Etiology
No single etiology has been identified for AN. Multiple biological, psychological, and social factors likely contribute to its development and persistence.
Evidence suggests that genetic factors are important in the development of AN. Heritability estimates range from 50% to 75% using a wide variety of assessment strategies.[38] Disturbances in neurobiologic functioning have been identified in people with AN, compared with control populations, and may have a role in development or maintenance of AN but it is difficult to distinguish brain changes that result from nutritional deficiencies in AN from those that contribute to the initiation of illness.
Psychosocial factors also likely play a significant role. Individuals tend to manifest anxiety disorders premorbidly, and the onset of illness often appears triggered by stressful life events, such as leaving home for university or childhood maltreatment such as bullying. The enormous psychological and biological changes accompanying puberty presumably also contribute, probably in multiple ways.
Pathophysiology
The pathophysiology associated with AN is linked to restricted eating and weight loss. Disturbances in all organ systems have been identified in underweight patients, including in cardiac, endocrine, hematopoietic, gastrointestinal, and renal functioning. Almost without exception, restoration of body weight to a healthy level reverses these abnormalities. However, AN generally develops during the time of life when peak bone mass is achieved and often interferes with bone development, leading to increased risk of osteoporosis later in life even if normal weight is eventually restored.[39][40]
Classification
Diagnostic and statistical manual of mental disorders, 5th edition (DSM-5-TR)[1]
The DSM-5-TR is a standardized system devised by the American Psychiatric Association to classify psychiatric disorders.
The DSM-5-TR criteria have been utilized throughout this topic for diagnostic purposes. However, the research data presented may be based on patients selected using previous editions of the DSM. The following are the key changes in the DSM-5-TR:
Removal of specific body mass index (BMI) or weight criteria to define significantly low weight; improved language to indicate that low weight determination should be made in the context of the individual’s age, sex, growth trajectory, etc
Improved language to include behavioral criteria to indicate reluctance to increase weight despite being at a significantly low weight
Addition of BMI criteria to assess severity
Removal of the criterion requiring amenorrhea.
The DSM-5-TR diagnostic criteria for AN are as follows:
Restriction of energy intake relative to requirements, leading to a significantly low weight in the context of age, sex, developmental trajectory, and physical health
Intense fear of gaining weight or persistent behavior that interferes with weight gain, even though at a significantly low weight
Disturbance in the way in which one's body weight or shape is experienced, undue influence of body weight or shape on self-evaluation, or persistent lack of recognition of the seriousness of the current low body weight.
Specific subtype:
Restricting subtype: no episodes of binge-eating or purging in the preceding 3 months; weight loss has been achieved by dieting, fasting, and/or excessive exercise
Binge-eating/purging subtype: recurrent episodes of binge-eating or purging behavior (i.e., self-induced vomiting or the misuse of laxatives, diuretics, or enemas) in the preceding 3 months.
Specific level of severity for adults. The level of severity can be increased based on other indicators such as medical instability and illness duration:
Mild: BMI ≥17 kg/m²
Moderate: BMI 16-16.99 kg/m²
Severe: BMI 15-15.99 kg/m²
Extreme: BMI <15 kg/m².
Children and adolescents: There are special considerations in determining whether or not a child or adolescent is underweight, and the use of the US Centers for Disease Control and Prevention (CDC) BMI-for-age percentile can be useful. The CDC has suggested that a BMI below the 5th percentile suggests underweight status, but children and adolescents above the 5th percentile may be significantly underweight in terms of their expected weight gain. Use of an individual growth chart can be very helpful in determining if the child is on his or her own personal growth trajectory.
International classification of diseases, eleventh revision (ICD-11)[2]
ICD-11 defines anorexia nervosa by significantly low body weight for the individual's height, age and developmental stage, not due to another health condition or to lack of available food. A commonly used threshold is BMI of less than 18.5 kg/m² in adults and BMI for age under the 5th percentile in children and adolescents. Rapid weight loss (e.g., more than 20% of total body weight within 6 months) may replace the guidance for low body weight when other diagnostic criteria are also met.[2]
Other required features for diagnosis include a persistent pattern of restrictive eating or behaviors that are aimed at achieving a low body weight including reducing energy intake, increasing energy expenditure through exercise or use of laxatives or vomiting. Also an excessive preoccupation with weight or shape, repeatedly weighing or measuring or avoiding tight clothing. Specifiers for underweight status are based on a lower BMI conferring a poorer prognosis and so divide into anorexia with normal body weight, with significantly low body weight or with dangerously low body weight.[2]
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