Characterisation of Chronic Pain Syndrome in Patients With Functional Neurological Disorder

ABSTRACT

Background

Functional neurological disorders (FNDs) are characterised by the presence of motor, sensory, or cognitive neurological symptoms not explained by a recognised neurological disorder. Chronic pain is not part of the diagnostic criteria for FND, but these two conditions frequently coexist. Our objective was to evaluate the frequency and mechanism‐based characteristics of chronic pain symptoms in a single‐center cohort of patients with FND.

Methods

Data were collected prospectively using a standardised questionnaire and a face‐to‐face interview.

Results

In this study, 63 consecutive patients with FND were interviewed, 54 (86%) of whom reported chronic pain. The onset of pain was associated with a traumatic event in 41% of patients. Pain was described as permanent in 65% of patients, primarily localised to the lower limbs (83%), but actually affecting several other body regions in the vast majority of cases (89%). Pain intensity averaged 5 to 6 out of 10, and its impact on daily activities was rated at 5.5 out of 10 on the Brief Pain Inventory. The description of pain symptoms was more consistent with neuropathic pain or cognitive symptoms of central sensitization than with nociceptive pain or emotional symptoms of central sensitization.

Conclusion

Chronic pain is the most frequently reported symptom by patients, even more than other clinical symptoms specific to FND. This pain has a significant impact on the daily lives of patients.

Significance

Chronic pain is very common in patients with functional neurological disorders and is characterised by highly varied pathophysiological mechanisms. Given its frequency, heterogeneity, and potential impact on other aspects of the functional neurological disorder and on its prognosis, improved management of chronic pain is needed in these patients, particularly through the optimisation of symptomatic treatment strategies.

1. Introduction

A wide range of terms has been used to describe symptoms not explained by disease, but none fully meets clinical and conceptual needs. Conversion disorder or hysteria though long established, is not etiologically neutral and implies an unproven psychological mechanism, while other labels such as ‘functional,’ ‘dissociative,’ or ‘psychogenic’ each carry different limitations. Terms like ‘non‐organic’ or ‘non‐epileptic’ are common but define patients by what they lack rather than what they have. ‘Medically unexplained’ is widely used but considered problematic because it is vague, suggests diagnostic uncertainty, and implies that psychological explanations are not medical (Stone et al. 2011; Garcin et al. 2023). The term functional neurological disorder (FND) is now widely accepted. Introduced into the DSM‐5 in 2013 (American Psychiatric Association 2013), this term best reflects the current understanding of the pathophysiology and is most easily accepted by patients and clinicians (Stone et al. 2002). According to the DSM‐5 (American Psychiatric Association 2013), FND is a condition characterised by (A) one or more symptoms of altered voluntary motor or sensory function, (B) being not better explained by another medical or mental disorder, (C) with clinical findings providing evidence of incompatibility between the symptom and recognised neurological or medical conditions, and (D) that cause clinically significant distress or impairment in social, occupational, or other important areas of functioning. Symptoms are genuinely experienced by patients without any neurological lesion that could explain this symptomatology (Bennett et al. 2021; Aybek and Perez 2022). The main presentations are limb weakness or paralysis, non‐epileptic seizures, movement disorders (tremor, dystonia, spasms), speech disturbances, and loss of vision or skin sensitivity (Hallett et al. 2022). Conversely, although frequently associated with FND, pain is not considered a core manifestation of this disorder.

FNDs represent approximately 6%–15% of outpatient neurology consultations, and incidence rates vary between 10 and 22 per 100,000 per year (Ahmad and Ahmad 2016; Carson and Lehn 2016; Finkelstein et al. 2025). There are numerous comorbidities or overlaps between FNDs and various pathologies that can be accompanied by chronic pain or fatigue (Carle‐Toulemonde et al. 2023). The links between chronic pain and FND are therefore probably closer than initially thought.

A recent systematic review analysed all studies reporting the presence of chronic pain in adults with FND and the diagnosis of FND in patients with chronic pain (Steinruecke et al. 2024). This meta‐analysis included 64 studies. While data supporting a diagnosis of FND in patients with chronic pain were scarce, the percentage of patients with FND reporting pain was estimated at 55% in a total cohort of 4272 patients. The comorbidity of FND with specific pain syndromes, such as complex regional pain syndrome, irritable bowel syndrome, and fibromyalgia, has been discussed. However, the pathophysiological mechanisms were not specifically addressed in this review, as data in this area are extremely limited. Therefore, the objectives of our study were to reassess the prevalence of pain in a new series of patients with FND, but above all to investigate the underlying mechanisms of pain presented by these patients. The mechanisms of chronic pain have been classified by the Terminology Working Group of the International Association for the Study of Pain into three categories: nociceptive, neuropathic (Merskey et al. 1994), and nociplastic (Kosek et al. 2016). These mechanisms also encompass various clinical, affective, or cognitive aspects of central sensitization to pain (Fitzcharles et al. 2021). In the patients included in our series who presented with chronic pain, these different mechanisms were evaluated using a standardised questionnaire and an individual interview.

2. Methods

This prospective monocentric descriptive study was approved by the local ethics committee of Avicenne University Hospital and registered on Clinicaltrial.gov (NCT06149728) in November 2023. For each participant, consent to participate was obtained before inclusion. Patients who presented consecutively between January and April 2024 to the tertiary reference center for FND at Avicenne University Hospital in Bobigny (France), and whose diagnosis of FND was confirmed after a thorough neurological examination by the experts at this center according to DSM‐5 criteria (American Psychiatric Association 2013) (see introduction), were included in this study. Exclusion criteria were: (i) age less than 18 years; (ii) presence of another pathology that could cause pain; (iii) refusal to participate in the study or inability to communicate refusal to participate.

Patients meeting the inclusion criteria were interviewed face‐to‐face. All interviews were conducted in the same way using a structured questionnaire, with the same questions always asked in the same order by the same interviewer (SA). Face‐to‐face interviews were preferred as they allow for a better understanding of the questions by the patient and for more accurate and reliable responses, reducing the risk of incomplete questionnaires.

First, the main clinical symptoms were recorded and classified. Regarding painful symptoms, the diagnosis of chronic pain was made if the symptoms persisted for more than 3 months. In case of chronic pain, the following data were collected:

• –Presence of a physical or psychological trauma that preceded the onset of pain.
• –The fact that painful symptoms were permanent (even with paroxysms) or intermittent, or were predominant during the day or the night.
• –The location of painful symptoms, classified into eight distinct anatomical regions (distal lower limb/proximal lower limb/trunk/distal upper limb/proximal upper limb/head/cervical region/pelvic region).
• –Pain intensity (in the present moment and minimal, mean and maximal pain in the last 24 h) and pain interference with seven functions (general activity, mood, walking ability, normal work, relations with other people, sleep, and enjoyment of life) rated on a 0–10 scale according to the Brief Pain Inventory (BPI).
• –Pathophysiological characteristics of pain according to a mechanism‐based approach to clinical symptoms divided into five categories of an original classification developed by the authors and derived from a study currently under review for publication. (1) nociceptive descriptors (throbbing, pulling, squeezing, heaviness); (2) neuropathic descriptors of small‐fibre sensitization (burning sensation, aggravated by warm or cold); (3) neuropathic descriptors of large‐fibre sensitization (electric shocks, tingling, aggravated by pressure); (4) affective descriptors (exhausting, obsessive, unbearable, depressing); (5) associated with symptoms of cognitive impairment (concentration, memory, or sleep disturbances).
• –Pain modulating factors that triggered, worsened or improved pain (application of heat or cold, mobilisation, maintaining a position, being touched or massaged, physical effort, rest, anxiety or stress, fatigue, distraction).
• –Analgesic treatments received and their efficacy (percentage of pain relief obtained and rate of responders, defined by pain relief ≥ 50%).

2.1. Statistical Analysis

Categorical variables are presented as numbers (percentages of the total, %) and quantitative variables as means [standard deviation (SD)]. After verifying the normality of the data distribution using the Kolmogorov–Smirnov test, statistical comparisons between patients with and without pain were performed using the chi‐square test for categorical variables and the Student's t‐test for quantitative variables, with the significance level set at p < 0.05. Furthermore, in patients with pain, the comparison of scores obtained for the seven interference domains of the BPI was performed using a nonparametric repeated measures ANOVA (Friedman test) with Dunn's post hoc tests, as these data did not follow a normal distribution.

3. Results

Between January and April 2024, 69 patients referred to the tertiary reference center for FND at Avicenne University Hospital met the inclusion criteria and were interviewed. However, six patients were subsequently excluded because the diagnosis of FND could not be made with certainty. Thus, 63 patients were included in the analysis.

The demographic characteristics of the group are presented in Table 1.

TABLE 1.

Demographic characteristics.

Demographic characteristics	Total (n = 63)	Patients with pain (n = 54)	Patients without pain (n = 9)	p
Female gender (n, %)	50 (79%)	44 (81%)	6 (67%)	p = 0.36a
Age (years)	44 [±16]	44 [±15]	44 [±23]	p = 0.98b
Age at symptom onset (years)	37 [±14]	36 [±16]	37 [±14]	p = 0.86b
Duration of symptoms (years)	7.4 [±9.3]	7.7 [±9.6]	4 [±4.8]	p = 0.24b
Active employment (n, %)	19 (30%)	16 (30%)	3 (33%)	p = 1a
Disability pension (n, %)	22 (35%)	21 (39%)	1 (11%)	p = 0.44a

Note: Categorical variables are presented as numbers (percentages of the total, %) and compared using the Chi‐square test^a, while quantitative variables are presented as means (standard deviation [SD]) and compared using the Student t‐test^b. No significant difference between the pain and non‐pain groups was observed (p > 0.05).

Among the 63 patients included, 19 patients were in active employment (30.2%). Four patients were on short‐term sick leave (6.3%), and 40 patients were temporarily or permanently unable to work (63.5%), including 22 patients who received disability pension.

Chronic pain was the most common symptom (54 of the 63 patients included, 86%), followed by motor deficit in 44 patients (70%), and chronic fatigue in 32 patients (51%). All symptoms presented by patients are shown in Figure 1.

FIGURE 1.

Symptoms presented by patients (n = 63). The symptoms included in the functional neurological disorder are shown in grey, while the comorbidities are shown in black.

No difference was found between the 54 patients with pain and the nine patients without pain regarding the different demographic characteristics studied (Table 1).

The onset of pain was concomitant with the occurrence of a traumatic event (physical or psychological trauma, surgery, abrupt change in daily life or infectious episode) in 22/54 patients (41%).

Pain was described as permanent by 35/54 patients (65%) and 46/54 patients (85%) reported that their pain could be as severe at night as during the day.

Pain was mainly located in the lower limbs (83% in the distal region and 74% in the proximal region) (Figure 2). Of the 54 patients with chronic pain, six patients (11%) had pain restricted to a single area, 37 patients (69%) had pain in multiple regions (between two and seven regions), and 11 patients (20%) had widespread pain, extending to the whole body.

FIGURE 2.

Pain location in the patients with chronic pain (n = 54). On the body representation on the right, the darker the colour, the higher the frequency of painful symptoms in the area.

At the moment of the interview, pain intensity was 5/10 [±3] on average, while minimal pain was 3/10 [±2], mean pain was 6/10 [±2], and maximal pain was 7/10 [±3] in the last 24 h. The scores of pain interference according to the seven categories of the BPI are presented in Table 2.

TABLE 2.

Brief Pain Inventory scores in patients with chronic pain (0 = no interference and 10 = interferes completely) (n = 54).

Category	Mean score (/10) [±SD]
General activity	6.2 [±2.9]
Mood	5.1 [±3.3]
Walking ability	6.3 [±3.0]
Normal work	6.9 [±3.5]
Relations with other people	4.8 [±3.3]
Sleep	5.4 [±3.5]
Enjoyment of life	3.9 [±3.3]
Average score	5.5 [±2.6]

These scores differed between categories (p < 0.0001, Friedman test). Post hoc Dunn tests showed that:

• –The ‘General activity’ score was higher than the ‘Relations with other people’ score (p < 0.001) and the ‘Enjoyment of life’ score (p < 0.001).
• –The ‘Walking ability’ score was also higher than the ‘Relations with other people’ score (p < 0.05) and the ‘Enjoyment of life’ score (p < 0.01).
• –The ‘Normal work’ score was higher than the ‘Mood’ score (p < 0.01), the ‘Relations with other people’ score (p < 0.001) and the ‘Enjoyment of life’ score (p < 0.001).
• –Other comparisons between categories did not reach significance.

The pathophysiological pain characteristics, according to the five categories detailed in the methods are presented in Table 3.

TABLE 3.

Pain characteristics in patients with chronic pain (n = 54).

Nociceptive descriptors n = 36 patients (67%)

Throbbing n = 23 (43%)

Pulling n = 22 (41%)

Squeezing n = 24 (44%)

Heaviness n = 14 (26%)

Neuropathic descriptors of small‐fibre sensitization n = 34 patients (63%)

Burning sensation n = 24 (44%)

Aggravated by warm n = 14 (26%)

Aggravated by cold n = 18 (33%)

Neuropathic descriptors of large‐fibre sensitization n = 45 patients (83%)

Electric shocks n = 36 (67%)

Tingling n = 35 (65%)

Aggravated by pressure n = 17 (31%)

Affective descriptors n = 37 patients (69%)

Exhausting n = 19 (35%)

Obsessive n = 10 (19%)

Unbearable n = 24 (44%)

Depressing n = 19 (35%)

Associated with symptoms of cognitive impairment n = 47 patients (87%)

Concentration disturbances n = 42 (78%)

Memory disturbances n = 28 (52%)

Sleep disturbances n = 38 (70%)

The percentage of patients observed in the different categories differed significantly (p = 0.012, Chi‐square test), with fewer patients having neuropathic descriptors of small‐fibre sensitization (63%), nociceptive descriptors (67%), or affective descriptors (69%) than neuropathic descriptors of large‐fibre sensitization (83%) or pain associated with symptoms of cognitive impairment (87%).

The factors that triggered, worsened, or improved pain are presented in Table 4. Clearly, pain was aggravated by physical effort, fatigue, and anxiety or stress, and relieved by rest or distraction.

TABLE 4.

Pain modulating factors in patients with chronic pain (n = 54).

	Pain triggering or worsening factors	Pain improving factors	p (Chi‐square test)
No factor identified	n = 10 patients (19%)	n = 12 patients (22%)	p = 0.63
Heat application	n = 6 (11%)	n = 10 (19%)	p = 0.28
Cold application	n = 8 (15%)	n = 7 (13%)	p = 0.78
Mobilisation	n = 8 (15%)	n = 5 (9%)	p = 0.37
Maintaining a position	n = 10 (19%)	n = 3 (6%)	p = 0.04
Being touched or massaged	n = 3 (6%)	n = 4 (7%)	p = 0.70
Physical effort	n = 32 (59%)	—	—
Rest	—	n = 22 (41%)	—
Fatigue	n = 22 (41%)	—	—
Anxiety or stress	n = 13 (24%)	—	—
Distraction	—	n = 6 (11%)	—

Since they experienced chronic pain, 51/54 patients (94%) received either pharmacologic or non‐pharmacologic treatment for analgesic purposes. The average pain relief provided by these treatments was 38%. Details of the treatments tried by patients and the rate of responders (defined by pain relief ≥ 50%) are shown in Table 5. The only treatments that were efficacious in at least 20% of patients who tried were nonsteroidal anti‐inflammatory drugs (NSAIDs) (27% of responders), psychotherapy (23%), and minor or major opioids (20%).

TABLE 5.

Therapies tried by patients (n = 51) for chronic pain treatment and their efficacy.

	Number of patients (%) who received the treatment	Number of patients (%) who responded to the treatment
Drug treatments
Nonsteroidal anti‐inflammatory drug	n = 11 (22%)	n = 3/11 (27%)
Paracetamol	n = 22 (43%)	n = 1/22 (4%)
Minor or major opioids	n = 20 (39%)	n = 4/20 (20%)
Antidepressants	n = 18 (35%)	n = 0/18 (0%)
Antiepileptics	n = 8 (16%)	n = 1/8 (13%)
Muscle relaxants	n = 3 (6%)	n = 0/3 (0%)
Medical cannabis	n = 1 (2%)	n = 0/1 (0%)
Antimigraine drugs	n = 3 (6%)	n = 0/3 (0%)
Lidocaine patches	n = 3 (6%)	n = 0/3 (0%)
Non‐drug treatments
Physiotherapy	n = 37 (73%)	n = 5/37 (13%)
Osteopathy	n = 6 (12%)	n = 1/6 (17%)
Occupational therapy	n = 2 (4%)	n = 0/2 (0%)
Psychomotricity	n = 1 (2%)	n = 0/1 (0%)
Balneotherapy	n = 17 (33%)	n = 1/17 (6%)
Cryotherapy	n = 1 (2%)	n = 0/1 (0%)
Acupuncture	n = 10 (20%)	n = 0/10 (0%)
Dry needling	n = 1 (2%)	n = 0/1 (0%)
Hypnosis	n = 14 (27%)	n = 1/14 (7%)
Sophrology	n = 6 (12%)	n = 1/6 (17%)
Reflexology	n = 1 (2%)	n = 0/1 (0%)
Psychotherapy	n = 13 (25%)	n = 3/13 (23%)
Transcutaneous electrical nerve stimulation	n = 3 (6%)	n = 0/3 (0%)

4. Discussion

The objectives of this study were to reappraise the prevalence of pain in a new series of patients with FND and to focus on the underlying mechanisms of pain presented by these patients.

The first point was the very high prevalence of chronic pain in patients with FND, since it was the main symptom described by our patients, more frequent than motor deficit, fatigue or other sensory complaints. A systematic review estimated the frequency of painful symptoms in patients with FND to be approximately 55% with a range from 20% to 87% (Steinruecke et al. 2024). In our study, the frequency of chronic pain was at the upper end of this range (86%), confirming that the question of pain in FND patients is probably underestimated in clinical practice. In this regard, a revision of the DSM‐5 has been proposed, adding a category of FND ‘with prominent pain’ (Maggio et al. 2020). The frequency of pain in FND is higher than in corresponding neurological diseases that FND mimics (Steinruecke et al. 2024). For example, pain is more common in patients with non‐epileptic seizures than in epileptic seizures, to the extent that some authors have suggested considering pain as a diagnostic criterion to differentiate between these two entities (Kerr et al. 2017).

The influence of biopsychosocial factors on the existence of pain is difficult to determine in this study, particularly due to the small number of patients included who did not present with pain, which limits the relevance of comparisons made between subgroups of patients with or without pain. Age and gender did not influence the association of pain with FND, which has been previously shown (Thomas et al. 2013; Linden 2020). Social integration difficulties were not specifically linked to the presence of chronic pain, as evidenced by the fact that about a third of our patients receive a disability pension, a result also observed in other cohorts of patients with FND (Carson et al. 2011). However, 41% of our patients linked the onset of their painful symptoms to traumatic events. Although the presence of psychological stress is no longer a diagnostic criterion for FND since 2013 (Demartini et al. 2016), these events remain indisputable predisposing and precipitating factors. The same is true for primary chronic pain syndromes, such as fibromyalgia or irritable bowel syndrome (Häuser et al. 2011; Yavne et al. 2018; Carle‐Toulemonde et al. 2023), whose diagnostic criteria are fulfilled in 10%–20% of patients with FND (Steinruecke et al. 2024). Given their many similarities, the question of whether there is a clinical entity encompassing FND and primary chronic pain, or whether it is an overlap syndrome or a matter of comorbidities, is a subject of debate (Carle‐Toulemonde et al. 2023).

The main originality of this study lies in the detailed clinical description of pain symptoms. Many of the reported characteristics are not specific to any particular pain syndrome. In temporal terms, the pain was described as fairly constant, present day and night. In spatial terms, the pain was mainly localised in the lower limbs, but in reality, it was present in several regions in more than two‐thirds of patients, or even totally diffuse. In terms of intensity, with average values of 5–6/10, the pain can be considered moderate (Jensen et al. 2017), which is similar or maybe slightly higher than reported in previous studies (Věchetová et al. 2018; Van der Feltz‐Cornelis et al. 2020; Gandolfi et al. 2021).

Regarding the impact of pain on daily functioning, this was assessed using the BPI. Our results show that this impact is similar to the intensity of the pain, with an average score of 5.5/10. Concerning the different BPI interference items, some domains were more specifically affected by pain: daily physical activities were more impacted than mood or pleasure, reflecting a greater physical than emotional impact of pain in patients with FND.

Regarding the analysis of pain symptom descriptors, our study did not allow us to clearly categorise the type of pain mechanism presented by our patients. Pain was associated with neuropathic rather than nociceptive descriptors, and more cognitive than affective aspects characterised central sensitization for the nociplastic dimension. However, there is an overlap between neuropathic descriptors and nociplastic conditions, since neuropathic‐like symptoms can characterise central sensitization, as shown, for example, by the difficulty in establishing a differential diagnosis between these two situations using questionnaires on neuropathic pain (Lefaucheur 2025).

Aggravating factors of pain included physical activities as well as anxiety or stress. Thus, pain in patients with FND appears complex and particularly difficult to categorise, although the impact of effective treatments may be a major confounding factor. The widespread nature of the pain and its association with mood or cognitive disorders would mean that the nociplastic nature of the pain is at the forefront (Fitzcharles et al. 2021). However, nociplastic pain is not synonymous with central sensitization (Häuser and Kosek 2026). On the other hand, if we consider at least the presence of neuropathic descriptors, in our study as in a previous one (Jimenez et al. 2019), pain associated with FND does not even appear to be solely a problem of central sensitization., which enriches the hypothesis of a continuum of different intrinsic mechanisms that may contribute to the emergence of a chronic pain syndrome in patients with FND.

One of the most interesting aspects of this study is the very high number of patients who received specific pain treatment. This clearly indicates that chronic pain syndrome, even of moderate intensity and impact on daily life, was clinically significant and warranted specific treatment. Furthermore, it is important to highlight the wide variety of treatments implemented, including numerous non‐pharmacological techniques. In the literature, interventions based primarily on physiotherapy and psychotherapy have generally proven ineffective (Reuber et al. 2007; Nielsen et al. 2017; Gandolfi et al. 2021, 2022). Conversely, psychotherapy proved to be one of the most effective procedures for our patients (nearly a quarter of patients responded), as did certain medications, such as NSAIDs and opioids. This further demonstrates the complexity of chronic pain mechanisms in patients with FND, as the most effective treatments in our study tended to alleviate the affective and nociceptive aspects of pain, while these descriptors were the least frequently reported by our patients. This could therefore indicate that these characteristics were less prevalent in our patients, as they were better controlled by specific treatments, at least in a significant number of them. Finally, the overall pain relief rate achieved with the various therapies was relatively low (38%) in our patients. This underscores the need to optimise pain management in patients with NFD, which could be achieved through a better clinical characterisation of their pain symptoms.

4.1. Study Limitations

This study has several important limitations that must be considered. First, the recruitment period was short, due to the interviewer's availability, and this resulted in a relatively small number of patients. In the review of Steinruecke et al. (2024), the number of patients exceeded 100 in nearly half of the studies analysed. In particular, the very small number of patients without pain limits the scope of comparisons with the vast majority of patients suffering from pain. Furthermore, this single‐center study took place within a department specialising in FND. Referral to a tertiary care center may have led to the inclusion of patients with more severe, chronic, and treatment‐resistant forms of FND, who may also experience more intense pain than the general FND population. Consequently, the final population may not be representative of all patients with FND. Second, pain is a subjective experience, and its measurement presents a significant challenge. In this study, we used self‐assessment scales (numerical rating scale, BPI) rather than objective measures, because these measures are more relevant for assessing provoked pain or sensitization phenomena, rather than spontaneous ongoing pain. However, the reliability and validity of these scales can vary from patient to patient due to individual differences in pain perception and tolerance. Although we attempted to control for these variables using standardised questionnaires, patients may interpret and respond to the scales differently, introducing variability into the data. Studies based on self‐reporting or interviews are often subject to recall bias. Finally, the design of this study also did not allow for the inclusion of a control group or external comparators.

5. Conclusion

Our study provides important insights into chronic pain syndrome in patients with FND. First, chronic pain is very common in these patients, and second, its clinical presentation and underlying mechanisms are highly variable. Given that the presence of chronic pain, even of moderate intensity, associated with FND alters its prognosis (Glass et al. 2018; Gelauff et al. 2019; Rauline et al. 2023), the management of pain presented by these patients is critical, but is often neglected or inappropriate. A better characterisation of the underlying mechanisms, which probably cannot be reduced to a central sensitization process for all patients, justifies being undertaken on the basis of clinical symptoms in order to optimise and personalise therapeutic strategies for each individual patient.

Author Contributions

Sarah Azzopardi: investigation, conceptualization, writing – original draft preparation. Béatrice Garcin: conceptualization, methodology, writing – reviewing. Francine Mesrati: conceptualization, writing – reviewing. Jean‐Pascal Lefaucheur: conceptualization, methodology, writing – reviewing. Sophie Ng Wing Tin: conceptualization, methodology, writing – original draft preparation.

Funding

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.