Injury to the lingual nerve is a well-recognized risk associated with certain routine dental and oral surgical procedures. The assessment and management of a patient with a traumatic lingual nerve neuropathy requires a logical and stepwise approach. The proper application and interpretation of the various neurosensory tests and maneuvers is critical to establishing an accurate diagnosis. The implementation of a surgical or nonsurgical treatment strategy is based not only on the established diagnosis, but also a multitude of variables including patient age, timing and nature of the injury, and the emotional or psychological impact.
Key points
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Injury to the lingual nerve is a well-recognized risk associated with certain routine dental and oral surgical procedures.
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The assessment and management of a patient with a traumatic lingual nerve neuropathy requires a logical and stepwise approach.
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The proper application and interpretation of the various neurosensory tests and imaging is critical in establishing an accurate diagnosis.
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When indicated, surgical repair of an injured lingual nerve can restore protective sensation in most patients.
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Nonsurgical management of traumatic lingual nerve neuropathies are helpful in optimizing the quality of life for patients with chronic pain or functional limitations.
Injury to the peripheral trigeminal nerve is a well-recognized risk associated with certain routine dental and oral surgical procedures. The lingual nerve can be injured as a result of iatrogenic mechanisms (odontectomies, orthognathic surgery, bone graft harvesting, tumor or cyst surgery, local anesthetic injections). The nature and character of the neurosensory dysfunction following such injuries is highly variable and not necessarily injury specific. This factor underscores the importance of the initial evaluation and continued assessment of the neurologic status for those patients who present with a traumatic neuropathy of the lingual nerve.
Similar to other pathologic conditions, the assessment and management of a patient with a neurosensory disturbance requires a logical and stepwise approach. This approach includes a detailed history, clinical neurosensory examination, development of a working diagnosis, and a treatment strategy. The initial evaluation and clinical examination are by far the most important because all subsequent examinations will be compared with it to determine if there has been a change in the sensory function over time. The time-honored practice of using serial neurosensory examinations to assist in stratifying the various objective and subjective findings associated with a traumatized lingual nerve is essential in establishing a diagnosis and a treatment plan. In this article, we present a stepwise approach to the evaluation, diagnosis, and treatment of a traumatic lingual nerve neuropathy.
Initial evaluation
During the initial interview, it is essential that the timing of the injury be ascertained. Sensory alterations that recover within 4 to 6 weeks are likely neuropraxia-type injuries that often have an excellent prognosis. Patients with a more prolonged schedule of recovery or improvement (2–3 months) represent a more involved injury (axonotmesis type), where the degree spontaneous recovery can be very variable. Those patients who present with no signs of sensory function return beyond 3 months are indicative of a more serious injury (neurotmesis type) and are associated with a poor prognosis for spontaneous recovery. Regardless of the nature of the injury, if the time from injury exceeds 12 months the likelihood of a favorable outcome after surgical treatment (if required) is greatly diminished.
Information about the mechanism of injury can also be very relevant in certain clinical scenarios. For example, whereas injection-related injuries are rarely addressed with surgical treatment, witnessed nerve injuries often require surgical treatment as soon as possible.
Patients should be initially segregated based on whether their altered sensation is a decreased sensation (anesthesia, paresthesia) or a painful sensation ( Figs. 1 and 2 ). The importance of this initial distinction is centered on the fact that the treatment and evaluation of these entities is very different. In that regard, the patient’s description of the sensory disturbance can be very helpful in discerning the type of disorder. Descriptors such as numbness, tingling, swollen, and tightness are suggestive of a decreased level sensation, whereas terms like stinging, burning, and tenderness are more suggestive of a painful neuropathy. The clinician also has to be aware that patients may be understandably angry about their condition and motivated to exaggerate their symptoms for medicolegal or other reasons. Therefore, it is important to differentiate what patients report as “painful” or “disturbing” from a numbness that is annoying or frustrating. Also, the patient’s perception of a functional deficit is relevant because this factor typically motivates patients to consider surgical treatment. , Patients with a lingual nerve injury should be questioned regarding the presence of tongue biting; drooling; burning of the tongue; speech difficulties; difficulty in chewing, swallowing, or drinking; an inability to distinguish between excessively hot and cold foods; difficulty with kissing or being intimate with a partner; and pain or limitation when performing routine dental hygiene. If present, these types of functional impairments can also trigger feelings of an altered self-image and difficulties with socialization and depression. Patients should also be queried as to whether the pain or numbness is progressive in nature or has improved since the injury.
Physical examination and neurosensory testing
The initial neurosensory clinical examination is the most important tool in determining viable treatment options. A detailed history of the injury along with current complaints followed by subjective and objective testing will organize patients into groups where treatment decisions can be made.
Patients with decreased tongue sensation are those who experience a loss of sensation without pain. These patients either have a diminished response to a painful or nonpainful stimulus (parasthesia) or are completely without sensation (anesthesia). Patients who present with anesthesia need to be stratified based on the degree of functional impairment (ie, mild, moderate, severe). ,
Patients with painful lingual nerve neuropathies can present with either stimulus-induced pain or spontaneous pain. Stimulus induced pain is characterized as hyperalgesia (an exaggerated response to a sharp pinprick), allodynia (a painful sensation to a stimulus that normally does not cause pain, for example, eating or brushing the teeth), or hyperpathia (a delayed or prolonged response to a normally painful stimulus). Pain that is spontaneous in nature can be intermittent or constant.
After determining which treatment algorithm is most appropriate for the patient, a focused clinical examination is important to establish a diagnosis and develop a treatment plan. Palpation of the posterior lingual plate may elicit a trigger response characterized by a shooting or painful sensation at or distal to the injured site (Tinel’s sign). An evaluation of the contralateral “normal” side will serve as a control for the entire neurosensory examination.
Neurosensory testing in the setting of anesthesia or paresthesia is divided into 3 levels.
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Level A evaluates for tactile direction and 2-point discrimination. Tactile direction is assessed by using a cotton swab across the dorsal and ventral aspects of the tongue. This test is repeated and compared with the control side. Two-point discrimination testing measures spatial acuity. A small caliper or a contact wheel will provide 2 points of surface contact ( Fig. 3 ) A 2-point discrimination of greater than 10 mm is a reliable indicator of sensory impairment in the majority of patients. If the direction determination and the 2-point discrimination tests are within the normal range, then no further sensory challenges are indicated. If level A testing is abnormal, then level B testing is indicated.
Fig. 3 Neurosensory testing instruments. - 2.
Level B testing estimates the level of contact detection, using a set of Semmes Weinstein pressure aesthesiometers (von Frey’s fibers) (see Fig. 3 ). The test site threshold is deemed abnormal if it is 250% less sensitive compared with the control value or greater than 2 standard deviations above the published normal mean values. Only a few axons require regeneration to achieve a normal result. Therefore, patients with test site sensitivity comparable with the control site are designated as being mildly impaired and there is no need to continue testing. If level B testing is abnormal, then level C testing is indicated
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Level C testing assesses the affected domain’s response to painful stimuli (mechanical and thermal). Responses to mechanical pain stimuli can be induced by a pinprick (see Fig. 3 ). It is important to recognize that the response to this type of testing can be subjective and difficult to quantify. Alternatively, an algometer (a spring-loaded sharp probe) can be used to quantify the force needed to generate pain throughout the lingual nerve domain and compared with the control side. Thermal testing is the final level C neurosensory test. A probe is warmed (>40 °C) or cooled (<20° C) using beakers of hot or cold saline or any other method of temperature regulation and placed over the test and control site for thermal evaluation. A heated stimulus of 50 °C against normal tissue will evoke an intolerable pain response without causing tissue damage. Patients with a significantly high threshold for level C testing are considered severely impaired, whereas those with a relatively normal response to level C testing will maintain the level B diagnosis of moderately impaired.
Because the lingual nerve also contains specialized sensory fibers that relay taste information, the evaluation of taste is also indicated. Taste function is assessed by applying a cotton applicator that is soaked in either a sugar or salt water solution to the dorsal surface of the tongue in a blinded fashion. The response to taste challenges should be compared with the contralateral side or control side. The patient should rinse their mouth with plain water between challenges to remove any residual chemical.
Neurosensory testing for the patient with neuropathic tongue pain also involves all 3 levels of testing (A, B, and C); however, the objective of neurosensory testing for patients with pain is to establish the character and degree of the stimulus-evoked pain. Level A testing involves applying a nonpainful mechanical stimulus (ie, brush stroke, soft touch), which may elicit a painful response (allodynia). The frequency, duration, and intensity of the elicited pain is also recorded. Level B testing evaluates for the presence of hyperpathia, which is an exaggerated response to pain. These patients can exhibit an explosive, radiating, or poorly localized pain with a delayed occurrence and can last for an extended period of time. As with level A testing, assessments of the frequency, duration, and intensity of the pain are also recorded. Level C testing in the patient with pain involves the use of a noxious stimuli (sharp point, hot and cold metal probes) to assess the presence of an exaggerated response to pain.
Imaging
The history and physical examination of patients with a nerve injury are the most important part of developing a working diagnosis and making treatment decisions. However, certain imaging techniques can be useful in the assessment of a nerve injury. Although the panoramic radiograph offers limited information owing to the 2-dimensional nature of the study, identifying a foreign body in the region of the lingual nerve may useful. A computed tomography scan will localize a foreign body in 3 dimensions or demonstrate a violation of the lingual cortical plate. However, this imaging modality provides limited information about the condition of an injured nerve.
MRI has been used in various anatomic locations to assess the integrity of large diameter nerves and characterize pathology. A high-resolution MRI may offer some helpful information with regard to the condition of the injured lingual nerve. A change in nerve diameter may be appreciated owing to Wallerian degeneration of the nerve distal to the site of injury, or an acute change in nerve position or shape owing to retraction or neuroma formation may be visualized. Magnetic resonance neurography using diffusion imaging and isotropic 3-dimensional fast imaging with steady state precession has demonstrated a good level of correlation with preoperative neurosensory testing and intraoperative findings. This novel imaging modality may enable early identification and treatment of an operable lingual nerve injury and result in improved outcomes. It is important to note that these imaging modalities can and should only be used as a supplement to a thorough history and clinical neurosensory examination.
Treatment guidelines
The decision to proceed with microsurgical treatment is established on an individual basis and depends on the specific presentation and clinical course for each patient (see Figs. 1 and 2 ). Surgical repair should be considered when the disability is of concern to the patient and there is clinical evidence of moderate, severe, or complete sensory impairment, dystrophic ageusia, or neuropathic pain of peripheral origin. As one follows the serial neurosensory examination over time, careful attention is directed at the extent and character of sensory recovery. The Medical Research Council Scale, a well-defined guideline for assessing sensory function for extremity injuries, was established by Mackinnon and Dellon ( Table 1 ). This scale has been adapted by other investigators to grade sensory recovery in the domain of the trigeminal nerve. , Based on the response to several neurosensory measurements, a score is assigned that can range from S0 (no recovery) to S4 (complete recovery). If there is clinical evidence of spontaneous restoration of useful protective sensation (Medical Research Council Scale score of ≥3), then surgical intervention is usually not indicated because both treatment end points are similar.
