Neuropathic orofacial pain: Facts and fiction

Signs and symptoms, quality of life and psychosocial data in 1331 post-traumatic trigeminal neuropathy patients seen in two tertiary referral centres in two countries

Background

Post‐traumatic trigeminal neuropathy (PTN) is a disturbance of function or pathological change of the trigeminal nerve branches following trauma and has an important impact on patient's quality of life (QoL).

Objectives

To provide diagnostic data on PTN and illustrate differences in aetiology, injured nerve, pain distribution, sensory profile and QoL between PTN subgroups.

Methods

1331 patients with painful or non‐painful PTN were retrospectively reviewed in two centres, extracting demographic data, time and cause of trauma, clinical findings including signs and symptoms, basic neurosensory testing, imaging modalities, treatments, and QoL or psychosocial assessment.

Results

More females were represented (70%) than males. The inferior alveolar nerve was most frequently damaged (60%) followed by the lingual nerve (28%). Wisdom teeth removal was considered the main cause (48%). Pain was reported in 63% of patients and pain frequency increased with age without clinically significant gender differences. Numbness was reported in 50% of PTN patients. Neurosensory testing showed larger affected dermatome involvement in persistent injuries, with no differences between the non‐painful and painful PTN groups. Patient clustering indicated different sensory profile distributions when stratified according to aetiology or affected nerve branch. High interference with lifestyle was reported (78%), and patients suffering from painful PTN had worse QoL and psychosocial outcomes.

Conclusion

Patients with painful PTN had different clinical profiles and lower QoL scores than those with non‐painful PTN. Sensory profiles may provide important prognostic and therapeutic information; however, more research is needed to assess the clustering procedure and link these clusters to therapeutic guidelines.

MiR-101 promotes pain hypersensitivity in rats with chronic constriction injury via the MKP-1 mediated MAPK pathway

This study was performed to characterize the effect of microRNA‐101 (miR‐101) on the pain hypersensitivity in CCI rat models with the involvement of mitogen‐activated protein kinase phosphatase 1 (MKP‐1) in spinal cord microglial cells. The mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) in the developed CCI models were determined to assess the hypersensitivity of rats to mechanical stimulation and thermal pain. To assess inflammation, the levels of interleukin (IL)‐1β, IL‐6 and tumour necrosis factor‐α (TNF‐α) in the spinal dorsal horns of CCI rats and lipopolysaccharide (LPS)‐activated microglial cells were examined. miR‐101 and MKP‐1 gain‐ and loss‐of‐function experiments were conducted in in vivo and in vitro settings to examine the roles of miR‐101 and MKP‐1 in CCI hypersensitivity and inflammation. The results showed that miR‐101 was highly expressed in the spinal dorsal horn and microglial cells of CCI rat models. Furthermore, overexpression of miR‐101 promoted the pain hypersensitivity in CCI rat models by reducing MWT and TWL. The overexpression of miR‐101 also promoted inflammation in LPS‐exposed microglial cells, as indicated by increased levels of IL‐1β, IL‐6 and TNF‐α. MiR‐101 was shown to target MKP‐1, inhibiting its expression. Moreover, miR‐101 promoted pain hypersensitivity in CCI rat models by inhibiting MKP‐1 expression and activating the mitogen‐activated protein kinase (MAPK) signalling pathway. Taken together, miR‐101 could potentially promote hypersensitivity and inflammatory response of microglial cells and aggravate neuropathic pain in CCI rat models by inhibiting MKP‐1 in the MAPK signalling pathway.

Therapeutic Effect of Tetrapanax papyriferus and Hederagenin on Chronic Neuropathic Pain of Chronic Constriction Injury of Sciatic Nerve Rats Based on KEGG Pathway Prediction and Experimental Verification

Background

Hederagenin is one of the main components of Tetrapanax papyriferus, and Tetrapanax papyriferus is one of the ingredients of Danggui Sini decoction. To explore whether Tetrapanax papyriferus and hederagenin can alleviate mechanical pain, thermal hyperalgesia, and cold pain at the same time, we comprehensively investigated the effects of two drugs on the levels of p38 MAPK phosphorylation, TRP proteins, and IL1β, IL6, and TNF-α in serum.

Methods

Firstly, we obtained pain-related targets and performed KEGG pathway enrichment on these targets. Then, 42 SD rats were separated randomly into six groups: sham operation group, CCI group, pregabalin group, mecobalamin group, Tetrapanax papyriferus group, and hederagenin group. All drugs were given orally. Rats in the sham operation group and CCI group were gavaged with saline. Rats in the pregabalin group were given pregabalin, while rats in the mecobalamin group were given mecobalamin. Rats in the Tetrapanax papyriferus group were given Tetrapanax papyriferus, while rats in the hederagenin group were given hederagenin. Besides, we conducted behavioral tests including acetone test, hot plate experiment, and von Frey filaments, and then dorsal root ganglion neurons were taken out on the 21st day after operation. Then, western blot, ELISA, and hematoxylin-eosin staining were conducted.

Results

Rats in the CCI group were more sensitive to hyperalgesia and allodynia to mechanical and thermal stimuli, as well as cold pain. All four drugs could relieve these pains. Pregabalin, mecobalamin, and Tetrapanax papyriferus can reduce the levels of IL1β, IL6, and TNF-α in serum compared to those of the CCI group. The expression of TRPM8, TRPA1, TRPV1, TRPV4, and phosphorylated p38 MAPK in DRG increased evidently on the 21st day after the operation in the CCI group. All four drugs could reduce the expressions of TRPM8, TRPA1, TRPV1, TRPV4, and phosphorylated p38 MAPK in dorsal root ganglion compared to those of the CCI group.

Conclusion

Tetrapanax papyriferus and hederagenin relieved sciatica by reducing inflammation levels, inhibiting p38 MAPK phosphorylation, and decreasing the levels of dorsal root ganglion proteins.

Chronic facial pain: different comorbidities and characteristics between neuropathic and nonneuropathic conditions

OBJECTIVE

The aim of this study was to investigate the association between comorbidities and chronic diseases and neuropathic and nonneuropathic orofacial pain diagnoses to suggest subclassifications of disease.

STUDY DESIGN

This was a cross-sectional, retrospective, case-control study. We evaluated 174 patients with orofacial pain and 132 controls by using a systematic protocol that consisted of medical history and demographic, pain, and orofacial characteristics. Patients were grouped according to their diagnosis-neuropathic or non-neuropathic pain; medical comorbidities; and exclusion criteria. Analyses included Z-score normalization, χ² test, Fisher's exact test, 1-way analysis of variance (ANOVA), Student t test, Pearson's correlation coefficient, 2-step clustering, and logistic regression at 95% confidence level.

RESULTS

Functional chronic diseases were prevalent and correlated with pain and orofacial features. Three groups were identified in the cluster analysis: neuropathic facial pain, other orofacial pain syndromes, and fibromyalgia/temporomandibular disorders (TMDs). Logistic regression showed that hypothyroidism and gastritis were predictors for nonneuropathic orofacial conditions. Psychiatric diseases and gastritis were more prevalent among patients with generalized pain syndromes and TMDs and less prevalent among patients with neuropathic pain.

CONCLUSIONS

Functional comorbidities were associated with orofacial and dental features and may correspond to multimorbidity states in patients with chronic orofacial pain. The findings support the hypothesis that nonneuropathic orofacial pain syndromes could be functional disorders.

Imaging of orofacial pain

BACKGROUND

Orofacial pain is a common complaint, with an estimated 75% of cases caused by dental disease, specifically a diseased pulp. A small percentage of orofacial pain cases will require specialist referral most commonly to oral medicine specialists or oral and maxillofacial surgeons from a dental perspective, or otolaryngologists or neurologists from a medical perspective.

IMAGING MODALITIES

Following a thorough history and clinical examination, imaging is often required to narrow the differential diagnosis or answer a specific query related to the final diagnosis. A range of imaging modalities can be used to evaluate orofacial pain including dental panoramic tomography (DPT), intraoral radiographs, cone beam computed tomography (CBCT), multidetector computed tomography (MDCT), ultrasonography (US), magnetic resonance imaging (MRI) and nuclear medicine.

IMAGING PROTOCOLS

This paper provides a guideline outlining imaging protocols for categories of facial pain divided into: (a) unilateral odontalgia; (b) unilateral facial pain; (c) combined unilateral odontalgia and facial pain; (d) trigeminal neuralgia; (e) trigeminal neuropathic pain with or without other sensory, autonomic or motor features; (f) temporomandibular joint disorders and associated pain; (g) referred pain and (h) non-specific orofacial pain.

CONCLUSION

Imaging for orofacial pain should be tailored to answer a specific query related to the aetiology of the reported pain. This should result in a specific diagnosis or narrowing of the differential diagnosis as possible causes of orofacial pain are eliminated. Choosing the correct imaging modality and protocol based on the pain category is important for efficient and effective pain diagnosis and management.

The Orofacial Pain Clinic Questionnaire (EDOF-HC) in the evaluation and diagnosis of orofacial pain

ABSTRACT Background: Diagnostic tools are necessary for the anamnesis and examination of orofacial pain, in order to fulfill diagnostic criteria and to screen potential causes of pain. Objective: To evaluate the Orofacial Pain Clinic Questionnaire (EDOF-HC) in the assessment and diagnosis of orofacial pain. Methods: Overall, 142 patients were evaluated and classified according to the criteria of the International Headache Society and International Association for the Study of Pain. All of them were evaluated with the EDOF-HC questionnaire, which consists of the orofacial and medical history, as well as the orofacial examination. Data were statistically analyzed with chi-square test and Bonferroni correction, one-way ANOVA with Tukey post hoc test, the two-step cluster and decision tree methods. Results: There were diferences in pain descriptors, pain in maximum mouth opening, number of trigger points, and history of previous surgery between the groups, which were classified into trigeminal neuralgia, burning mouth syndrome, temporomandibular disorders and trigeminal posttraumatic neuropathic pain with classification analysis. Conclusions: The EDOF-HC is a clinical supportive tool for the assessment of orofacial pain. The instrument may be used to support data collection from anamnesis and examination of patients according to the diagnostic criteria of most common orofacial conditions. It is also useful in the investigation of local and systemic abnormalities and contributes for the diagnosis of conditions that depend on exclusion criteria.

The Therapeutic Effectiveness of Full Spectrum Hemp Oil Using a Chronic Neuropathic Pain Model

BACKGROUND

Few models exist that can control for placebo and expectancy effects commonly observed in clinical trials measuring 'Cannabis' pharmacodynamics. We used the Foramen Rotundum Inflammatory Constriction Trigeminal Infraorbital Nerve injury (FRICT-ION) model to measure the effect of "full-spectrum" whole plant extracted hemp oil on chronic neuropathic pain sensitivity in mice.

METHODS

Male BALBc mice were submitted to the FRICT-ION chronic neuropathic pain model with oral insertion through an incision in the buccal/cheek crease of 3 mm of chromic gut suture (4-0). The suture, wedged along the V2 trigeminal nerve branch, creates a continuous irritation that develops into secondary mechanical hypersensitivity on the snout. Von Frey filament stimuli on the mouse whisker pad was used to assess the mechanical pain threshold from 0-6 h following dosing among animals (n = 6) exposed to 5 μL of whole plant extracted hemp oil combined with a peanut butter vehicle (0.138 mg/kg), the vehicle alone (n = 3) 7 weeks post-surgery, or a naïve control condition (n = 3).

RESULTS

Mechanical allodynia was alleviated within 1 h (d = 2.50, p < 0.001) with a peak reversal effect at 4 h (d = 7.21, p < 0.001) and remained significant throughout the 6 h observation window. There was no threshold change on contralateral whisker pad after hemp oil administration, demonstrating the localization of anesthetic response to affected areas.

CONCLUSION

Future research should focus on how whole plant extracted hemp oil affects multi-sensory and cognitive-attentional systems that process pain.

Differential Diagnosis of Chronic Neuropathic Orofacial Pain: Role of Clinical Neurophysiology

Orofacial pain syndromes encompass several clinically defined and classified entities. The focus here is on the role of clinical neurophysiologic and psychophysical tests in the diagnosis, differential diagnosis, and pathophysiological mechanisms of definite trigeminal neuropathic pain and other chronic orofacial pain conditions (excluding headache and temporomandibular disorders). The International Classification of Headache Disorders 2018 classifies these facial pain disorders under the heading Painful cranial neuropathies and other facial pains. In addition to unambiguous painful posttraumatic or postherpetic trigeminal neuropathies, burning mouth syndrome, persistent idiopathic facial and dental pain, and trigeminal neuralgia have also been identified with neurophysiologic and quantitative sensory testing to involve the nervous system. Despite normal clinical examination, these all include clusters of patients with evidence for either peripheral or central nervous system pathology compatible with the subclinical end of a continuum of trigeminal neuropathic pain conditions. Useful tests in the diagnostic process include electroneuromyography with specific needle, neurography techniques for the inferior alveolar and infraorbital nerves, brain stem reflex recordings (blink reflex with stimulation of the supraorbital, infraorbital, mental, and lingual nerves; jaw jerk; masseter silent period), evoked potential recordings, and quantitative sensory testing. Habituation of the blink reflex and evoked potential responses to repeated stimuli evaluate top-down inhibition, and navigated transcranial magnetic stimulation allows the mapping of reorganization within the motor cortex in chronic neuropathic pain. With systematic use of neurophysiologic and quantitative sensory testing, many of the current ambiguities in the diagnosis, classification, and understanding of chronic orofacial syndromes can be clarified for clinical practice and future research.

Exploring the time course of painful post-traumatic trigeminal neuropathy: a pilot study

Patients suffering from painful post-traumatic trigeminal neuropathy (PTTN) often report circadian variations in pain. This pilot study aimed to assess the daily evolution of pain in patients fulfilling the following criteria: PTTN diagnosed based on ICHD-3 criteria; >18 years of age; no impairment in communication and signed informed consent. Primary study outcome was a self-declared quantitative assessment of pain intensity using an 11-point numerical scale (0–10) for 8 consecutive days. Impact on oral function and quality of life was also assessed, using psychometric questionnaires GOHAI and HADS. Eleven patients with PTTN were recruited (mean age: 66.1 ± 6.8 years old). Mean pain intensity was 3.6 ± 0.99. Mean pain intensity increased progressively and significantly during the day, from 1.8 ± 1.3 to 4.6 ± 2.3 (p < 0.0001). Mean HADS score was 7.8 ± 2.7. Mean GOHAI score was weak (35/60). In conclusion, PTTN seems to follow a circadian rhythm.

Alterations in grey matter density and functional connectivity in trigeminal neuropathic pain and trigeminal neuralgia: A systematic review and meta-analysis

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Activation likelihood estimation (ALE) shows to be a verified method to meta-analyze heterogeneous imaging results.

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From a broad variety of key regions, structural and functional changes were repeatedly found in the thalamus, the cingulate cortex and the middle frontal gyrus in TN/TNP patients.

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Future research should focus on these regions of interest in order to improve diagnostic imaging in TN/TNP.

Background

Various studies reported changes in grey matter volumes and modifications in functional connectivity of cortical and subcortical structures in patients suffering from trigeminal neuralgia (TN) and trigeminal neuropathic pain (TNP). This study meta-analyzed the concordant structural and functional changes in foci and provide further understanding of the anatomy and biology of TN/TNP.

Methods

Relevant articles on magnetic resonance imaging (MRI) and functional MRI in TN/TNP, published before August 2018, were searched for on PubMed and Embase. Following exclusion of unsuitable studies, a meta-analysis was performed using activation likelihood estimation (ALE).

Results

In total, 322 paper were identified, 11 of which could be included based on the predefined inclusion and exclusion criteria. Eight papers, totaling 279 subjects, discussing structural changes and four papers, totaling 102 subjects, discussing functional changes were included (i.e., one paper investigated both structural and functional alterations). ALE analysis showed that in TN/TNP, grey matter decreases are found in the thalamus, (anterior) cingulate gyrus, bilateral striatum, the superior-, middle- and transverse temporal gyrus, subcallosal gyrus, the bilateral insular cortex, the pre- and postcental gyrus, the middle frontal gyrus bilaterally and the anterior cerebellar lobe. Grey matter increases were seen in the periaqueductal grey (PAG). Increased resting state functional organization was found within the bilateral middle- and superior frontal gyri, the (posterior) cingulate cortex and the thalamus/pulvinar.

Conclusions

Structural and functional changes meta-analyzed in this paper may contribute to elucidating the central pathophysiological mechanisms involved in TN/TNP. These results may be used as biomarkers to predict the response to medication and, ideally, in the future to offer personalized treatments.

MC4R Is Involved in Neuropathic Pain by Regulating JNK Signaling Pathway After Chronic Constriction Injury

Background

Neuropathic pain can develop after nerve injury, when deleterious changes occur in injured neurons and glia cells. Melanocortin 4 receptor (MC4R) is involved in the regulation of pain due to its high expressions in brain. Moreover, MC4R could mediate the c-Jun N-terminal kinase (JNK) signaling pathway, but whether the MC4R-regulated JNK signaling pathway participated in neuropathic pain after chronic constriction injury (CCI) is still unclear.

Methods

A total of 128 Sprague-Dawley rats were allocated into four experiment groups: the SHAM group, CCI + NaCl group, CCI + HS group, and CCI + SP + HS group. For the CCI + NaCl group, the sciatic nerves were ligated. For the SHAM group, an identical manner to the CCI without ligation was performed. For CCI + HS and CCI + SP + HS groups, rats were injected with MC4R inhibitor (HS014) and HS014 plus JNK inhibitor (SP600125), respectively, from days 3 to 14 after CCI. Paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) were used to assess the nociceptive behavior. ELISA was used to detect the levels of inflammatory cytokines. qRT-PCR and Western blots (WB) were utilized to examine the mRNA and protein expressions of JNK signaling pathway-related genes. Meanwhile, the expression levels of MC4R and p-JNK were further evaluated by immunohistochemistry (IHC) and immunofluorescence (IF) experiments. Finally, in order to confirm the in vivo results, astrocytes were isolated and transfected with MC4R-overexpression plasmid. Furthermore, the protein expressions of JNK signaling pathway-related genes were tested by WB.

Results

It was showed that the values of PWL and PWT were significantly increased in CCI + HS group and CCI + SP + HS group compared with CCI + NaCl group. The increased interleukin-6 (IL-6), IL-1β, and tumor necrosis factor-α (TNF-α) secretion in CCI + NaCl group was lowered by HS and SP + HS. MC4R, p-JNK, ATF3, and c-Jun levels were up-regulated with CCI surgery, but down-regulated with HS and SP + HS treatments. Moreover, the IHC and IF results further revealed that MC4R and p-JNK expressions in CCI + NaCl group were remarkably higher than those in HS group and HS + SP group. In vitro data also indicated that HS, SP, and SP + HS could down-regulate the expressions of MC4R, p-JNK, ATF3, and c-Jun in M1830 astrocytes.

Conclusion

Our findings indicated that MC4R is involved in neuropathic pain by regulating JNK signaling pathway after CCI.

How May Placebo Mechanisms Influence Orofacial Neuropathic Pain?

The conceptualization of placebo has changed from inactive pills to a detailed understanding of how patients' perception of receiving a treatment influences pain processing and overall treatment outcome. Large placebo effects were recently demonstrated in chronic neuropathic pain, thereby opening the question of whether placebo effects also apply to orofacial neuropathic pain. In this article, we review the new definitions, magnitude, and social, psychological, neurobiologic, and genetic mechanisms of placebo effects in pain, especially neuropathic pain, to illustrate that placebo effects are not simply response bias but psychoneurobiological phenomena that can be measured at many levels of the neuroaxis. We use this knowledge to carefully illustrate how patients' perceptions of the treatment, the relationship with the health care provider, and the expectations and emotions toward a treatment can influence test and treatment outcome and potentially skew the results if they are not taken into consideration. Orofacial neuropathic pain is a new research area, and we review the status on definition, diagnosis, mechanisms, and pharmacologic treatment of neuropathic pain after trigeminal nerve injury, as this condition may be especially influenced by placebo factors. Finally, we have a detailed discussion of how knowledge of placebo mechanisms may help improve the understanding, diagnosis, and treatment of orofacial neuropathic pain, and we illustrate pitfalls and opportunities of applying this knowledge to the test of dental treatments.

Postdural Puncture Headache Treated With Epidural Blood Patch and Subsequent Resolution of Atypical Odontalgia: A Case Report

A 37-year-old gravida 5 para 4 with gestational hypertension underwent uneventful repeat cesarean delivery and tubal ligation under combined spinal-epidural anesthesia. On postoperative day 3, she developed a postural headache, unrelieved by caffeine, ibuprofen, and hydration. On postoperative day 8, she returned to the hospital for treatment. An epidural blood patch was administered, providing immediate and complete relief of her headache and bilateral upper and lower dental pain, which she noted arising concomitantly with her headache. We suspect bilateral traction on cranial nerve V2 and V3, secondary to intracranial hypotension after dural puncture, as the cause of her pain.

Electrical Synapses are Involved in Orofacial Neuropathic Pain

Accumulated evidences suggest important roles of glial GAP-junctions in pain. However, only a few studies have explored the role of neuronal GAP-junctions or electrical synapses in neuropathic pain (NP). Therefore, the present study explores the role of connexin 36 (Cx36) in NP using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model in rat. A significant increase in Cx36 labeling was observed in the medullary dorsal horn (MDH) of CCI-IoN-lesioned compared to sham rats. The expression of Cx36 in CCI-IoN-lesioned rats revealed a rostroventral gradient of punctuate labeling within lamina IIo of the MDH. Cx36-positive somata and processes were also observed in MDH laminae IIi and III-V. These somata were mostly of the Gamma aminobutyric acid (GABA) and occasionally Glycine transporter 2 (GlyT2) cell subtypes. Moreover the GABA cell subtypes are highly coupled in lamina IIo as revealed by the intense Cx36 staining in this lamina. Pharmacological Cx36 blockade by intracisternal administration of mefloquine decreased significantly the mechanical allodynia observed in CCI-IoN-lesioned rats. Altogether, our findings demonstrated that Cx36 play an important role in mechanical allodynia by coupling GABA cells. Increasing cell coupling by enhancing Cx36 expression favors neuropathic pain while disrupting this coupling alleviates it. This mechanism may constitute a novel target for the treatment of orofacial mechanical allodynia.

Comparison of orofacial thermal sensitivity assessed with simple devices and sophisticated equipment

BACKGROUND

Simple thermal devices providing reliable data are needed to detect somatosensory disturbances in non-specialized clinical settings. Currently, evidence is lacking about their use. Therefore, the aim was to compare the assessment of perceived thermal sensitivity/pain in healthy humans with a state-of-the-art thermotester and with simple inexpensive customized thermal aluminium devices.

METHODS

Twenty healthy volunteers participated in the study. The infraorbital region and the tip of the tongue were tested with the Medoc Pathway thermotester and simple aluminium thermal devices, with temperatures varying between 5-50°C. A numerical rating scale (NRS) from 0-50-100 was used for rating the perceived thermal sensitivity/painfulness. A control experiment was performed with 10 of the participants to test the potential impact of temporal summation of thermal stimuli with the use of temperature ramps (Medoc) compared with static temperature (simple devices).

RESULTS

In the original experiment, the scores from the thermotester stimulation were higher than the scores for stimulation with the simple thermal devices with mean NRS differences between devices of 7.2 and 10.2 for the two tested regions. In the control experiment, the mean NRS differences decreased to -0.3 and 2.2, respectively.

CONCLUSIONS

Provided that temporal summation of the thermal stimuli was avoided, there were only minor differences in perceived thermal sensitivity/painfulness between the two different assessment methods at both test sites. Therefore, the simple thermal devices can be useful for assessment of thermal sensitivity in clinical settings without access to expensive sophisticated equipment. However, more sophisticated equipment is needed for threshold measurements.

SIGNIFICANCE

This study indicates that simple thermal devices to some extent can be used in a comparable way with sophisticated thermal stimulators. Therefore, they can be useful in clinical practice, where access to further equipment is lacking.