On which side does uncrossed hemispheric deviation of the mouth occur in relation to the affected brain hemisphere?

Uncrossed Hemispheric Deviation of the Mouth

In uncrossed hemispheric deviation, the mouth deviates to the SAME side as the brain lesion (ipsilateral deviation), which occurs when the lesion affects the corticobulbar fibers BEFORE they decussate in the lower pons.

Understanding the Neuroanatomy

The direction of mouth deviation depends critically on WHERE the lesion interrupts the corticobulbar pathway:

Crossed (Contralateral) Deviation - Most Common Pattern

• Supranuclear lesions above the pons cause the mouth to deviate AWAY from the lesion (contralateral to the affected hemisphere) 1, 2
• This occurs because the corticobulbar fibers have already crossed (decussated) before reaching the facial nucleus 1
• In stroke patients, tongue and mouth deviation to the side of limb weakness (contralateral to lesion) occurs in 29% of cases 2
• The deviation is always to the side of the limb weakness in typical supranuclear lesions 2

Uncrossed (Ipsilateral) Deviation - The Exception

• Lesions in the ventral pons BEFORE the corticobulbar fibers decussate cause ipsilateral deviation 1
• The contralateral corticobulbar fibers pass through the medial part of the ventral pons, while ipsilateral fibers pass through the more lateral aspect 1
• The decussation generally occurs at the pontomedullary junction, though individual variance exists 1
• When both mouth deviation and hemiparesis occur on the SAME side (ipsilateral to each other, contralateral to lesion), this indicates the lesion affected the fibers prior to their decussations 1

Clinical Algorithm for Localization

Step 1: Determine the side of mouth/tongue deviation

• Note whether deviation occurs with protrusion or at rest 2

Step 2: Assess associated motor findings

• Check for hemiparesis and note which side 1, 2
• Examine for facial droop (supranuclear 7th nerve palsy occurs in 100% of cases with tongue deviation) 2
• Test for Babinski sign 1

Step 3: Localize based on deviation pattern

• If deviation is CONTRALATERAL to hemiparesis: Lesion is above the pons (cortical, subcortical, or upper brainstem) 2
• If deviation is IPSILATERAL to hemiparesis (both on same side relative to each other): Lesion is in the ventral pons before decussation 1

Step 4: Assess for associated bulbar symptoms

• Dysphagia occurs in 43% of patients with tongue deviation 2
• Dysarthria occurs in 90% of patients with tongue deviation 2

Important Clinical Pearls

Lesion Characteristics Associated with Deviation

• Most common in non-lacunar stroke subtypes (56% of cases) 2
• Cortical infarctions show deviation in 55% of cases 2
• Large subcortical infarctions show deviation in 45% of cases 2
• Arm weakness is significantly associated with presence of tongue deviation 2

Hemispheric Considerations

• The right hemisphere controls attention to the left side of space, and right hemisphere lesions cause left-sided neglect 3, 4, 5
• Left neglect after right brain damage is more frequent and severe than right neglect after left brain damage 4
• Approximately 95% of right-handed individuals and 70% of left-handed individuals have left hemisphere language dominance 3

Common Pitfalls to Avoid

• Do not assume all mouth deviations are contralateral to the lesion - paramedian pontine lesions can cause ipsilateral deviation 1
• Check for fasciculation and atrophy - their absence suggests supranuclear rather than nuclear or infranuclear pathology 1
• Consider previous stroke history - tongue deviation occurs more frequently in patients with previous stroke on the contralateral side 2
• Remember that 5% of normal controls show tongue deviation - clinical context is essential 2