Not all psychiatrists recognize this explicitly, but there is an assumption in psychiatric practice that mental disorders are arranged in a hierarchy with those at the top being more serious than those lower down. The hierarchy has often been represented in a pyramid introduced some 50 years ago by two academics working in Edinburgh.
At the bottom of the pyramid are states of personal distress (in the original account called dysthymic states) characterised by symptoms such as anxiety, irritability, low mood and non-specific physical symptoms. In the so-called neurotic disorders the symptoms are still recognizable as part of distress, but one or more of them seems to have taken on a life of its own – out of proportion to other symptoms of distress and perhaps distorted or expressed in an unusual way. Higher up the pyramid are delusional (psychotic) states, characterised by the emergence of symptoms that are not part of mainstream experience – like holding delusional beliefs or hearing voices. At the very top are extreme states where mental life is so disrupted that a stable sense of self is difficult to find.
This arrangement represents a hierarchy in another sense, that of progressive deterioration in the organisation of mental function so that those higher up can have symptoms from any of the states lower down in the hierarchy without their diagnosis needing to change, but not vice versa.
The hierarchy is nodded at in the arrangement of the two standard diagnostic manuals – current editions DSM5 and ICD11 – in which schizophrenia and related disorders appear before severe mood disorders and then move on to the neurotic disorders.
Over time, perhaps especially in the UK, it has become common practice to talk about severe mental illness, initialised (always a bad sign) as SMI, and SMI has come to mean “any psychotic disorder”. This was never a good idea. In many branches of medicine the diagnosed disorder can be a pointer to severity but the coupling is not so strong that diagnosis can be taken as a severity statement. Some diabetes is more severe than some cancer and some COPD is more severe than some leukaemia.
So, what do we mean by severity? The answer takes us into matters that are more important to patients than the classificatory ordering of symptom states – for example impact on life expectancy, quality of life, ability to function physically (impairment or handicap) or socially (participation) come into the frame.
Is there a tension between these two ways of looking at severity? The UK government’s vaccination policy in the COVID pandemic certainly suggests some lack of clarity about what severe mental illness is and why it matters. The official order of priority for receiving the vaccine is described in a series of Priority Groups, defined by likelihood that COVID infection will be serious and could lead to death. Thus, age defines the top groups. Priority Group 6 includes adults aged 16 to 65 years in an at-risk group defined by clinical conditions one of which, in an uncomfortable change of grammar in the list, is “are severely mentally ill”. Further clarification comes in the COVID-19 Green Book Chapter 14a which defines severe mental illness as (again the weird change of grammar) “Individuals with schizophrenia or bipolar disorder, or any mental illness that causes severe functional impairment.” A recent commentary in The Lancet described the rationale as follows: “Patients with severe mental illness, which is patients with psychotic disorder, bipolar disorder, or severe depression, are at a significantly increased risk of being hospitalised or dying from COVID.”
So, the Green Book definition takes it both ways – severity as psychosis and severity as illness causing severe (not defined) functional impairment – whereas the research underlying the decision seems to refer only to the diagnosis-based definition of severity. The advantage of the latter approach is that it is logically consistent with the other priority groups, taking likelihood of hospitalisation or death as the indication for vaccination, and there are at least some plausible mechanisms for such poor outcomes. The disadvantage is that equating severe mental illness with psychosis is clearly wrong and leads to a sense of injustice among those whose illness is severe by quite reasonable more person-centred criteria. The disadvantage of the former (Green Book) approach is that while it acknowledges a wider definition of severity of illness – including, as recommended by some experts, people such as those who have eating disorders or have been given a diagnosis of personality disorder – it is no longer clear why severity thus defined puts somebody in a priority group for vaccination.
It is difficult to escape the conclusion that there is something inherently wrong with the term Severe Mental Illness (SMI). When equated with psychosis it fails to recognize that severity is not adequately captured by diagnosis. When the initials are dropped and it refers to any mental illness, it lacks specificity as a criterion for decision-making. It would be better replaced with a more accurate and specific terminology.