The first two aims of this study were to describe the clinical characteristics of patients with the DSM-IV diagnosis eating disorder NOS and establish its severity with reference to bulimia nervosa. This required recruiting a large and representative patient sample and assessing it using standardised measures of eating disorder-specific and general psychopathology. This was done. The diagnostic composition of the sample was as expected with over half the patients receiving the diagnosis eating disorder NOS, a third meeting diagnostic criteria for bulimia nervosa, and the remainder having anorexia nervosa. These figures are similar to those from three other studies of well-diagnosed outpatient samples of adults with an eating disorder (Martin et al., 2000
; Ricca et al., 2001
; Turner & Bryant-Waugh, 2004
The patients with eating disorder NOS had longstanding eating problems, the mean duration being over eight years. Their EDE ratings showed that they displayed the psychopathology characteristic of anorexia nervosa and bulimia nervosa, and it was comparable in severity to that seen in bulimia nervosa. This remained the case even after the subthreshold cases of anorexia nervosa and bulimia nervosa had been removed. The eating disorder NOS patients resembled those with bulimia nervosa in many other ways: for example, in age, gender, ethnicity, marital status and occupation. In addition, they had similarly raised levels of comorbid general psychiatric symptoms, although their alcohol intake was not as high. Almost a quarter had a history of anorexia nervosa and over third had a history of bulimia nervosa illustrating the cross-diagnostic temporal movement that is common among people with eating disorders (Fairburn & Harrison, 2003
; Milos, Spindler, Schnyder, & Fairburn, 2005
The third aim of the study was to determine whether the high relative prevalence of eating disorder NOS might be due in part to the presence within the diagnosis of cases closely resembling anorexia nervosa or bulimia nervosa, cases that might be better re-diagnosed as such (Fairburn & Bohn, 2005
). The availability of EDE ratings on the full sample allowed us to examine this possibility by systematically relaxing the diagnostic criteria for anorexia nervosa and bulimia nervosa along lines already proposed in the literature while still preserving the core clinical concepts (e.g., that patients with anorexia nervosa be actively maintaining an unequivocally low weight, and that patients with bulimia nervosa experience repeated episodes of binge eating). It emerged that, with one exception, none of the adjustments either in isolation or in combination had much impact on the relative prevalence of eating disorder NOS which remained at 50% or more. The exception involved the expansion of the concept of a binge to include any episode of eating associated with a sense of loss of control irrespective of the amount eaten. When this change was made, together with all the other changes, it resulted in an increase in the relative prevalence of bulimia nervosa and corresponding drop in the prevalence of eating disorder NOS but even so over a third of the patients retained the diagnosis eating disorder NOS. Only one other study has systematically investigated the impact of adjusting the diagnostic criteria for anorexia nervosa and bulimia nervosa (Thaw, Williamson, & Martin, 2001
) and it used a convenience sample that was likely to have been atypical in composition. Nevertheless similar findings emerged.
It should also be noted that the high relative prevalence of eating disorder NOS was not due to the presence of cases of binge eating disorder as just seven patients met its diagnostic criteria—this figure rose to nine if the minimum average frequency of binge eating was reduced to one day per week. This low prevalence of binge eating disorder is consistent with findings from other samples; for example, two of the three patient samples referred to above reported prevalence figures for binge eating disorder and in both cases it was less than 10% (Martin et al., 2000
; Ricca et al., 2001
This study had certain strengths. First, the two-site catchment area sampling frame meant that the patients were likely to have been representative of many other outpatient samples of adults with eating disorders. Second, the cases were well characterised. Leading measures of psychopathology were employed, and the use of operational EDE-based diagnostic criteria enabled diagnostic thresholds to be adjusted in a systematic and replicable way. Third, although there was multiple statistical testing, we chose not to adjust our significance level when comparing the eating disorder NOS cases with those with bulimia nervosa. This favoured the identification of false positives rather than false negatives (i.e., raising Type I rather than Type II error). This was a conservative strategy since it made more likely the detection of statistically significant differences between the groups. If we had reduced our significance level to the 1% level, none of the observed differences would have been statistically significant.
In terms of limitations, the sample did not include patients at the two far ends of the weight spectrum (i.e., those with a BMI below 16.0 or of 40.0 or above). Patients with a BMI below 16 would have been likely to fulfil diagnostic criteria for anorexia nervosa and those with a BMI of 40 or more would have been likely to be cases of eating disorder NOS as patients with bulimia nervosa are rarely so heavy. Data from the entire Leicester eating disorder service for the three-year period 2003–2005 indicate that both weight groups are uncommon (8%
respectively, of the overall referrals). If they had been included in the present sample, and given their likely diagnoses, the diagnostic distribution would have been 11% anorexia nervosa, 31% bulimia nervosa and 58% eating disorder NOS, figures that differ little from those of the actual study sample. The exclusion of these two small subgroups of patients will have had very little effect on the study's findings since these concern the characteristics of the patients with eating disorder NOS, almost all of whom were included. One other point about the sample should be noted. The study was of adults: its findings cannot necessarily be generalised to adolescents. This said, data on such patients suggest that eating disorder NOS is common among them too (Chamay-Weber, Narring, & Michaud, 2005
; Nicholls, Chater, & Lask, 2000
Four main conclusions may be drawn from this study. First, it is confirmed that eating disorder NOS is the most common eating disorder diagnosis encountered in adult outpatient settings. Second, the psychopathology of eating disorder NOS closely resembles that of anorexia nervosa and bulimia nervosa. Third, across a wide range of clinical variables, eating disorder NOS is comparable in severity to bulimia nervosa. Fourth, the high relative prevalence of eating disorder NOS is not attributable to the existence within the diagnosis of cases closely resembling anorexia nervosa or bulimia nervosa, nor is it due to the presence of cases of binge eating disorder.
The implications of the findings are important, especially with regard to the classification of eating disorders. If NOS diagnoses are intended to be truly “residual”, and by implication few in number, then the clinical state (or states) currently embraced by the diagnosis eating disorder NOS ought to be reclassified as one or more specific forms of eating disorder, especially since it (or they) are as severe as the established eating disorder bulimia nervosa. From these findings it seems reasonable that a small proportion of these cases be re-diagnosed as cases of anorexia nervosa or bulimia nervosa, or binge eating disorder if it becomes a recognised diagnostic entity. However, the great majority differ in their precise clinical presentation whilst still having the psychopathology that characterises anorexia nervosa and bulimia nervosa. How their clinical picture should be classified is a matter of debate (Beumont, Kopec-Schrader, Talbot & Touyz, 1993
; Fairburn & Bohn, 2005
; Murphy, Perkins, & Schmidt, 2005
; Palmer & Norring, 2005
). Our general clinical experience and our knowledge of the cases that comprise the present sample suggest that most might be best characterised as “mixed” because the clinical features of anorexia nervosa and bulimia nervosa are present but combined in subtly different ways to those seen in the two currently specified syndromes. Indeed, it has been suggested that such cases could be designated as belonging to a new diagnostic category, perhaps termed “mixed eating disorder” (Fairburn & Bohn, 2005
). Alternatively, subcategories of eating disorder NOS could be sought, although we would argue against the delineation of new subgroups unless there is a strong case for doing so. It has been stated that “In the last resort all diagnostic concepts stand or fall by the strength of the prognostic and therapeutic implications they embody” (Kendell, 1975
). If this view is accepted, then there is at present no case for subdividing eating disorder NOS since almost nothing is known about the course of these cases or their response to treatment.
The findings also have important practical implications. They highlight the need for studies that recruit broader samples than at present. Research needs to address the whole range of eating disorders seen in clinical practice, not just anorexia nervosa and bulimia nervosa. The recruitment of complete transdiagnostic samples (i.e., those including patients with anorexia nervosa, bulimia nervosa and all forms of eating disorder NOS) would be of special value it would permit the entire scheme for classifying eating disorders to be examined afresh. As noted elsewhere (Fairburn & Bohn, 2005
), the collection of good transdiagnostic data, particularly cross-diagnostic information on course and response to treatment, is needed if clinically informative subdivisions are to be identified.
From the clinical point of view there is also a pressing need for studies of the treatment of eating disorder NOS as these patients have been neglected to date. Existing treatments for anorexia nervosa and bulimia nervosa might benefit them, given that they share the same distinctive psychopathology (Fairburn, Cooper, & Shafran, 2003
), but this needs to be shown to be the case.