Here you can see two images from a patient suffering from left hypoglossal (12th cranial) nerve palsy. In the first image, the tongue deviates to the left side (the side on which 12th cranial nerve is affected) on protrusion. In the second image, when the tongue is kept within the oral cavity, mild wasting on the left side of tongue is noted.
Deviation to the affected side is also noted in trigeminal nerve (5th cranial nerve) motor palsy. On the other hand, in cases of facial (7th cranial) and vagus (10th cranial) nerves, the deviation occurs to the normal (unaffected side).
In case of 12th cranial nerve palsy, one should look for long tract signs such as pyramidal tract signs, which could indicate an intrinsic brain stem lesion. In pure cranial nerve palsy, the causes are extra-axial. Common causes include diabetic cranial neuropathy, infections, tumors, etc.