Deciphering Persistent Orofacial Pain - PIFP

Deciphering Persistent Orofacial Pain - Persistent Idiopathic Facial Pain

Persistent non-dental orofacial pain is common yet poorly characterised, and its pathophysiology remains largely unknown. Two clinically similar entities are distinguished: post-traumatic trigeminal neuropathic pain (PTNP), in which peripheral nerve damage is demonstrable, and persistent idiopathic facial pain (PIFP), in which peripheral findings are typically absent - pointing to an altered central processing of trigeminal nociceptive input. This monocentric study characterises the peripheral and central mechanisms of PIFP by comparing PIFP patients with age- and sex-matched healthy controls using a combination of non-invasive peripheral and central approaches.

The study comprises two work packages. The peripheral work package uses quantitative sensory testing (standardised DFNS protocol) to test the hypothesis that PIFP patients retain an intact peripheral nervous system. The central work package uses functional MRI with standardised trigemino-nociceptive stimulation to characterise brainstem network dynamics, testing the hypothesis that PIFP patients exhibit a central trigeminal processing disturbance at the brainstem level. In PIFP patients, the functional MRI is repeated before and after a local-anaesthetic nerve block in the pain area, additionally distinguishing patients whose pain is completely abolished by the block from those with persistent pain despite it. The overarching aim is to characterise the pathophysiological basis of PIFP and thereby advance the mechanistic understanding of persistent orofacial pain.

Study Overview

• Status: Not yet recruiting
• Conditions: Persistent Idiopathic Facial Pain (PIFP)

Detailed Description

Background and Rationale:

Persistent non-dental orofacial pain is common but poorly understood, and its underlying pathophysiology remains largely undefined. Two clinically similar entities can be distinguished. Painful post-traumatic trigeminal neuropathy (PTNP) is characterized by persistent pain accompanied by demonstrable somatosensory changes (negative signs such as hypoesthesia and hypoalgesia, and/or positive signs such as allodynia and hyperalgesia) in the painful area, typically following nerve injury. Persistent idiopathic facial pain (PIFP; formerly "atypical facial pain") is a constant, dull, poorly localized facial pain that does not follow the distribution of a single trigeminal branch, may cross the midline over time, and is not associated with a neuralgiform pain component; clinical and radiographic examinations are unremarkable. Because the two conditions are clinically nearly identical, patients frequently attribute the pain to dental causes, and unnecessary dental treatment or tooth extraction can aggravate and chronify the pain.

The absence of somatosensory deficits in PIFP suggests that altered central processing of trigeminal nociceptive input, rather than a purely peripheral mechanism, contributes to the pain. This is consistent with the concept of nociplastic pain, in which centrally altered sensory processing and pain modulation drive an augmented pain response. Prior high-resolution brainstem functional MRI (fMRI) work demonstrated that PIFP patients show stronger activation of the spinal trigeminal nucleus in response to standardized trigeminal nociceptive stimulation compared with healthy controls. However, fMRI alone cannot distinguish central sensitization (top-down) from a peripheral drive maintained by ongoing nociceptor input. This study therefore combines quantitative sensory testing (QST) with a trigemino-nociceptive fMRI paradigm that incorporates a peripheral nerve block, in order to characterize the peripheral and central contributions to PIFP.

Objectives and Hypotheses:

The overarching objective is to describe and characterize the pathophysiology of PIFP by comparing PIFP patients with age- and sex-matched healthy controls using non-invasive psychophysical and neuroimaging methods. The central hypothesis is that PIFP patients exhibit a disturbance of central trigeminal nociceptive processing at the brainstem level and in other critical trigeminal pain-processing regions, such as the thalamus and insula, that distinguishes them from healthy controls. A secondary hypothesis is that, within PIFP patients, a peripheral anesthetic nerve block will separate individuals whose pain is driven peripherally (pain abolished during the block) from those whose pain is maintained centrally/nociplastically (pain persists despite cutaneous numbness), and that these subgroups differ in their central nociceptive processing.

Study Procedure:

Participants are assigned to groups by clinical diagnosis; there is no randomization or blinding. Two components are applied to all participants: standardized psychophysical/sensory characterization and task-based fMRI with standardized electrical trigemino-nociceptive stimulation. Patients are recruited through the headache/facial pain outpatient clinic; healthy volunteers are recruited via public postings and local internet platforms. The study plans to enroll 30 patients with PIFP together with 30 age- and sex-matched healthy controls for comparison. Patients are recruited through the headache/facial pain outpatient clinic; healthy volunteers are recruited via public postings and local internet platforms.

Clinical and Psychophysical Characterization:

All participants complete standardized questionnaires for deep phenotyping, including the Patient Health Questionnaire (PHQ-9), central sensitization inventory (CSI), brief pain inventory (BPI), short form-36 health survey (SF-36), and custom instruments covering general health, medication and stimulant use, dental history, and oral hygiene, as well as detailed pain history, pain character and intensity, prior treatments, and potential triggering events. Patients undergo a standardized extraoral and intraoral dental examination, including palpation of the trigeminal exit points and masticatory apparatus and standardized oral health indices. Quantitative sensory testing is performed in all participants over the trigeminal dermatomes bilaterally, following the standardized protocol of the German Research Network on Neuropathic Pain (DFNS). Thermal thresholds are assessed with a thermal sensory analyzer, and mechanical stimuli with von Frey filaments and Pinprick stimulators.

Functional MRI Paradigm:

Task-based fMRI is acquired on a 3T scanner using a brainstem-optimized high-resolution protocol. Standardized nociceptive stimulation is delivered via an MR-compatible constant-current stimulator coupled to MR-safe electrodes positioned over the second (V2) and third (V3) branches of the trigeminal nerve bilaterally. After determination of individual electrical pain thresholds, stimulation intensity is set at 3 times the pain threshold and capped at 10 mA to remain tolerable and safe. Four stimulation sites, bilateral V2 and V3, are presented in randomized order with repeated trials across sessions, and participants rate perceived pain intensity after the first trial and for every 10 trials on each stimulation site. This paradigm probes the functional representation of trigeminal nociceptive input in the spinal trigeminal nucleus, thalamus, insula, and related brainstem structures.

PIFP patients who consent are scanned twice with this paradigm: before and after a local-anesthetic nerve block administered in the painful area by a dentist. This design distinguishes patients in whom the pain is abolished while the area is anesthetized (suggesting a predominantly peripheral, nociceptor-driven mechanism) from those in whom cutaneous numbness is achieved but the pain persists (suggesting a central, nociplastic mechanism).

Outcome Measures:

The primary outcome is the difference in the fMRI response to standardized trigemino-nociceptive stimulation within trigeminal pain-processing regions - including the spinal trigeminal nucleus, thalamus, and insula - in the following comparisons: (i) PIFP patients versus healthy controls; and (ii) within PIFP patients, the difference in activation before versus after the local-anesthetic nerve block, and between patients whose pain is completely abolished by the block versus those with persistent pain despite the block. Secondary outcome includes: (1) comparison of deep phenotyping and QST parameters between PIFP patients and healthy controls

Statistical Analysis:

The primary analysis requires 20 complete datasets per cohort. Based on previous studies with this population and sequence, approximately 25% of enrolled participants are expected to yield unusable data (dropout, excessive head motion, or incidental findings). Neuroimaging data are analyzed using a general linear model with statistical parametric mapping, contrasting the painful side versus the control side, and comparing groups and pre/post nerve-block conditions. For the fMRI data, a family-wise error-corrected threshold of PFWE < 0.05 will be applied at the whole-brain level, except for a priori regions of interest - the spinal trigeminal nucleus, thalamus, and insula - for which small-volume-corrected PFWE < 0.05 will be used. Psychophysical and QST parameters are compared between groups using parametric or non-parametric tests as appropriate after testing distributional assumptions, with a two-sided significance level of p < 0.05. Analyses are performed after completion of data collection; no interim analyses are planned.

• Study Type: Observational
• Enrollment (Estimated): 60

Contacts and Locations

• Study Contact: Name: Arne May, MD, PhD; Phone Number: +49 40 74105 9189; Email: a.may@uke.de
• Study Contact Backup: Name: Kuan-Po Peng, MD; Phone Number: +49 40 74102 7305; Email: k.peng@uke.de

Study Locations

• Germany
  • Hamburg, Germany, 20246
    • University Medical Center Hamburg-Eppendorf
    • Contact: Kuan-Po Peng, MD; Phone Number: +49 40 74102 7305; Email: k.peng@uke.de

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

• Sampling Method: Non-Probability Sample

Study Population

Patients with PIFP will be recruited from the headache/facial pain outpatient clinic of the University Medical Center Hamburg-Eppendorf

Description

Inclusion Criteria:

• Healthy controls: written informed consent.
• PIFP cohort: written informed consent, and a diagnosis of PIFP according to the ICHD-3 and ICOP diagnostic criteria.

Exclusion Criteria:

• Contraindication to MR scanning
• Anxiety disorders, including claustrophobia
• Other somatic diseases, including other pain disorders
• Other primary headache disorders and medication-overuse headache
• Skin lesion , infection, or scarring at the stimulation sites preventing safe electrode placement
• Healthy volunteers: any history of persistent pain or primary headache, including in first-degree relatives
• History of psychiatric disease and/or addiction
• Pregnancy or lactation
• Any regular medication; in PIFP patients, occasional acute analgesic use (NSAID, fewer than 8 days per month) is permitted
• Use of acute analgesic within 24 hours before the experiment

Study Plan

How is the study designed?

Number of groups / cohorts

2

Cohorts and Interventions

Group / Cohort
Patients with persistent idiopathic facial pain (PIFP); Patients with persistent idiopathic facial pain
Age- and sex-matched healthy controls (HC)

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Difference in brain functional neuroimaging response to standardized trigemino-nociceptive stimulation between PIFP patients HC	BOLD signal change (contrast estimates, arbitrary units) during standardized trigemino-nociceptive stimulation versus baseline, compared between PIFP patients and HC in a priori defined regions of interest and at whole-brain level.	Baseline (MRI session before local anesthesia)
Difference in brain activation before versus after the local-anesthetic nerve block, and between complete responders and non-responders	BOLD signal change (contrast estimates, arbitrary units) during standardized trigemino-nociceptive stimulation, compared within participants before versus after the local-anesthetic nerve block, and between participants whose pain is completely abolished by the block and those with persistent pain despite the block	Pre-block MRI at baseline and post-block MRI, both on the same day 1

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Comparison of deep phenotyping and quantitative sensory testing (QST) parameters between the study cohorts (PIFP versus HC).	Standardized DFNS quantitative sensory testing parameters (thermal and mechanical detection and pain thresholds), compared between PIFP patients and HC.	Baseline (single assessment before local anesthesia)

Collaborators and Investigators

• Sponsor: Universitätsklinikum Hamburg-Eppendorf
• Investigators: Principal Investigator: Arne May, MD, PhD, Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf

Publications and helpful links

General Publications

• Mehnert J, Basedau H, Sturm LM, Nielsen T, Jensen RH, May A. Functional brainstem representations of the human trigeminal cervical complex. Cephalalgia. 2023 May;43(5):3331024231174862. doi: 10.1177/03331024231174862.
• Ziegeler C, Schulte LH, May A. Altered trigeminal pain processing on brainstem level in persistent idiopathic facial pain. Pain. 2021 May 1;162(5):1374-1378. doi: 10.1097/j.pain.0000000000002126.
• May A, Ciancia V, Basedau H. Visualizing nociplastic pain: functional hyperexcitability in neuropathic and idiopathic facial pain syndromes. J Headache Pain. 2025 Oct 13;26(1):211. doi: 10.1186/s10194-025-02133-w.

Study record dates

Study Major Dates

• Study Start (Estimated): September 1, 2026
• Primary Completion (Estimated): June 30, 2029
• Study Completion (Estimated): October 31, 2029

Study Registration Dates

• First Submitted: August 18, 2026
• First Submitted That Met QC Criteria: August 25, 2026
• First Posted (Actual): August 27, 2026

Study Record Updates

• Last Update Posted (Actual): August 27, 2026
• Last Update Submitted That Met QC Criteria: August 25, 2026
• Last Verified: August 1, 2026

More Information

Terms related to this study

• Keywords: fMRI; nerve block; local anesthesia; atypical facial pain; PIFP
• Other Study ID Numbers: 2024-101254-BO-ff-PIFP

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: Individual participant data (IPD) will not be shared with other researchers. Data are collected and analyzed in pseudonymized form, and results will be reported and published only in aggregated, anonymized form that does not permit identification of individual participants. The pseudonymization key is held exclusively by the study leadership and is deleted as soon as the research purpose allows. Consistent with the informed consent obtained from participants and the approved data protection concept, no de-identified individual-level datasets will be made available externally.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No