Where in the brain or head could a tumor be located if it causes facial asymmetry with smiling?

Tumor Locations Causing Facial Asymmetry with Smiling

A tumor causing facial asymmetry with smiling is most commonly located along the facial nerve (CN VII) pathway, including the brainstem (pons), cerebellopontine angle, internal auditory canal, temporal bone, or parotid gland. 1

Primary Anatomical Locations

Brainstem (Pons)

• The facial nerve nucleus resides in the pons, and tumors at this level produce ipsilateral facial weakness affecting all facial muscles including the forehead. 2
• Brainstem gliomas can compress or infiltrate the intrapontine portion of the facial nerve, causing facial asymmetry that becomes apparent with voluntary movements like smiling 3
• Look for the characteristic "crossing pattern": ipsilateral facial paralysis with contralateral limb weakness, which confirms brainstem localization 2
• Brainstem tumors typically cause gradual or fluctuating facial weakness rather than acute paralysis, often accompanied by diplopia, ataxia, or contralateral weakness 2

Cerebellopontine Angle (CPA) and Internal Auditory Canal (IAC)

• The CPA and IAC are the second most common locations, with 75-90% of tumors being vestibular schwannomas and 3-5% being meningiomas. 1
• Facial nerve schwannomas can present as masses mimicking acoustic neuromas in this region 4
• Less common entities include epidermoid lesions, lipomas, facial neuromas, metastatic tumors, lymphomas, and rarely angioleiomyomas 1
• These tumors may present with facial spasms, hemifacial spasm, or facial tic in addition to weakness 4

Temporal Bone

• Tumors within the temporal bone can affect the facial nerve as it courses through the fallopian canal 1
• Facial nerve schwannomas occur in any segment from the brainstem to the neuromuscular junction, with temporal bone involvement being common 4, 5
• Aggressive basal cell carcinomas in the periauricular region can invade the temporal bone and directly involve the facial nerve 6

Parotid Gland

• Intraparotid tumors, including facial nerve schwannomas, can cause facial asymmetry 5
• Malignant tumors (squamous cell carcinoma, adenoid cystic carcinoma, melanoma, basal cell carcinoma) can demonstrate perineural spread along the facial nerve 1
• Aggressive basal cell carcinomas can invade the parotid gland with concomitant facial nerve involvement 6

Skull Base Locations

• Cavernous sinus tumors (most commonly angioleiomyomas in this location) can cause facial sensation deficits and diplopia, though facial motor weakness is less typical. 1
• Sellar/suprasellar lesions typically present with visual deficits and endocrinopathies rather than facial asymmetry 1

Perineural Tumor Spread

• The trigeminal (CN V) and facial (CN VII) nerves are the most commonly affected by perineural tumor spread from head and neck malignancies. 1
• Squamous cell carcinoma (cutaneous and mucosal), adenoid cystic carcinoma, melanoma, lymphoma, and basal cell carcinoma most commonly demonstrate perineural spread 1
• Subtle imaging clues include nerve enhancement, nerve enlargement, foraminal expansion, or muscle volume loss 1

Diagnostic Approach

• Contrast-enhanced MRI of the brain, face, and neck tailored to the temporal bone is the primary imaging modality for evaluating facial nerve pathology. 1
• MRI with diffusion-weighted imaging (DWI) is mandatory for brainstem processes, with thin-section coronal DWI detecting 25% more brainstem lesions than standard axial sequences 2
• CT with high-resolution bone algorithm windows is complementary for characterizing osseous changes of the skull base and neural foramina 1
• FDG-PET/CT may be useful for detecting perineural tumor spread and determining response to therapy 1

Critical Clinical Distinction

• Peripheral facial nerve paralysis from brainstem or facial nerve tumors always involves the forehead muscles ipsilaterally, whereas supranuclear (cortical) lesions spare the forehead due to bilateral cortical innervation of upper facial muscles. 2
• This distinction is essential for localizing the lesion and guiding appropriate imaging 2