On the nosology of formal thought disorder

To the Editor

Current diagnostic criteria for formal thought disorder (FTD) in schizophrenia (Diagnostic and Statistical Manual of Mental Disorders–Fifth Edition [DSM-5]; American Psychiatric Association, 2013) do not reflect (1) the general consensus in the literature for it being associated with neurocognitive deficits or (2) the distinction between positive and negative FTD. This situation should be rectified.

In terms of nosology, FTD is currently classified as simply a psychotic feature in the DSM-5. Classification as a psychotic feature traditionally reflects the observations that FTD severity fluctuates between acute and chronic states, much like hallucinations. However, there is evidence suggesting that unlike psychotic symptoms, the impairments observed in patients with FTD never truly recover even when FTD is in remission, i.e., semantic memory deficits (Leeson et al., 2005). Problems with language production are also well-documented in FTD, with evidence suggesting that language comprehension problems are already present early in the illness (Wood et al., 2007).

The aforementioned work on semantic and language impairments in FTD highlights an underlying continuity of FTD-related impairments. Such a situation is more akin to the classification for cognitive symptoms and differs from the transient nature of psychotic symptoms under which FTD is currently classified. Consequently, it might be time to consider revisiting the nosological debate surrounding FTD, and look at re-classifying it as a neurocognitive phenomenon.

Another pertinent reason for change is that the current criteria do not account for the well-observed differences between positive and negative FTD, at both levels of underlying mechanisms and prognostic outcome. The change in nosology is important as it results in clearer diagnostic criteria that better reflect the nature of the symptom as well as schizophrenia itself. This in turn will help better inform treatment and rehabilitation options for individuals with FTD.

While we acknowledge that these top-level changes unfortunately do not happen quickly, there are some measures that can be adopted in the interim. First, it would be beneficial to avoid use of the term ‘disorganised thoughts’ in positive symptom classifications of schizophrenia to avoid confusion with ‘thought disorder’. This would better facilitate the identification of FTD as being of a different nature. Second, distinctions between positive and negative FTD should continue to be recognised and adopted. Finally, continued examination of the cognitive mechanisms underlying individual FTD symptoms, particularly longitudinal work, would help in further fine-tuning the growing understanding of the disorder and strengthen the case for nosological change.