We have presented an argument for the importance of cognitive impairment in schizophrenia and have suggested that cognitive impairment should be represented in the diagnosis for schizophrenia. However, there are several considerations regarding how it should be included. We propose that the following criterion should be considered for the diagnosis of schizophrenia: “a level of cognitive functioning suggesting a consistent severe impairment and/or a significant decline from premorbid levels considering the patient's educational, familial, and socioeconomic background.” Diagnosticians should consider all aspects of cognitive impairment in this definition but should be alerted that, in general, schizophrenia patients may have particularly severe deficits in the cognitive domains of memory, attention, working memory, reasoning and problem solving, processing speed, and social cognition.1
It is not uncommon for some aspects of cognition to be unimpaired in the context of severe impairments in other areas, with an overall level of impairment in the severe range. A statement that the assessment of cognitive function must consider the patients' background was included to avoid overdiagnosing schizophrenia in individuals whose environments deprive them of their ability to develop cognitive abilities. If in the event that DSM-V
changes to a completely dimensional approach to the symptoms of psychosis,43
cognitive impairment should be one of the key dimensions.
This change in DSM will potentially increase the point of rarity with other psychoses. It is likely that some patients diagnosed with schizophrenia who have little or no cognitive impairment have treatment responses and courses of illness that are more consistent with a diagnosis of affective disorder. If this is the case, it will benefit clinicians to change their expectations based upon this revised diagnosis. On the other hand, some patients diagnosed with affective disorders and severe cognitive impairment may follow the longitudinal course and treatment response of patients with schizophrenia. One of the important research questions that will need to be addressed is whether patients whose diagnosis changes based upon the new criteria are more likely to have genetic and other biological indicators consistent with the new diagnosis.
Changing the DSM criteria for schizophrenia to include cognitive impairment will also force clinicians to consider the cognitive impairment of their patients, which has been largely ignored among clinical psychiatrists. This change would thus direct clinicians' attention toward the aspect of the disorder that is the largest determinant of long-term functioning. It may also help develop the pathway for new treatments to improve this fundamental component of the illness and force educational systems to teach clinicians how to recognize cognitive impairment and improvement.
However, the implementation of this change in DSM will present several challenges. If this criterion is included in the criteria for schizophrenia, it will be crucial to consider how cognition will be measured by clinicians and researchers making a diagnosis. It is unrealistic to expect that all patients with schizophrenia would receive formal neuropsychological testing by psychologists, which is time consuming and expensive. In most treatment settings, these costs are prohibitive. However, if cognitive paradigms were developed that were able definitively to separate diagnostic entities, a case could be made that this testing is essential to patient diagnosis and treatment planning. Unfortunately, as discussed above, we are not yet at this stage.
A second consideration regarding the use of cognitive impairment as a criterion for schizophrenia is that current cognitive performance is affected by factors unrelated to cognitive decline in patients with schizophrenia such as level of education and environments that are variably conducive to normal learning.23
Some patients may have very poor cognitive functioning due to factors unrelated to schizophrenia while other patients may have cognitive performance that is in the “normal range” despite significant decline from premorbid levels. How will diagnosticians determine how schizophrenia may interact with these factors to result in a patient's current cognitive levels? Since not all patients are defined as “impaired” on cognitive tests, it is important to emphasize that the criterion will be met if a patient's current cognitive performance represents a “decline from premorbid cognitive functioning”. On average, the longitudinal course of cognitive function in patients with schizophrenia appears to decline at least one full SD from childhood. During childhood and adolescence, patients who will eventually develop schizophrenia perform about 0.5 SDs below their peers who will not develop schizophrenia.28,29,31
Immediately prior to the onset of psychosis, patients who are about to develop schizophrenia demonstrate a worsened cognitive function, such that the average person at ultra high risk for schizophrenia disorders who will eventually convert to psychosis performs about 1 SD below healthy controls.30,34
It will be important in these cases for diagnosticians to determine whether there has been a decline in cognitive functions from expected cognitive levels based upon antecedent factors such as parental education, early school performance, and reading level. It will be essential for diagnosticians to collect a complete history on the cognitive performance of each patient, including how the patient's current cognitive performance compares to early school performance and any academic, intelligence, or cognitive testing that was performed during premorbid and prodromal periods. Further, a patient's level of cognitive performance will need to be compared with other members of the patient's family and sociocultural background, if available. In some cases, testing would benefit this assessment. In other cases, the amount of cognitive impairment in a patient would be clearly obvious and in direct contrast to early cognitive competence in an individual. Finally, because cognitive impairment in affective disorders in the context of clinical exacerbation may be difficult to distinguish from schizophrenia cross-sectionally, longitudinal assessment will be important for an accurate diagnosis. While this historical and longitudinal data collection may initially appear to add burden, if indeed the level and course of cognitive deficit is crucial not only to diagnosis but also to prognosis and treatment planning, it is likely that this “front-loading” of clinical care may actually reduce clinical burden in the form of improved treatment response and long-term functioning.
Third, while clinician judgment will be an important component of assessing cognition in schizophrenia, recent data suggest that clinicians cannot be the sole source of information for making this determination. The challenge that arises here is that many patients with schizophrenia will not have enough contact with other people for someone to be able to report reliably on their usual level of cognitive functioning. Patients without available informants will need to have additional assessments such as more extensive interviews or an actual cognitive assessment, which is the most informative method for collecting cognitive information about a patient.
As discussed above, if a patient is assessed during a period of clinical exacerbation, cognitive impairment may be very similar in patients with schizophrenia and those with affective psychoses.10,14,15
Thus, for the patient to meet the criterion of cognitive impairment, it will be important for the cognitive deficits to be stable throughout a long period of illness. This would help to differentiate the cognitive impairment found in schizophrenia from those in affective psychoses. However, it will also result in delays in definitive diagnoses in cases where cognitive impairment is present in the context of symptom exacerbation.
In sum, we have recommended for consideration that a criterion for consistent severe cognitive impairment be added to the DSM diagnosis of schizophrenia. There are several challenges for this suggestion to meet acceptance by the research and clinical communities. Research is needed to determine: if such a criterion will increase the point of rarity between schizophrenia and other diagnostic entities; if clinicians are able assess cognition reliably with brief formal assessment instruments or interview-based methods; and if the inclusion of such a criterion will improve the value of the diagnosis of schizophrenia for prognosis and treatment outcomes.