Brainstem (Pons) Infarction
The presentation of left facial weakness and numbness with right-sided body numbness localizes to the left pons, representing a crossed brainstem syndrome (likely Millard-Gubler syndrome or variant). 1, 2
Anatomical Localization
The key distinguishing feature is the "crossed" pattern of deficits:
- Ipsilateral facial involvement (left side) indicates disruption of the facial nerve nucleus or fascicles in the pons, which are located in the brainstem before the facial nerve crosses 1, 2
- Contralateral body numbness (right side) results from involvement of the medial lemniscus or spinothalamic tract, which have already crossed at lower levels 2, 3
This crossed pattern (ipsilateral cranial nerve deficit + contralateral body deficit) is pathognomonic for a brainstem lesion and cannot occur with cortical or internal capsule strokes 1.
Why Not Other Locations?
Cortical or hemispheric strokes are excluded because:
- A left cortical or internal capsule stroke would produce contralateral deficits affecting both face and body on the same (right) side, not a crossed pattern 1, 4
- Cortical lesions produce purely contralateral deficits 1
- The forehead would typically be spared in supranuclear (cortical) facial palsy due to bilateral cortical innervation of upper facial muscles 4
Cerebellar infarcts are excluded because:
- Cerebellar strokes cause ipsilateral ataxia and coordination problems, not contralateral sensory deficits or facial weakness 4
- Pyramidal signs only occur with cerebellar infarcts when there is brainstem compression, which would present with altered consciousness and cranial nerve palsies 4
Specific Pontine Syndrome
This presentation is consistent with Millard-Gubler syndrome or a variant:
- Classic Millard-Gubler involves the ventral caudal pons, causing ipsilateral facial nerve palsy, ipsilateral abducens palsy, and contralateral hemiparesis 2, 3
- Your patient's presentation with sensory rather than motor findings on the contralateral side represents a variant involving sensory pathways 2
- Isolated pontine infarction accounts for 7% of all ischemic strokes and can present with subtle contralateral limb symptoms 3, 2
Diagnostic Confirmation
Urgent MRI with diffusion-weighted imaging (DWI) is essential:
- MRI is superior to CT for detecting acute pontine infarcts, which can be as small as 4mm and easily missed 1, 5
- The lesion will typically involve the basis pontis and possibly tegmentum 3
- Initial imaging may be read as negative, requiring careful re-evaluation with attention to the brainstem 5
Critical Pitfall to Avoid
Do not misdiagnose this as Bell's palsy:
- Isolated facial palsy from pontine infarct can mimic Bell's palsy, but the presence of contralateral body symptoms immediately distinguishes this as a brainstem stroke 5
- Bell's palsy would not cause contralateral body numbness 5
- The acute presentation with associated neurological symptoms (right-sided numbness) mandates urgent stroke evaluation 6, 5