One step forward …

First, ICD-11 has made some promising progress. It has introduced a model for classifying depressive symptoms according to ‘clusters’ of affective, cognitive–behavioural and neurovegetative symptoms. These clusters broadly align with the symptom domains of the ACE (activity, cognition and emotion) model of mood disorders (Fig. 1), which is based on Kraepelin and Weygandt's conception of will (volition), thoughts and feelings.^{Reference Malhi, Irwin, Hamilton, Morris, Boyce and Mulder3}

Fig. 1 Symptoms of depression according to clusters in ICD-11 and domains in the ACE (activity, cognition and emotion) model.^{Reference Malhi, Irwin, Hamilton, Morris, Boyce and Mulder3}

Shaded sections indicate the symptom cluster to which each symptom belongs within the ICD-11 taxonomy (affective is denoted by light blue, cognitive–behavioural by middle blue and neurovegetative by dark blue) and the ACE model (emotion is denoted by light blue, cognition by middle blue, activity by dark blue). Note that irritability is an important transdiagnostic symptom that is diagnostic of mania and features strongly in depression in young people but is not captured specifically in either of the main classificatory schema for depression in adults.

The ACE classification of depressive symptoms is an important advance in the conceptualisation of mood disorders and the use of diagnoses in clinical practice. This is because understanding the functional domains that are affected by a depressive episode in an individual allows for more precise tailoring of treatments and more granular tracking of clinical changes over time. For this reason, the ACE model was included in recent binational guidelines,^{Reference Malhi, Bell, Bassett, Boyce, Bryant and Hazell4} which have since gained widespread recognition.

Two steps back

In contrast to the sophistication introduced into the classification of depressive episodes (clearly a step forward), simultaneously ICD-11 has taken two steps back.

First, after 30 years of research since ICD-10, the symptomatic characterisation of manic episodes remains much the same. Puzzlingly, the clustering of symptoms used to subdivide and refine the depressive syndrome has not been extended to mania, despite manic symptoms readily fitting the affective, cognitive–behavioural and neurovegetative labels. Indeed, the ACE model for mood disorders can be used to group all the symptoms of bipolar disorder (i.e. both depressive and manic symptoms). This is because the spectrum of symptoms of both depression and mania are simply extremes of more continuous dimensional changes. Using a classification based purely on polar opposites (e.g. diminished and heightened sleep, energy and pleasure respectively) and ignoring any intermediate states fails to capture the clinical complexity of real-world phenotypes.

Another important omission is that ICD-11 has provided no means of capturing unipolar mania, namely manic symptoms that occur in the absence of any depressive episodes. This means that this entity cannot be diagnosed and therefore will not be counted in future epidemiological research. Instead, such presentations of mania will automatically be subsumed within bipolar disorder. In other words, clinicians can diagnose depression on its own, and depression occurring in conjunction with mania automatically becomes part of bipolar disorder, but mania on its own has no distinct status and is considered the same as bipolar disorder. This asymmetry is common to both ICD-11 and DSM-5.

But surely the publication of ICD-11 nearly three decades after ICD-10, and almost a decade after DSM-5, was an opportunity to make improvements rather than recreate a previously flawed conceptualisation. However, an argument often put forward in defence of the adopted approach is the need to harmonise the two taxonomies.^{Reference Malhi, Bell, Bassett, Boyce, Bryant and Hazell4} This would perhaps be acceptable if the classification being emulated had achieved optimal accuracy, but it hasn't. Further, harmonisation alone should not be the dominant driver of nosology, especially if the data argue against it, and the two systems of classification have distinct histories and objectives. Indeed, the contested nature of diagnosis per se and the contrasts between the two systems reflect underlying contradictions. Yet, some contrasts are promoted: for example, dimensions and clusters have been introduced to describe depression in ICD-11, even though this is not in keeping with the approach used by DSM-5.

A second step back concerns the arbitrary separation of hypomania and mania, and thereby the subtyping of bipolar disorder. Once again, with a view to following DSM-5's lead, ICD-11 has partitioned mania and given credence to the existence of subtypes of bipolar disorder – specifically, bipolar I and II. Within the diagnostic criteria for hypomania, the only criteria that distinguish hypomania from mania are the duration and severity of symptoms. Thus, ICD-11 acknowledges that there is no real difference in kind between hypomania and mania, and that the term hypomania is essentially used to mean less severe mania. This again highlights the classificatory inconsistencies in ICD-11 as regards depressive and manic symptomatology. For instance, although depression is considered a dimensional construct with respect to severity, there is no subtype of hypodepression to describe less severe forms of depressive disorders. In contrast, mania is partitioned arbitrarily on the basis of severity, instead of the clustering of symptoms into domains as applied to depression, which arguably would have provided greater specificity.

These inconsistencies are first confusing, especially when both mood states (mania and depression) appear in the same disorder, and second illogical, as there seems to be no cogent argument as to why mania is split according to severity but depression is not. Further, the inclusion of bipolar II disorder in ICD-11 does not advance our understanding of mood disorders and does not meaningfully inform clinical management.

Finally, although ICD attempts to demarcate hypomania from mania, it does not clearly define its boundary with normality, and indeed puts fewer restrictions on diagnosing hypomania than DSM-5.^{Reference Angst, Ajdacic-Gross and Rössler5} At the same time, the occurrence of hypomania in the absence of a history of other types of mood episode is insufficient to warrant an independent mood disorder diagnosis. This curious state of affairs further weakens both the theoretical argument for subtyping bipolar presentations and the clinical value of assigning a diagnosis of bipolar II disorder.