Introduction

As the theme of this chapter demonstrates, the schizophrenia and affective disorder spectra overlap across multiple dimensions. The nature and extent of this common ground has long been subject to debate, and itself reflects broader questions about the conceptualization of the major psychotic disorders. In this light, overlaps in clinical, epidemiological, cognitive, genetic/neurobiological and other relevant areas serve both to better delineate the specific disorders, and also to further our understanding of the nature of severe mental illness. One area that may shed light on both spectra, and on the nature of the liability to develop them, involves physical comorbid conditions. This chapter focuses on the issue, first by reviewing representative evidence that rates of certain related physical disorders are elevated in both schizophrenia and in affective illness. Poor glucose regulation will be emphasized in this discussion as an example of a physical condition that occurs in both spectra and may be inherent to one or both disorders. While physical disorders may influence many features of psychiatric illness (e.g., their course, their nature or the severity of clinical symptoms), common elements in their potential to modulate liability for schizophrenia and affective illness will be emphasized, along with conceptual and clinical implications of the comorbid relationships.