Atlas of Facial Nerve Surgeries and Reanimation Procedures (eBook)

█Introduction

The facial nerve is developmentally derived from the second pharyngeal, or branchial arch. The second pharyngeal arch is also called the hyoid arch because it contributes to the formation of the lesser horn and upper body of the hyoid bone (the rest of the hyoid is formed by the third arch). The facial nerve supplies motor and sensory innervation to the muscles which are also derived by the second pharyngeal arch, including the muscles of facial expression, the posterior belly of the digastric, stylohyoid, and stapedius. The motor division of the facial nerve is derived from the basal plate of the embryonic pons, while the sensory division originates from the cranial neural crest.

The anterior two-thirds of the tongue is derived from the first pharyngeal arch, which gives rise to fifth cranial nerve (CN). So the lingual branch of the mandibular division (V3) of CN V carries nontaste sensation (pressure, heat, texture) from the anterior part of the tongue via general visceral afferent fibers, whereas the taste sensation from the anterior two-thirds is carried by the chorda tympani branch of CN VII via special visceral afferent fibers.

The facial nerve with its complex course, branches, and its relationship with the surrounding structures develop during the first 3 months of prenatal life, although the nerve is not fully developed before 4 years of life. So, to clearly understand the embryological develop­mental pattern of facial nerve along with its surrounding structures, it can be divided into categories based on gestational weeks of the embryo.

0 to 4 Weeks of Gestation

The facial nerve development starts as a “facioacoustic primordium or crest” attached to the metencephalon, just rostral to the otic vesicle after the third week of gestation. It is a collection of neural crest cells which finally becomes facial nerve (CN VII). At this stage it is just a collection of neural tissue without any branches or ganglia.

In the 4th week (embryo size 4.8 mm), the facial nerve divides into two branches. The main branch becomes the main trunk of facial nerve and it terminates into mesenchyme. The smaller branch becomes the chorda tympani nerve. It travels anteriorly to the first pharyngeal pouch and enters the mandibular arch.

At size 6 mm, the nerve approaches the epibranchial placode, and appearance of the large, dark nuclei of neuroblasts mark the development of the geniculate ganglion (Fig. 1.1).

Weeks 5 to 6 of Gestation: 8 to 18 mm

At 7 mm of embryo length, the branches in the form of greater superficial petrosal nerve and the ganglion in the form of geniculate ganglion start to appear. The chorda tympani enters the mandibular arch and terminates near a branch of the mandibular nerve, which finally becomes the lingual nerve.

At 8-mm size stage, facial nerve motor nucleus is developed.

In the 5th week (10-mm length) itself, the facial nerve gives off another branch in the form of postauricular branch, very close to chorda tympani, which travels to the posterior digastric premuscle mass. At this stage, the facial nerve completely separates from acoustic nerves along with appearance of small nervus intermedius, which becomes a separate entity approximately in the 7th week.

At 14-mm size embryo, the greater superficial petrosal nerve and the geniculate ganglion are fully developed. The greater superficial petrosal nerve while developing courses anteriorly lies on the lateral aspect of the developing internal carotid artery where it is joined by the deep petrosal nerve and both continue as a single nerve called “the nerve of the pterygoid canal.” It terminates in the tissue that will become the future pterygopalatine ganglion.

In the middle of the 6th week (16- to 17-mm length), a branch arises from the anterior aspect of the geniculate ganglion and courses posteroinferiorly to the superior ganglion of glossopharyngeal nerve. The chorda tympani and lingual nerves finally reach the developing submandibular ganglion.

As far as the development of facial muscles is concerned, it goes along with the development of facial nerve. When the embryo is between 10 to 18 mm in length, the mesenchymal tissue starts differentiating into the posterior digastric complex (the stapedius muscle, the stylohyoid muscle, and the posterior belly of the digastric muscle).

Weeks 7 to 9 of Gestation: 18 to 58 mm

At 18-mm length, the nervus intermedius enters into the brainstem between the facial nerve motor root and vestibulocochlear nerve. The chorda tympani and lingual nerves unite to become one nerve to enter the submandibular gland.

By the time the embryo is 19-mm long, all of the peripheral branches of developing facial nerve lie close to the deep surfaces of the myoblastic laminae that will form the facial muscles. At the end of the 7th week, all peripheral branches have got segregated and can be identified separately.

In the 22-mm embryo, muscles of posterior digastric complex, namely, the stapedius muscle, the posterior belly of the digastric muscle, and stylohyoid muscles, are developing. A branch which developed from anterior aspect of geniculate ganglion in the 6th week is reduced to a communication as the tympanic plexus and the lesser petrosal nerve develop from CN IX. Out of all peripheral branches, the temporal, zygomatic, and upper buccal branches lie superficial to the parotid primordium, while the lower buccal, mandibular, and cervical branches lie deeper to parotid primordium, as the parotid gland itself is in the stage of development. Many of the facial muscles have developed at this time as well, including frontalis, zygomaticus major and minor, buccinators, and depressor anguli oris.

At the same time (around 18-mm embryo length), the parotid gland has begun to develop from the parotid bud, appearing as an evagination from the lateral oral cavity area. At this stage, it is called parotid primordium.

In the 8th week (32- to 49-mm length), the fallopian canal (FC) starts developing around the facial nerve in the form of a sulcus. The facial muscles, namely, the orbicularis oculi, the levator anguli oris, and the orbicularis oris muscles, develop by week 8 as well.

In the 9th week (50- to 60-mm length), rest of the facial muscles, namely, auricularis anterior, corrugator supercilii, platysma, and levator labii superioris alaeque nasi muscles, appear.

Weeks 10 to 15 of Gestation: 58- to 80-mm Length

Most of the peripheral branches of the facial nerve are developed at this stage. Extensive communications form between developing facial nerve and branches of the trigeminal nerve and also between the nervus intermedius, the VIII nerve and the motor root of the VII nerve. The vertical course of facial nerve starts developing at this stage and its location at this stage is far anterior to the external and middle ear structures than it is seen in the adult.

Weeks 11 to 15 of Gestation: 80- to 146-mm Length

The third branch from geniculate ganglion, namely, external petrosal nerve, arises from distal to the geniculate ganglion and courses with a branch of the middle meningeal artery. Branches also arise from the facial nerve between the stapedius and the chorda tympani nerves. All these branches along with those of CNs IX and X provide sensory innervation to the external auditory canal. These branches also develop communication with the zygomaticotemporal nerve. Communications develop with the lesser occipital and transverse cervical nerves, which are the branches of cervical nerves. The horizontal portion of the facial nerve at this stage lies close to the developing otic capsule.

At 14 or 15 weeks of gestation, the geniculate ganglion is fully developed, and relationship of facial nerve to middle ear structures is fully developed.

Week 16 of Gestation Till Birth

As the enlargement and development of middle ear is in progress, the facial nerve remains more superficial and anterior in relation to the auricle than it is in the adult. All important communicating branches of facial nerve are developed at this stage.

By 26 weeks, with the ossification and growth of the outer layer of periosteal bone of FC, the closure of the sulcus starts at this stage.

By 35 weeks, the geniculate ganglion separates from the epitympanic rim.

At birth, the facial nerve has completely developed into a consistent structure. Its anatomy and location approximate with that of the adult; however, at its exit through the stylomastoid foramen (SMF) it is still superficially located. The deeper location of exit of facial nerve at SMF occurs along with the parallel development of mastoid tip which goes on till about 4 years of age.

█Development of the Fallopian Canal

The FC enclosing the intratemporal part of facial nerve courses from the internal acoustic meatus (IAM) to the SMF. The intratemporal part of facial nerve includes the labyrinthine (LS), tympanic (TS), mastoid segments (MS), and two genus of the facial nerve. The...