Fibrous dysplasia of bone (FD) / McCune-Albright syndrome (MAS) is a rare congenital bone disorder affecting one or multiple bones, caused by a mosaic somatic mutation of the GNAS gene. In some cases, it may be associated with endocrine or cutaneous abnormalities. The spectrum of bone disease is broad, ranging from isolated monostotic fibrous dysplasia to complete skeletal involvement. Functional prognosis can be complex due to pain, bone deformities, and fracture risk. The disease may initially be identified through non-specific clinical signs such as pain. Indeed, bone pain has been reported in up to 81% of adults and 49% of children, mainly affecting the lower limbs and the spine, with highly variable pain intensity that does not always correlate with the extent of bone lesions. This pain may persist throughout life and impact patients' daily activities.
In the general population, it is well known that chronic musculoskeletal pain following events such as surgery or fractures can be associated with central sensitization, a neurophysiological phenomenon characterized by hyperreactivity of the central nervous system, along with impaired modulation of pain through descending inhibitory pathways, a normally protective mechanism that becomes reduced. The pathophysiology of bone pain in FD/MAS remains poorly studied and poorly understood. The presence of central sensitization, reduced pain modulation, and hypersensitivity to everyday stimuli are rarely described but suggested by the existence of chronic pain often lasting many years. The mixed characteristics of pain experienced (nociceptive, neuropathic, inflammatory, or nociplastic) are also poorly defined. To date, no study has explored pain in FD/MAS using a psychophysical approach in comparison with a control population.
Our hypothesis is that patients with FD/MAS exhibit central sensitization with reduced pain modulation. This exploratory pilot study aims to investigate, through psychophysical approaches, the pathophysiological mechanisms underlying pain in FD/MAS.
Male or female over 18 years of age with fibrous dysplasia/McCune-Albright syndrome diagnosed by a rheumatologist.
Mentally and legally able to provide informed consent to participate in the study.
Affiliated with a health insurance system.
Male or female over 18 years of age.
Medical and/or surgical history considered by the investigator or delegated physician to be incompatible with study procedures (e.g., amputation or physical limitation preventing completion of pain assessment tests).
Recurrent pain at sites planned for stimulation during thermal and mechanical testing (forearms or palms).
Presence of anxiety and/or depression defined as Hospital Anxiety and Depression Scale (HADS) score >11.
Use of analgesic medication within the week preceding inclusion.
Parallel
Visit 1 at the center
Other interventionPain assessment and quality of life evaluation
Central sensitization tests, measurement of the Conditioned Pain Modulation (CPM) effect
The patients are seated, the ATS thermode associated with the Pathway is applied to the dominant forearm. From the baseline value of 32°C, the Pathway delivers a "Pain 60 / Test stimulus" for 10 seconds, and the patient scores the pain on a visual numerical scale from 0 to 100. Then, the Pathway delivers a "Pain 60 / Test stimulus" for 30 seconds, and the patient scores the pain on the same scale. Fifteen minutes after the end of the two stimulations, the patient immersed the non-dominant arm for 60 seconds in a water bath at 46.5°C. Then a second identical sequence of 10 and 30 second stimulations was performed, with the scores recorded after each stimulation on the visual numerical scale from 0 to 100. The CPM effect is measured by taking the difference between the pain scores on the visual numerical scale before and after immersion.
Visual Analog Scale
This scale allows the patient to rate the pain on a 100-mm horizontal line with a movable cursor. It ranges from 0 mm (0 cm), corresponding to "no pain," to 100 mm (10 cm), corresponding to "worst pain imaginable." Participants indicate, using the cursor, the point on the line that best represents their level of pain. The distance (in mm or cm) between 0 and the participant's mark determines the intensity of pain.
The Brief Pain Inventory Questionnaire (BPI)
This self-assessment scale allows the patient to characterize the pain in terms of its intensity and its psychosocial repercussions by means of 11 scales rated from 0 (no pain or does not bother) to 10 (the most horrible pain you can imagine or completely bothers you). The BPI also asks the patient to black out the painful areas on a diagram (front and back) and to put an "S" if the pain is on the surface or a "P" if it is deep. The questionnaire also asks the patient to indicate what treatment or medication he or she is taking for the pain and what percentage improvement has been obtained.
Measurement of the threshold of sensitivity and pain perception induced by a thermal stimulus (hot and cold) at the Pathway - Médoc®
The measurements will be performed using an ATS thermode applied to the dominant arm of the patients. The Pathway-Medoc system associated with the thermode allows, from a base value of 32°C, to deliver adjustable temperature peaks (in the hot or in the cold and according to a regular slope of 1°C) and controlled by fast feedback, which allows to adapt to the different sensitivity thresholds of the C and A fibers. This device will be used to evaluate: the sensitivity threshold to heat, the sensitivity threshold to cold, the pain threshold to heat and the pain threshold to cold. The determination of each threshold will be established by an average of three measurements.
Measurement of mechanical pain thresholds and mechanical temporal summation
Mechanical pain thresholds and mechanical temporal summation will be assessed using standardized mechanical stimulation. Mechanical pain thresholds will be measured with calibrated PinPrick stimulators using a modified method of limits, with stimuli applied to the palm of the dominant hand (or non-dominant if necessary). Thresholds will be calculated as the geometric mean of ascending and descending series. Mechanical temporal summation will be assessed using a 180 g Von Frey filament applied to the forearm. Pain intensity after a single stimulus and after a train of 10 stimuli at 1 Hz will be rated on a 0-10 visual analog scale. The temporal summation ratio will be calculated as the ratio between repeated and single stimulus pain ratings. Matched healthy volunteers will be tested at the same sites as patients.
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