In a supranuclear CN XII palsy, the tongue deviates toward which side?

In a supranuclear CN XII palsy, the tongue deviates toward which side?

• A. Toward the lesion
• B. Away from the lesion
• C. No deviation
• D. Variable deviation

Answer: (B)

In supranuclear CN XII palsy the tongue shows weakness on the side opposite the brain lesion because the corticobulbar input to the hypoglossal nucleus is largely contralateral. With a unilateral upper motor neuron lesion, the muscles on the opposite side lose input and become weak. When the tongue is protruded, the intact, stronger side pushes the tongue toward the weaker side, so the tongue deviates away from the lesion. This differs from a lower motor neuron CN XII palsy, where the deviation would be toward the side of the lesion.

In supranuclear CN XII palsy the tongue shows weakness on the side opposite the brain lesion because the corticobulbar input to the hypoglossal nucleus is largely contralateral. With a unilateral upper motor neuron lesion, the muscles on the opposite side lose input and become weak. When the tongue is protruded, the intact, stronger side pushes the tongue toward the weaker side, so the tongue deviates away from the lesion. This differs from a lower motor neuron CN XII palsy, where the deviation would be toward the side of the lesion.