Introduction

The facial nerve is the seventh cranial nerve (CN VII). This nerve carries visceral and branchial motor signals as well as general and special sensory signals. It innervates the muscles of facial expression while supplying parasympathetic innervation to the mucous membranes of the nasopharynx, hard and soft palate, and the lacrimal, submandibular, and sublingual glands. Additionally, the facial nerve relays the sensation of taste from the anterior portion of the tongue and general sensation from the skin of the concha of the auricle and a small area behind the ear. The branchial motor fibers constitute the largest fibers of the facial nerve.[1] 

The chorda tympani branches off of the facial nerve, just superior to the stylomastoid foramen. After branching off the seventh cranial nerve, the facial nerve, the chorda tympani, pierces the tympanic cavity and enters the posterior canaliculus. It then descends close to the spine of the sphenoid bone and merges with a branch of the mandibular nerve, the lingual nerve.[2]

Structure and Function

The chorda tympani forms from fibers from two brain stem nuclei: the superior salivatory nucleus and the solitary nucleus. The superior salivatory nucleus is located in the pontine tegmentum in the brainstem and innervates the submandibular gland and sublingual gland.[3] The solitary nucleus is in the medulla oblongata of the brainstem. It is the central visceral sensory relay station within the brain.[4]

The chorda tympani leaves the intracranial space through the internal auditory canal with the seventh cranial nerve. Immediately after branching off the facial nerve, the chorda tympani runs through the posterior colliculus. It then travels through the middle ear and across the tympanic membrane. The chorda tympani goes between the malleus and incus and re-emerges anterior to the middle ear cavity. It then enters medially into the temporomandibular joint through the petrotympanic fissure. Finally, the chorda tympani joins the lingual nerve after exiting the petrotympanic fissure in the infratemporal fossa.[5]

The chorda tympani carries special visceral afferent fibers that relay taste sensations from the anterior two-thirds of the tongue. Additionally, it carries parasympathetic general visceral efferent fibers to the submandibular ganglion that innervate the sublingual and submandibular glands.[6] The chorda tympani's efferent parasympathetic fibers cause vasodilation of blood vessels in the tongue.[7]

Embryology

The facial nerve formation starts in the third week of development. It is a nerve from the second branchial or pharyngeal arch. The muscles of the face also derive from this arch. These muscles include but are not limited to the occipitofrontal, stylohyoid, posterior belly of the digastric, stapedius, and auricular muscles. The facial nerve innervates all of these muscles. The chorda tympani, a branch of the facial nerve, begins to formate in embryos around week 5 of development.[8]

Blood Supply and Lymphatics

The labyrinthine artery, a branch off the anterior inferior cerebellar artery, is believed to supply the facial nerve proximal to the geniculate ganglion. Subsequently, a petrosal branch of the middle meningeal artery is thought to supply the nerve as it reaches the stylomastoid foramen. Finally, the stylomastoid branch of the posterior auricular artery has two branches that supply the remaining portions of the facial nerve.[9]

Nerves

The chorda tympani carries the taste sensation from the anterior portion of the tongue to the brain via the middle ear. It also transports efferent secretomotor innervation to both the sublingual and submandibular glands.[10]

Physiologic Variants

The chorda tympani branches off at the mastoid segment of the facial nerve; however, there are anatomical variations in which portion of this segment the branching can occur. In most cases (approximately 70%), it is located at the mid-third of the mastoid segment. In about 20% of individuals, it arises in the proximal third, and in about 10% of individuals, it leaves the facial nerve at the distal third of the mastoid segment.[11]

Surgical Considerations

During surgery of the middle ear, it is common to have damage to the chorda tympani. However, it is essential to note that damage to the chorda tympani generally has little long-term clinical relevance.[12] Individuals with chorda tympani damage due to surgery usually do not report taste deficits. Additionally, there are no taste decrements when the chorda tympani nerve is anesthetized; this suggests that the taste system can compensate for the losses in chorda tympani input. The phenomenon of ‘taste constancy’ results in hypotheses that when the chorda tympani innervation gets removed, this disinhibits certain cells that receive excitatory inputs from other gustatory nerves from the nucleus of the solitary tract, or tractus solitarius. This thinking explains a study that showed that when the chorda tympani was anesthetized, there was an increase in the perceived intensity of quinine on the circumvallate papillae, which ultimately suggests that neuroplasticity can compensate for the partial taste loss that occurs with chorda tympani nerve loss.[13]

Clinical Significance

A vestibular schwannoma or acoustic neuroma may result in the loss of function of the chorda tympani on the ipsilateral side; this is often also associated with paralysis of ipsilateral facial muscles and impaired secretions of the lacrimal, submandibular, and sublingual glands. A complete loss of taste is extremely rare as it would require a bilateral injury to the nerves in the oropharyngeal region. A decreased taste sensitivity or distortions in taste perception can result from drug use and a variety of medications and is even associated with viral infections, psychiatric disorders, and Bell Palsy. Conditions accompanied by inflammation are suggested to be treated with a corticosteroid to reduce the potential damage of any nerve swelling.[14]

A range of conditions that are associated with inflammation of the middle ear can affect or cause injury to the chorda tympani. Various inflammatory middle ear diseases can have damaging effects on the chorda tympani. These conditions include disorders like otitis media and cholesteatoma. These conditions result in structural signs of degeneration that can be observed using light and electron microscopy. This nerve damage has also been demonstrated to result in taste disturbances.[15] 

Cholesteatomas are cysts that can be harmful and occur in the middle ear. They can be congenital and arise from embryologic rests of epithelium that are left behind in the middle ear during development. Cholesteatomas can also be acquired and also develop due to eustachian tube dysfunction and chronic negative middle ear pressure. The pressure creates a suction-like effect on the middle ear and causes the eardrum to collapse into the middle ear, ultimately resulting in a cholesteatoma or cyst formation.[16]