A prospective observational study comparing rates of medical instability between adolescents with typical and atypical anorexia nervosa.

Background: Recognition of atypical anorexia nervosa (AAN) has challenged underweight as a defining factor of illness severity in anorexia nervosa (AN). The present study aimed to compare rates of medical instability in adolescents with underweight (AN) and non-underweight (AAN) anorexia nervosa.

Methods: The study examined assessment data from specialist eating disorder services in the UK between January and December 2022. Participants (n = 205) aged 11-18 years were recruited across eight eating disorder clinics and diagnosed with AN (n = 113) or AAN (n = 92) after clinical assessment. Parameters associated with risk of medical instability were compared between AN and AAN groups, using t tests and regression analysis.

Results: Rates of bradycardia and hypotension did not differ significantly between AN and AAN groups (p = 0.239 and p = 0.289). Although white blood cell counts were lower in the AN group, rates of leukopaenia could not be statistically compared as a result of there being too few counts in at least one group. No incidences of hypophosphataemia were found in the sample. A significant regression equation was found for percentage median body mass index, but not rate of weight loss, as a predictor of blood pressure, serum phosphorous and magnesium.

Conclusions: Our findings indicate that medical instability occurs across a range of body weights in young people with AN and AAN. Although certain parameters of risk such as blood pressure, serum phosphorous and magnesium may be worsened at lower weight, both AN and AAN are serious mental health conditions that can lead to medical instability.