What does it mean if a diagnosis is not included in the DSM?
It means that, as of 1994, there was not sufficient data to justify its inclusion in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). Just because a category is not included in DSM-IV does not necessarily mean that it is not worthy of being a focus of research or treatment.
There are many ways diagnoses have ended up in the Diagnostic and Statistical Manual of Mental Disorders (DSM). During the time of the publication of Diagnostic and Statistical Manual of Mental Disorders, First Edition and Second Edition (DSM-I and DSM-II), 1952 and 1968 respectively, there was very little empirical data available about psychiatric disorders. Since the primary purpose of the early versions of the DSM was to standardize data collection administratively and to facilitate communication among clinicians, the diagnostic makeup of these early versions represented a consensus of those disorders that were seen by psychiatrists in the United States in the 1950s and 1960s. After the publication of Diagnostic and Statistical Manual of Mental Disorders, Third Edition (DSM-III), and a subsequent expansion of research in psychiatry, attempts have been made to make the DSM as empirically-based as possible. While DSM-III and Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revise (DSM-III-R) relied primarily on expert consensus that was informed by the experts' familiarity with then-current psychiatric research, DSM-IV was developed based on a comprehensive review of the literature.
As the DSM has become increasingly more informed by research, so has the basis for inclusion of new categories in the DSM. As stated above, originally categories were included because they were felt to represent what psychiatrists were treating. In later editions of the DSM (DSM-III-R and DSM-IV), new categories were only considered for inclusion if there was significant data available to allow critical consideration of the relevant merits and risks of inclusion. In contrast, some categories that date back to older editions of the DSM may have relatively little empirical data. Thus, because the data requirement for consideration of new categories has become more stringent, some proposed categories that were ultimately rejected may have had more data available than grandfathered categories already in the DSM.