The cranial nerves – CN VII

Introduction
The CN VII comprises motor, sensory and parasympathetic fibres [Figure 23]. The motor fibres innervate the muscles responsible for mimicry. The motor nucleus is located in the pons and consists of an area responsible for sending stimuli to the musculature around the mouth. This part receives unilateral crossed fibres from the cerebral cortex. The part of the facial nerve nucleus which is responsible for innervation of the muscles around the eyes is innervated from both hemispheres.
The sensory fibres of the CN VII transmit taste stimuli from the anterior two-thirds of the tongue. The parasympathetic fibres innervate the parotid, mandibular, sublingual and lacrymal glands.

Figure 23

Procedure

• Observe the patient’s face and determine whether it is symmetric. Slight asymmetry at rest is physiological.
• Ask the patient to:
  • raise their eyebrows
  • close their eyes tightly [Figure 24]
  • puff up their cheeks
  • pout their lips
  • show their teeth.
• Note whether the patient is able to carry out these movements symmetrically.
• Observe any spontaneous facial mimicry (for example, the loss of any left / right differences when the patient laughs).
• Ask the patient whether they have experienced any loss of taste. If the patient indicates that this is the case or if you have other reasons to suspect loss of
  CN VII function, you can hold filter papers (e.g. coffee filters) soaked in a salt or sugar solution against the side of the patient’s tongue and have the patient taste them. The patient must taste before putting their tongue back in their mouth. 
• Ask the patient whether he / she is oversensitive to loud noises (hyperacusis). This symptom can also arise with loss of CN VII function and may be due to paralysis of the stapedius muscle.

Figure 24

Interpretation

• A lesion above the brainstem nuclei (= a central lesion) results in loss of function of nerves that feed the muscles around the mouth, on the opposite side to the lesion [Figure 23]. The muscles around the contralateral eye will also be less strong, but will not be completely immobilised (buccal branch paresis).
  A lesion in the facial motor nucleus or the peripheral nerve will result in loss of innervation of the ipsilateral muscles to the forehead, around the eyes and the mouth (buccal, temporal and zygomatic branch paresis, Bell’s palsy).
• If there is CN VIII loss of function as well as loss of CN VII function, you should consider the possibility of a cerebellopontine angle tumour.
• If the lesion is proximal to the branching of the chorda tympani, there will be ipsilateral loss of taste. 
• If the lesion is bilateral, consider polyradiculoneuropathy (Guillain-Barré syndrome), poliomyelitis, myasthenia gravis or myopathies.
• If the mimetic function is (almost) symmetric again when the patient laughs or cries, a lesion higher than the nuclei should be suspected.