Friday, 25 July 2014

Utility without validity

Utility and validity

A frequent concern is, if psychiatric disorders don’t actually describe “natural kinds” i.e. clearly separate illnesses such as differentiating pneumonia from a heart attack then this lack of validity means they have no usefulness in treatment or prognosis (utility).

Think of colours

Western and Japanese culture recognises 7 main colours (excluding black and white) other cultures recognise smaller number e.g.  some in Africa recognise 2 main colours. When tested all people from all cultures can recognise differences in tone/ colour when tested. (So clearly separate different colours can be reliably recognised).

Differences between colours are not absolute e.g. orange eventually becomes yellow but the borderline is indistinct. Differences between neighbour colours is more difficult e.g. orange/yellow or indigo/ violet than differences between clearly separate colours e.g. orange/violet.

So this is a spectrum phenomenon (but with an external validated criterion of light frequency/wavelength) that we categorise into separate colours, not by the external criterion but how it appears to us (so the wavelength is assigned to a subjectively decided colour). The number of categories is culturally determined and we can reliably differentiate between colours/ tones if on clearly different points of spectrum but not if close together on spectrum.

We categorise colours despite it actually being a spectrum because we find it useful for purposes of description. We categorise based on archetypes e.g. of orange, of yellow, not clearly separated. The archetype is of the greenest green or the bluest blue etc Note there is no assumption of “clustering” i.e. that in nature, light frequencies/wavelengths encountered will be more common either each named archetype/ category or that the prevalence of light frequency/wavelengths will be commoner in the centre of the archetype/categories light frequency/ wavelength optimum value.

Note Pantone system of allocating number and name codes to varying shades of colour is a form of categorisation using archetypes.

Application to mental disorders

If you assume that mental activities/ behaviours/ symptoms exist on a spectrum (multi-dimensional rather than single dimensional) and are not separated dichotomously from each other or from “normal” range (i.e. are not separate “natural kinds” and not valid separate categories)

You can still apply categories to these dimensions in a similar way to colours above.

These categories form “archetypes” which to be useful need to be
  • Reliably distinguished (neighbour categories harder to distinguish than distant categories)
  • Provide useful information in terms of other associated features, prognostic frequencies (not absolute but increased and decreased rates), guide to treatment based on research, use for stats etc
  • We can expect that differences between “neighbouring” archetypes may be less than those with “distant” archetypes
  • That marginal cases may exist that don’t fit closely into these archetypes (c.f. index levels of caseness in S.C.A.N. interview schedule) but that the optimum number of categories is not necessarily fixed but depends on other factors (c.f. different no. of colours in different cultures) see debates between lumpers and splitters

Now how might this work in practise?

Looking at 2 dimensions: psychotic symptoms and presence/absence of mania. Now some cases of psychosis may have mild manic type symptoms intermittently but overall are closer to the psychosis, no mania archetype. Let’s place them at different part of the spectrum (equivalent to red and violet in he rainbow). Let’s look at one aspect: the effectiveness of Lithium and antipsychotics in treating the presenting picture. We can get the following picture (derived from Cochrane Reviews and clinical practise).

Antipsychotic effective?
Lithium Effective?
Mania, no psychosis
Mania with psychosis
Psychosis, no/little mania

I’ve not described some of the other differences e.g. tendency to different ages of onset, different social class prevalence, different levels of and type of cognitive impairment etc.

We can see from the point of view of a prescriber it is important to distinguish presence of mania and psychosis and to allocate to appropriate category (one can argue the important question is manic yes/no? if no, psychotic yes/ no, give appropriate treatment. Note for a prescriber the individual symptom in this case is unimportant, e.g. auditory hallucination, persecutory delusion, insomnia, what is more important is to which class the symptom in context with other symptoms places the patient into. 

Another important distinguishing feature could be outcome/prognosis. Very few general medical illnesses give an absolute prognosis e.g. 100% guaranteed to recur in a specific time frame. They often give a range of probabilities of outcome.

The AESOP study in the UK looked at people with a First Episode psychosis. They found that people who presented with Mania had 3-4 times better chance of symptom and functional recovery at 3 and 10 years than people presenting with Schizophrenia/ Schizoaffective disorder (a mixture of both psychosis and mood disorder). (From a lecture given at the General Adult Psychiatry Conference in Manchester in 2013).

So can you reliably identify different points of the spectrum? After all, if people confuse red for green what’s the use of a colour system (and of course some people with colour blindness do have difficulty differentiating certain colours but the prevalence of this problem is low enough for us to continue using colours for many purposes).

 One way of looking at this is what’s called a kappa concordance figure. In the UK we use ICD10 as our official diagnostic system. Field trials showed the kappa figures for schizophrenia overall as a category were very good 0.77-0.84 and for bipolar, manic disorders to be 0.69-0.77. (kappa figures 0.61-0.8 are regarded as substantial agreement between separate raters). This suggests that different clinicians have a high chance of reliably agreeing between themselves what is a manic state and a psychotic state without mania.

So we can see how even if psychosis is like a multidimensional spectrum disorder and not divided into clearly separate disorders then it can still be useful to recognise different points of the spectrum if they are associated with different treatment responses and outcomes.

In short, utility without validity.

However this utility of making the diagnosis depends on how reliably the diagnosis can be made by different clinicians and whether there is sufficient difference in treatment and/or prognosis/outcome for it to be worthwhile. This needs to be made for each diagnosis.

Also, within this psychosis spectrum many people don’t come close to either archetype (equivalent to yellow or green) so assumptions of treatment response and prognosis are less likely to apply to them.

At this point we start to get “how-cannery”, how can these broadbrush archetypes actually predict treatment response, outcomes etc? How can people with such varying content even within the same symptom (the wide variety of different types of persecutory delusions for example)  let alone multiple types of symptoms have the same range of probabilities of response to treatment and outcome? Setting aside the fact that the brain outputs are by far the most complex of any bodily organ outputs  (compare the heart which basically pumps blood), the fact is that research and clinical practice demonstrates that this is indeed the case. A hypothetical problem is solved by experience in the real world. Of course in psychiatry and general medicine also, people with the same diagnosis have different responses to treatment and outcome.

People not using the medical model often have other ways of describing problems that they find more useful to them when helping people with their problems

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