7th nerve palsy short case

The facial nerve, CN VII, is the seventh paired cranial nerve. In this article, we shall look at the anatomical course of the nerve, and the motor, sensory and parasympathetic functions of its terminal branches.

The facial nerve is associated with the derivatives of the second pharyngeal arch.

Motor: Innervates the muscles of facial expression, the posterior belly of the digastric, the stylohyoid and the stapedius muscles.

Sensory: A small area around the concha of the auricle.

Special Sensory: Provides special taste sensation to the anterior 2/3 of the tongue.

Parasympathetic: Supplies many of the glands of the head and neck, including:

Submandibular and sublingual salivary glands.
Nasal, palatine and pharyngeal mucous glands.
Lacrimal glands.

Anatomical Course

The course of the facial nerve is very complex. There are many branches, which transmit a combination of sensory, motor and parasympathetic fibres.

Anatomically, the course of the facial nerve can be divided into two parts:

Intracranial – the course of the nerve through the cranial cavity, and the cranium itself.

Extracranial – the course of the nerve outside the cranium, through the face and neck.

Intracranial

The nerve arises in the pons, an area of the brainstem. It begins as two roots; a large motor root, and a small sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate nerve).

The two roots travel through the internal acoustic meatus, a 1cm long opening in the petrous part of the temporal bone. Here, they are in very close proximity to the inner ear.

Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal. The canal is a ‘Z’ shaped structure. Within the facial canal, three important events occur:

Firstly the two roots fuse to form the facial nerve.

Next, the nerve forms the geniculate ganglion (a ganglion is a collection of nerve cell bodies).

Lastly, the nerve gives rise to:

Greater petrosal nerve – parasympathetic fibres to mucous glands and lacrimal gland.
Nerve to stapedius – motor fibres to stapedius muscle of the middle ear.
Chorda tympani – special sensory fibres to the anterior 2/3 tongue and parasympathetic fibres to the submandibular and sublingual glands.

The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen. This is an exit located just posterior to the styloid process of the temporal bone.

Extracranial

After exiting the skull, the facial nerve turns superiorly to run just anterior to the outer ear.

The first extracranial branch to arise is the posterior auricular nerve. It provides motor innervation to the some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle.

The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and inferiorly into the parotid gland (note – the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve).

Within the parotid gland, the nerve terminates by splitting into five branches:

Temporal branch

Zygomatic branch

Buccal branch

Marginal mandibular branch

Cervical branch

These branches are responsible for innervating the muscles of facial expression.

Upper Motor vs Lower Motor

Symptoms according to the level of injury to cranial nerve VII

Clinical Relevance: Damage to the Facial Nerve

The facial nerve has a wide range of functions. Thus, damage to the nerve can produce a varied set of symptoms, depending on the site of the lesion.

Intracranial Lesions

Intracranial lesions occur during the intracranial course of the facial nerve (proximal to the stylomastoid foramen).

Fig 1.3 – Right sided weakness of the muscles of facial expression, due to facial nerve paralysis.

The muscles of facial expression will be paralysed or severely weakened. The other symptoms produced depend on the location of the lesion, and the branches that are affected:

Chorda tympani – reduced salivation and loss of taste on the ipsilateral 2/3 of the tongue.

Nerve to stapedius – ipsilateral hyperacusis (hypersensitive to sound).

Greater petrosal nerve – ipsilateral reduced lacrimal fluid production.

The most common cause of an intracranial lesion of the facial nerve is infection related to the external or middle ear. If no definitive cause can be found, the disease is termed Bell’s palsy.

Extracranial Lesions

Extracranial lesions occur during the extracranial course of the facial nerve (distal to the stylomastoid foramen). Only the motor function of the facial nerve is affected, therefore resulting in paralysis or severe weakness of the muscles of facial expression.

There are various causes of extracranial lesions of the facial nerve: