29 Facial Nerve Palsy

A facial nerve palsy is a catastrophic event for a patient. It should be remembered that idiopathic palsy (Bell’s palsy) is the commonest type and the majority of these patients have complete facial neve recovery. Early identification of the cause, appropriate treatment to avoid and limit long-term changes, and eye care are the tenets of optimal management.

29.1 Anatomy

The facial nerve is broadly divided into intracranial, intratemporal and extratemporal segments. Anatomy of the intracranial portion of the facial nerve is complex. Briefly, voluntary control is initiated by supranuclear inputs arising from the cerebral cortex projecting to the facial nucleus. Cell bodies of the upper facial motor nerves giving rise to the frontal branch receive bilateral cortical inputs, and neurons to the remainder of the facial nucleus receive contralateral cortical innervation. This explains why supranuclear (central) lesions affecting the facial nerve will not paralyse the forehead on the affected side, resulting in a unilateral facial paralysis with forehead sparing. Spontaneous facial movements are centrally transmitted via the extrapyramidal system, which involves diffuse axonal connections between multiple regions including the basal ganglia, amygdala, hypothalamus and motor cortex. The facial nuclei contain the cell bodies of facial nerve lower motor neurons, which exit the brainstem at the cerebellopontine angle, where it is joined by the nervus intermedius.

Both the facial and vestibulocochlear nerves enter the internal auditory meatus of the temporal bone simultaneously with the facial nerve located superior to the vestibulocochlear nerve. The facial nerve enters the fallopian canal which consists of labyrinthine, tympanic and mastoid segments. The labyrinthine segment is the narrowest segment. The geniculate ganglion resides within the distal part of the labyrinthine segment of the facial nerve. It gives rise to the first branch of the facial nerve—the greater petrosal nerve—which carries visceral motor parasympathetic fibres to the lacrimal gland. Two other branches—external petrosal nerve and lesser petrosal nerve—extend from the geniculate ganglion to provide innervation to the middle meningeal artery (sympathetic) and parotid gland (parasympathetic), respectively. The junction of the labyrinthine and tympanic components of the fallopian canal is formed by an acute angle (genu). The tympanic segment connects with the mastoid segment at the second genu and the facial nerve gives off three branches (stapedius, sensory branch of the facial nerve, chorda tympani) within this region. The chorda tympani is the terminal branch of the nervus intermedius.

The extratemporal component of the facial nerve starts when the facial nerve exits the stylomastoid foramen. It is relatively superficial in children and thus post-auricular incisions must be carefully planned because the trunk of the facial nerve is a subcutaneous structure at this level. However, in adults, the facial nerve is protected laterally by the mastoid tip, tympanic ring and mandibular ramus. After exiting the stylomastoid foramen, the facial nerve gives off motor branches to the posterior belly of digastric, stylohyoid, and the superior auricular, posterior auricular and occipitalis muscles. The facial nerve then travels along a course anterior to the posterior belly of the digastric and lateral to the external carotid artery and styloid process before dividing into its main motor branches (frontal, zygomatic, buccal, marginal mandibular, cervical) at the posterior edge of the parotid gland. The facial nerve trunk is usually identified approximately 1 cm deep and just inferior and medial to the tragal pointer.

The frontal branch is usually situated within the temporoparietal fascia. Therefore, injury to this nerve can be avoided if a subcutaneous plane superficial to the temporoparietal fascia or a deep plane on the surface of the deep temporoparietal fascia or between the superficial and deep layers of the deep temporal fascia was maintained during facelift procedure or parotidectomy. The marginal mandibular branch of the facial nerve, which innervates the depressor anguli oris musculature, is posterior to the facial artery and usually located above the inferior border of the mandible. The buccal branches become superficial as they emerge from the anterior border of the parotid gland lying beneath the superficial musculoaponeurotic system (SMAS).

Communication with the vestibulocochlear nerve occurs within the internal auditory meatus, with the otic ganglion and sympathetic afferents from geniculate ganglion branches, and with the auricular branch of the vagus nerve from a branch of the mastoid segment of the facial nerve. Extracranially, there are communications with the glossopharyngeal, vagus, greater auricular and the auriculotemporal nerves and multiple communications with branches of the trigeminal nerve. These interconnections explain the mastoid, ear, face and neck pain associated with herpes zoster and Bell’s palsy, and the referred otalgia, face, occipital, throat, and neck pain which may occur with malignant disease.

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