Numb chin syndrome

INTRODUCTION — Numb chin syndrome (NCS) was initially described in the 1830s by Charles Bell when he noted an absence of sensation in the left lower lip of a patient he was seeing for breast cancer. During her examination, he noted a palpable mass extending along the angle of her jaw, presumably into the mental foramen. The term "syndrome of the numb chin" did not arrive, however, until the 1960s when Calverley and Mohnac described five patients with chin numbness from various malignancies [1]. Though it is now understood that NCS may arise from a variety of conditions, it is the known potential association with malignancy that is most concerning.

Given the relatively small area involved, it is possible for both patients and clinicians to underemphasize this symptom. It is the goal of this topic to improve the understanding of the widespread differential of this potentially ominous sign. This topic will review the anatomy, clinical features, differential diagnosis, evaluation, and prognosis for NCS.

NERVE ANATOMY — NCS is characterized by numbness or decreased sensation of the chin and lower lip that occurs within the region innervated by the mental nerve or inferior alveolar nerves, which are terminal branches of the mandibular nerve, one of the divisions of the trigeminal nerve.

Moving distally, the inferior alveolar nerve travels through the medial surface of the mandible though the mandibular canal, dividing into the incisive and mental branches just anterior to the first premolar tooth (figure 1 and figure 2). The incisive branch innervates the first premolar, canine, and incisor teeth along with the associated vestibular gingiva. The mental nerve passes through the mental foramen on the anterior mandible, typically below the second premolar tooth, before dividing into two or three branches that supply sensation to the skin of the chin, the mucous membrane of the lower lip, and the buccal gingivae of the mandibular teeth and premolars [2,3]. These branches typically stop at midline [4].

Moving proximally, the inferior alveolar nerve is a branch off the posterior portion of the mandibular nerve, which passes through the foramen ovale before connecting to the trigeminal ganglion in Meckel's cave. This sends afferent sensory trigeminal fibers into the lateral pons, coursing dorsomedially to the principal sensory and spinal trigeminal nuclei in the brainstem.

PATHOPHYSIOLOGY — NCS can occur from a lesion anywhere along the course of the trigeminal nerve, including proximally within the Gasserian ganglion or even the nerve cell bodies within the pons [5]. Lesions can be broadly divided into peripheral lesions (involving the mandible or direct nerve infiltration, inflammation, or compression) and central lesions (involving the base of skull, leptomeninges, or brainstem). In the case of malignancy-associated NCS, neuropathy can be from direct infiltration of the nerve or mandible, such as from melanoma of the oral mucosa, or metastasis from a distant neoplasm.

In some cases, patients with a known history of malignancy will present with NCS, but there will be no obvious mass in the mandible or skull base. It seems likely that some of these cases are related to leptomeningeal seeding, as suggested by the following observations:

●In a retrospective study of 36 patients with NCS who were evaluated with panoramic radiographs, CT of the brain and base of skull, bone scintigraphy, and cerebrospinal fluid (CSF) analysis, 22 percent had normal imaging but malignant cells in the CSF consistent with leptomeningeal seeding [6].

●In a report of four patients who presented with NCS as the first sign of intracranial metastases, the CSF showed no evidence of cellularity [7]. At autopsy, each patient had meningeal or cranial nerve involvement by the tumor, often with evidence of widespread meningeal involvement [7].

●Even in the absence of a known mass lesion, some patients with NCS respond to focal radiotherapy to the mandible, possibly suggestive of small clinically and radiologically undetectable mandibular metastases [8,9].

There have been other mechanisms proposed to explain NCS in patients with known distal neoplasm but no obvious mass on imaging. As an example, cervical lymphadenopathy in the deeper upper cervical region might exert pressure on the inferior alveolar nerve as it passes beneath the border of the lateral pterygoid to enter the mandibular canal [10]. In addition, some cases of NCS may be a paraneoplastic process, with antibodies directed against a yet-unknown antigen in the nervous system. In support of this, a case of buccal neuropathy ("numb cheek" syndrome) was associated with the anti-Hu antibody [11]. Pathogenesis remains uncertain, however, as the patient went on to be diagnosed with small cell lung cancer four months later.

CAUSES — NCS has been described as the initial manifestation of several conditions including giant cell arteritis [12,13], thalamic infarct [14], multiple sclerosis [15,16], multiple myeloma [17], leukemia/lymphoma [18,19], posttransplant lymphoproliferative disorder [20], HIV (due to Burkett lymphoma) [21], and numerous malignancies (table 1 and table 2 and table 3).

Malignancy — Metastatic lesions to the oral and maxillofacial (OMF) region are rare, comprising only about 1 to 3 percent of OMF malignancies [22-25], and are often evidence of a widespread disease [26]. Within OMF metastases, the mandible is probably the most frequent site affected. In a review of 765 cases of OMF metastases reported in the English and Chinese literature, the mandible was the most commonly affected area (36 percent of patients), with numbness of the lower lip occurring in 14 percent of cases [22].

Almost all types of tumors can metastasize to the OMF region [22]. In a series of 673 cases of oral metastases, the major primary tumor sites in males included lung, kidney, liver, and prostate [26]. In females, metastases from breast, female genital organs, kidney, and the colorectum were more common. In a systematic review of 136 patients presenting specifically with NCS associated with malignancy, the most frequent primary tumor was breast cancer (40 percent), followed by lymphoma (21 percent), prostate cancer (7 percent), and leukemia (5 percent) [27].

Cases of bilateral NCS have been reported more frequently with hematologic than solid malignancies, possibly because hematologic malignancies infiltrate the central nervous system more readily than solid malignancies [19]. In patients with NCS secondary to malignancy, NCS is the presenting symptom in 47 percent [8]. In patients with a known history of malignancy, NCS often indicates tumor recurrence or progression [3,28].

Outside of metastatic lesions, NCS may be the first manifestation of a primary oropharyngeal tumor either affecting the mental or inferior alveolar nerve or the surrounding mandible [29]. Direct extension with perineural tumor invasion may occur with local malignancies such as lip melanoma, squamous cell carcinoma, lymphoma, and myeloma [30].

Schwannomas and malignant peripheral nerve sheath tumors can arise from Schwann cells around the trigeminal nerve [31], while glial tumors may rarely arise from the trigeminal nerve root entry zone [32]. These tumors present with altered sensation and facial pain along the affected divisions of the trigeminal nerve. (See "Peripheral nerve tumors".)

Dental and traumatic injury — The most common cause of NCS is probably dental, traumatic, and iatrogenic injury to the mental and inferior alveolar nerves. Iatrogenic injury can occur with numerous procedures, including removal of impacted teeth or cysts, local anesthetic blocks, bone grafting, endodontic therapy, orthognathic surgery, and dental implant placement. Between 9 and 69 percent of patients undergoing orthognathic surgery describe some paresthesias of the lip or chin, depending on the procedure [33-35]. Numbness confined to the mental or inferior alveolar distribution has been described with bisphosphonate-related osteonecrosis of the jaw [36,37]. Therefore, in an oncologic patient requiring intravenous bisphosphonates for possible bone metastases, both metastases and osteonecrosis should be considered as a cause of new-onset NCS [37]. (See "Medication-related osteonecrosis of the jaw in patients with cancer".)

Outside of orthognathic surgery, removal of the third molar has one of the highest incidences of injury to the inferior alveolar nerve, estimated at 1 to 12 percent [38]. This incidence may increase as high as 20 to 60 percent if there is intraoperative exposure or tooth contact with the inferior alveolar nerve [38-40]. Fortunately, most of these cases have gradual improvement in their sensation. After third molar removal with injury to the inferior alveolar nerve, only about 1 percent of patients have permanent numbness [41], with the risk increasing to 30 percent if there is intraoperative exposure of the inferior alveolar neurovascular bundle [39].

With age, mandibular atrophy may increase the risk of mental nerve injury [42]. After the loss of teeth, there is a slow process of alveolar resorption that may eventually lead to exposure of the mental foramen, sometimes leading to pressure trauma to the mental nerve. This may be noted in older adult patients wearing ill-fitting dentures that slide or move over an exposed mental foramen during chewing [43,44]. There have been several cases of orthodontic treatment associated with NCS or lower lip paresthesias, presumably related to movement of the apices of the molars in relation to the inferior alveolar canal and neurovascular bundle [45].

The location of the mental nerve branches also places them at risk for injury following procedures to the mucosal lower lip, such as minor salivary gland biopsies [46]. One of the more unusual cases of traumatic injury as a cause of NCS was a patient who scalded his tongue with hot food [47]. This resulted in an inflammatory reaction of the lingual nerve with some apparent retrograde inflammation affecting the mandibular branch of the trigeminal nerve. The patient had numbness over the ipsilateral tongue, mandibular gingiva, teeth, and lower lip with paresthesia that eventually spread to the chin and angle of the jaw. Magnetic resonance imaging (MRI) demonstrated enhancement and enlargement of the mandibular nerve. The numbness improved over a week, and follow-up MRI showed resolution of the mandibular nerve abnormalities [47].

Even in cases of NCS after a known dental procedure, the possibility of malignancy is still present. A case report described a female presenting with NCS a few months after placement of a dental implant [48]. Evaluation revealed widespread metastatic breast cancer with peri-implant lesions. There have been multiple case reports of primary and metastatic tumors around dental implants, sometimes mimicking benign implant complications such as peri-implantitis [48-50]. Though no cause and effect with implants have been established, some authors have theorized that the trauma of implant placement and the subsequent peri-implant inflammation may contribute to the development of tumors in the region [48,51].

Toxin or drug induced — In case reports, numbness confined to the mental or inferior alveolar distribution was associated with mefloquine treatment just prior to travel [52]. Another report described a patient on allopurinol who had transient NCS and left hand hypesthesia that resolved two weeks after discontinuing the medicine [53].

Other medication or toxic exposures have been linked with more patchy or extensive involvement of the trigeminal nerve. As examples, interferon-alpha has been associated with trigeminal neuropathy, especially affecting V2 and V3 [54]. Exposure to trichloroethylene, an organic solvent, has been associated with a subacute trigeminal neuropathy and other cranial neuropathies [55]. Some authors have suggested that it is actually dichloroacetylene, a degradation byproduct of trichloroethylene and known neurotoxin to the trigeminal sensory nucleus, which is responsible for the trigeminal neuropathy [56].

Infection — Isolated unilateral and bilateral trigeminal neuropathies may occur rarely as manifestations of syphilis [57,58]. Isolated mental neuropathy has also been described with Lyme disease [59] and herpes simplex virus [60].

NCS caused by lymphoma has been described in patients with HIV infection [61,62] and was the initial presentation of HIV in one patient [21].

Inflammatory/autoimmune — In rare cases, NCS may be the initial presenting symptom of multiple sclerosis [15,16]. In addition, there are case reports of NCS with sensorimotor mononeuritis multiplex [63], NCS with giant cell arteritis [12,13], post-hepatitis B vaccination mental neuropathy [64], and NCS as part of a multifocal sensory neuropathy associated with anti-GQ1b antibody [65].

Trigeminal sensory neuropathy has been associated with a wide variety of connective tissue diseases including systemic lupus erythematosus, Sjögren's disease, scleroderma, rheumatoid arthritis, mixed connective tissue disease, and dermatomyositis [66-68]. In a review of patients with connective tissue disease who developed a trigeminal sensory neuropathy, 15 percent of patients had numbness only in the mandibular distribution on one side [66].

Isolated unilateral and bilateral trigeminal neuropathies can occur with sarcoidosis [69-71]. A sarcoid granuloma may present with progressive painful trigeminal neuropathy on one side and resemble a peripheral nerve sheath tumor on imaging [69,71]. Amyloid infiltration of the gasserian ganglion can be associated with patchy unilateral or bilateral loss of facial sensation, especially in the maxillary and mandibular divisions [72-77]. This may be associated with pain, mimicking trigeminal neuralgia [74].

In one small report, patients with chronic inflammatory demyelinating polyneuropathy and those with a severe diabetic polyneuropathy frequently had electrophysiologic evidence of a sensory neuropathy involving the mandibular nerve, but this was primarily subclinical [78].

Sickle cell disease — Patients with severe mandibular bone pain from vaso-occlusive pain may experience numbness of the chin, which in some cases may recover very slowly (18 months in one patient) [79]. NCS related to sickle cell disease may be due to thrombosis of the vasa nervorum supplying the inferior alveolar or mental nerves or to compression and ischemia of the inferior alveolar nerve from periosteal inflammation within the mandibular canal [29,80].

Cerebral ischemia — Ischemic stroke may present with isolated sensory deficits, including numbness restricted to the face and hand (cheiro-oral syndrome), but it is rare to have isolated facial numbness as a result of cerebral ischemia. There is one report of a case of lower lip and chin numbness as the presenting complaint for a small contralateral thalamic infarct affecting the ventral posteromedial nucleus [14].

Craniofacial fibrous dysplasia — Rare cases of mandibular fibrous dysplasia have been reported to be associated with numbness with or without pain within the mental nerve distribution [81,82]. Surgical decompression of the inferior alveolar nerve has resulted in improvement in both pain and sensation [83].

CLINICAL FEATURES — NCS involves reduced or absent sensation in an area over the chin and lower lip within the distribution of the mental or inferior alveolar nerves (figure 1 and figure 2). Sensory loss may be associated with paresthesia, dysesthesia, or pain. Cases of NCS associated with malignancy tend to be "dead-type" numbness, with pain occurring in less than 10 percent of patients [27]. When pain does occur, however, it may take on any quality, including neuralgic-type pain, mimicking trigeminal neuralgia [84,85]. Motor symptoms and altered taste sensation are rare. NCS tends to be unilateral but may be bilateral in 10 to 15 percent of cases [6,27].

In a series of 36 patients with malignancy-associated NCS, other neurologic findings on examination were present in only 8 patients (22 percent) [6]. Patients with skull-based metastases tended to have abnormal findings related to other cranial nerves, especially the sixth and seventh cranial nerves. Patients with leptomeningeal metastases had more subtle findings such as areflexia, which was noted in four of eight patients with leptomeningeal metastases [6]. In cases of mandibular metastases, the patient may also complain of swelling of the gums, loosening of teeth, and jaw pain [26]. Pathologic fractures are uncommon.

EVALUATION — In the absence of a known, temporally-related dental or iatrogenic etiology, patients with unexplained new-onset numbness in the area of the mental or inferior alveolar nerve require careful evaluation with imaging and laboratory studies to exclude malignancy and other serious conditions.

History and examination — The initial evaluation should include assessment for any history of dental procedure/trauma to the region, systemic malignancy (even remote), and local or systemic symptoms concerning for malignancy. Malignancies can mimic common dental conditions such as temporomandibular joint pain, dental infection, or osteomyelitis. There may be swelling of the gums, pain, and loosening of the teeth.

The examination should include detailed neurologic testing of the cranial nerves to document the distribution of trigeminal territory sensory loss and any associated cranial nerve abnormalities.

Imaging — When the cause of NCS is uncertain or unexplained, imaging is mandatory. It can be valuable to discuss the case with the radiologist to help plan the type and sequence of imaging studies. The choice of different imaging studies may depend upon the clinical setting:

●For patients with NCS and no history of trauma, orofacial surgery, or dental procedures, we suggest a contrast-enhanced MRI with special attention to the length of the trigeminal nerve and its branches extending through the face and mandible. An MRI of the brain with gadolinium may identify leptomeningeal or demyelinating disease but will not adequately visualize the trigeminal nerve, mental foramen, or mandible. An MRI of the brain with a dedicated trigeminal nerve protocol will help assess the nerve root entry zone and much of the nerve itself in greater detail but still may not visualize the mandible and mental foramen in detail. In patients with a negative MRI of the brain, an MRI of the face and mandible or MR neurography of the inferior alveolar nerve may be helpful [86-88].

●A positron emission tomography (PET) scan is typically the next step if the MRI of the brain and mandible are negative and suspicion for malignancy is still present. In cases of neurolymphomatosis specifically, PET may have a slightly higher sensitivity than MRI [89].

●CT is the optimal study in the setting of suspected traumatic injury [90]. A CT of the head and neck is superior to MRI for demonstrating bony anatomy and can assess the mandible for masses, especially around the mental foramen and inferior alveolar canal. However, the nerve itself is better imaged with MRI [91]. For patients with known trauma or iatrogenic injury to the inferior alveolar nerve and any concern for posttraumatic neuroma, some radiologists advocate imaging with high-resolution three-dimensional volume-rendering magnetic resonance neurography [92].

●Patients who present to a dentist with a complaint of NCS may be evaluated initially with a panoramic dental radiograph. However, this study is inadequate since malignancies may mimic osteomyelitis or periapical infections on dental radiographs, and further imaging is often required.

Any mass found on imaging can be further evaluated by biopsy if necessary. A CT of the chest, abdomen, and pelvis, fluorodeoxyglucose PET or CT, or whole-body scintigraphy with technetium (Tc99m) can assess for the primary malignancy or more diffuse metastases.

Laboratory studies — For patients with no clear history of trauma to the nerve and negative or indeterminate imaging (see 'Imaging' above), we suggest a broader evaluation for metabolic, infectious, and inflammatory causes, including cerebrospinal fluid analysis (see 'Cerebrospinal fluid analysis' below) and blood and serologic studies such as:

●Complete blood count

●Erythrocyte sedimentation rate (see "Acute phase reactants", section on 'Erythrocyte sedimentation rate')

●C-reactive protein (see "Acute phase reactants", section on 'C-reactive protein')

●Antinuclear antibodies (see "Measurement and clinical significance of antinuclear antibodies")

●Antibodies to extractable nuclear antigens including Ro/SSA and La/SSB (see "The anti-Ro/SSA and anti-La/SSB antigen-antibody systems")

●Glycated hemoglobin (hemoglobin A1C) (see "Clinical presentation, diagnosis, and initial evaluation of diabetes mellitus in adults", section on 'A1C')

●Angiotensin converting enzyme assay (see "Neurologic sarcoidosis", section on 'Diagnostic approach')

●Monoclonal protein with immunofixation (see "Laboratory methods for analyzing monoclonal proteins")

●Lactate dehydrogenase level

●Peripheral blood smear if there is suspicion for lymphoma or leukemia (see "Evaluation of the peripheral blood smear")

Depending upon clinical suspicion, an evaluation for infectious causes may include the following:

●HIV testing (see "Screening and diagnostic testing for HIV infection")

●Lyme serology (see "Nervous system Lyme disease", section on 'Peripheral neuropathy')

●Syphilis testing (see "Neurosyphilis", section on 'Diagnosis')

●Herpes simplex virus testing (see "Epidemiology, clinical manifestations, and diagnosis of herpes simplex virus type 1 infection")

Cerebrospinal fluid analysis — For patients with no clear history of trauma to the nerve and negative or indeterminate imaging (see 'Imaging' above), we suggest a lumbar puncture for cerebrospinal fluid (CSF) analysis. Evaluation of the CSF may reveal the presence of malignant cells, though the absence of cellularity does not rule out malignancy. In a review of 61 cases of malignancy-related NCS with lumbar puncture results, malignant cells in the CSF were present in 12 patients (20 percent), with solid tumors in six patients and hematologic malignancies in the remaining six [29]. Repeated CSF testing may be useful when there is suspicion for hematologic malignancy if initial findings are negative for malignant cells. Both cytology (cytomorphologic examination by light microscopy) and flow cytometry should be performed with each CSF test, as the combination is more sensitive than cytology alone [93]. (See "Secondary central nervous system lymphoma: Clinical features and diagnosis", section on 'Lumbar puncture'.)

Electrodiagnostic studies — For patients with sensory symptoms outside the mental nerve distribution, electromyography with nerve conduction studies including blink studies may be useful to assess the extent of trigeminal nerve dysfunction.

TREATMENT — The treatment of NCS depends upon the underlying cause.

●Traumatic or surgical causes – With traumatic or surgical injury to the trigeminal nerve, such as removal of the third molar tooth, most patients will recover sensation within the first three months [94]. Therefore, the patient can be observed with follow-up visits, including a sensory examination, at one week, one month, and three months after the injury. Those with a persistent objective sensory deficit at three months can be referred to a microneurosurgeon for discussion of potential surgical options [41]. Timing is important because the nerve injury processes of wallerian degeneration, necrosis, and scar tissue formation probably become irreversible after 9 to 15 months [41]. Limited data suggest a higher success of nerve repair if the nerve is surgically repaired within nine months of injury, with progressively decreasing success rates thereafter [41].

●Inflammatory and immune-mediated causes – With inflammatory or immune causes of NCS, a trial of glucocorticoids may be warranted. Pulse therapy with methylprednisolone was associated with gradual symptom improvement in a case of NCS related to multiple sclerosis [16], while oral prednisone and prednisolone were associated with gradual improvement of pain and numbness in cases with giant cell arteritis [12,13]. In contrast, prednisone was administered to 38 patients with trigeminal sensory neuropathy related to connective tissue disease, and only two had improvement in pain [66]. Treatment of the connective tissue disease generally had no effect on the numbness [66].

●Malignancy – In cases of NCS associated with malignancy, treatment of the underlying neoplasm may improve the symptoms of NCS. In a review of 95 cases of malignancy-related NCS treated predominantly with radiotherapy and/or chemotherapy, there was full or partial resolution of NCS in 64 and 20 percent of patients, respectively [29].

Neurogenic pain with NCS may improve with pharmacologic approaches. Some patients with painful neoplastic trigeminal neuropathy had initial improvement with carbamazepine or amitriptyline [84]. Gabapentin and pregabalin have also been effective for select cases of neuropathic pain associated with NCS and other trigeminal neuropathy [54,95]. Anecdotally, the author has also had success with topical creams combining amitriptyline, ketamine, and sometimes lidocaine in cases of terminal branch neuropathies of the trigeminal nerve.

PROGNOSIS — As with treatment, the prognosis of NCS depends upon the cause:

●With iatrogenic injury to the mental or inferior alveolar nerve, most have gradual improvement in sensation over the first three months. However, numbness that persists after one year is likely to be permanent [96].

●With trigeminal neuropathy related to connective tissue disorders, recovery is uncommon. In one report of 66 patients with long-term follow-up (median 5 years), marked improvement was noted in only 3 percent, and no patient had full recovery of sensation [66].

●The presence of NCS related to malignancy has a poor prognosis, often heralding onset, progression, or recurrence of the neoplasm [6,8,97]. In a review of 16 studies with follow-up data for 84 patients with NCS occurring in the setting of malignancy, the overall mortality was 79 percent, with a weighted mean survival of approximately seven months [27].

SUMMARY AND RECOMMENDATIONS

●Definition and anatomy – NCS is characterized by numbness or decreased sensation of the chin and lower lip that occurs within the region innervated by the mental nerve or inferior alveolar nerves, which are terminal branches of the mandibular nerve, one of the divisions of the trigeminal nerve (figure 1 and figure 2). (See 'Nerve anatomy' above.)

●Pathophysiology – NCS can occur from a lesion anywhere along the course of the trigeminal nerve, including proximally within the Gasserian ganglion or even the nerve cell bodies within the pons [5]. Lesions can be broadly divided into peripheral lesions (involving the mandible or direct nerve infiltration, inflammation, or compression) and central lesions (involving the base of skull, leptomeninges, or brainstem). (See 'Pathophysiology' above.)

●Common causes – NCS and other types of trigeminal neuropathy can be due to several etiologies. These include neoplastic (table 1), inflammatory (table 2), and infectious, traumatic, dental, or toxic causes (table 3). (See 'Causes' above.)

●Clinical features – NCS involves reduced or absent sensation in an area over the chin and lower lip within the distribution of the mental or inferior alveolar nerves (figure 1 and figure 2). Sensory loss may be associated with paresthesia, dysesthesia, or pain. (See 'Clinical features' above.)

●Diagnostic evaluation – In the absence of a known dental or iatrogenic etiology, patients with unexplained new-onset numbness in the area of the mental or inferior alveolar nerve require careful evaluation with imaging to exclude malignancy and other serious conditions.

•Imaging – For patients with acute onset of NCS and no obvious cause, we suggest contrast-enhanced MRI with special attention to the length of the trigeminal nerve and branches extending through the face and mandible.

•Laboratory testing – For patients with no clear history of trauma to the nerve and negative or indeterminate imaging, we suggest a broader evaluation for metabolic, infectious, and inflammatory causes, including cerebrospinal fluid analysis, blood, and serologic studies as discussed above. (See 'Evaluation' above.)

●Treatment and prognosis – The treatment and prognosis of NCS depend upon the underlying cause. (See 'Treatment' above and 'Prognosis' above.)