By Dr Robert Baylis, Consultant in Pain Medicine and Pain Medicine Expert Witness.
Chronic pain and psychiatric disorders are not separate clinical territories. In the most complex personal injury and clinical negligence claims, they overlap, and that overlap is frequently the most contested part of the case. Understanding how the diagnostic frameworks relate, where they diverge, and what they mean for medico-legal evidence is a necessary and practical requirement for anyone instructing experts in these cases.
Introduction
For much of the twentieth century, chronic pain was treated as either a symptom of tissue injury or, where tissue injury was absent, a psychiatric phenomenon. The two specialities operated in parallel, with limited shared language and significant clinical disagreement. That model has been replaced, but its legacy persists in litigation. Some of the diagnostic conflict that routinely arises between pain medicine and psychiatry experts is a direct consequence of this historical divide, not a reflection of genuine clinical disagreement about the patient in front of them.
The purpose of this article is to explain how the current diagnostic framework works, how the relevant conditions relate to one another, and what that means for the quality and interpretation of expert evidence. The aim, in short, is to explain it to the lawyers.
A Brief History of a Long Confusion
The International Association for the Study of Pain was founded in 1974. At that point, and for decades afterwards, chronic pain had no independent diagnostic status in mainstream classification systems. Under ICD-8 and ICD-9, it was coded only as a symptom. There was no dedicated section, no structured classification, and conditions like fibromyalgia, chronic back pain, and neuropathic pain sat in different chapters with no structural relationship between them.
The IASP developed its own taxonomy manual to fill this gap. In clinical pain services through the 1990s, individual presentations were coded using the IASP system rather than ICD, a parallel classification that reflected how far mainstream coding had fallen behind clinical reality. ICD-10 did not resolve this. Chronic pain remained under “R52 – Pain, not elsewhere classified,” the physical versus psychological divide structurally embedded, even as clinical understanding was moving away from it.
ICD-11, adopted in 2019, changed this fundamentally.
How ICD-11 Changes the Framework and Why It Matters for Litigation
Chronic pain now has a dedicated chapter in ICD-11 (MG30 to MG37) and a clear definition: pain that persists or recurs for longer than three months. The biopsychosocial model is explicitly endorsed, and this classification has been adopted by NHS and NICE, with increasing emphasis on psychological evaluation and treatment.
Two practical consequences follow. First, chronic pain can now be recognised as a diagnosis in its own right, not merely a symptom attached to a structural finding. Second, dual coding is now encouraged as NHS and NICE best practice, meaning that a psychiatric comorbidity sits alongside the pain diagnosis rather than in competition with it. A report carrying both a Chronic Primary Pain diagnosis and a recognised psychiatric comorbidity is not displaying clinical uncertainty, it is applying current diagnostic standards correctly.
For legal professionals evaluating competing expert opinions, that distinction matters. The historical expectation, that pain medicine and psychiatry should produce a single, agreed diagnosis, is no longer clinically accurate. The current standard is dual coding, not diagnostic competition.
The Diagnostic Landscape: Four Conditions and Their Relationship
The most complex pain presentations in litigation typically involve a cluster of diagnoses that are clinically related but frequently treated as if they were alternatives. They are not.
- Chronic Primary Pain (ICD-11) is defined as persistent or recurrent pain for more than three months, with significant emotional distress or functional disability, not better explained by another chronic pain condition. It explicitly bridges pain medicine and psychiatry, reframing pain as a brain-based disorder of perception and emotion explicable through central sensitisation and altered pain processing. Psychiatric comorbidity, depression, anxiety, trauma-related disorders, is common. First-line treatments are largely psychological: CBT, ACT, mindfulness, and antidepressants. In cases where pain medicine and psychiatry intersect, Chronic Primary Pain is the anchor diagnosis.
- Somatic Symptom Disorder (DSM-5) focuses on the psychological response to somatic symptoms, including pain, emphasising psychological amplification, catastrophising, and health anxiety. Importantly, it abandons the previous “medically unexplained symptoms” terminology. That has direct medico-legal relevance: the absence of a clear biological explanation no longer places a presentation in a diagnostic vacuum. SSD addresses the meaning and interpretation of pain, not its aetiology alone.
- Bodily Distress Disorder (ICD-11) is ICD-11’s replacement for somatisation disorder, covering distressing bodily symptoms not fully explained by a known medical condition, accompanied by excessive preoccupation and health-related anxiety. It bridges Chronic Primary Pain and SSD, incorporates both attentive and affective dysregulation, and converges with pain medicine around central sensitisation and interoceptive dysfunction. It avoids the physical versus psychological framing that has historically generated unnecessary clinical conflict.
- Dissociative Neurological Symptom Disorder, what neurologists more commonly call Functional Neurological Disorder (FND), involves altered voluntary motor or sensory function incompatible with recognised neurological disease. It is common in presentations involving fibromyalgia and chronic pain syndromes, reflecting a non-dualistic model in which pain and conversion symptoms share common neurocognitive mechanisms. It is a neurological formulation, not a psychiatric one and its appearance alongside a pain medicine diagnosis should not be read as conflict.
Why These Diagnoses Coexist Rather Than Compete
CPP, SSD, BDD, and DNSD share a common neurobiological substrate: dysregulated pain modulation networks, predictive coding errors, and dysfunctional descending inhibitory control. That shared neurobiology validates shared treatment pathways, CBT, ACT, antidepressants, and avoidance of long-term opioid prescribing, across all four conditions.
Where multiple experts from different specialities recommend the same treatments despite using different diagnostic labels, that convergence is clinically significant. It is consistent with a shared underlying mechanism, and it is evidence that apparent diagnostic disagreement is one of framing rather than substance.
A Framework for Evaluating Complex Presentations
A four-dimensional framework allows the clinician, and the court, to address complex presentations systematically.
- The nociceptive–neuropathic axis: is there a clear tissue or nerve injury source? Where the biological explanation is absent or disproportionate, Chronic Primary Pain enters the analysis.
- The affective–cognitive axis: are there excessive worries, avoidance behaviours, or health anxiety? Where present and prominent, BDD or SSD is relevant alongside the pain diagnosis.
- The sensorimotor–dissociative axis: are there positive functional neurological signs or dissociative episodes? DNSD requires consideration and its presence does not displace the pain diagnosis.
- The social-contextual axis: what is maintaining the disability? Benefits, work stress, social isolation, and ongoing litigation are all documented factors in functional impairment, operating across all four diagnoses. For litigation purposes, this is often the most significant axis of all.
The Same Patient, Different Labels: What This Looks Like in Practice
The abstract point about speciality-specific labelling becomes clearest when illustrated by a real presentation. Consider a claimant involved in a road traffic collision, able to drive away from the scene, who subsequently develops a complex range of unexplained somatic symptoms: chronic pain, fatigue, poor concentration, numbness and paraesthesia, motor weakness, and loss of coordination, alongside anxiety, depression, inability to cope, and health anxiety.
Pain medicine reaches diagnoses of Chronic Primary Musculoskeletal Pain and possible DNSD, recognising possible SSD or BDD while deferring to psychiatry on those formulations. Psychiatry and psychology, reviewing the same patient, reach diagnoses of adjustment disorder and SSD or BDD. The two specialities appear to disagree.
On closer review, the pain experts conclude that Chronic Primary Pain would not be sustainable as a standalone label, that SSD or BDD more comprehensively explains the symptom profile. That is not a concession. It is the diagnostic hierarchy working as it should: the pain formulation provided the anchor; the psychiatric formulation provided the more complete account of what was driving the presentation.
For lawyers, the practical lesson is this: where pain medicine and psychiatry experts reach different primary diagnoses for the same presentation, the first question is not which expert is right. It is whether the diagnoses are actually incompatible, or whether they are different formulations of the same underlying disorder.
What to Look for in an Expert Report
In UK services, patients with complex pain presentations move between specialties. Pain services use Chronic Primary Pain as the primary label. Psychiatry may formulate the same presentation as BDD or SSD. Neurology may diagnose DNSD where motor or sensory symptoms predominate. Each speciality is looking at the same problem from a different perspective.
From a pain management perspective, the hierarchy is consistent: CPP is the anchor diagnosis; BDD and SSD describe the cognitive and emotional responses; DNSD reflects functional motor and sensory expression. Together they describe body-brain miscommunication, a single dysregulated network expressing itself through different symptom profiles, assessed by different specialities using different frameworks.
A report that understands and articulates this relationship is far more useful to the court than one that defends a single diagnostic label against competing formulations.
Conclusion
Pain is not merely a sensory signal. It is a complex, context-dependent brain state. CPP, BDD, SSD, and DNSD are now best understood as reflecting a continuum of disordered centralised and effective pain processing. The historic divide between psychiatry and pain medicine is giving way to a neuropsychiatric model of embodied distress, where pain and the distress that accompanies it are linked as different expressions of a single dysregulated network.
The absence of proportional tissue pathology does not render a presentation medically incoherent. The presence of multiple diagnostic labels from different specialities does not automatically indicate expert conflict. And the convergence of treatment recommendations across those diagnoses is clinically meaningful, not coincidental.
The challenge for legal professionals is not to resolve which speciality’s label is correct. It is to instruct an expert who can explain the relationship between them and translate that relationship, accurately and accessibly, for the court.
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