June 5, 2017
by Dr. Foad Elahi and Dr. Robert Green
Often referred to as “numb chin syndrome”, mental nerve neuropathy is a sensory neuropathy characterized by an altered sensory perception along the distribution of the mental nerve. Patients may present with anesthesia, paresthesia, dysesthesia, neuropathic pain or a combination of these symptoms. The impact that these symptoms can have on a patient cannot be underestimated. There is a substantial psychosocial burden placed on these patients. Interruption of daily social functions such as eating, drinking, speaking, kissing, applying makeup, shaving and sleeping can have a significant negative impact on the patient’s quality of life. 1
There are a variety of etiologic factors that cause a mental nerve neuropathy, and invasive dental procedures are the most common. 2 Inflammatory periapical dental pathoses, benign odontogenic cysts and tumours and infections are also causative factors. 2 BRONJ, a condition being seen with increasing frequency, may also lead to a painful neuropathy. 3 Neoplasia, both benign and malignant lesions, can occur in a considerable number of patients presenting with an initial complaint of a numb lip and chin. Metastatic breast, lung and prostate cancers, locally invasive squamous cell carcinoma and lymphoproliferative cancers such as lymphoma and leukemia are the most common. 2,4 These lesions can involve the mandible, base of skull, or occur intracranially. 2 Systemic medical conditions may also present as an isolated mandibular nerve neuropathy, or as part of a disease process, such as vasculitis, which can occur secondary to rheumatoid arthritis, cerebro-vascular accident, connective tissue disorders, multiple sclerosis, sarcoidosis, Sjogren’s syndrome, syphilis and diabetes mellitus. 1
Iatrogenic injuries following invasive dental procedures are by far the most common cause of neuropathy, typically affecting the inferior alveolar nerve. It occurs primarily due to the proximity of root apices to the nerve, and as a result, the removal of impacted third molars has the highest incidence of nerve related complications from dental extractions. The incidence for nerve injury ranges from .41% to 8.1% for temporary paresthesias and .014%-3.6% for permanent injury. 5 The mandibular second molar apices have been reported to be the closest to the mandibular canal as compared with the first molar and premolars, and dental procedures involving the second molar may have a higher incidence of inferior alveolar nerve injury. 6 Major oral surgical procedures, such as sagittal split osteotomies for orthognathic surgery and facial trauma can also result in neuropathic nerve injuries, however the incidence is low. 7
Dental implant placement can lead to injury in several ways – direct injury from surgical instruments or the implant itself, or indirectly by compression of bone adjacent to the canal. 8 The incidence of non-painful neurosensory disturbances range from .6% to 36% and it is suggested that the incidence of a painful neuropathy is around 8%. 9
Nerve injuries can occur as a result of endodontic procedures. Apical infections, trans-apical injection of hypochlorite, over-instrumentation, and extrusion of endodontic filling materials that may cause both chemical and physical injury. 9 Persistent pain after successful endodontic treatment was found to occur in 3%-13% of cases. In surgical endodontics, chronic neuropathic pain may reach 5% of cases. 9
Although rare, nerve blocks with local anesthesia can injure the nerve directly with the needle, or cause vascular damage creating a hematoma that compresses the nerve. 10 Toxicity of the local anesthetics, particularly 4% solutions, may play a role in nerve injury. 10 The incidence of this injury is rare, somewhere between 1 in 160, 571 to 1 in 26, 762. 11
The course after an inferior alveolar injury is quite variable. A patient may have spontaneous resolution of symptoms, or have a residual functional deficit or rarely, develop debilitating pain. 12 The inferior alveolar nerve may recover completely within three months when the injury is minor with no continuity loss. 12 The nerve may recover partial function within six to eight months following a partial or complete transaction, largely because the injured sections remain within the mandibular canal which favours spontaneous regeneration of the proximal stump towards the distal stump. 12 However, if the site of injury is external to the mandibular canal, such as the mental nerve, this may not apply. 12 If a patient develops a painful neuropathy, early intervention is important for optimal physiologic and functional recovery. 13 There is increasing evidence that unmanaged, ongoing pain leads to sensitization and changes in the peripheral and central nervous systems that may help establish chronic pain. 9 The transition from acute to chronic pain occurs in discrete pathophysiological steps involving multiple signalling pathways. The duration and intensity of the initial stimulus leads to both peripheral and central sensitization that synergistically exacerbates pain perception. A multimodal therapeutic approach is best suited to target the complex mechanisms leading to the transition from acute to chronic pain. Pain physicians who treat chronic neuropathic pain usually initiate multimodality and multidisciplinary managements that includes antidepressant, antiepileptic medications with local desensitization methods along with utilizing biofeedback, coping and other neurophysiological techniques. In addition, they recommend procedures like nerve blocks, radiofrequency nerve ablation, cryo-nerve ablation and chemical neurolysis if the nerve responsible for transduction and transmission of pain is easily accessible.
We present the management of two unique cases of mental nerve neuropathy subsequent to dental extraction. After a period of failed conservative medical management and repetitive successful nerve blocks at the mental foramen, it was decided to proceed with radiofrequency nerve ablation. In both cases, there was a demonstrated significant decrease in pain.
A 64-year-old woman underwent a right second molar extraction in April of 2011 with subsequent development of insidious pain and numbness around the right chin and right lower lip the day following the procedure. Multiple dental visits ensued with no sign of tooth remnant or infection clinically. NSAID’s and short acting opioids did not provide any pain relief. After a year of persistent pain and unpleasant numbness, the patient was finally referred to the pain clinic. She reported her pain as located along the right side of the chin and lower lip, associated with numbness, tingling and at times a constant burning pain with intensity rated 10 on a numerical scale. Mastication was reported to exacerbate the pain. The unremitting pain was affecting her sleep, impacting negatively on her quality of life. Treatment with indomethacin, hydrocodone and acetaminophen provided partial pain alleviation, whereas gabapentin provided no pain relief. On physical examination, significant tenderness to light touch and hyperalgesia to pinprick was noted in the area supplied by the mental nerve on the right side. No motor weakness was evident. A diagnosis of neuropathic pain was made and multimodal approach was initiated at the first visit with a combination of antiepileptic and antidepressant medications, topical Lidoderm cream and instruction on desensitization techniques. At initial follow-up, only a partial improvement was noted. Consequently, a series of right mental nerve blocks were trialed with near 100% pain reduction. In light of the patient’s persistent, unbearable pain complaints, and a worsening quality of life, it was decided to perform a trial of right mental nerve radiofrequency ablation (RFA). Under fluoroscopic guidance and local anesthesia at the skin injection site and a sterile technique, a 50 mm RFA needle with a 4 mm active tip (Baylis Medical Co., Cooksville, Ontario) was then inserted into the region of the mental foramen. Using a continuous radio-frequency current, the right mental nerve was ablated at 80 degrees Celsius for 90 seconds. The patient tolerated the procedure without any noted complications. Significant symptomatic improvement with an absence of pain, numbness and tingling was reported at the initial follow-up, and at the 8 month follow-up, a modest, yet tolerable complaint of right lower lip numbness was reported. Overall, the patient was extremely satisfied with the near 100% pain relief and quality of life improvements resultant from treatment with RFA.
An 80-year-old man was referred to the pain clinic with chronic, non-radiating pain and numbness around the right chin and lower lip that began shortly after the removal of a right third molar in July 2010. The pain was characterized as burning and described as “like a cold sore”. Pain intensity was rated as eight out of 10 on a numerical pain scale and persisted in this intensity since onset. Pain exacerbation was associated with consumption of hot and cold foods or drinks and with shaving. Over the course of two years, the patient was treated by several specialists, including other pain physicians and neurologists. During that time period, topical medications such as capsaicin and Lidoderm creams were trialed, but with no pain relief. Pregabalin and tramadol decreased the pain intensity to six out of 10 on the numerical pain scale. Despite this modest pain alleviation, the patient remained extremely frustrated and continued to ask for better pain control. On examination, the initial inspection revealed markedly more facial hair on the right chin, consistent with the patient’s purported difficulty shaving due to pain. Palpation of the right lower lip, chin, associated buccal tissues and gingivae revealed pain to light touch. The history and physical findings were suggestive of right mental nerve neuropathy secondary to the patient’s previous extraction. A series of diagnostic right mental nerve blocks were performed with a near 100% pain reduction. Considering the positive diagnostic nerve block responses, the decision was made to perform RFA of the right mental nerve using the same procedural protocol as in case 1. During follow-up examinations, the patient reported complete pain relief for six months duration, as well as marked decrease in adjuvant pain medication usage. Overall, the patient was extremely satisfied with the pain relief and quality of life improvements resultant from treatment with RFA.
Painful traumatic trigeminal neuropathy is extremely difficult to manage successfully. Pharmacologic management is initiated and the main class of medications used remain antiepileptic drugs and tricyclic antidepressants. 9 One class of drug is initiated and if not successful, the next class is instituted. If these strategies fail, combination therapy is started (combine an antidepressant with an anticonvulsant). In contrast to the traditional 50% pain reduction for clinical significance, research has shown that about a 30% reduction represents meaningful pain relief with neuropathic pain. 9 If the previous trials fail, opioids and opioid combinations may be added. The role of surgery in the management of non-painful neuropathies is well established and nerve repair may improve the level of sensation in injured patients. 9 Repair within one year shows good success rates and about 50% of repaired nerves recover with complete sensory function by seven months. 14 However, the efficacy of surgery for trigeminal neuropathies is unclear. Most patients who seek relief from trigeminal neuropathic pain by surgical repair do not show permanent pain relief after surgery. 15 Nerve repair surgery is effective in resolving or decreasing pain in some patients, whereas it has no effect on pain relief in others. 15
Chronic pain management physicians and patients are always searching for long-term solutions rather than short-lived interventions. One important non-surgical modality is radiofrequency ablation. The pain relief with radiofrequency ablation has been attributed to the thermocoagulation of the nerve fibres. This would imply that vital nerves are required for success of this treatment. Diagnostic blocks are performed before considering RFA to identify the proper nerve target for ablation, and to assess the degree of pain control. It has been demonstrated that primary afferent input is critical for maintaining neuropathic pain in peripheral nerve injury.16 Histologically, radiofrequency lesions appear as a local tissue burn, with destruction of the nerve architecture and a Wallerian degeneration. Pain recurrence after initial successful radiofrequency ablation is due to nerve regeneration, and can be managed by a repeat ablation.
We believe this is the first report of the use of radio frequency ablation for mental nerve neuropathy following dental extractions. Early recognition of mental nerve neuropathy in conjunction with accurate etiologic identification is crucial, as early pain management may prevent the transition from and acute to a chronic pain state. OH
Oral Health welcomes this original article.
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About the Authors
Dr. Foad Elahi obtained his medical degree in 1990 from University of Shahid Beheshti Medical Sciences in Tehran, Iran and was a neurosurgeon for 14 years. He has been practicing pain medicine for nearly a decade now. He previously was assistant professor at the University of Iowa. Currently, he is the president of the California Centre of Pain Medicine and Rehabilitation.
Dr. Robert Green is a staff member in the Dept of Surgery, Hamilton Health Sciences and maintains a private practice in Stoney Creek, Ontario.