Hypoglossal nerve (CN) XII
The hypoglossal nerve functions includes supplying all intrinsic and extrinsic muscles of the tongue except the palatopharyngeus (vagus). The nerve may be damaged by infiltrating malignancies of the pharynx and surgery in the region of the carotid bifurcation. Injury produces paralysis of the ipsilateral side of the tongue, with wasting and fasciculation; the tongue deviates towards the affected side.
Tongue movements are assessed by rapid pointing and withdrawing the tongue (figure 43a,b), and during speech.
Power is assessed by pushing the tongue into the cheek, and against resistance applied to the outside of the cheek (figure 44a,b). In lower motor neurone paralysis, there is muscle wasting and, on putting out the tongue, it deviates to the side of the lesion (figure 45).
The ninth to twelfth cranial nerves can be damaged collectively in the brain stem, producing a bulbar palsy, severely affecting swallowing. However, these muscles are bilaterally innervated and require bilateral cortical lesions to produce this affect from upper motor neuron damage (pseudobulbar palsy).
Cranial nerve autonomic fibers are carried in 3,7,9,10. Damage affects lacrimation, salivation, swallowing and gut motility. Pupillary reflexes may be lost, as are the normal heart rate changes with: respiration (sinus arrhythmia), hyperventilation (30 breaths per minute for 20 seconds should raise the heart rate by 12 beats per minute), Valsalva manoeuvre (reduction of heart rate), carotid sinus massage (reduction of heart rate), postural change from lying to standing (a rise of 20–30 beats per minute). The latter reflex reduces blood pressure fall, but this may be up to 10 mm of mercury in a normal elderly person.