Laboratory Studies
Note the following:
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Twenty-four–hour urine collection for catecholamines: Although functioning tumors are rare, if a secreting paraganglioma is suspected, obtain a 24-hour urine collection for catecholamines to screen for a secreting lesion. [4, 19, 23] If catecholamine level results are positive, perform a metaiodobenzylguanidine (MIBG) scan (see MIBG scanning). [16]
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Vanillylmandelic acid (VMA), or 4-hydroxy-3-methoxymandelic acid [19]
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Metanephrine [19]
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Consider routine preoperative blood work such as a complete blood count (CBC) and basic metabolic panel
Imaging Studies
Radiologic studies are essential in the evaluation of a patient with a suspected parapharyngeal space (PPS) mass. [4, 7, 8, 9, 10] Performing these studies before considering biopsy is important because, given the differential diagnosis of a PPS lesion, one can often make a diagnosis on the basis of imaging studies without the need for fine-needle aspiration biopsy or open biopsy.
Imaging studies should answer the following questions:
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Is the mass prestyloid or poststyloid?
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What is the relationship to the parotid gland?
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What is the relationship to the great vessels?
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What are the soft tissue characteristics of the tumor?
The differential diagnosis of a PPS mass can be greatly narrowed by determining whether the mass arises from the deep lobe of the parotid and whether the mass originates in the prestyloid or poststyloid space. [11] The parapharyngeal fat pad is located in the prestyloid space. Prestyloid lesions cause medial displacement of the parapharyngeal fat pad and are located anterior to the great vessels. [13, 10] Poststyloid lesions displace the parapharyngeal fat pad anteriorly and laterally, between the mass and the pterygoid muscles. [13, 10]
Computed tomography (CT) scanning and magnetic resonance imaging (MRI) have equal efficacy in localizing the lesion to the prestyloid or poststyloid space. Both studies have advantages and disadvantages when compared to each other. [4, 11] CT scanning or MRI is used as a starting point in the evaluation of a PPS mass, and, sometimes, only one test may be needed. Angiography is reserved for enhancing lesions. [10] Ultrasonography does not provide adequate resolution of the PPS and is not indicated in the workup of PPS lesions. [28]
CT scanning
CT scanning can localize a PPS mass to the prestyloid or poststyloid space. It may also demonstrate whether the mass arises from the deep lobe of the parotid; a fat plane between the parotid and the mass suggests an extraparotid origin. [10]
CT scanning is superior to MRI in demonstrating the presence of calcifications and bony involvement. [9]
With contrast infusion, the relationship of the mass to the great vessels may be appreciated, although enhanced poststyloid lesions may be difficult to separate from the carotid artery. [10]
CT scanning is inferior to MRI in delineating soft tissue characteristics of the tumor, particularly in large lesions in which the tumor margin may be difficult to distinguish from the fascia of adjacent musculature. [11] CT scanning also carries the risk of exposure to ionizing radiation and intravenous contrast. [4] Nevertheless, CT scanning remains widely used because of its wide availability and lower cost. CT scanning may be the only study required because most PPS masses are prestyloid lesions arising from the parotid gland. Because of its superior soft tissue and vascular resolution, a subsequent MRI is recommended to further evaluate all poststyloid masses or, if malignancy is suspected, to evaluate the extent of malignancy and to rule out intracranial extension. [11]
Magnetic resonance imaging
MRI is superior to CT scanning in its ability to ascertain the soft tissue characteristics of parapharyngeal space (PPS) tumors. [4, 11] MRI can be used to differentiate between tumor and muscle, and it has greater resolution in defining the great vessels and their relationship to tumor. [4, 11] Intracranial extension is better delineated on MRI.
The diagnosis can often be made on the basis of characteristic MRI findings. Paragangliomas have been described as having a "salt-and-pepper" appearance on MRI because of numerous flow voids within the lesion. [7] Schwannomas show greater enhancement on T2-weighted images, enhance with gadolinium, and lack flow voids. [12]
MRI scanning is probably the screening test of choice to evaluate a PPS lesion because of the greater soft tissue resolution and vascular information obtained. [23] However, its cost is significantly greater, the ability to determine bone involvement is poor, and it may be contraindicated in certain patients (eg, those with pacemakers, those with certain hardware, those who are claustrophobic).
The information obtained from both CT scanning and MRI is complementary, and both studies should be performed in the evaluation of extensive lesions or when malignancy is suspected.
Angiography
Angiography is recommended in the workup of most vascular lesions. [4] Angiography is also used if malignancy is suspected and if carotid sacrifice is possible during resection. [23] If carotid resection is considered, angiography is combined with balloon occlusion testing to measure cerebral blood flow (see Balloon occlusion test in Other Tests).
Angiography is useful in the evaluation of poststyloid lesions to demonstrate their relationship to the great vessels and to distinguish between neurogenic and vascular lesions; however, this distinction can usually be made with MRI. [12]
Findings on angiography may be diagnostic of neurogenic lesions. Carotid body tumors are usually located at the bifurcation and cause splaying of the carotid bifurcation, called the lyre sign. [12]
Glomus vagale tumors, because of their association with the vagus nerve, lie laterally and posteriorly to the carotid system, displacing this anteriorly and medially. [16]
Schwannomas appear as avascular masses displacing the carotid system anteriorly. [12] Metastases (eg, renal cell carcinoma) often appear quite vascular if they take their blood supply from the external carotid. Most vascular lesions should be evaluated with angiography to delineate their extent and blood supply; feeding vessels may be identified and may be embolized preoperatively to facilitate resection.
CT angiography and magnetic resonance angiography are emerging as alternatives to conventional arteriography. These tests are becoming more widely used, but they may relegate conventional arteriography to those instances in which the blood supply remains in question, balloon occlusion is required, or embolization is being considered.
Other Tests
Balloon occlusion test
The balloon occlusion test measures the effect of internal carotid artery occlusion on cerebral blood flow (CBF) and the adequacy of the contralateral circulation. It is indicated when findings on imaging studies suggest carotid involvement or when resection of the lesion carries a high risk of intraoperative carotid artery injury. [4, 12, 6]
Angiography is performed, and the internal carotid artery is occluded for 10-15 minutes using a balloon-tipped catheter. In patients who develop neurologic symptoms, the test is abandoned, and no further evaluation is performed. These patients are considered at high risk for stroke and should undergo nonoperative therapy, subtotal resection with carotid preservation, or revascularization prior to resection. In some settings, blood flow can be quantitated using substances such as xenon. [4]
In patients without neurologic symptoms during balloon occlusion, xenon-enhanced CT scanning is performed to quantitate CBF. CBF is evaluated both before and after balloon occlusion; xenon gas is inhaled, it diffuses rapidly into the bloodstream, and it causes perfused areas of the brain to become radiopaque. Patients with diminished perfusion on xenon-enhanced CT scanning (approximately 25% of patients) are considered at mild-to-moderate risk of stroke with prolonged carotid occlusion. Interposition grafting of the internal carotid artery is recommended if the artery is to be sacrificed.
A 4% risk of neurologic sequelae exists from the balloon occlusion test itself, so the procedure is reserved only for cases in which the carotid is suspected to be involved or at risk.
Nuclear scanning
If screening for a functional paraganglioma by urinary VMA and metanephrine levels is positive, obtain a metaiodobenzylguanidine (MIBG) scan. The radioisotope MIBG has a molecular structure similar to that of norepinephrine and is used to trace catecholamine uptake and storage. Alternatively, an octreotide scan can be used to identify paragangliomas and metastasis, with the tracer binding to somatostatin receptors two and five (SSTR2, SSTR5). [29]
Metastatic workup
If a metastatic lesion is suspected, the primary tumor should be sought by performing a full clinical evaluation, panendoscopy, and a full metastatic workup as directed by the clinical examination findings.
Diagnostic Procedures
Biopsy
Under most circumstances, a presumptive diagnosis can be made on the basis of the findings of the imaging studies described above. Do not consider a biopsy of a parapharyngeal space (PPS) prior to obtaining results from the radiologic studies, to ensure that the mass is not a vascular lesion. Complete surgical excision is the mainstay of treatment and is recommended for both diagnostic and therapeutic purposes.
Fine-needle aspiration biopsy (FNAB) may be a useful adjunct when the mass is readily accessible, either transcervically or transorally, and may provide useful information if a diagnosis of malignancy is suspected. However, if imaging studies suggest a vascular lesion, FNAB provides little if any useable information and is not indicated.
Incisional biopsy should be considered only if the patient is not an operative candidate and FNAB findings are inconclusive and if a diagnosis of malignancy or lymphoma is strongly suspected.
Transoral open biopsy has been described but carries a significant risk of hemorrhage and of contamination of the pharyngeal mucosa by tumor, which requires excision of that site during subsequent definitive resection. Most surgeons do not support this practice.
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Parapharyngeal space tumor manifesting as an oropharyngeal mass.
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Parapharyngeal space tumor manifesting as a neck mass.
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CT scan of prestyloid space mass. The lesion causes medial displacement of the parapharyngeal fat pad and represents a benign salivary gland tumor.
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T2-weighted MRI of a prestyloid space mass. This mass is confluent with the deep lobe of the parotid gland; the parapharyngeal fat pad is displaced medially. These findings are consistent with an origin from the deep lobe of the parotid.
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CT scan of a poststyloid space mass. Note the anterior and lateral displacement of the parapharyngeal fat, characteristic of poststyloid lesions. This lesion enhances and proved to represent a carotid body tumor.
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MRI of a poststyloid mass. Note anterior displacement of the parapharyngeal fat and multiple flow voids within the tumor itself, consistent with a diagnosis of paraganglioma.
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Angiogram of carotid body tumor. These are found between the carotid bifurcation and cause splaying of the internal carotid artery and external carotid artery resulting in the lyre sign.
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Angiogram of glomus vagale tumor. Because the vagus nerve lies laterally and posteriorly to the carotid system, these appear as vascular lesions displacing the internal carotid artery and external carotid artery anteriorly and medially.
