Anorexia Nervosa (AN) is a serious psychiatric illness that carries the highest mortality rate of any mental health disorder. It is characterized by restricted energy intake, an intense fear of gaining weight, and a distorted perception of body weight or shape. Acute care decisions, such as hospitalization, focus on preventing immediate medical catastrophe caused by malnutrition. While low body weight is a defining feature, the decision to admit a patient for inpatient treatment is based on factors beyond a single number.
Understanding BMI and Severity Levels
The Body Mass Index (BMI) is a calculation based on height and weight, and it is the primary metric used to categorize the severity of Anorexia Nervosa in adults. Clinical guidelines, such as those in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), use specific BMI thresholds to classify the illness. These classifications help clinicians determine the baseline severity of the weight component of the disorder.
For adults, a Mild case of AN is specified by a BMI of 17 kilograms per square meter (kg/m²) or greater, while a Moderate case falls between 16 and 16.99 kg/m². A Severe diagnosis is given for a BMI between 15 and 15.99 kg/m², and an Extreme diagnosis is specified by a BMI below 15 kg/m². These numbers provide a clinical snapshot of the degree of underweight, but they do not automatically dictate the need for hospitalization. For children and adolescents, the use of BMI percentiles, rather than absolute BMI values, is necessary because their bodies are still developing.
A common threshold for considering hospitalization in adolescents is a weight less than 75% of the expected body weight for their age and height, or a BMI at or below the 5th percentile. While a low BMI indicates medical risk, current weight is only one component of the full clinical picture. A patient with a “Severe” BMI may be medically stable enough for outpatient treatment if they show no other signs of acute distress.
Acute Medical Instability: The Decisive Factors
Hospitalization for Anorexia Nervosa is primarily driven by acute medical instability, which often overrides the BMI alone. The goal of acute inpatient medical hospitalization is to stabilize life-threatening physiological complications arising from starvation. These complications can cause sudden cardiac events or permanent organ damage.
Cardiovascular compromise is a concern, indicated by severe bradycardia (a resting heart rate typically below 45 beats per minute). Hypotension, or low blood pressure (particularly a systolic reading below 80 mmHg), is another sign of instability. Orthostatic changes—a significant drop in blood pressure or sharp increase in heart rate when moving to standing—signal a high risk of fainting and warrant immediate intervention.
Electrolyte abnormalities are significant, as starvation depletes essential minerals like potassium, phosphate, and magnesium necessary for heart and nerve function. Low potassium (hypokalemia) can trigger fatal cardiac arrhythmias. A rapid rate of weight loss (e.g., losing 10-15% of body weight in a short period) indicates a high risk for refeeding syndrome upon nutritional rehabilitation, requiring close medical monitoring.
The Continuum of Treatment Settings
Acute medical hospitalization is the highest level of care, focused narrowly on medical stabilization to remove immediate danger. Once vital signs and electrolyte levels are stable, and the immediate risk of death is mitigated, the patient transitions to a lower level of specialized eating disorder care. This transition is necessary because acute medical units are not designed for long-term psychological and nutritional rehabilitation.
The next step down is often a Residential Treatment Center (RTC), where patients live at the facility and receive 24-hour supervision and intensive therapy. They must be medically stable enough not to require daily intravenous treatments or continuous cardiac monitoring. Following this, patients progress to a Partial Hospitalization Program (PHP), attending treatment for most of the day before returning home. The lowest levels are the Intensive Outpatient Program (IOP) or standard Outpatient Care, offering a few hours of treatment several times a week.
When Hospitalization is Needed Regardless of BMI
The medical need for hospitalization is not exclusively tied to a low BMI, a concept highlighted by Atypical Anorexia Nervosa. A patient with Atypical AN meets all the behavioral and psychological criteria for the disorder—restriction, intense fear of weight gain, and body image disturbance—but their current weight is not classified as underweight. Despite a “normal” BMI, these individuals may have experienced rapid weight loss, which can be as medically dangerous as a low starting weight.
Rapid or significant weight loss is independently associated with severe medical complications like bradycardia and electrolyte imbalance, regardless of the patient’s current size. A patient with Atypical AN who has lost weight quickly can still present with life-threatening vital sign instability requiring emergency medical admission. The focus of acute care is not to reach a specific BMI number, but to halt the dangerous physiological trajectory and stabilize the patient before further medical decline occurs.