The anatomy and complexity of the nerve supply of the head, neck, and face in the adult is perhaps best understood in light of its development in the embryo, as the face forms around the primitive mouth (the stomodeum). Initially, the stomodeum is surrounded caudally by the mandibular arch (which is supplied by the mandibular nerve), laterally on each side by the maxillary processes (which are supplied by the maxillary division of the trigeminal nerve), and rostrally by the forebrain capsule, from which develops the frontonasal process (which is supplied by the first division of the trigeminal nerve, the ophthalmic nerve). The frontonasal process grows down into the primitive stomodeum from the forebrain capsule, and eventually this will form the nose of the mature embryo (Fig. 17-1). The two maxillary processes grow inward from either side and join together below the primitive nose, as shown, and they then form the rostral margin of the primitive mouth. Thus, in the mature face, the forehead, eyebrows, upper eyelids, and nose are supplied by the first ophthalmic division of the trigeminal nerve. The lower eyelid, cheek, and upper lip are supplied by the second division (i.e., the maxillary nerve), and the lower lip, chin, mandibular, and temporal regions are supplied by the third division, mandibular nerve. Because of the disproportionate growth of the cranial cavity in humans, these dermatomal distributions are distorted cranially, with the result that some skin innervated by the cervical plexus is drawn up over the angle of the mandible onto the face and posteriorly over the occipital area and the scalp as far forward as the vertex (Fig. 17-2).

The trigeminal nerve is the predominant sensory nerve of the head and face, and is also the motor supply for the muscles of mastication. It originates from the pons. The sensory fibers derive from the cells of the semilunar temporal bone. As it enters the pons, it divides into upper and lower roots. Three branches arise, the ophthalmic V1, maxillary V2, and the mandibular V3 divisions. Both the ophthalmic and maxillary divisions consist exclusively of sensory fibers, whereas the mandibular nerve also has a motor component (Table 17-1).

The semilunar ganglion, also known as the Gasserian ganglion, lies posteromedially in the middle cranial fossa at the junction of its floor and the cavernous sinus just anterior to the ridge of the petrous temporal bone. The ganglion invaginates the dura and therefore lies in a dural pouch—Meckel’s cave—which contains cerebrospinal fluid (CSF).

In its intracranial course, the trunk of the ophthalmic nerve does not lend itself to regional anesthesia. The intraorbital branches of the nasociliary nerve are blocked by retrobulbar block. The intraorbital branches, anterior ethmoidal and infratrochlear, can also be blocked in the orbit. The terminal divisions in the forehead and nose are suitable for peripheral nerve blocks of the scalp and face (6).

The ophthalmic nerve is the smallest division of the trigeminal nerve. It supplies the cornea, conjunctiva, ciliary body, and the iris. The lacrimal gland; mucous membranes of the nasal cavity; the skin of the eyebrow, eyelids, and forehead; and the nose are also supplied by this division. The frontal nerve is the largest branch and may be regarded as the continuation of the first division of the trigeminal nerve. As it enters the orbit through the superior orbital fissure, it proceeds forward between the levator palpebrae superioris and the periosteum. The two branches, the supraorbital and the supratrochlear nerve supply sensory innervation to the forehead and anterior scalp (Fig. 17-3). The supraorbital nerve and vessels emerge from the supraorbital foramen and continue superiorly on the anterior portion of the forehead and scalp. The supraorbital nerve trunk divides into a deep and a superficial branch. The deep branch courses superiorly and laterally, running parallel to
the superior temporal line of the skull in the loose areolar tissue between the galea and the pericranium. The terminal branches of the deep supraorbital nerve branch pierces the galea near the coronal suture to supply scalp sensation. The superficial division lies medially at its origin and quickly divides into multiple branches that pierce the frontalis muscle. These smaller branches pass cephalad to supply the forehead and up to 3.5 cm of the frontal scalp.

The supratrochlear nerves exit the orbit at the superior orbital rim through a notch, above the trochlea and medial to the supraorbital notch. At the supraorbital rim, the nerves penetrate the corrugator muscle and the frontalis muscle. The nerves supply cutaneous sensation to a central vertical strip of forehead and to the medial upper eyelid (7).

The maxillary nerve is purely sensory and is called the infraorbital nerve when it reaches the infraorbital fossa. It divides into four branches: the external nasal, the internal nasal, the inferior palpebral, and the superior labial. The nerve can be easily accessed superficially. The posterior superior alveolar branch arises from the trunk of the nerve just before it enters the infraorbital groove. It enters the infratemporal surface of the maxilla communicating with the middle superior alveolar branch, giving off branches to the lining membrane of the maxillary sinus and branches to the molar teeth. The middle superior alveolar branch arises from the nerve in the posterior part of the infraorbital canal and runs downward and forward in a canal along the lateral wall of the maxillary sinus. The anterior superior alveolar nerve branches off just as it exits the infraorbital foramen and descends in a canal in the anterior wall of the maxillary sinus. It communicates with the middle superior alveolar branch, which supplies the mucous membrane of the anterior part of the inferior meatus and the floor of the nasal cavity, communicating with the branches from the sphenopalatine ganglia. The external nasal branches supply the skin of the nose and of the septum mobile nasi and joins with the terminal branches of the nasociliary nerve. The superior labial branches, the largest, descend behind the quadratus labii superioris and are distributed to the skin of the upper lip, the mucous membrane of the mouth, and the labial glands. They are joined, immediately below the orbit by filaments from the facial nerve, forming with them the infraorbital plexus (7). A computed tomography (CT)-guided imaging study demonstrated that the infraorbital foramen is located approximately 2 cm lateral to the midline in most children at the level of the inferior rim of the orbit (8). The superior anterior part of both

septum and lateral wall of the nose receive contributions from the anterior ethmoidal branch of the ophthalmic nerve. The entire hard palate is supplied by the maxillary nerve via the sphenopalatine ganglion.

The palatine nerves are distributed to the roof of the mouth, soft palate, tonsil, and lining membrane of the nasal cavity. Most of their branches are derived from the sphenopalatine branches of the maxillary nerve. There are three main branches: the anterior, middle, and posterior branches.

The anterior palatine nerve descends through the pterygopalatine canal and emerges in the hard palate through the greater palatine foramen. It passes in a groove in the hard palate as far as the incisor teeth. It supplies the gums, and the mucous membrane and glands of the hard palate.

The middle palatine nerve emerges through one of the minor palatine foramen and provides the sensory supply to the uvula, tonsils, and the soft palate.

The posterior palatine nerve descends through the pterygopalatine canal and emerges through a special opening behind the greater palatine foramen. It supplies the sensory branches to the soft palate, tonsils, and uvula. The middle and posterior palatine branches join the tonsillar branch of the glossopharyngeal to form a plexus (cirrus tonsillaris) around the tonsils (7).

The mandibular nerve emerges from the cranial cavity through the floor of the middle cranial fossa, via the foramen ovale, to enter the infratemporal fossa. This fossa is a rectangular compartment bounded anteriorly by the posterior wall of the maxilla and posteriorly by the styloid apparatus and carotid sheath. The lateral wall is the ramus of the mandible, and the medial wall is composed anteriorly of the lateral pterygoid plate of the sphenoid bone and posteriorly by the constrictor muscles of the pharynx. It has no floor, but its roof is the floor of the middle cranial fossa. In the infratemporal fossa, the mandibular nerve divides into its terminal branches.

The glossopharyngeal nerve supplies the posterior third of the tongue and the oropharynx from its junction with the nasopharynx at the level of the hard palate. It supplies the pharyngeal surfaces of the soft palate and the epiglottis, and the pharyngeal wall, as far down as the pharyngoesophageal junction at the level of the cricoid cartilage (C6).

The vagus nerve supplies sensation to the larynx. The undersurface of the epiglottis and the laryngeal inlet down to the vocal folds are supplied by the internal laryngeal branch of the vagus. This nerve reaches the larynx by piercing the thyrohyoid membrane, which joins the thyroid to the hyoid cartilages. Below the cords, the larynx and trachea are supplied by the recurrent branch of the vagus that ascends in the neck, in the groove between the trachea and esophagus. The recurrent laryngeal nerve also supplies motor function to all the intrinsic muscles of the larynx (except the cricothyroid muscle), and bilateral motor block produces loss of phonation and loss of ability to close the glottis.

The cervical plexus contributes to the supply of both the deep and the superficial structures of the neck. The first cervical nerve, C1, is a motor nerve to the muscles of the suboccipital triangle and has no sensory distribution to skin. The skin of the neck is supplied in a sequential dermatomal pattern (like the trunk) by the cutaneous branches of C2–C4 by both anterior and posterior primary rami. The cervical plexus is formed by the ventral rami of the upper four cervical nerves. Their dorsal and ventral roots combine to form spinal nerves as they exit through the intervertebral foramen. The anterior rami of C2 through C4 form the cervical plexus. The cervical plexus lies behind the clavicular head of the sternocleidomastoid, giving out both the superficial and deep branches. The superficial cervical plexus wraps around the belly of the clavicular head of the sternocleidomastoid to form four branches: (a) the lesser occipital, (b) the great auricular, (c) the transverse cervical, and (d) the supraclavicular nerves. The great auricular nerve is the largest of the ascending branches. It arises from the second and third cervical nerve roots, winds around the posterior border of the sternocleidomastoid and, after perforating the deep fascia, ascends behind the clavicular head of the sternocleidomastoid beneath the platysma to the parotid gland, where it divides into an anterior and posterior branch. The anterior branch (ramus anterior; facial branch) is distributed to the skin of the face over the parotid gland and communicates in the substance of the gland with the facial nerve. The posterior branch (ramus posterior; mastoid branch) supplies the skin over the mastoid process and on the back of the auricle, except at its upper part; a filament pierces the auricle to reach its lateral surface, where it is distributed to the lobule and the lower part of the concha. The posterior branch communicates with the smaller lesser occipital, the auricular branch of the vagus, and the posterior auricular branch of the facial nerve (7).

In the region of the scalp, the nerves of supply have long superficial upward courses. Four sensory nerves pass in front of the ear to the scalp (supratrochlear and supraorbital from V1, zygomaticotemporal from V2, auriculotemporal from V3), and four pass behind the ear (great auricular and greater, lesser, and least occipital nerves from cervical plexus) (Figs. 17-2 and 17-3).

Local anesthetics injected into the ganglion can produce profound analgesia in certain pain states, such as tic douloureux (but only for short periods), and neurolytic blocks with alcohol were used in the past by some therapists for this condition (5,12,13). The pain relief is obtained at the price of hemifacial and corneal analgesia, with saliva often dribbling out of the ipsilateral, numb side of the mouth. Such blocks, for long-term pain relief, are rarely performed by pain specialists today (Chapters 42 and 45).

Gangliolysis, or thermogangliolysis, has great therapeutic implications because of its proven results (14). For gangliolysis, an insulated needle is placed through the foramen ovale. Electrical stimuli are delivered, and the needle tip is adjusted until paresthesias are elicited in the area of pain and a radiofrequency lesion is made, using a thermistor. This appears to be a successful maneuver in those 30% of tic douloureux patients who do not benefit from carbamazepine or other medication therapy, and approximately 80% will get at least 1 year’s relief of pain. An alternative to thermal coagulation for tic douloureux is provided by “bathing” the trigeminal ganglion in glycerol (0.1–0.3 mL) (15). Proponents of this technique claim the relief is as satisfactory as thermal coagulation, with a greater degree of safety with regard to the production of excessive neural damage. However, it is claimed that glycerol gangliolysis is less likely to produce corneal anesthesia than are radiofrequency lesions of the first division, and so it may have some advantages (16). Gangliolysis has improved success over neurolytic block, surgical neurectomy, and rhizotomy therapies. However, which of the gangliolysis methods is superior is still debated among the proponents of the different techniques.