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Anatomy and clinical signs of lesion of the hypoglossal nerve
1. Big picture
The hypoglossal nerve, cranial nerve XII, is a pure motor nerve supplying the tongue muscles. In the exam, the most important point is not just anatomy, but localization:
A hypoglossal lesion can be:
| Type of lesion | Site | Tongue deviation | Atrophy/fasciculation |
|---|---|---|---|
| Central / supranuclear lesion | Corticobulbar tract above the hypoglossal nucleus | Away from the lesion | Absent |
| Peripheral / lower motor neuron lesion | Hypoglossal nucleus, fascicle, or nerve | Toward the lesion | Present |
The key muscle is the genioglossus, because it protrudes the tongue and has special supranuclear innervation. This is why tongue deviation is so useful clinically.
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