength of this covering maintains the contour and patency of the airway. The composite layer also prevents any disease process in the airway to extend into the deep spaces of the neck.

- 1. A. Defect in the pharyngobasilar fascia, named as the sinus of Morgagni, exists in the superior part on both sides. Through this defect, the eustachian tube and the levator veli palatini enter the nasopharyngeal space (Fig. 3.9.4). This defect is a potential gateway of infection and malignancy from the nasopharynx into the deep neck spaces.
  2. B. Relations of the nasopharynx.

Fig. 3.9.4 Axial T2W image shows the components and anatomical boundaries of nasopharynx.

The posterior relation of the nasopharynx is the longus colli muscles and the retropharyngeal space. The lateral relations are the parapharyngeal spaces. On the anterior aspect are the nasal cavities (Fig. 3.9.3). The skull base forms the superior relation.

- 1. C. Key components of the nasopharynx.

There are three anatomical components of the nasopharynx that are clinically relevant and deserve special mention (Fig. 3.9.4).

1. 1. The fossa of Rosenmuller: This fossa is located in the posterosuperior aspect of the nasopharynx on both sides. Nasopharyngeal cancer usually originates from this region. This fossa is also called the lateral pharyngeal recess, and lesions here are clinically occult.
2. 2. The torus tubarius: This mucosal elevation is located anterolateral to the fossa of Rosenmuller and in the superolateral wall of the nasopharynx on both sides. The mucosal elevation is caused by the medial opening of the Eustachian tube (cartilaginous part). The levator veli palatini, few minor salivary glands and lymphoid tissue lie beneath the torus tubarius as well. The levator veli palatini, the tensor veli palatini and the salpingopharyngeus are responsible for opening up the nasopharyngeal opening of the eustachian tube during swallowing of food to equate air pressure between the nasopharynx and middle ear cavities.
3. 3. Adenoids: Posterosuperior wall of the nasopharynx is rich in lymphatic tissue in children. This lymphatic tissue is called the “nasopharyngeal tonsil” or “adenoid”. Enlargement of adenoid is observed in early childhood. The maximum size is reached by 3–6 years. Beyond the age of 6, the adenoids undergo atrophy.

Enlarged adenoid in adults should trigger search for infection and lymphoproliferative conditions. Nasopharyngeal carcinoma is another differential.

Pathologies in the nasopharynx

The common diseases that are prevalent in the nasopharynx are mentioned in the following.

1. Congenital
- Thornwaldt cyst
- Persistent canalis basilaris medianus
- Craniopharyngeal canal
- Fossa navicularis
- Transsphenoidal encephalocoele
- Teratoma
- Haemangioma
2. Inflammatory
- Adenoid hypertrophy
- Retropharyngeal abscess
- Mucous retention cyst
- Tendinitis of the longus colli
3. Neoplastic
- Nasopharyngeal carcinoma (NPC)
- Nasopharyngeal lymphoma
- Nasopharyngeal rhabdomyosarcoma
- Minor salivary gland tumours

Since NPC is the most important pathology in this region, we will discuss it first and then go on to describe the others.

Nasopharyngeal carcinoma

Introduction

The incidence of NPC varies around the world and is common in certain ethnic groups. NPC is more prevalent in the Mongoloid population in China and Southeast Asia. The Arctic (Eskimos) and north African region are also endemic for this disease. This disease is pathologically a squamous cell carcinoma (SCC), the peak incidence of which is in the fourth to fifth decades. The incidence ratio of males to females is 3:1.

Though NPC is the most common malignant tumour of the nasopharynx, its incidence worldwide remains low at 0.7% of all cancers. The disparate geographic distribution suggests underlying genetic modification in certain ethnic groups that make them susceptible to NPS.

Pathology

Many factors have been incriminated for the etiopathogenesis of NPC which include genetic and environmental factors as well as infection. The disease is an interplay of several of these factors and they are listed in the following:

1. Certain human leukocyte antigens are more prone, e.g., HLA-A2, HLA-B17 and Bw26.
2. Foods containing nitrosamines and polycyclic hydrocarbons as preservatives.
3. Infection with Epstein–Barr virus (EBV). EBV infects epithelial cells by inclusion of clonal viral DNA into the host genome which can then result in malignant transformation.

Carcinogenesis of NPC demonstrates interplay of the aforementioned factors. It is highly possible that all or some of these factors are coconspirators in the etiopathogenesis and each is not singularly responsible for development of NPC.

NPC is different from other carcinomas in the head and neck regions in epidemiological and histopathologic features. The management strategies and response to therapy are also very different from the other carcinoma of the head and neck regions.

The histopathological classification of NPC as proposed by WHO is as follows:

1. Type I is keratinizing squamous cell carcinoma (SCC).
2. Type II is nonkeratinizing SCC.
1. a. Differentiated.
2. b. Undifferentiated.
3. Basaloid.

The endemic form of NPC that is largely prevalent in China, Southeast Asia, North Africa and the Arctics are EBV related and of the undifferentiated form of nonkeratinizing SCC.

Clinical presentation and appropriate imaging

Clinical presentation of NPC varies from asymptomatic to nasal obstruction, change in voice, nasal bleed and neck adenopathy. In fact, the commonest cause of neck adenopathy in endemic populations is NPC.

The physician depends only on endoscopy for evaluation of the nasopharynx. Nasal endoscopy is a great modality for diagnosis and sampling but underestimates the true extent of the tumour. Also, approximately 10% of nasopharyngeal cancers can be missed on endoscopy. For detection of small lesions, delineation of the exact extent and staging of the tumour as well as the nodes, some form of cross-sectional imaging is essential.

The appropriate imaging modality in NPC depends on the clinical stage. Contrast-enhanced MRI (CEMRI) is usually the mainstay due to its ability to detect soft-tissue extension. Diagnostic precision is of paramount importance in this region to detect subtle cranial nerve involvement and skull base extension, both of which are served well by CEMRI. CEMRI is efficient in detecting lymph nodes in the neck as well. Posttreatment imaging is another scenario where CEMRI has been proven to be extremely reliable for diagnosing subtle residual disease or complete response. In the posttreatment setting, 18-FDG-PET-CT has also been used.

NPC with nodes greater than 6 cm or nodes below the lower border of the cricoid cartilage is at high risk of metastatic disease. In this scenario, some form of whole-body imaging is required, and 18-FDG-PET-CT is appropriate. Use of 18-FDG-PET-CT has also been advocated in primary staging and to judge response.

The recommendations for appropriate imaging in NPC are as follows:

1. Staging of primary as well as the nodes is by CEMRI. In the author’s department, noncontrast CT (NCCT) sections are obtained as in addition for detecting subtle bony skull base disease.
2. Posttreatment response is by CEMRI.
3. In cases with increased risk of systemic disease (nodes greater than 6 cm and/or nodes below the inferior border of the cricoid), 18-FDG-PET-CT is recommended.

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