Atypical anorexia nervosa
Atypical anorexia nervosa is an eating disorder in which individuals meet all the qualifications for anorexia nervosa, including a body image disturbance and a history of restrictive eating and weight loss, except that they are not currently underweight. Atypical anorexia qualifies as a mental health disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), under the category Other Specified Feeding and Eating Disorders (OSFED). The characteristics of people with atypical anorexia generally do not differ significantly from anorexia nervosa patients except for their current weight.Atypical anorexia was not described in earlier editions of the DSM, which included a requirement that person to have a body weight no higher than 85% of normal. Patients with atypical anorexia were diagnosed with the DSM-4 qualification "eating disorder not otherwise specified" (EDNOS) until the DSM-5 was released in 2013. Prior to DSM-5, EDNOS made up the majority of eating disorders diagnoses, making it difficult to estimate the prevalence of atypical anorexia during this period.The term atypical anorexia was historically used to describe the restrictive eating habits of some people with autism. The DSM-5 superseded this term with the avoidant restrictive food intake disorder (ARFID) diagnosis.

Signs and symptoms
Current consensus is that atypical anorexia patients are at risk for the same medical complications of anorexia nervosa.Evidence from a study conducted at the University of California San Francisco Eating Disorders Program suggests that atypical anorexia patients are equally likely as anorexia nervosa patients to develop secondary side effects, including bradycardia (decreased heart rate), amenorrhea (stopping of the menstrual period), and electrolyte imbalances.

Treatment
Treatment for atypical anorexia is very similar to treatment for anorexia nervosa. Increasing calorie intake to a healthy amount is a crucial part of treatment, but patients with severe atypical anorexia are at risk for refeeding syndrome during early recovery, so it is recommended that they are treated in an inpatient facility and slowly adjusted to increased calorie intake by 100-200 additional calories per day. Treatment may also include a variety of therapies that help a patient deal with the depression, anxiety, and other mental symptoms that arise from the eating disorder.In the US, treatment may be complicated by the need to get health insurance plans to pay.  Medical coding may be incorrect on requests or may be rejected because payers incorrectly evaluated it under the separate criteria for anorexia nervosa.

Epidemiology
It is difficult to gauge the true prevalence of atypical anorexia pre-2013 because patients were lumped together under the EDNOS diagnosis. Evidence suggests that atypical anorexia is more prevalent than anorexia nervosa, but individuals experiencing it are less likely to receive care. For example, one prospective study of 196 women found a prevalence of 2.8% for atypical anorexia, compared to only 0.8% for anorexia nervosa by the age of 20.


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