Download Pictorial review of the imaging anatomy and common pathology of

Pictorial review of the imaging anatomy and common
pathology of the parapharyngeal space.
Poster No.:
C-1092
Congress:
ECR 2014
Type:
Educational Exhibit
Authors:
B. S. Purohit , R. Hermans , K. Op de beeck ; SINGAPORE/SG,
1
2
2 1
2
Leuven/BE
Keywords:
Infection, Cysts, Cancer, Diagnostic procedure, MR, CT, Head and
neck
DOI:
10.1594/ecr2014/C-1092
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Page 1 of 32
Learning objectives
This educational exhibit aims to acquaint the reader with:
1. The imaging anatomy of the parapharyngeal space (PPS).
2. Approach to imaging issues of PPS lesions.
3. The displacement patterns of PPS fat that are essential for accurate diagnosis and
appropriate treatment of pathology arising in this region.
3. Common differentials of true PPS lesions and their imaging features.
Background
•
The PPS is a central fat-filled space in the deep face and suprahyoid neck
extending from the skull base to the level of the hyoid bone. Since it largely
contains fat, it is always well visualised on CT and MR imaging of the head
and neck.
•
The PPS is bordered by the pharyngeal mucosal space (PMS) medially,
masticator space (MS) laterally, parotid space (PS) posterolaterally, and
the retropharyngeal space (RPS) posteromedially. The PPS is further
subdivided into the prestyloid and retrostyloid compartments.
•
The presence of fat in the PPS allows the radiologist to identify displacement
patterns of the PPS and to use a logical algorithm to identify the true space
of origin of tumours in this region.
•
Although tumours of the PPS are rare, their diagnosis and treatment options
may be challenging. Differentiation of a prestyloid lesion from a retrostyloid
lesion is crucial for narrowing the differential diagnosis.
•
The most common lesion in the PPS are tumours arising from the
deep parotid lobe, extending in the prestyloid compartment of the PPS.
Neurogenic tumours and paragangliomas are the commonest primary
lesions of the retrostyloid PPS. Primary prestyloid PPS lesions are rare,
nd
however, lipomas, atypical 2 branchial cleft cyst (BCC) and ectopic
salivary gland tumours may be seen.
Page 2 of 32
Findings and procedure details
Anatomy of the PPS:
The PPS is an inverted cone-shaped space that extends from the level of the skull base
upto the hyoid bone on either side of the pharynx.
The anatomic boundaries of the PPS are as follows:
•
Superior: The temporal bone lateral to the attachment of the pharyngobasilar
fascia and medial to the foramen ovale and foramen spinosum. None of the
skull base foramina open within the boundaries of the PPS (Fig. 1).
•
Inferior: The greater cornu of hyoid bone and the posterior belly of the
digastric muscle. The PPS is continuous with the posterior aspect of the
submandibular space (SMS) at this level (Fig. 1).
•
Medial: The buccopharyngeal fascia which covers the pharyngobasilar
fascia and constrictor muscles. The PMS lies medial to the PPS and the
RPS lies posteromedial to the PPS (Fig. 2).
•
Lateral: The fascia overlying the pterygoid muscles (MS) and the
sphenomandibular ligament. The PPS communicates laterally with the PS
through the stylomandibular tunnel (Fig. 1, 2).
•
Anterior: The pterygomandibular raphae which extends from the medial
pterygoid plate to the mylohyoid line on the lingual surface of the mandible.
•
Posterior: The tensor-vascular-styloid fascia (TVS) overlying the tensor veli
palatini (TVP) muscle extends from the medial pterygoid plate to the styloid
process (Fig. 2). It subdivides the PPS into the prestyloid and retrostyloid
compartments. Some authors describe the retrostyloid PPS separately as
the carotid space [1-3].
Contents of the PPS:
•
Prestyloid PPS: Contains fat, the internal maxillary and ascending
pharyngeal artery, pterygoid venous plexus and may contain ectopic salivary
gland tissue.
•
Retrostyloid PPS: Contains the internal carotid artery (ICA), internal jugular
vein (IJV), cranial nerves IX-X-XI-XII and the sympathetic plexus. Lymph
Page 3 of 32
nodes of the deep cervical chain are found only below the level of the
posterior belly of the digastric muscle [1-5].
Fig. 1
References: Bela Purohit
Fig.1: Graphic representation of the coronal spatial anatomy and craniocaudal extent of
the PPS. SB-Skull base, SZMS-Suprazygomatic masticator space, MS-Masticator space,
Page 4 of 32
PMS-Pharyngeal mucosal space, PS-Parotid space, SMS-Submandibular space, SLSSublingual space, OC-Oral cavity.
Fig. 2
References: Bela Purohit
Fig. 2: Graphic representation of the axial spatial anatomy of the suprahyoid neck and
the PPS. BS-Buccal space, RPS-Retropharyngeal space.
Page 5 of 32
Radiological anatomy of the PPS:
•
The fat-filled PPS is well appreciated on both axial and coronal CT and MR
images. The PPS appears hypodense with fat attenuation on CT images
and hyperintense on T1W MR images. In normal circumstances, the PPS
should look symmetrical on both sides (Fig. 3, 4) [1-3].
•
MRI is often preferred over CT in the evaluation of PPS pathology because
of its superior contrast resolution [1].
•
In the axial plane, the prestyloid PPS is seen as a triangular fat-fillled space
widest at the level of the soft palate (Fig. 3). The pharyngobasilar may be
seen on MRI as a hypointense line outlining the pharyngeal mucosa [1].
•
The TVS cannot be routinely identified on MRI. Its course can be traced by
drawing an imaginary line from the TVP muscle upto the styloid process [1].
•
The lateral fascial border of the PPS is not identifiable on MRI. Its cranial
attachment lies just medial to the foramen ovale and the exiting mandibular
nerve (Fig. 4) [1].
•
The ICA and IJV appear as flow voids on T1W and T2W MR images, just
medial to the syloid process (Fig. 3) [1].
Page 6 of 32
Fig. 3
References: Department of Radiology, University Hospitals, Leuven
Fig. 3: Axial T1W MR image showing spatial anatomy of the PPS. The central fat-filled
PPS (asterisk) is surrounded by the PMS medially (solid black arrow), MS anterolaterally
(arrowhead) and the PS posterolaterally (thin black arrow). The ICA in the retrostyloid
PPS is seen as a flow void (thin white arrow).
Page 7 of 32
Fig. 4
References: Department of Radiology, University Hospitals, Leuven
Fig. 4: Cor T1W MR image showing the craniocaudal extent of the PPS. Superiorly, the
PPS (asterisk) extends to the skull base, just medial to the opening of the foramen ovale
(white arrow). Inferiorly, the PPS merges with the SMS (thin black arrow). The PPS is
related to the PMS medially (solid black arrow) and the MS laterally (arrowhead).
Page 8 of 32
Clinical and diagnostic approach to PPS lesions:
•
Since the PPS has relatively few internal structures, primary lesions of the
PPS are rare. The PPS is more commonly displaced or infiltrated by lesions
arising in the surrounding spaces [1-4].
•
Small tumours of the PPS may be incidental findings whereas large tumours
may cause dysphagia, ear fullness, jaw pain and cranial nerve palsy. The
deep location of these tumours may hinder their clinical evaluation and
may lead to delay in diagnosis. Large lesions may cause internal bulging
of the naso/oro-pharyngeal wall or swelling in the parotid region or in the
submandibular region [1, 5].
•
The clinican as well as the radiologist must be aware of the fact that the
cranio-caudal extent of the PPS can make it function as an 'elevator shaft'
through which infection or tumour from adjacent spaces can spread to the
skull base [3].
•
CT and MR are the cornerstones in the initial evaluation of PPS lesions [5].
The main goals of imaging are:
1.
2.
To identify the true space of origin of a PPS lesion.
To identify if a primary PPS lesion lies within the prestyloid or retrostyloid
compartment.
To find imaging clues leading to the likely differential diagnosis.
To charcterise the lesion as aggressive or non-aggressive.
If the lesion appears aggressive, to identify adjacent spaces of spread,
intracranial extension and perineural spread.
To assess for vascularity of lesions, especially, those which may need
biopsy.
To provide a road map for image-guided biopsy, if needed.
3.
4.
5.
6.
7.
Displacement patterns of PPS fat:
The PPS is surrounded by four key spaces (PMS, RPS, MS, PS) of the suprahyoid
neck (SHN). The direction of displacement of the PPS by a mass lesion arising from a
surrounding space can be a key finding in determining its space of origin (Fig. 5) [1-5].
PS mass lesion pushes PPS anteromedially.
MS mass lesion pushes PPS posteromedially.
PMS mass lesion pushes PPS laterally.
Page 9 of 32
Lateral RPS mass pushes PPS anterolaterally.
Retrostyloid PPS mass pushes prestyloid PPS anteriorly.
Fig. 5
References: Bela Purohit
Fig. 5: Graphic representation of the vectors of displacement of the PPS fat (asterisk) by
surrounding space lesions.
Some examples of displacement patterns of PPS fat:
Page 10 of 32
Fig. 6
References: Department of Radiology, University Hospitals, Leuven
Fig. 6: Axial T2W MR image in a patient with pleomorphic adenoma arising from the deep
lobe of the left parotid gland. The black arrow indicates the vector of displacement of the
left-sided PPS fat by the mass. Normal right PPS is shown for comparison (white arrow).
Page 11 of 32
Fig. 7
References: Department of Radiology, University Hospitals, Leuven
Fig. 7: Axial T2W MR image in a patient with sarcoma of the right MS (asterisk). The
black arrow indicates the vector of displacement of the right-sided PPS fat by the mass.
Normal left PPS is shown for comparison (white arrow).
Page 12 of 32
Fig. 8
References: Diagnostic Imaging, National University Hospital, National University
Hospital - SINGAPORE/SG
Fig. 8: Axial contrast-enhanced T1W MR image in a patient with right-sided
nasopharyngeal carcinoma. The black arrow indicates the vector of displacement of the
right-sided PPS fat by the mass. Normal left PPS is shown for comparison (white arrow).
Page 13 of 32
Fig. 9
References: Diagnostic Imaging, National University Hospital, National University
Hospital - SINGAPORE/SG
Fig. 9: Axial T2 fat-saturated MR image in a patient with right-sided oropharyngeal
carcinoma (asterisk) and an enlarged metastatic right retropharyngeal node. The
black arrow indicates the vector of displacement of the right-sided PPS fat by the
retropharyngeal node. Normal left PPS is shown for comparison (white arrow).
Page 14 of 32
Fig. 10
References: Department of Radiology, University Hospitals, Leuven
Fig. 10: Axial T2W MR image in a patient with a right vagal schwannoma. The black arrow
indicates the vector of displacement of the right-sided prestyloid PPS fat by the mass.
The retrostyloid mass displaces the right styloid process anteriorly (arrowhead). The ICA
is displaced anterolaterally. Normal left PPS is shown for comparison (white arrow).
Imaging findings in PPS lesions:
Page 15 of 32
We retrospectively reviewed PPS lesions detected on CT and MR at the University
Hospitals, Leuven from 2002-2013. This pictorial review illustrates common examples of
pathologies arising in the PPS.
Prestyloid PPS lesions:
1. Salivary gland tumours:
•
These constitute the majority of neoplasms in the prestyloid PPS. These
tumours arise from ectopic salivary gland rests in the prestyloid PPS.
However, more commonly they extend from the deep lobe of the parotid and
extend exophytically into the PPS (Fig. 6).
•
Pleomorphic adenoma/benign mixed tumour is the commonest salivary
gland tumour. Malignant tumour like mucoepidermoid carcinoma and
adenoid cystic carcinoma are uncommon.
•
A tumour is considered to be primary to the PPS if it is completely
surrounded by PPS fat. A deep parotid lobe tumour often appears dumbellshaped. It is connected to the deep lobe of the parotid gland and is seen to
widen the stylomandibular tunnel. It pushes the PPS fat anteromedially (Fig.
6).
•
Pleomorphic adenoma usually appears as a well-circumscribed,
heterogeneous mass on CT. On MRI, it appears hypointense on T1W
images with marked hyperintensity on T2W MR images. Heterogeneous
post contrast enhancement may be seen (Fig. 6, 11, 12, 13). Haemorrhage,
necrosis and calcifications may occur (Fig. 11).
•
Complete surgical removal is the treatment of choice for a pleomorphic
adenoma in view of high tendency for recurrence and the less common but
likely possibility of malignant conversion (malignant mixed tumour) [1-4, 6].
Page 16 of 32
Fig. 11
References: Department of Radiology, University Hospitals, Leuven
Fig. 11: Axial T1W MR image in a patient with a pleomorphic adenoma arising from minor
salivary glands in the right PPS. The tumour (asterisk) is isointense to muscle with a
hyperintense focus within, likely due to haemorrhage (arrow).
Page 17 of 32
Fig. 12
References: Department of Radiology, University Hospitals, Leuven
Fig. 12: Axial T2W MR image in the same patient as in Fig. 11. The tumour (asterisk)
shows heterogeneously hyperintense signal. There is no connection with the deep lobe
of the right parotid gland (arrowhead).
Page 18 of 32
Fig. 13
References: Department of Radiology, University Hospitals, Leuven
Fig. 13: Axial contrast-enhanced T1W MR image in the same patient as in Fig. 11, 12.
The tumour (asterisk) shows heterogeneous post-contrast enhancement. There is a well
maintained fat-plane separating the tumour from the deep lobe of the right parotid gland
(arrows).
2. Lipomas:
Page 19 of 32
•
Primary lipomas of the PPS are rare and liposarcomas rarer still. Some of
lipomas may arise from the parotid space and extend into the PPS.
•
Lipomas usually appear as well-circumscribed lesions within the prestyloid
PPS. Large lipomas may cause internal bulging of the lateral pharyngeal
wall. They show fat attenuation on CT and hyperintense signal on TIW
MR images (Fig. 14). Focal enhancement within a lipoma is suspicious for
malignancy ie liposarcoma [1-3, 7].
Fig. 14
References: Department of Radiology, University Hospitals, Leuven
Page 20 of 32
Fig. 14: Axial T1W MR image in a patient with a lipoblastoma in the left PPS. The
lipoblastoma (asterisk) appears well-circumscribed and causes inward bulging of the left
oropharyngeal wall. It appears typically uniformly hyperintense.
3. Branchial cleft cyst:
•
The prestyloid PPS is a likely but rare location for an atypical 2nd branchial
cleft cyst (BCC).
•
An uncomplicated BCC usually appears well circumscribed with thin walls
and fluid contents on CT and MRI (Fig. 15). Thickened irregular walls may
be due to infection.
•
In the absence of infection, an atypical cystic lesion in the PPS may be
suspicious for metastatic necrotic adenopathy from papillary carcinoma of
the thyroid or squamous cell carcinoma of the pharyngeal mucosal space [1,
3, 4, 8].
Page 21 of 32
Fig. 15
References: Department of Radiology, University Hospitals, Leuven/ Reference 1.
Fig. 15: Axial T2W MR image in a patient with an atypical 2nd BCC in the left PPS.
The uncomplicated BCC (asterisk) is well circumscribed with thin walls and shows fluid
intensity. It causes mild inward displacement of the left lateral oropharyngeal wall.
Retrostyloid PPS lesions:
1. Neurogenic tumours:
Page 22 of 32
•
Neurogenic tumours (schwannomas and neurofibromas) are the most
common primary tumours of the retrostyloid PPS. Retrostyloid tumours
tyically displace the styloid process anteriorly, ICA anteromedially and the
prestyloid PPS anteriorly.
•
Schwannomas commonly arise from the vagus nerve. CT shows a
hypodense fusiform, well-circumscribed mass with mild homogeneous
enhancement (Fig. 16). Schwannomas often appear hyperintense on T2W
MR images (Fig. 10). Heterogeneous signal on T1W and T2W images may
be due to cystic degeneration or intralesional haemorrhage. Schwannomas
are relatively hypovascular lesions (which often helps to differentiate them
from paragangliomas). Malignant transformation is rare [1-3, 5, 9].
•
The imaging features of a solitary neurofibroma overlap those of a
schwannoma. Neurofibromas tend to be more hypodense on CT and less
enhanced as compared to schwannomas on post contrast CT and MR
images. Neurofibromas may show the classical 'target-sign' on axial T2W
images [5, 9].
Page 23 of 32
Fig. 16
References: Department of Radiology, University Hospitals, Leuven
Fig. 16: Axial contrast-enhanced CT in a patient with left vagal schwannoma.
The schwannoma (asterisk) appears homogeneously hypodense with poor contrast
enhancement. The left prestyloid PPS is displaced anteriorly (black arrow), the left ICA is
displaced anteromedially (arrowhead) and the left IJV (white arrow) is compressed and
displaced posterolaterally.
Page 24 of 32
2. Paragangliomas:
•
Paragangliomas/ glomus tumours in the head and neck arise from
chemoreceptor cells located at three sites; at the nodose ganglion of the
vagus at the skull base (vagal paraganglioma), at the carotid bifurcation
(carotid paraganglioma/ carotid body tumour) and at the level of the jugular
foramen (jugular paraganglioma).
•
Glomus tumours are typically hypervascular and show marked post contrast
enhancement on both CT and MRI (Fig. 17). A 'salt-pepper' appearance
is common on T1W MR images due to focal areas of haemorrhage and
tortuous vessels causing flow voids. Carotid body tumours typically splay the
internal and external carotid artery (ECA) away from each other (Fig. 18).
•
Conventional angiography helps to confirm the diagnosis and may be
required for pre-operative embolisation [1-3, 5, 10].
Page 25 of 32
Fig. 17
References: Department of Radiology, University Hospitals, Leuven
Fig. 17: Axial contrast-enhanced CT image in a patient with left carotid body tumour. The
well-circumscribed mass (asterisk) is located at the left carotid bifurcation and marked,
homogeneous post contrast enhancement.
Page 26 of 32
Fig. 18
References: Department of Radiology, University Hospitals, Leuven
Fig. 18: Coronal contrast-enhanced CT image in the same patient as in Fig. 17. The left
carotid body tumour (asterisk) causes typical splaying of the left ICA (black arrow) and
the left ECA (white arrow).
Page 27 of 32
Secondary lesions of the PPS:
•
The PPS is often secondarily involved by tumour and infection arising in the
PMS, MS and PS. Agressive masticator space neoplasms like sarcomas
and PMS neoplasms like squamous cell carcinomas and lymphomas readily
infiltrate the PPS [1-4].
•
Secondary involvement of the PPS is often seen in benign
muticompartmental lesions like lymphangioma and hemangioma [1, 3].
•
Parapharyngeal abscess is rare in the modern era of effective antibiotics.
The most common scenario is post acute tonsillitis with extension of a
peritonsillar abscess through the buccopharyngeal fascia into the PPS (Fig.
19). The exact extension of the abscess into the adjacent spaces must be
described so as to guide effective drainage [1, 3].
Page 28 of 32
Fig. 19
References: Department of Radiology, University Hospitals, Leuven
Fig. 19: Axial contrast-enhanced CT in a child with acute left tonsillitis leading to left
peri-tonsillar abscess. A rim-enhancing centrally hypodense/liquefied abscess is seen
extending into the left PPS (black arrow). Normal right PPS is shown for comparison
(white arrow).
Page 29 of 32
Conclusion
Take home points:
1. A clear understanding of the spatial anatomy of the PPS is crucial for an accurate
diagnostic approach towards pathology arising in this region.
2. Lesions arising from adjacent spaces displace the PPS fat in a particular way; in
combination with the imaging characteristics of the mass, this information helps to narrow
down the list of differential diagnosis.
3. When tumour or infection is detected in the PPS, the entire PPS from the skull base
to the hyoid is imaged to ascertain the point of origin.
4. The commonest lesions in the prestyloid PPS are deep parotid lobe tumours,
nd
ectopic salivary gland tumours, lipomas and atypical 2 BCCs. Neurogenic tumours and
paragangliomas are the commonest lesions of the retrostyloid PPS.
Personal information
Dr. Bela Purohit, MD, DNB, FRCR.
Singapore.
E-mail: [email protected]
Prof. Dr. Robert Hermans, MD, Ph.D.
Leuven, Belgium.
Email: [email protected]
Dr. Katya Op de beeck, MD.
Leuven, Belgium.
E-mail: [email protected]
Page 30 of 32
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