Version May 2024
INTRODUCTION — Surgeons operating in the head and neck area must have an in-depth understanding of the regional anatomy to achieve good surgical outcomes and avoid complications. The surgical risk of injury to nerves or other relevant anatomic structures should be discussed with the patient preoperatively, and informed written consent should be obtained. (See "Informed procedural consent".)
This topic will discuss how to identify the anatomical danger zones in the head and neck area to avoid injuries during cutaneous surgery. Anatomic danger zones for facial injection of soft tissue fillers are discussed separately. (See "Anatomic danger zones for facial injection of soft tissue fillers".)
TYPES OF NERVE INJURY — There are three major types of nerve injury based upon the severity of nerve disruption and loss of axonal continuity: neurapraxia, axonotmesis, or neurotmesis [1-3]. Each type has its own healing process, consequences, and management [4].
Neurapraxia — Neurapraxia is segmental demyelination without interruption of axonal continuity. It is the most superficial type of nerve injury and may be caused by ischemia, blunt trauma, compression, or traction. Neurapraxia causes a temporary conduction block. Wallerian degeneration (degeneration of the axon and myelin sheath distal to the injury site) does not occur. Spontaneous recovery within weeks to a few months is the rule.
Axonotmesis — In axonotmesis, the axon is disrupted and Wallerian degeneration occurs. Axonotmesis is commonly seen in crush and stretch injuries. Functional recovery is expected through axonal regeneration. The Schwann cells and an intact endoneurial tube play a key role in the regeneration process. Recovery is generally achieved over several months.
Neurotmesis — Neurotmesis usually involves complete nerve transection, including myelin, axon, and various layers of connective tissue, with ensuing Wallerian degeneration. Spontaneous recovery is unlikely without surgical repair [3]. The risk of synkinesis (involuntary activation of a muscle when another muscle is activated) is highest with neurotmesis because of loss of nerve conduit integrity, which normally guides accurate nerve regeneration.
CONSEQUENCES OF NERVE INJURY
Sensory nerve injury — Paresthesia, dysesthesia, and/or anesthesia may result from sensory nerve injury depending on the extent of the injury and the healing process:
●Paresthesia is an abnormal sensation such as tingling, pain, or burning and can be an early or late manifestation of a nerve injury.
●Dysesthesia, an abnormal unpleasant sensation, is often a late finding in sensory nerve injury as the nerves are healing and are more easily stimulated.
●Anesthesia or numbness is often an early and immediate result of nerve injury.
It should be noted that temporary injury to the nerve roots may occasionally occur in performing regional nerve blocks for surgical or cosmetic procedures. Nerve block of the supraorbital, infraorbital, and mental nerves targets the nerve roots at their origin from the skull. The accidental injection of anesthetics into the foramen may result in nerve compression and ischemia, with an ensuing neurapraxia. As the needle enters the foramen, the patient typically experiences a much greater pain than expected with routine numbing injections. Under these circumstances, the needle should be immediately withdrawn.
Due to redundancy of sensory nerves, injury is often reversible, although the healing process can be stressful to the patient. It may require many months for full sensation to return. Neurapraxia secondary to nerve compression in the setting of regional nerve block usually resolves completely within weeks to months.
Motor nerve injury — Injuries to motor nerves result in either partial paralysis (paresis) or total paralysis of the target muscles. This can lead to a functional compromise that can be cosmetically disfiguring and functionally challenging. Extirpation of cancers with perineural invasion may lead to nerve injury. In addition, motor nerve injuries have been reported in 0.3 to as high as 20 percent of facelifts [5,6].
Motor nerve injuries will have different consequences depending upon the location of the injury along the nerve. Proximal injuries will have a more widespread effect than distal injuries where bifurcations of the trunks have already occurred.
DANGER ZONES OF THE HEAD AND NECK — There are eight danger zones in the head and neck area where the motor or sensory nerves travel more superficially or nerve roots emerge from the skull foramina (picture 1A-B). Key structures to avoid during surgery include [7]:
●The supraorbital and supratrochlear branches of the ophthalmic branch of the trigeminal nerve and accompanying blood vessels
●The infraorbital branches of the maxillary branch of the trigeminal nerve and the infraorbital vessels
●The mental branch of the mandibular branch of the trigeminal nerve
●The temporal (frontal) branch of the facial nerve
●The marginal mandibular branch of the facial nerve
●The buccal and zygomatic branches of the facial nerve and the parotid duct (Stensen's duct)
●The posterior triangle of the neck, including the great auricular, lesser occipital, spinal accessory, supraclavicular, phrenic, and suprascapular nerves
●The parotid gland
The eight danger zones in the head and neck area are described in detail below, beginning with sensory, motor, and mixed motor and sensory locations.
SENSORY NERVES — The major sensory nerves of the face are branches of the trigeminal nerve.
Trigeminal nerve — Surgical injury to the roots of the three main branches of the trigeminal nerve may cause widespread numbness of the areas they innervate (figure 1). However, this type of injury occurs rarely because of the deep location, immediate branching, and redundancy of these nerves.
Supraorbital and supratrochlear nerves — The first danger zone is located at the undersurface of the upper orbital rim and includes a circumferential area of approximately 1.5 cm (picture 1B). This area includes the supraorbital and supratrochlear branches of the ophthalmic branch of the trigeminal nerve.
The zone can be identified by palpating the supraorbital notch located 2 to 3 mm superior to the supraorbital rim in the midpupillary line (approximately 2.5 cm lateral to midline) [8]. The supraorbital and supratrochlear nerve and vessels travel in a cephalic direction to supply blood and sensation to upper eyelid and mid forehead.
The supraorbital nerve emerges from the supraorbital foramen and travels deep beneath the corrugator supercilii, then penetrates the frontalis muscle to provide sensation to the upper eyelid and mid forehead. Clinically, differentiation between nerves and vessels is facilitated by the observation that nerve branches have a sinewy, white, flat appearance, whereas vessels have a more rounded tubular appearance, with arteries visibly pulsating on examination. The supraorbital nerve originates approximately 2.1 to 3.5 cm from midline and 0 to 0.5 cm above the orbital rim. Branches of this nerve originate deep and enter the subcutaneous plane by piercing through the frontalis muscle at a mean distance of 2.6 cm above the orbital rim, with an average depth of 0.3 cm [9].
The supratrochlear nerve lies approximately 1 cm medial to the supraorbital notch. It penetrates the corrugator supercilii and frontalis and provides sensation to the medial upper eyelid, forehead, and glabella. The supraorbital and supratrochlear nerves, in relation to the corrugator supercilii muscle, give off an average of 7.7 supraorbital branches and 5.1 supratrochlear branches emerging from the orbit. The majority of the supraorbital branches (60 percent) become intramuscular branches, whereas the majority of the supratrochlear branches (68 percent) become superficial [10]. Therefore, surgery resection of the corrugator supercilii may be expected to result in supraorbital numbness, while surgery above the corrugator may result in sensory damage to the supratrochlear branches.
Injury to the supraorbital or supratrochlear nerve at this proximal location may have extensive sensory effects on the eyelid, conjunctiva, and forehead.
Infraorbital nerve — The second danger zone is located at the infraorbital foramen on the down-sloping surface of the maxillary bone, approximately 6 to 7 mm below the lower orbital rim in the midpupillary line and includes a circumferential area of approximately 1.5 cm (picture 1B) [8,11].
This area includes the infraorbital trunk of the maxillary branch of the trigeminal nerve and the infraorbital vessels. The infraorbital nerve and vessels emerge from the foramen and travel in a circumferential direction to supply blood and sensation to the ipsilateral lower eyelid, mid cheek, nasal sidewall, and upper lip. The main trunk of the nerve lies deep below the facial muscles and is rarely encountered in routine facial surgery.
Damage to the nerve root in this area may cause widespread sensory deficits in the midface.
Mental nerve — The third danger zone is located at the mental foramen of the mandible in the midpupillary line, and includes a circumferential area of approximately 1.5 cm (picture 1B). This area overlies the mental trunk of the mandibular branch of the trigeminal nerve.
The zone is often identified by a slight dimple in the skin approximately 2.5 cm lateral to the midline and 1.3 cm superior to the inferior mandibular margin [8]. The mental nerve and vessels emerge from the foramen and travel in a circumferential direction to supply blood and sensation to the ipsilateral chin, mandible, and lower lip.
Surgical injuries in this area occur infrequently because the nerve root lies at the level of the bony foramen. However, one cadaveric study demonstrated that near the central face, the infraorbital and the mental nerves are intermingled with the buccal branch and marginal mandibular branch of the facial nerve, respectively (picture 2) [12]. After the intermingling, it becomes impossible to distinguish the motor branches from the sensory ones.
MOTOR NERVES — The major motor nerves of the face are branches of the facial nerve. The surgeon should keep in mind that facial nerves vary not only in rami but also in location among patients and even between each side of the head and neck in the same patient. Thus, due to the variability of the location of the nerves relative to visible anatomical landmarks, understanding the surgical plane where the facial nerve is found is most helpful in avoiding injuries [13].
Facial nerve — The facial nerve has multiple branches and redundancies [12]. Cadaveric studies have demonstrated diversity in facial nerve branching not only from cadaver to cadaver but even from hemiface to hemiface within the same specimen [12].
Redundancy allows preservation of function and partial to full recovery in some cases of iatrogenic facial nerve injury. Some studies have shown that functional recovery of moderate to severe facial nerve paralysis is hastened and improved by physical therapy, including functional training of facial mimic muscles and facial massage [14,15]. However, consultation with an experienced microsurgeon specializing in nerve repair is advised when obvious functional impairment is first noted [16].
Temporal (frontal) nerve — The fourth danger zone involves the path of the temporal (frontal) branch of the facial nerve, from the superior aspect of the parotid gland to the lateral forehead (picture 1A). The nerve leaves the parotid parenchyma and travels toward the tail of the eyebrow to innervate the frontalis muscle. A line drawn from 0.5 cm below the tragus to approximately 1.5 cm lateral to the tail of the brow identifies the main course of the nerve [17].
Alternatively, the location of this nerve was described in 19 cadavers as lying in a zone at the zygomatic arch ranging between 21.5 to 35.4 mm medially from the root of the helix and arcing toward a location 10 mm lateral to the supraorbital notch in a range 2.8 to 25 mm above the supraorbital ridge (figure 2) [18].
The nerve travels most superficially over the zygomatic arch, which is considered its most vulnerable point. However, cadaveric studies have shown that the temporal branch often has multiple rami at the zygomatic arch [19]. This redundancy may help to avoid permanent and widespread ipsilateral forehead paralysis if one of the rami is injured during surgery.
The temporal branch of the facial nerve is thought to travel just below the superficial musculoaponeurotic system (SMAS); however, several studies have shown that it often lies deep to the SMAS, below a separate parotid-temporal fascia [20,21]. An unproductive intraoperative search for the temporal nerve just below the SMAS during the SMAS facelift procedure provides some reassurance that the nerve is traveling in a deeper plane.
When cephalad to the zygomatic arch, the plane of the temporal nerve has been found to be deep to the temporoparietal fascia [22]. Therefore, operating just above the SMAS when caudal to the zygomatic arch or deep to the deep temporal fascia when cephalad to the arch will allow one to avoid injury by being either above the nerve or below it, respectively.
Damage to the nerve roots deep in this area may cause motor deficits including the inability to raise the ipsilateral brow. In a review of 616 patients who underwent Mohs surgery for removal of a tumor located in the temporal nerve danger zone, nerve damage occurred in 28 patients (4.5 percent), most of whom had complete nerve paralysis with complete lack of frontalis muscle activation noted on follow-up visits [23]. Factors associated with an increased risk of nerve damage included tumors ≥3 cm, recurrent tumors, tumors with high-risk pathologic features, postoperative defect ≥3 cm, and immunosuppression.
In the author's experience, iatrogenic brow ptosis resulting from a distal nerve injury in the temple region is likely to resolve after several months via muscle recruitment from redundant innervations to the area and or subsequent spontaneous reinnervation. If the nerve is severed intraoperatively (eg, during perineural tumor resection) an immediate direct brow lift may be performed to avoid brow ptosis during the recovery phase [24].
Marginal mandibular nerve — The fifth danger zone involves the path of the marginal mandibular branch of the facial nerve from the mandible to the lower face (picture 1A). The danger zone begins just anterior to the facial artery, where the nerve courses most superficially as it travels over the facial artery and vein. The marginal mandibular nerve is vulnerable at this site, because of its superficial course and the reduced thickness of the superficial musculoaponeurotic system.
In most cases, the marginal mandibular nerve emerges from the inferior border of the parotid gland, travels along the angle and the inferior border of the mandible before arising anterior and superficial to the facial artery and vein at the anterior border of the masseter muscle [25]. Laterally, the nerve usually courses below the mandibular border until it reaches the facial artery, then it courses above the border as it travels medially toward the lower lip [26]. Contrary to common belief that the marginal mandibular is a single branch, the nerve travels toward the oral commissure as one or two branches and often terminates in more than two branches to innervate the target muscles of the lower lip and chin [12,25-27].
Injury to the nerve may be more common with jowl and neck liposuction than with other procedures due to the superficial nerve location at the mandibular border. Damage to the nerve root in this area may cause paralysis of the muscles that depress the corner of the mouth (depressor anguli oris and depressor labii inferioris muscles). Blunt trauma from liposuction procedures may cause neurapraxia. In the author's experience, such neurapraxia resolves spontaneously in weeks to months.
Buccal and zygomatic nerves and parotid Stensen's duct — The sixth danger zone involves the path of the buccal and zygomatic branches of the facial nerve as well as the parotid Stensen's duct (picture 1A and figure 3). The nerves and duct leave the parotid parenchyma and travel medially toward the central face. The buccal branch and parotid duct travel in an imaginary line from the tragus toward the mid upper lip area; the zygomatic branch travels superiorly toward the ipsilateral medial canthus. The duct and one or several of the buccal branches of the facial nerve travel together and can be injured deep in this area. The parotid duct travels from the parotid parenchyma over the masseter muscle and dives medially through the buccinator muscle anterior to the buccal fat pad to empty just above the second maxillary molar.
A cadaveric study of the anatomy of the buccal nerve in this danger zone revealed that, on average, the buccal nerve was found 3 cm lateral to the angle of the mouth and about 5 mm medial to the medial aspect of the masseter muscle [28]. It was found 7 mm from the point of entrance of the parotid duct into the buccinator muscle and had an average distance of 5.5 mm from the facial artery. In another study, 17 of 20 cadaveric facial sides had a single buccal branch of the facial nerve, whereas 3 had two branches [29]. In 16 of 20 sides, the nerve was inferior to the duct as it emerged from the parotid gland, whereas in the other 4 the nerve crossed the duct, usually from superior to inferior. The crisscrossing points of the facial buccal nerves and the parotid duct (picture 1A) have been described as lying within a 30 mm radius that is located with a focal point located superolateral to the oral commissure and is 12 mm above and 53 mm lateral to the oral commissure [30].
Injury to the nerve roots deep in this area may cause motor deficits including the inability to tightly close the eye and an asymmetric smile. Damage to the buccal nerve will result in asymmetry as well as difficulty in chewing due to paralysis of masticatory muscles and flaccidity of the buccal mucosa. An asymmetric smile may occur with damage to the buccal or the zygomatic nerve, since both provide innervation of the lip elevator muscles; however, with more distal injuries, the redundancy between these nerves may facilitate preservation of function and appearance.
Management of buccal nerve injuries may require direct repair and/or static suspensory procedures to restore baseline symmetry. Partial to full recovery may be expected over time. Nerve grafting or repair may be an option for better recovery [31].
Damage to the parotid duct in this area may lead to edema, pain, sialocele, fistula, abscess, and infection. If injury to the parotid duct is noted intraoperatively, attempt at immediate repair (distal ductal injuries) or ligation (proximal ductal injuries) should be performed by an experienced surgeon [32]. In the event of delayed presentation, reanastomosis or grafting may be performed. If surgical repair is not feasible, parotid fistulas and sialoceles may be treated with conservative therapy, including oral antisialagogues, botulinum toxin A injections, parenteral feedings, and pressure dressings [33-41].
MIXED MOTOR AND SENSORY NERVE
Posterior triangle of the neck — The seventh danger zone involves the nerves located in the posterior triangle of the neck (picture 1A). Superficial surgery in this area may cause injury to the cutaneous sensory nerves of the cervical plexus, particularly the great auricular (GAN) and lesser occipital (LON) nerves (figure 4A). Deep surgery carries a risk of injury to the spinal accessory nerve (SAN), supraclavicular nerve, phrenic nerve, and suprascapular nerve (figure 4B) [42]:
●Posterior triangle – The posterior triangle is bordered posteriorly by the anterior aspect of the trapezius muscle, anteriorly by the posterior aspect of the sternocleidomastoid muscle (SCM), and inferiorly by the middle third of the clavicle. The posterior triangle includes the upper occipital triangle, which has similar anterior and posterior boundaries but is bordered inferiorly by the omohyoid muscle and contains the SAN and cervical plexus. One can draw a line from the external auditory meatus inferiorly 6.5 cm to locate Erb's point on the posterior border of the sternocleidomastoid muscle (SCM). Erb's point approximates the point where the four branches of the cervical plexus (GAN, LON, transverse cervical, and suprascapular nerve) exit from the posterior border of the SCM [43-45]. The cutaneous nerves run on the superficial surface of the SCM in the investing fascia, while the SAN is found deep in this area, approximately 1 to 2 cm superior to this point [46].
●The supraclavicular triangle – The supraclavicular triangle is the inferior portion of the posterior triangle separated from the upper occipital triangle by the omohyoid muscle. Multiple motor and sensory nerves run within the supraclavicular triangle, and four cutaneous branches of the cervical plexus can be found here (supraclavicular, lesser occipital, great auricular, and transverse cervical nerves). In addition, trunks of the brachial plexus with motor and sensory innervations to the upper extremities pass between the anterior and middle scalene muscles in this location. The superior trunk of the brachial plexus formed by C5 and C6 lies most superficially and as close to 1 cm beneath the skin in some individuals. This is the trunk most likely encountered in dermatologic surgery in this area and results in an inability to raise the arm or flex the elbow, a development of adduction and internal rotation contractures, and a possibly paralytic supination of the forearm [42].
Visualization of the superficial and deep nerves is critical when operating in the posterior triangle of the neck especially in open neck dissection procedures. Damage to the SAN may result in impaired motor function and chronic pain.
Management of SAN injury often requires nerve repair or grafting for optimal recovery and is a challenging surgical endeavor.
Parotid gland — The eighth danger zone includes the area of the parotid gland (picture 1A). The parotid gland has an inverted cone shape and is located in the preauricular area, bounded by the tragal cartilage and the ramus of the mandible posteriorly, the angle of the mandible inferiorly, the masseter muscle anteriorly, and the level of the tragus superiorly. The gland lies deep to the superficial musculoaponeurotic system and is invested by its own parotid fascia covering the surface of the gland.
Parotid gland injury is a well-recognized complication of parotidectomy, facelift procedures, tumor resection, and penetrating trauma to the parotid gland [36,47-49]. Two main types of parotid injuries occur in cutaneous surgery: damage to the parotid sheath and parenchyma or damage to the parotid duct. Iatrogenic damage to the parotid sheath and parenchyma may be unintentional or intentional, such as in superficial parotidectomies. Damage to the parotid duct may occur with extirpation of tumors and sharp trauma. Frequently, damage to the parotid gland or duct is not noticed intraoperatively and the patient presents within hours to days with signs and symptoms suggestive of parotid injury, such as pain and swelling worsened by eating or drinking, infection, sialocele, and/or salivary fistula. (See 'Buccal and zygomatic nerves and parotid Stensen's duct' above.)
SUMMARY AND RECOMMENDATIONS
●Danger zones of the head and neck for nerve injury – There are eight danger zones in the head and neck area where the motor or sensory nerves travel more superficially, the parotid gland and duct are located, or nerve roots emerge from the skull foramina (picture 1A-B):
•The supraorbital and supratrochlear branches of the ophthalmic branch of the trigeminal nerve and accompanying blood vessels
•The infraorbital branches of the maxillary branch of the trigeminal nerve and the infraorbital vessels
•The mental branch of the mandibular branch of the trigeminal nerve
•The temporal (frontal) branch of the facial nerve
•The marginal mandibular branch of the facial nerve
•The buccal and zygomatic branches of the facial nerve and the parotid duct (Stensen's duct)
•The posterior triangle of the neck, including the great auricular, lesser occipital, spinal accessory, supraclavicular, phrenic, and suprascapular nerves
•The parotid gland
Key structures to avoid are the branches of the trigeminal and facial nerves (figure 1), the spinal accessory nerve, the great auricular nerve (figure 4A-B), the parotid (Stensen's) duct, and the parotid gland (figure 3). (See 'Danger zones of the head and neck' above.)
●Sensory nerve injury – Injury to sensory nerves can cause a mixture of three different types of effects: paresthesia, dysesthesia, and/or anesthesia. The area involved and the duration of these symptoms depend upon the extent of the injury and the healing process.
●Motor nerve injury – Injuries to motor nerves result in either partial paralysis (paresis) or total paralysis of the target muscle. This can lead to a functional compromise that can be cosmetically disfiguring and functionally challenging. (See 'Motor nerves' above.)
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