What is the level of the lesion in a patient with a pontine infarct presenting with left upper motor neuron (UMN) 7th cranial nerve palsy and left hemiparesis?

Anatomical Localization of Pontine Infarct with Left UMN 7th Nerve Palsy and Left Hemiparesis

Direct Answer

The lesion is located in the ventromedial region of the LEFT lower pons, specifically affecting the basis pontis after the corticofacial and corticospinal fibers have already crossed the midline. 1

Anatomical Reasoning

This clinical presentation represents an ipsilateral pattern where both the facial weakness and hemiparesis occur on the same side (left), which is unusual and requires specific anatomical explanation:

Why Both Deficits Are Ipsilateral (Left-Sided)

• The corticofacial fibers that control the lower face cross the midline ABOVE the level of the facial nucleus in the pons 1
• After crossing, these fibers descend through the contralateral (left) ventromedial pons before reaching the facial nucleus 1
• A left ventromedial pontine lesion therefore affects the already-crossed corticofacial fibers, producing left-sided UMN facial palsy 1
• The same lesion simultaneously affects the corticospinal tract (which has not yet decussated at the pontine level), producing left-sided hemiparesis 1

Specific Anatomical Location

• The lesion must be in the VENTROMEDIAL portion of the lower pons (basis pontis) 1
• Paramedian pontine infarcts in this location characteristically produce faciobrachial dominant hemiparesis with dysarthria 2
• The ventromedial location explains why both motor deficits appear ipsilateral to the lesion 1

Clinical Syndrome Classification

This presentation fits the ventromedial pontine syndrome:

• Ventral pontine infarcts produce motor involvement ranging from mild to severe hemiparesis, often with bilateral ataxia and dysarthria 3
• Three-fourths of patients with ventral pontine infarcts show clinical evidence of usually mild tegmental dysfunction 3
• Basal pontine infarcts commonly present with dysarthria and hemiparesis with upper extremity predominance 2

Important Caveat About "Classic" Alternating Syndromes

This case does NOT represent a classic alternating pontine syndrome (like Millard-Gubler), despite having cranial nerve and motor involvement:

• Classic alternating syndromes show CONTRALATERAL hemiparesis with IPSILATERAL cranial nerve palsies 4
• Millard-Gubler syndrome specifically involves the ventral caudal pons and produces ipsilateral 6th and 7th nerve palsies with contralateral hemiparesis 5
• Only 4 out of 36 patients with isolated pontine infarcts had true alternating deficits, and these never corresponded to classic pontine syndromes 3

Vascular Territory

• This pattern is most consistent with basilar artery branch disease affecting the paramedian pontine perforators 2, 3
• Basilar artery branch disease accounts for 44% of isolated pontine infarcts and is particularly associated with large ventral infarcts 3
• Paramedian pontine infarcts are usually due to thrombosis of perforating arteries 2

Prognostic Implications

• Upper pontine lesions have significantly better outcomes than lower pontine lesions 2
• Recovery is good in two-thirds of patients with isolated pontine infarcts 3
• The favorable outcome relates to the level of the pontine lesion and its effect on the corticospinal tract 2