Diagnostic labels in functional disorders

Abstract

Diagnostic labels in functional disorders are explored with particular reference to functional neurological disorder (FND). Use of the term ‘functional’ is discussed, and is compared with alternative terminology. Relevant considerations including avoidance of inaccurate nomenclature and dualist labels, and the impact of diagnostic label choice on individual patient experience and broader service provision are considered.

Introduction

This article was written at the request of the editors to discuss the diagnostic labels used for functional disorders. As a neurologist with an interest in functional neurological disorders, I have centred the discussion on the terminology we use for neurological conditions. The issues raised, however, are largely generalisable to functional disorders affecting other bodily systems too.

I will begin by discussing diagnostic labels specifically, both preferred and alternative labels. I will then explore terminology in functional disorders more broadly, including the potential pitfall of over-diagnosis, and the potential for standardisation of terminology across specialties to improve clarity and to streamline services.

Functional disorders in neurology: current preferred terminology

Functional neurological symptom and functional neurological disorder (FND) are well-established terms in neurology. Functional neurological symptom describes a symptom associated with pathophysiological brain network activity; in other words, a symptom related to the way that a brain functions. When one or more functional neurological symptom(s) becomes severe or intrusive enough to reach a threshold, neurologists may diagnose an individual with FND. The level of the threshold is discussed later in this article.

Although adoption of the diagnostic label of ‘FND’ is relatively recent, use of the term ‘functional’ in relation to physiological explanations of neurological symptoms actually dates back to the 19th century.¹ Other terms have been used over the years, fluctuating in popularity, but within neurology ‘functional’ is now the preferred term in most situations requiring the use of a medical label. The DSM-5-TR reflects this change: it uses the term functional neurological symptom disorder; an update from the DSM-4, which used conversion disorder.

The term ‘functional’ is also considered appropriate by patients. In 2002, for example, 86 patients’ opinions of the term ‘functional’ were compared with their opinions of synonyms including ‘psychosomatic’ and ‘medically unexplained’: in the context of functional weakness, ‘functional’ weakness was perceived by patients as having far fewer negative connotations, and as being less offensive, than the alternatives.²

Alternative terminology for functional neurological symptoms or disorder

An appropriate diagnostic label must be clear, accurate, and avoid stigmatising the patient who receives it. This has been explored with nuance in relation to functional disorders.^3,4 Some alternative labels are discussed below:

• •Psychogenic symptoms and conversion disorder. These labels imply that a patient has developed bodily symptoms through psychological mechanisms, or because mental distress has been converted into bodily distress, and are usually unhelpful. Although some patients develop functional symptoms in the context of psychological distress such as past trauma or anxiety, others do not: physical illness or injury, for example, are also clear risk factors. It is incorrect to assume that all patients with functional symptoms experience significant psychological risk factors.¹ Moreover, these labels can be stigmatising: a recent French language study noted that labels implying psychological rather than physical causation of symptoms were associated with higher levels of offence felt by volunteer participants, and with lower levels of willingness to care for patients by healthcare professionals.³ There is a broader issue around why conditions thought to be psychological provoke these negative responses; that is beyond the scope of this discussion.
• •Somatic symptom disorder. This term sits separately to functional neurological symptom disorder in the DSM-5-TR. The labels are not synonymous, but there are significant overlaps when the diagnostic criteria are applied. The label somatic symptom disorder describes ‘physical symptoms accompanied by excessive thoughts, emotion, and/or behaviour related to the symptom, which causes significant distress and/or dysfunction’. The word ‘excessive’ is challenging to apply in practice: what is a ‘normal’ amount of thought, amount or behaviour to focus on a disabling and potentially life-changing bodily symptom?⁵
• •Persistent physical symptoms. In contrast with ‘somatic symptom disorder’, ‘persistent physical symptoms’ is a physically focused term used to describe persistent bodily symptoms that last at least a few months and cause significant distress.³ The term is deliberately broad and is applied based on symptoms rather than underlying diagnosis. While this has some benefits, it inevitably brings a lack of clarity and the potential for conflation of aetiologically different conditions. There is also evidence that although patients find labels of ‘physical’ symptoms less stigmatising than psychological labels described above, they may also view them as more likely to be fixed and therefore less amenable to treatment.³
• •Functional somatic disorder. The labelling problems discussed in the three points above illustrate the inherent problem of using dualist labels which attempt to separate ‘mind and body’; the term ‘functional’ largely bypasses this. The term functional somatic disorder has also been proposed as one that ‘should occupy a neutral space within disease classifications, favouring neither somatic disease aetiology, nor mental disorder’.⁴
• •Medically unexplained symptoms. The term ‘medically unexplained’ is another inaccurate label. Functional symptoms are in fact not unexplained: there is an established and continually expanding evidence base for the mechanisms involved in symptom generation. These include, but are not limited to: variability in interoceptive sensitive and accuracy (awareness of internal bodily states),⁶ in predictive coding (how brains predict what is going to happen based on prior beliefs and new sensory input),⁷ and in levels of self-directed attention,⁸ dissociation,⁹ the effects of heightened sympathetic drive, and associations between these mechanisms and neurodivergence.¹⁰ FND is a complex condition, not an unexplained one. Moreover, FND is not a nebulous label for uncategorisable neurological symptoms: FND has clear diagnostic criteria,^11,12 and common patterns of functional symptoms recur, eg typical motor, sensory, gait patterns and seizures. There is also extensive understanding around risk factors for developing FND (ie aetiology), mechanisms leading to the symptoms of FND, and treatment approaches. FND is no more or less medically unexplained than any other neurological condition. Moreover, a diagnosis of ‘medically unexplained symptoms’ frequently – and logically – does not stop patients searching for explanations; instead, it often perpetuates a loop of further consultations, further investigations, and the risk of resulting iatrogenic harm, eg from incidental findings on investigations or from non-evidence-based interventions.
• •Symptom-based disorder. This term is problematic in the same vein as ‘medically unexplained symptoms’: FND is no more or less defined by being based on symptoms than many other neurological conditions such as, for example, Parkinson’s disease and migraine.
• •Non-epileptic seizures or attacks. These are synonyms for functional, or dissociative, seizures. They describe what a patient does not have, rather than what he or she does have: they are therefore unclear and a rather empty way of telling an individual their diagnosis.
• •Dissociative neurological symptom disorder. This term is used to classify functional neurological symptoms in the ICD-11. This is appropriate for those patients for whom dissociation is clear – such as almost all functional seizures, or a numb limb from which the owner feels detached, for example – but may be less helpful for patients with other functional phenotypes. Where dissociation is a dominant causative mechanism, it can be useful to discuss symptoms through this lens and to name accordingly. On the other hand, however, a functional tremor, for example, may be better framed though discussion of attentional direction than of dissociation.

Consistency in terminology

Of course, patient preference is important: some patients have long-established diagnoses using the terms above and, if they are comfortable with them, there is no strong reason to change them. (In my experience, this is most commonly the case for non-epileptic seizures or attacks.) For new diagnoses, consensus around the most helpful diagnostic labels is valuable. Moreover, consistency in terminology facilitates epidemiological research. Variability in terminology leads to inaccurate or uncoded symptoms in medical records¹³; in turn, that tends to lead to under-estimation of population prevalence of functional symptoms, with all the implications around healthcare usage and resource allocation that this holds. Realistically, without coded data it is almost impossible to prove what most clinical doctors know from their experience: functional disorders are extremely common, but services to diagnose, to provide evidence-based interventions, and to facilitate self-management and ongoing support are not.^14,15

The risks of over-diagnosis

I have described some benefits of naming functional disorders. Within neurology, we have seen a significant increase in the extent to which neurologists make – and name – these disorders, and in the most part this is positive. However, there is a danger that the pendulum is starting to swing too far the other way, with over-diagnosis sometimes creeping in to replace under-diagnosis. This excess medicalisation has just as much potential to damage patients’ wellbeing. The ICD-11 dissociative neurological symptom disorder is helpfully clear on this, with a sub-section entitled bBoundary with normality (threshold)’. This states that ‘transient alterations in sensory or cognitive functions that can accompany intense engagement in work or sports or intense emotional states and transient difficulties in coordinating movements … are relatively common and do not result in significant impairment’. The mechanisms generating these transient symptoms may be the same as those generating more persistent functional symptoms, but the symptoms themselves do not reach a threshold for a medical diagnosis. In neurology, for example, reassurance around ‘benign sensory symptoms’ is common and usually appropriate.

Another over-diagnosis to avoid is where functional symptoms are clearly a result of another – primary – symptom. For example, it is normal to experience motor or cognitive inhibition in the context of pain; this does not merit a second diagnosis of FND (which tends to distract from treatment of the underlying problem). Similarly, neurologists commonly note functional neurological symptoms in the context of migraine: a typical migraine aura will last up to an hour (usually less); neurological symptoms persisting beyond this in the context of an otherwise typical migraine are not uncommon and are usually functional. There is often no benefit to diagnosing FND in addition to migraine, however; this is only useful where the functional symptoms become the main and more persistent problem, impeding normal life and function. In my experience, explaining the mechanisms causing symptoms to patients in this situation can be helpful and reassuring in these situations; adding a name is often not needed and is potentially harmful.

Finally, it is important that neurologists differentiate between functional neurological symptoms and functional neurological disorder. Once symptoms reach a threshold of impact on life, we diagnose FND. The DSM-5-TR defines this threshold as the point at which the symptom(s) ‘causes significant distress, psychosocial impairment, or warrants medical evaluation’. The ICD-11 defines the threshold for dissociative neurological symptom as when ‘symptoms significantly impair family, interpersonal, occupational, or other important areas of psychosocial functioning’. There is no benefit to diagnosing patients with a ‘disorder’ before this threshold is reached, and in fact diagnosing FND for relatively mild symptoms risks trivialising the diagnosis for those more severely affected.

Functional disorders outside neurology: a unified multisystem condition?

Non-neurological symptoms generated by similar pathophysiological mechanisms to those recognised in FND are common and I believe will be discussed elsewhere in this journal edition. Sometimes distinct terminology is used to name a diagnosis; sometimes it is not. The term ‘functional’ is used by gastroenterologists as well as by neurologists: functional gastrointestinal disorders are a category in the ICD-11. Functional urological disorders are a broad set of conditions, but one in which the relationship between urological symptoms and other functional conditions is being explored.¹⁶ Functional pain syndromes – also known as chronic primary pain conditions – are also well-known.

Many patients I see with FND have associated non-neurological symptoms. The lack of a unifying terminological label is problematic: patients often have a long and somewhat overwhelming list of diagnoses; my patients often report that when we can reduce these to a single, multisystem functional disorder, it makes them feel more manageable and facilitates a unified approach to treatment. Standardising this practice would – I think – bring clarity for both patients and treating clinicians. Over time it would surely also contribute to better integration of services for this group of patients: might a multispecialty functional disorders service be a more efficient and effective way to assess and treat patients than through multiple disparate single specialty services, each with their own specialty-specific terminology?

Summary

The words that we use for any diagnosis should be clear, helpful and non-stigmatising. Standardised terminology is beneficial for patients and clinicians and facilitates population-level work. Despite its limitations, ‘functional’ is a non-dualist and non-stigmatising term that is already well-established in neurology and some other specialties; extending it to other specialties would improve clarity, whether though the terms ‘functional disorder (or symptom)’ or ‘functional somatic disorder (or symptom)’. Over time, moving towards an understanding of functional disorders as multi-system, albeit with inter-individual variability in the extent to which different systems are affected, would facilitate development of cross-specialty services with all the benefits that this would entail.

CRediT authorship contribution statement

Marianne Novak: Writing – original draft.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Marianne Novak is employed as a consultant neurologist working within functional neurological disorder (FND) services. She also reports a relationship with the Association of British Neurologists and serving as chair of its Advisory Group on FND and Neuropsychiatry. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.