Response

I read with great interest the Editorial by Harland et al regarding the necessity of rethinking the diagnostic hierarchy in psychiatry. ^{Reference Harland, Phillips-Owen, Conlan and Owen1} The authors compellingly argue that reliance on descriptive, checklist-based diagnostic systems, such as the DSM and ICD, has led to a phenomenon of ‘diagnostic flattening’. By reducing complex psychiatric syndromes to horizontal criteria, modern classification systems have compromised clinical formulation, obscured aetiological precedence and spawned an artificial inflation of comorbidities. I strongly support the authors’ call to re-establish a hierarchical approach. Indeed, the problem of diagnostic flattening is by no means unique to neurodevelopmental conditions like ADHD or ASD; it reflects a systemic issue that equally complicates diagnosis and clinical decision-making at the psychiatric–somatic interface – notably in chronic orofacial pain.

In response to the pitfalls of diagnostic flattening, frameworks such as the research domain criteria (RDoC) and the hierarchical taxonomy of psychopathology (HiTOP) were introduced. ^{Reference Cuthbert and Insel2,Reference Kotov, Krueger, Watson, Achenbach, Althoff and Bagby3} While RDoC bypasses arbitrary categorical boundaries by organising neurobiological and behavioural units of analysis, it presents a distinct clinical challenge: the ‘deconstruction’ of pathology into a horizontal matrix of domains. By breaking down clinical presentations into isolated biological or cognitive variables without explicit diagnostic prioritisation algorithms, deconstructed domains risk creating data-flattening – offering rich mechanistic details without clear rules for clinical decision-making. Conversely, quantitative classification models like HiTOP offer robust statistical groupings of overlapping symptoms, yet they still face challenges in defining criterion-standard rules to resolve when overlapping somatic and psychiatric symptoms over-explain one another.

In the field of chronic orofacial pain, the phenomenology of clinical presentations strikingly illustrates this diagnostic dilemma. ^{Reference Nagamine4} Patients frequently report persistent oral burning, atypical tooth pain or deep facial discomfort in the complete absence of detectable structural pathology. At the early stages, these presentations are driven by subtle peripheral or localised mechanisms, such as mucosal small-fibre dysfunction or peripheral trigeminal nerve sensitisation. However, over time, the clinical phenomenology shifts: persistent nociceptive input gives way to central adaptive neuroplasticity, altered sensory processing and secondary psychological responses.

To standardise such complex presentations, the field has increasingly relied on descriptive systems, most notably the International Classification of Orofacial Pain (ICOP). ⁵ While ICOP has brought much-needed standardisation, its checklist-style architecture inherently risks flattening complex pathophysiology. When conditions such as burning mouth syndrome, persistent idiopathic facial pain or painful temporomandibular disorders are evaluated purely through ICOP criteria alongside DSM/ICD checklists, patients are routinely assigned multiple, overlapping diagnostic labels. ^{Reference Ziegeler, Beikler, Gosau and May6,Reference Fischer, Tsoumpas, Chana, Feltbower and Aggarwal7}

Crucially, this diagnostic flattening spawns a phenomenon strikingly parallel to the inflation of psychiatric comorbidities seen in ADHD and ASD. In neurodevelopmental disorders, flat checklists frequently lead to secondary diagnoses of anxiety, mood or behavioural disorders without establishing whether these are core developmental features or secondary distress. Similarly, patients with chronic orofacial pain are frequently co-labelled with multiple psychiatric conditions – such as somatic symptom disorder, major depressive disorder or generalised anxiety disorder – because current classification systems lack explicit priority rules or standard criteria to decide whether psychiatric symptoms are secondary manifestations over-explained by primary pain mechanisms or distinct co-occurring conditions. ^{Reference Anita, Asnely Putri and Maulina8,Reference Rahardian, Putri and Maulina9} This horizontal listing of comorbidities fails to identify the primary mechanism maintaining the illness presentation – be it peripheral nociceptive pathology, central sensitisation, secondary maladaptive behavioural avoidance or psychosocial stress. Consequently, clinical practice in this domain risks unguided polypharmacy, unnecessary dental interventions and frequent treatment failures.

To address diagnostic flattening without falling into domain fragmentation, clinical practice across both psychiatry and psychosomatic medicine requires a practical, dynamic hierarchical formulation framework. Rather than assuming a historical gold-standard algorithm that has been empirically proven, a modernised hierarchy serves as an organising formulation tool. Building upon these principles, recent work in chronic orofacial pain proposes an integrative, five-layer framework – the adaptive hierarchical domain network – to organise mechanisms from biological susceptibility and peripheral drivers to central plasticity, behavioural adaptation and social embedding. ^{Reference Nagamine4} Simplified into key operational layers, clinical data can be organised vertically:

1. (a) biological and peripheral layer: local nociceptive drivers, structural somatic lesions or genetic/endocrine susceptibility;

2. (b) central processing layer: central sensitisation, altered neural network connectivity and altered predictive sensory processing;

3. (c) behavioural and psychological layer: cognitive biases, catastrophising and maladaptive avoidance behaviours;

4. (d) predispositional and contextual layer: personality traits, developmental history and social/environmental stressors.

This hierarchical framework is designed as a dynamic clinical formulation tool to help structure clinical priority, rather than a rigid taxonomy or an absolute diagnostic rule. Historically, strict diagnostic precedence sometimes led to underdiagnosis of real, treatable comorbid distress (e.g. withholding depression treatment in autistic individuals). The goal of a modernised hierarchy is not to ignore co-occurring symptoms, but to clarify which layer currently serves as the primary maintainer of illness so that interventions can be rationally prioritised.

In chronic orofacial pain, peripheral factors may initiate the illness but, over time, central neuroplastic shifts or behavioural reinforcement often become the primary drivers maintaining pathology. ^{Reference Nagamine4} Determining which layer maintains the illness relies on synthesising routine clinical tools – such as detailed history, physical examination and validated behavioural questionnaires – alongside specialised investigations where available (e.g. sensory testing or functional neuroimaging). While boundaries between adjacent levels (such as behavioural avoidance in Layer 3 and personality structure in Layer 4) inherently overlap, evaluating their primary relative contribution prevents unguided polypharmacy.

In conclusion, the challenges in chronic orofacial pain provide a striking parallel to general psychiatry, reinforcing the message of Harland et al. ^{Reference Harland, Phillips-Owen, Conlan and Owen1} By adopting dynamic hierarchical formulations alongside descriptive classifications, clinicians can better navigate complex psychiatric–somatic presentations without getting lost in checklist criteria or fragmented data domains.