Where is the lesion likely located in a patient with right‑sided simple‑partial seizures without impaired consciousness and a central right facial (cranial nerve VII) paresis?

Lesion Localization in Right Hemisphere Focal Seizures with Central Right Facial Nerve Paresis

The lesion is most likely located in the left frontal lobe, specifically involving the left primary motor cortex (face area) or the left supplementary motor area, causing contralateral (right-sided) simple partial seizures and contralateral (right-sided) central facial weakness. This presentation reflects a left hemisphere lesion producing right-sided symptoms, not a right hemisphere lesion as the question states.

Critical Anatomical Clarification

There appears to be a fundamental discrepancy in the question: right hemisphere focal seizures would not typically cause right-sided central facial paresis. 1 The facial nerve receives bilateral cortical innervation for the upper face but predominantly contralateral innervation for the lower face. 2, 1 A central (upper motor neuron) facial palsy affecting cranial nerve VII indicates a lesion in the contralateral motor cortex or corticobulbar pathways. 2

Most Likely Scenario: Left Hemisphere Lesion

• Left frontal cortex involvement (specifically the face area of the primary motor cortex or supplementary motor area) would produce both right-sided simple partial seizures and right-sided central facial weakness 3, 4
• The preserved consciousness during seizures indicates a simple partial seizure originating from a focal cortical region without spread to alter awareness 5
• Central facial palsy with forehead involvement distinguishes this from a peripheral (lower motor neuron) lesion 1

Anatomical Localization Algorithm

Step 1: Determine Upper vs Lower Motor Neuron Lesion

• Forehead involvement present = Central (UMN) lesion affecting contralateral motor cortex 1
• Forehead spared = Peripheral (LMN) lesion affecting facial nerve itself 2, 1

Step 2: Localize the Cortical Region

• Simple partial motor seizures originate from the primary motor cortex or adjacent premotor regions 5
• Face representation in the primary motor cortex is located in the lower third of the precentral gyrus 3
• Supplementary motor area involvement can also produce facial weakness with inverse automatic-voluntary dissociation 4

Step 3: Identify Associated Features

• No impairment of consciousness = Seizure focus remains localized without bilateral spread 5
• Isolated facial symptoms without other cranial nerve involvement = Cortical rather than brainstem localization 2
• No aphasia or language disturbance helps distinguish from Broca's area involvement 3

Specific Cortical Regions to Consider

Primary Motor Cortex (Face Area)

• Located in the lower third of the precentral gyrus (Brodmann area 4) 3
• Lesions here produce contralateral facial weakness and can generate focal motor seizures 3
• The face representation is lateral on the motor homunculus 3

Supplementary Motor Area

• Located on the medial surface of the superior frontal gyrus (Brodmann area 6) 4, 6
• Lesions can produce facial palsy with characteristic inverse automatic-voluntary dissociation 4
• Often associated with motor underutilization of the contralateral limbs 4

Premotor Cortex

• Located anterior to the primary motor cortex in the posterior frontal lobe 3
• Connected to the primary motor cortex via cortico-cortical fibers important for motor control 3

Diagnostic Workup Recommendations

Imaging Studies

• MRI brain with and without contrast is the gold standard for identifying structural lesions causing focal seizures 2
• Diffusion-weighted imaging (DWI) can identify acute ischemic lesions in the motor cortex 3
• FDG-PET imaging shows focal hypometabolism in the epileptogenic zone in 55-80% of extratemporal lobe epilepsy cases 2

Electrodiagnostic Studies

• EEG with focal spikes or sharp waves from the left frontal region confirms the epileptogenic focus 5
• Normal routine EEG does not rule out partial seizures; prolonged monitoring may be needed 5

Common Etiologies to Investigate

Structural Lesions

• Low-grade gliomas or other epilepsy-associated brain tumors are common in focal epilepsy 2
• Focal cortical dysplasia (FCD) appears as hypometabolism on PET, often more extensive than MRI findings 2
• Vascular malformations or prior stroke can cause epileptogenic foci 2

Acquired Lesions

• Previous cerebral insult resulting in static epileptogenic focus 5
• Trauma affecting the frontal cortex 2

Critical Pitfalls to Avoid

• Do not assume right hemisphere involvement based solely on right-sided symptoms; central facial palsy indicates contralateral cortical pathology 1
• Do not miss forehead examination to distinguish central from peripheral facial nerve lesions 1
• Do not delay MRI in any patient with new-onset partial seizures, as structural lesions are found in a significant percentage 5
• Do not assume normal routine EEG excludes epilepsy in patients with clinically suspected partial seizures 5
• Do not overlook subtle MRI abnormalities that may be better visualized with FDG-PET imaging 2