Introduction
The anatomy and embryology of the facial nerve are complex. A basic understanding of developmental anatomy is necessary to comprehend and anticipate variations encountered by the surgeon. The reader is referred to Embryology and Anomalies of the Facial Nerve and Their Surgical Implications (Sataloff, 1991) and the 2-part series, Phylogeny and Embryology of the Facial Nerve and Related Structures (Sataloff and Selber, 2003) for a comprehensive review of the development of the facial nerve and the associated development of the ear.1,2 Congenital anomalies of the auricle should raise suspicion for an associated facial nerve anomaly. The objective of this article is to outline the embryology of the facial nerve and its common clinical implications.
Developmental Anatomy
Mature facial nerve
While studying the embryology of the facial nerve, keep in mind the mature course and structure that is the end result of developmental events. The motor nucleus of the facial nerve is located in the reticular formation of the caudal pons. Upon leaving the motor nucleus, axons extend dorsally and medially, cranially and superficially, to bend around the abducens (sixth cranial nerve) nucleus. The fibers then exit the CNS between the olive and the inferior cerebellar peduncle.
The sensory root (nervus intermedius) consists of (1) central projections of neurons located in the geniculate ganglion (both general somatic fibers that synapse in the spinal nucleus of the trigeminal nerve and special afferent fibers that synapse in the nucleus solitarius) and (2) axons of parasympathetic neurons from the superior salivatory (lacrimal) nucleus. The nervus intermedius enters the CNS lateral to the motor root at the pontocerebellar groove.
After it exits the internal auditory canal, the facial nerve enters the middle ear where it bends posteriorly (first, or medial, genu) and courses horizontally through the middle ear. Just anterior to the lateral aspect of the horizontal semicircular canal, the facial nerve curves gently (the second genu) to form the vertical, or mastoid, segment that exits via the stylomastoid foramen.
Intracranial embryology of the facial nerve
Weeks 0-4 (0-6 mm)
The rhombencephalon (or hindbrain) is divided into the myelencephalon (caudal), which becomes the medulla oblongata, and the metencephalon (cranial), which becomes the pons and cerebellum. The facioacoustic (acousticofacial) primordium appears during the third week of life (4.2 mm crown-rump length [CRL]). It is attached to the metencephalon just cranial to the otic vesicle. The facial part of the acousticofacial primordium migrates cranial and ventral to end adjacent to the epibranchial placode, which is located on the dorsal and caudal aspect of the first branchial cleft.
By the end of the fourth week of gestation (4.8-6.5 mm CRL), the facial nerve splits into 2 parts, caudal and rostral trunks. The chorda tympani nerve exits rostrally and courses ventrally to the first pharyngeal pouch to enter the mandibular arch. Shortly thereafter, the nerve approaches the epibranchial placode, inducing the appearance of the large dark nuclei of neuroblasts that represent the future geniculate ganglion.
Weeks 5-6 (7-17mm)
Mesenchymal concentrations that form the cephalic muscles are seen in association with their nerves, while the epibranchial placode disappears and the geniculate ganglion is identifiable. The greater superficial petrosal nerve (GSPN) is present. The chorda tympani nerve enters the mandibular arch and terminates just proximal to the submandibular ganglion, near a branch of the trigeminal nerve that will become the lingual nerve. The posterior auricular nerve appears near the chorda tympani.
Complete separation of the facial and acoustic nerves is apparent, and a discrete nervus intermedius develops, making this an important temporal reference point for gestational disorders that affect both systems. The GSPN courses to the lateral aspect of the developing internal carotid artery (ICA), where it joins the deep petrosal nerve and continues as the nerve of the pterygoid canal. It terminates in a group of cells that will become the pterygopalatine ganglion. At this point, the most distal branches of the facial nerve are a loose network or interconnecting twigs.
Week 7 (18-31 mm)
The nervus intermedius is now smaller than the motor root and enters the brain stem between the vestibulocochlear nerve and the motor root of the facial nerve. The chorda tympani and lingual nerve unite proximal to the submandibular gland. The posterior auricular nerve now divides into cranial and caudal branches.
Several branches are visible in the peripheral portion of the seventh nerve. All of the peripheral branches lie deep to the myoblastic laminae that will form the facial muscles. At the end of the seventh week, the separations between the terminal branches continue to increase to the extent that all peripheral divisions can be identified.
The parotid gland is beginning to develop from the parotid bud at this stage. The temporal, zygomatic, and upper buccal branches are superficial to the parotid primordium, while the lower buccal, mandibular, and cervical branches are deeper. Multiple facial muscles appear at this time as well, including the zygomaticus major and minor, depressor anguli oris, buccinators, and frontalis.
Week 8 (32-49 mm)
A sulcus develops around the facial nerve that is the beginning of the fallopian canal. The orbicularis oris, levator anguli oris, and orbicularis oculi muscles appear.
Week 9 (50-60 mm)
Auricularis anterior, corrugator supercilii, occipital and mandibular platysma, and levator labii superioris alaeque nasi muscles appear. All the cranial nerves more closely resemble their adult relationships.
Weeks 10-15 (61-80 mm)
Extensive branching of the peripheral portions of the facial nerve occurs at this stage. Communication with the trigeminal nerve (via infraorbital, buccal, auriculotemporal, and mental branches) occurs in the perioral and infraorbital regions. The vertical portion of the facial nerve begins in the middle ear, and its overall relationship to external and middle ear structures is far more anterior than in the adult. Branches that will supply sensation to the external auditory canal arise between the stapedius and chorda tympani nerves.
Intricate connections between the superficial and deep lobes of the parotid and their relation to the facial nerve develop. By the fifteenth week the geniculate ganglion is fully developed, and the facial nerve's relationship to middle ear structures is more fully developed.
Week 16 to birth (146 mm)
All definitive communications of the facial nerve are established by the 16th week. At 26 weeks, ossification has progressed to partial closure of the previously formed sulcus into the fallopian canal.
In late fetal life, the fallopian canal is closed by bone in most areas, except in the anterior cranial portion, where it remains open to form the facial hiatus along the floor of the middle cranial fossa. At least 25%, and as many as 55%, of fallopian canals are dehiscent, with the most common location adjacent to the oval window.
At birth, the anatomy of the facial nerve approximates that of the adult, except for its exit through the more superficially located stylomastoid foramen. Adult anatomy will occur in this region as the mastoid tip develops after birth.
Congenital Facial Paralysis
Abnormalities of the facial nerve may occur in conjunction with malformations of the ear, in isolation without associated anomalies, or in conjunction with a variety of syndromes that include abnormalities elsewhere in the body.
In the newborn, the otolaryngologist evaluating a facial paresis or facial palsy must decide whether it is congenital or acquired. One in 2000 live births has a unilateral facial palsy, with a 90% spontaneous recovery rate. Approximately 75-80% of palsies in newborns are related to birth trauma. A history of forceps delivery, prolonged labor, ecchymosis over the mastoid, or hemotympanum raises suspicion for birth trauma.
The presence of bilateral facial paralysis, other cranial nerve deficits, or other anomalies suggests a developmental etiology. Early accurate diagnosis is important if the etiology is traumatic. In rare cases, surgery and facial nerve repair may be required in the newborn if the etiology is determined to be traumatic.
Evaluation of facial nerve paralysis includes evoked electromyogram (EEMG), CT scan, and electromyogram (EMG). If the etiology is traumatic, the nerve can be stimulated for 3-5 days postnatal; fibrillation potentials on EMG develop 14-21 days after birth. If the cause is not traumatic, treatment generally is delayed. Eye protection is rarely required in congenital facial paralysis.
In patients with congenital malformations, eliciting the fetal age at which development was arrested is usually possible. This allows for elucidation of the anatomy of the malformed structure based on its normal course of embryological development. Furthermore, if anomalies are present in other organ systems (in particular the kidney), they often reflect arrested development at the same time during development. In this way, the surgeon should be able to predict the location of the facial nerve, particularly in the case of middle ear malformation.
Most hereditary conditions that include facial paralysis are manifest at the time of birth (see Table 1, below). However, a few hereditary syndromes are associated with the development of facial paralysis later in life. In addition, many hereditary and congenital malformations are associated with abnormal facial nerve anatomy in the presence of normal nerve function. The otolaryngologist must be familiar with these conditions because abnormal development may place the nerve at increased risk of injury during otologic surgery.
Table 1. Developmental Syndromes Associated With Facial Nerve Abnormalities*Open table in new window
Table
| Syndrome | Facial Nerve Abnormality | Description |
| Bulbopontine paralysis with progressive sensorineural hearing loss |
|
|
| DiGeorge syndrome | Facial paralysis reported |
|
| Dominant craniometaphyseal dysplasia | Unilateral and bilateral facial paralysis reported |
|
| Hemifacial microsomia | Facial paralysis | Facial asymmetry, including unilateral microtia, macrostomia, and failure of mandibular ramus and condyle to form |
| Hereditary acoustic neuromas | Facial paresis and/or palsy |
|
| Melkersson-Rosenthal syndrome | Recurrent alternating facial paralysis | Recurrent alternating facial paralysis Edema of lips, face, and eyelids Cheilitis and fissured tongue Positive family history |
| Möbius syndrome |
|
|
| Osteopetrosis | Facial paralysis, which may be acute and recurring |
|
| Recessive craniometaphyseal dysplasia | Unilateral facial paralysis |
|
| Sclerostenosis |
|
|
| Sickle cell disease | Facial paralysis observed to occur during a crisis |
|
| von Recklinghausen neurofibromatosis | Facial paralysis possible from a neurofibroma of the facial nerve or secondary to encroachment by an acoustic schwannoma |
|
| Syndrome | Facial Nerve Abnormality | Description |
| Bulbopontine paralysis with progressive sensorineural hearing loss |
|
|
| DiGeorge syndrome | Facial paralysis reported |
|
| Dominant craniometaphyseal dysplasia | Unilateral and bilateral facial paralysis reported |
|
| Hemifacial microsomia | Facial paralysis | Facial asymmetry, including unilateral microtia, macrostomia, and failure of mandibular ramus and condyle to form |
| Hereditary acoustic neuromas | Facial paresis and/or palsy |
|
| Melkersson-Rosenthal syndrome | Recurrent alternating facial paralysis | Recurrent alternating facial paralysis Edema of lips, face, and eyelids Cheilitis and fissured tongue Positive family history |
| Möbius syndrome |
|
|
| Osteopetrosis | Facial paralysis, which may be acute and recurring |
|
| Recessive craniometaphyseal dysplasia | Unilateral facial paralysis |
|
| Sclerostenosis |
|
|
| Sickle cell disease | Facial paralysis observed to occur during a crisis |
|
| von Recklinghausen neurofibromatosis | Facial paralysis possible from a neurofibroma of the facial nerve or secondary to encroachment by an acoustic schwannoma |
|
*Modified from Sataloff, 1991
Table 2. Summary of the Derivatives of the Second Branchial Arch
Open table in new window
Table
| Pharyngeal Arch | Nerve | Artery | Muscles | Skeleton |
| II = Hyoid (Reichert cartilage) | Cranial nerve VII (Facial nerve) | Stapedial |
|
|
| Pharyngeal Arch | Nerve | Artery | Muscles | Skeleton |
| II = Hyoid (Reichert cartilage) | Cranial nerve VII (Facial nerve) | Stapedial |
|
|
Keywords
facial nerve embryology, facial nerve, developmental anatomy, embryology, congenital anomalies of the auricle
More on Facial Nerve Embryology |
| References |
References
Sataloff RT. Embryology and Anomalies of the Facial Nerve and Their Surgical Implications. Lippincott Williams & Wilkins Publishers; 1991.
Sataloff RT, Selber JC. Phylogeny and embryology of the facial nerve and related structures. Part I: Phylogeny. Ear Nose Throat J. Sep 2003;82(9):704, 707-10, 712 passim. [Medline].
Baxter A. Dehiscence of the Fallopian canal. An anatomical study. J Laryngol Otol. Jun 1971;85(6):587-94. [Medline].
Beddard D, Saunders WH. Congenital defects in the fallopian canal. Laryngoscope. 1962;72:112 - 115.
Donaldson JA, Anson BJ. Surgical anatomy of the facial nerve. Otolaryngol Clin North Am. Jun 1974;7(2):289-308. [Medline].
Gasser RF. The early development of the parotid gland around the facial nerve and its branches in man. Anat Rec. May 1970;167(1):63-77. [Medline].
Rasweiler CL. Surgical causes of facial paresis. In: Rubin RL, ed. The Paralyzed Face. Mosby-Year Book; 1991:101-103.
Sataloff RT, Selber JC. Phylogeny and embryology of the facial nerve and related structures. Part II: Embryology. Ear Nose Throat J. Oct 2003;82(10):764-6, 769-72, 774 passim. [Medline].
Schaitkin BM, Eisenman DJ. Anatomy of the facial muscles. In: The Facial Nerve. Thieme Medical Publishers; 2000:95-105.
Further Reading
Keywords
facial nerve embryology, facial nerve, developmental anatomy, embryology, congenital anomalies of the auricle
