Psychiatric syndromes other than dementia

There is wide variability in how psychiatry guidelines and textbooks address the question of cerebrospinal fluid (CSF) diagnostics in the screening of psychiatric disorders. A United States-based textbook confirms that there is no consensus about which laboratory investigation should be routinely performed in psychiatric patients, but with respect to CSF diagnostics, the differences are even more striking. A survey among European experts showed a wide variety of opinions regarding clinical use and criteria in various countries of Europe and worldwide: some psychiatrists, mostly university hospital-based, recommended performing CSF diagnostics in every patient first experiencing severe mental illness (SMI), but especially in patients from the schizophrenia spectrum, whereas others almost never perform CSF examinations themselves and usually refer patients to neurology departments if necessary. Minor neurologic signs are generally frequent in SMI, mainly in affective and schizophrenic disorders. Even with neurologic signs present, there are no clear guidelines regarding CSF evaluation, leaving doctors with experience-based decision making. However, the field is evolving. A recent review provides helpful yellow and red flags for differential diagnosis of SMI from autoimmune encephalitis; interestingly, minor CSF abnormalities are considered a red flag, suggesting that CSF should be routinely performed in acute psychiatric patients. There are reports of single cases identified as an established neurologic disorder: patients within affective and schizophrenic spectrum disorders systematically underwent CSF examination, and were rediagnosed based on CSF results. This was often to the surprise of the psychiatric doctors. Overall, an increasing number of psychiatrists believe that CSF is too rarely examined in psychiatric patients. This chapter provides an overview of differential diagnostic issues in SMI, particularly for new-onset cases. The general recommendations regarding CSF examination procedures can be found in other chapters of this book. Here we focus on specific aspects of differential diagnosis in SMI. Also, there will be an overview of admittedly limited CSF research efforts in psychiatric disorders, focusing on more recent CSF studies. CSF studies in SMI performed with state-of-the-art methods, for example proteomics or assessments of cytokines, were intriguing but difficult to interpret and required critical considerations regarding respective methodology, an undertaking which is outside the scope of this chapter.