Masticator Space Masses and Pseudomasses

The masticator space is divided into suprazygomatic and nasopharyngeal compartments by the zygomatic arch.

The nasopharyngeal compartment contains the ramus and posterior body of the mandible, the lingual and inferior alveolar branches of V3; the inferior alveolar artery and vein; and the muscles of mastication except the temporalis muscle. The suprazygomatic compartment contains the temporalis muscle.

The masticator space is separated from the parapharyngeal space by a fascial layer along the medial aspect of the medial pterygoid muscle that extends to the medial aspect of the foramen ovale.

Developmental lesions

• Capillary hemangioma: Presents in early infancy and demonstrates rapid growth with involution by adolescence. Look for phleboliths and intense enhancement on CT. Look for areas of signal void from associated high-flow vessels on MRI within proliferating capillary hemangiomas.
• Arteriovenous malformation: Look for areas of signal void on MRI from associated high-flow vessels.
• Lymphatic malformation: Usually fluid signal and attenuation, but may also have heterogeneous signal intensity and rapid enlargement from hemorrhage
• Epidermoid and dermoid cysts: Rare, but when they do occur, they tend to originate from the suprazygomatic compartment. Bony erosion on CT suggests dural involvement.

Inflammatory lesions

• Odontogenic abscess: The most common lesion of the masticator space. Uncontained infections may extend to the floor of the mouth, sublingual and submandibular spaces inferiorly and the suprazygomatic masticator space and skull base superiorly.
• Extension of sinus infection:
• Complication of parotid calculus disease:
• Osteomyelitis:
• Osteoradionecrosis: May mimic osteomyelitis. May be difficult to exclude coexistent tumor recurrence. Association of cortical defects distant from the position of original tumor should support osteoradionecrosis vs tumor recurrence.
• Inflammatory pseudotumor: Idiopathic. The fibrotic type demonstrates low T2-weighted signal and helps differentiate this lesion from malignancy.

Benign neoplasms

• Nerve sheath tumors: Most frequent benign tumors of the masticator space. Related to the V3 as it passes through the masticator space. May be extension of a well-circumscribed fusiform mass through the foramen ovale and adjacent middle cranial fossa. Usually intermediate on T1 and hyperintense on T2, but neurofibromas with a central fibrous core may have increased peripheral signal intensity and decreased central signal intensity on T2-weighted images.
• Meningioma:
• Osteoblastoma:
• Giant cell tumor:

Malignant neoplasms

• Chondrosarcoma: Arises from the temporomandibular joint
• Osteosarcoma:
• Local extension from the upper aerodigestive tract:
• Maxillary ameloblastoma: May extend posterolaterally into masticator space. Usually low grade, causing bony remodelling rather than destruction
• Perineural spread: Along trigeminal nerve. Look for smooth thickening and enhancement of the nerve, concentric expansion of the foramen ovale, obliteration of the Meckel cave and bulging of the cavernous sinus
• Metastatic disease:
• Rhabdomyosarcoma: Rare. Usually seen in childhood.
• Lymphoma:

Pseudolesions

• Masseteric hypertrophy: Enlargement of the masseter muscles. May obtain a history of tooth grinding. Bilateral in 50%. There is preservation of soft tissue planes, identical attenuation and signal to muscle, lack of pathological enhancement, associated pterygoid and temporalis muscle enlargement and hyperostosis at the site of masseteric attachment are useful in differentiating unilateral cases from more worrisome processes (e.g., lymphoma or leukemia).
• Denervation atrophy of the masticator muscles: Acute phase (<1 month): Increase in muscle size with high signal intensity on T2-weighted images and abnormal enhancement. Subacute phase (1–20 months): Abnormal high intensity on T1 and T2-weighted images and abnormal enhancement with moderate volume loss. Chronic phase (>20 months): Extensive fatty infiltration and volume loss without abnormal enhancement.Look for similarity of other muscles innervated by V3.
• Accessory parotid tissue: Lies superficial to the masseter muscle in ~20% of patients. Signal is identical to that of the parotid gland.

References

Connor SE, Davitt SM. Masticator space masses and pseudomasses. Clin Radiol. 2004 Mar;59(3):237-45.