Cognition and disability in schizophrenia: cognition‐related skills deficits and decision‐making challenges add to morbidity

Schizophrenia contributes 13.4 (95% UI: 9.9‐16.7) million years of life lived with disability to the global burden of disease. Its societal costs are immense, with costs derived from productivity loss even larger than direct treatment costs, a pattern observed across different countries and health care systems. Based on these data, disability reduction in schizophrenia is a priority, yet there are few effective treatments available.

Nonsocial and social cognitive impairment contributes substantially to reductions in everyday functioning and subjective quality of life in persons with schizophrenia. Green et al1 present and evaluate sophisticated models of the influence of nonsocial and social cognition on functioning, considering moderating variables (e.g., defeatist attitudes, motivation, reward sensitivity) as well as neurobiological correlates and their potential implications. Further, they thoroughly evaluate treatment efforts to date for these deficits, including pharmacological and remediation‐based approaches. Among these efforts are exercise interventions, which target physical fitness and have been shown to have beneficial effects on cognitive performance.

Just like with any other chronic disease process, there are multiple factors that contribute to the development of disability in schizophrenia. Obesity and health‐related comorbidities are common. Physical fitness is visibly impaired. The presence of these elements shows a correlation with cognitive impairments2.

One of the issues covered in less detail in Green et al's review is that of functional capacity (the ability to perform everyday functional skills) and its potential mediating effect between nonsocial and social cognition and functional outcomes. In several studies, functional capacity was found to be proximally related to impairments in everyday functioning, with the strongest predictor of deficits in this capacity generally being nonsocial cognition. In addition, when social functional capacity, generally referred to as social competence, is examined for its relationship to functional outcomes, it can be shown that some elements of social cognition predict performance on measures of social competence, which in turn predict informant ratings of everyday social functioning. Thus, impairments in nonsocial and social cognition may be a precursor to functional skills deficits, which then in turn predict impaired everyday outcomes across several domains.

In a related vein, we have recently documented that correlates of poor physical health and fitness are important determinants of disability in schizophrenia that interact with nonsocial and social cognition to complicate functional outcomes. The end result of these physical impairments might prevent people from even leaving their residences and may exacerbate limitations in functional capacity beyond those originating from nonsocial and social cognitive deficits, while generating additional roadblocks to effective deployment of everyday skills that the patients might possess.

We developed a model that integrates these different contributory paths into a unified model of disability in schizophrenia, attempting to isolate the pertinent individual factors (for example, symptoms, cognition, physical functioning) and their interactions, so that they can be approached in a synergistic manner3.

In analyses of data from the Suffolk County Mental Health Project, we examined the 20‐year course of weight gain and its impact on everyday functioning at the 20‐year follow‐up. We found that weight gain was progressive over the entire period, leading to over 50% of bipolar patients and 60% of schizophrenia patients having a body mass index in the obese range 20 years after diagnosis4, a striking change from 8% and 20%, respectively, at the time of first diagnosis.

In a separate examination of the everyday functioning of these same patients at the 20‐year follow‐up, we found that schizophrenia patients, who had a greater prevalence of obesity and worse cognitive performance, also had worse everyday functioning outcomes in terms of sustaining competitive employment and living independently5. For both patient samples, cognitive impairment and two indicators of physical functioning, waist circumference and the ability to rapidly and repeatedly rise from a chair (chair stands), were associated with competitive employment. When a logistic regression was used to predict employment, diagnosis accounted for 11% of the variance, with chair stands accounting for 9% and negative symptoms for an additional 5%. The diagnostic effect was likely associated with cognitive differences between the groups, but mobility limitations associated with obesity were excellent predictors of work outcomes. Modeling residential independence, only diagnosis accounted for variance in outcomes.

These findings do not cast any doubt on the importance of cognitive impairments for predictions of everyday outcomes. Rather, they likely suggest that cognitive impairments may contribute to the development of physical limitations. Obesity in schizophrenia is correlated with multiple impairments in nonsocial and social cognition6. On the nonsocial side, decision‐making regarding dietary choices has been shown to be impaired. Poor dietary quality is common among low socioeconomic status groups, including those with schizophrenia. Fruit and vegetable intake is uncommon compared to the rest of the population. Those dietary choices, combined with the consumption of highly processed energy‐dense food, foster obesity7.

These calorie‐dense, highly palatable foods are readily available in industrialized societies, requiring little effort in procurement and preparation. Patients with schizophrenia appear especially vulnerable to this environment, as they consume more food than mentally healthy people, and their food choices are poorer. In addition, very few patients follow a regular physical exercise routine8 and, amongst those who do, erroneous assumptions about what represents healthy “exercise” prevail. In addition, the same deficits in valuation judgments noted by Green et al in the performance of emotionally neutral problem‐solving tasks are present in food choices, with substantial tendencies toward short‐term reinforcement rather than delayed gratification and planned food choices.

Further, impairments in functional capacity, known to be driven by cognitive limitations, are also common in relation to food related skills. Several studies have shown that schizophrenia patients are impaired in their ability to plan for and shop for nutritious meals. Their actual performance of cooking skills is also impaired9. Using a series of laboratory‐based simulation tests, patients with schizophrenia manifested substantially more impairment in their ability to plan a meal, shop for ingredients, and actually cook the food than healthy controls. These functional deficits were correlated with the severity of negative, but not positive, symptoms, and with executive functioning, but not memory, deficits.

In conclusion, we suggest that cognitive limitations of people with schizophrenia not only correlate with disability directly, but contribute substantially to other skills deficits (functional capacity; social competence) that exacerbate disability outcomes. Poor health and fitness, which add variance to current cognitive assessments for the prediction of disability, can also be traced back to cognitive deficits. The flow‐forward cascade of impaired cognition, particularly in domains of reasoning and problem solving and reinforcement valuation, can lead to deficits in functional capacity which then lead to poor dietary and exercise choices, contributing to poor functional outcome.

Thus, influences on outcomes that appear to be unrelated to cognitive deficits may at least partially originate from cognitive limitations and respond to adequate cognitive enhancing treatments.