BJPsych Kaleidoscope November 2019

magazine-coverPharmacotherapy ‘or’ psychotherapy for schizophrenia? A hot debate for many is the evidence base of each.

On the one hand, the evils of ‘big pharma’ and corruption of trial data are often wheeled out; on the other, well is the evidence base for CBTp as strong as proponents sometimes make out? December’s Kaleidoscope reviews a paper that asks if we have been looking at apples and oranges insofar as the relative trials of each are usually against placebo or treatment as usual - not directly compared with each other.

The authors unpicked the underpinning characteristics of 80 studies involving over 18 000 total participants on medication, and 53 studies with over 4000 participants included in psychological interventions. Psychological trials had less severely unwell individuals with shorter illness histories, a longer intervention duration that was administered in addition to medication, and had higher risks of bias. Medication studies included larger number of participants recruited across a greater number of research centres, and participants included more men, in-patients, and older individuals.

Psychological trials were largely publically funded, pharmacological ones by the drug manufacturers; however, the two groups were considered equally conflicted insofar as the psychotherapy ones were often run by researchers with an allegiance to the treatment. So, yes, perhaps apples and oranges, but in clinical practice one would not expect individuals who are acutely psychotic to commence a psychological therapy, and the two treatment modalities are not in any way exclusive of each other. I support the authors’ concluding statement that ‘In the interest of patients, psychopharmacologists and psychotherapists should optimise their treatments rather than seeing them in competition’.

It is an intriguing observation that congenital blindness is protective against the development of psychosis; how is this the case, and what does it teach us about neuropathology?

The finding is all the more peculiar given how congenital blindness often comes from perinatal traumata or chromosomal disorders that have higher rates of psychosis. Kaleidoscope reviews a fascinating paper that addresses this through a Bayesian prediction error minimisation model. In Bayesian inference, prior predictions of the world are combined with actual sensory inputs to create a posterior probability distribution; the relative weight assigned to the prior compared with the sensory data determines how much the latter updates beliefs or gets ignored.

The authors give the analogy of speaking with people at a noisy party: it is far easier to talk to someone who is well known to you in such a setting, as you will have an already existing precise model of their speech against which to assimilate the received sensory input. Thus, the precision of your prior beliefs will be high, and one can more easily disregard prediction errors, and the likelihood of misunderstanding is lower. Taking this to psychosis, a Bayesian model has inappropriate weighting of irrelevant sensory stimuli, which erroneously update beliefs about the world. In the case of congenital blindness, they argue that this increases the precision and stability of higher-level ‘priors’ that protect against the computational deficits underpinning psychosis.

In other words, such congenitally blind individuals are forced to rely so much more on their other senses such that their model of the world becomes far more resilient to false inferences from ‘noisy’ or ambiguous perceptual input in other modalities, as is seen in schizophrenia. The authors extend their argument to later-life visual impairment that is associated with hallucinations, such as Charles-Bonnet syndrome. Here the brain has ‘developed normally’ in early life, and thus higher-level predictions of reduced visually precise input will be explained confidently but falsely, resulting in visual hallucinations. They take this on to neurodegenerative conditions such as Parkinson’s disease and Alzheimer’s dementia, which are often associated with complex psychoses including visual abnormalities.

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