Introduction

The lingual nerve is a sensory nerve that arises from the mandibular division of the trigeminal nerve (cranial nerve V). The lingual nerve is often in a common stem with the inferior alveolar nerve after the mandibular division enters the infratemporal fossa through the foramen ovale [1]. The lingual nerve separates from the inferior alveolar nerve and then descends anteriorly into the oral cavity. It travels adjacent to the medial surface of the mandibular ramus in the third molar region. As it does so, it innervates the mucous membrane of the anterior two-thirds of the tongue, the floor of the oral cavity, and the adjacent lingual gingiva [2]. Understanding the anatomy of the lingual nerve is crucial for performing safe surgical procedures in its area of distribution, mainly when extracting impacted mandibular third molars, as lingual nerve injury is one of the most serious complications of this procedure [1].

Structure and Function

The mandibular nerve is a mixed nerve containing afferent and efferent fibers and leaves the cranium through the foramen ovale [1]. The lingual nerve arises from the posterior branch of the mandibular nerve high in the infratemporal fossa and passes between the tensor veli palatini and lateral pterygoid muscles. It then emerges from between the lateral and medial pterygoid muscles anterior to the inferior alveolar nerve [3]. The nerve passes between the lateral surface of the medial pterygoid muscle and the mandibular ramus en route to the oral cavity. 

The lingual nerve reaches the oral cavity passing below the inferior border of the superior constrictor muscle in close proximity to the pterygomandibular raphe's attachment in the mandible. It courses anteriorly and inferiorly adjacent to the medial aspect of the mandibular ramus traveling through the vicinity of the mandibular third molar's roots. This relation is important during wisdom tooth extraction.

The lingual nerve continues to run anteriorly, but now it turns medially to reach the tongue, traveling between the hyoglossus and mylohyoid muscles. The nerve extends to the hyoglossus's lateral aspect and gets to its front border. The nerve loops under the submandibular duct as it passes toward the apex of the tongue [2]. The lingual nerve terminal branches provide somatic afferent innervation to the mucosa of the anterior two-thirds of the tongue [4]. During its trajectory through the floor of the mouth, the lingual nerve gives branches to innervate the lingual gingiva of the ipsilateral mandibular teeth and the floor of the mouth [3].

Embryology

The trigeminal nerve neurons originate from the neural crest cells from the cranial mesencephalon and caudal diencephalon.

Nerves

The chorda tympani nerve (branch of the facial nerve) joins the lingual nerve below the skull near the lower border of the lateral pterygoid muscle within the pterygomandibular space [3]. The lingual nerve transports special visceral afferent (SVA) fibers from the chorda tympani responsible for taste perception in the anterior two-thirds of the tongue, and general visceral efferent (GVE) fibers responsible for secretory and motor innervation of the sublingual and submandibular glands [3]. These preganglionic fibers synapse in the submandibular ganglion, suspended from the lingual nerve, and the postganglionic parasympathetic fibers enter the glands [5]. The lingual nerve also has anterior, middle, and posterior communicating branches with the hypoglossal nerve [1].

Other sensory branches of the mandibular nerve are the auriculotemporal nerve, the buccal nerve, and the inferior alveolar nerve; however, the inferior alveolar nerve also gives off a small motor branch, the mylohyoid nerve.

Physiologic Variants

Variations in the termination of the lingual nerve, specifically in its branching pattern, have been described. The lingual nerve has been found to divide into between one and four branches at its terminal end in the tongue [2]. The most common type of branching seen at the termination of the lingual nerve is two branches. Another variation is the distance the lingual nerve travels on the floor of the mouth before turning medially toward the tongue. Most lingual nerves are found to deviate toward the tongue from the floor of the mouth between the mesial of the first molar and the distal of the second molar [2]. The lingual nerve can also have connections with the mylohyoid nerves, occurring in up to 12.50% of cases [6]. When considering surgery in the infratemporal fossa or floor of the oral cavity for a patient, these possible variations in the course of the lingual nerve are extremely important to consider.

Surgical Considerations

The lingual nerve is one of the two nerves most commonly injured during oral surgery with the other being the inferior alveolar nerve [3]. Numerous procedures have been thought to cause lingual nerve damage, including mandibular resection, extraction of a third molar tooth, surgery of the salivary glands, excision of tumors, and lingual flap retraction [7][8].

The lingual nerve may also suffer an injury during suspension laryngoscopy, a procedure commonly done in otorhinolaryngology. The risk of damage to the nerve increases during this procedure when there is difficulty during the surgery and longer operating times [9].

An increase in the number of second molar implants has also increased the risk of injury to the nerve [3]. However, the lingual nerve most commonly gets damaged during inferior alveolar nerve block injections and third molar tooth removals [3][10]. Reports indicate that damage to this nerve occurs during the removal of the third molar between 0 and 23% of the time [11][12]. The wide variation in reported frequencies of this injury may be in part due to the variations seen in the course of the lingual nerve. Potential mechanisms for preventing lingual nerve injury during third molar surgery include avoiding lingual flap elevation and conducting tooth sectioning [8][13]. 

Injury to the lingual nerve most often results in temporary symptoms, including hyperaesthesia (increased sensitivity), anesthesia (complete loss of feeling),  hypoaesthesia (diminished sensitivity), and dysaesthesia (painful sensation) in the anterior two-thirds of the tongue [7]. Other complications from lingual injury include mechanical (due to biting) and thermal trauma if the tongue and lip are affected by paraesthesia, speaking difficulty, problems with tooth brushing, and sleep difficulty when there is allodynia [14]. Reports indicate that the nerve typically repairs itself within 6 months of damage [15]. If the nerve is injured during dental work and does not repair itself, treatment can be given, including physiotherapy, pharmacotherapy, and stellate ganglion block [16]. It can sometimes be repaired with a direct epineural repair or indirect (graft) neurorrhaphy, most commonly using the sural nerve [4].

The lingual nerve can also be the target for a nerve block during certain pathological conditions and procedures. Specifically, a lingual nerve block has been found to be useful for procedures and conditions occurring within the soft tissues of the tongue, including minor infections, traumatic lacerations, and pathological lesions [17]. Benefits of a lingual nerve block instead of an inferior alveolar nerve block include avoiding unnecessary anesthesia of the lip, chin, and buccal soft tissues [17]. This difference is an especially important consideration in patients with mental deficits, children, and the elderly, who present with traumatic lip biting [17].

Clinical Significance

As the lingual nerve travels from the infratemporal fossa to underneath the tongue, it can become entrapped by several structures, including fibers of the lateral pterygoid muscle, pterygospinous or pterygoalar ligaments, and the lateral pterygoid plate. Entrapment of the lingual nerve can cause numbness and loss of taste from the anterior two-thirds of the tongue, loss of sensation from the lingual gingiva, and pain with speech disorders [18].

Smoking can impact the somatosensory function of the lingual nerve, causing degeneration of the nerve fibers. Studies have indicated that smokers have a loss of non-noxious thermal stimuli, but not loss of sensation of mechanical or painful thermal stimuli [19]. A hypothesis to the cause of this sensory change is the degeneration of the thermoreceptors [19].

Lingual Nerve Injury during Dental Procedures

The lingual nerve can be damaged due to dental procedures, nerve blocks, or pathological tooth complications. The most common dental reason for lingual nerve injury is the extraction of mandibular wisdom teeth, followed by the administration of inferior alveolar nerve block and endodontic or periodontal complications [14]. A study showed that the lingual nerve was damaged in 89% of paraesthesia cases after the inferior alveolar nerve block [20].

Several reasons are believed to play a role in the lingual nerve's high susceptibility during the IAN block, which is even higher than the inferior alveolar nerve's. The lingual nerve tends to be unifascicular and has a thick perineurium, making it less capable of supporting trauma from edema and hemorrhage [14]. Furthermore, it is located at only 5 to 6 mm below the mucosa's surface and lacks osseous protection [14][21][22]. Another reason could be that the lingual nerve stretches taut when opening the mouth during the administration of the block and is fixed underneath the interpterygoid fascia. It may not be capable of moving when contacted by the needle, resulting in the nerve being directly penetrated by the needle, with the consequent damage [23].

When the lingual nerve is affected during a traumatic incident, the chorda tympani (facial nerve branch) may also be damaged, and dysgeusia and xerostomia may develop [14].

Other Issues

The lingual nerve is more likely to be damaged during the administration of the inferior alveolar nerve block than the inferior alveolar nerve. Even though neural injuries are rare, it is imperative that dental professionals have deep knowledge of the anatomy of the lingual nerve and an understanding of the IAN block technique to avoid complications.