9 Child and adolescent psychiatry9.7 Disorders in childhood and adolescence
9.7.8 Eating disorders in children and adolescents
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Written by Resident Doctor Committee · Published
Diagnosis
Weight in a young person is judged against age, sex and growth trajectory, not against adult body mass index (BMI) thresholds.
Disorder | Weight | Core features |
|---|---|---|
Anorexia nervosa, restricting type | Significantly low for age | Weight-lowering behaviour, fear of weight gain or shape disturbance, no bingeing |
Anorexia nervosa, binge-purge type | Significantly low for age | As above, with recurrent bingeing and purging |
Bulimia nervosa | Normal or raised | Recurrent bingeing with compensatory behaviour |
Binge eating disorder is recurrent bingeing without regular compensatory behaviour.
Low weight is judged by the BMI-for-age centile, and failure to gain expected weight can replace weight loss.
ICD-11 lets rapid substantial weight loss, such as over 20% in six months, stand in for the low-weight requirement.
Amenorrhoea is no longer required for diagnosis.
Once the low-weight criterion is met, bingeing and purging make the binge-purge subtype of anorexia, not bulimia nervosa.
An impulsive profile (substance misuse, shoplifting, self-harm, anger) is more common in bulimia nervosa, whereas starvation signs and an obsessional temperament point to anorexia.
Normal weight does not exclude an eating disorder or medical risk.
Avoidant/restrictive food intake disorder (ARFID) is restricted intake driven by sensory sensitivity, low interest in food or fear of choking or vomiting, not by weight or shape concerns.
ARFID and autism frequently coexist.
Assessment of physical risk
Height and weight, expressed as BMI for age (adjusted for age and sex), come first in suspected anorexia because they establish physical severity and risk.
Percentage median BMI = current BMI / median BMI for age and sex × 100, so 14.5 / 20.4 gives about 71%.
Parameter | High risk under 18 |
|---|---|
Weight loss | 1 kg or more a week for two weeks |
Percentage median BMI | Below 70%, with 70 to 80% also concerning |
Waking heart rate | Below 40 beats per minute, with 40 to 50 also concerning |
Temperature | Below 35.5°C tympanic |
Sit-up, squat, stand test | Cannot rise without using the arms |
Electrocardiogram | Prolonged corrected QT interval |
No single reassuring reading establishes safety, and admission rests on overall risk rather than an absolute BMI.
Physical complications
The blood picture of anorexia nervosa is a normocytic normochromic anaemia with leukopenia and sometimes thrombocytopenia.
It comes from starvation suppressing the marrow, which is hypocellular, not from a specific nutrient deficiency.
Malnutrition slows growth and puberty, and catch-up can be incomplete after prolonged illness.
NICE recommends considering bone density assessment after one year of underweight, or earlier for bone pain or recurrent fractures.
Refeeding syndrome
Starvation depletes total body phosphate, potassium, magnesium and thiamine even when first serum results are normal.
On refeeding, insulin moves potassium, phosphate and magnesium into cells, so all three fall in the first days, while sodium and calcium usually stay in range.
Hypophosphataemia is the main early biochemical marker of refeeding syndrome.
The syndrome can cause fluid overload and cardiac, respiratory and neurological complications.
Feeding starts promptly under a specialist protocol with thiamine and planned electrolyte monitoring, because underfeeding also harms.
In a trial of adolescents and young adults at 60% median BMI or above, starting at 2000 kilocalories restored stability faster than 1400, without more electrolyte disturbance.
Psychological treatment
The National Institute for Health and Care Excellence (NICE) guideline NG69 sets the order of treatment.
Diagnosis | First line | If unacceptable, contraindicated or ineffective |
|---|---|---|
Anorexia nervosa | Anorexia-nervosa-focused family therapy, typically 18 to 20 sessions over a year | Individual eating-disorder-focused cognitive behavioural therapy (CBT-ED) or adolescent-focused psychotherapy |
Bulimia nervosa | Bulimia-nervosa-focused family therapy | Individual CBT-ED |
Binge eating disorder | Guided self-help, as for adults | Group CBT-ED |
In family therapy, parents take temporary charge of nutrition and hand autonomy back as recovery allows.
No single kind of family causes anorexia, so families are treated as partners in recovery.
Medication must not be the only treatment, and no medicine is licensed in the UK for anorexia nervosa.
Fluoxetine is licensed for bulimia only in adults, so adolescent use is off-label and not first line.
Family therapy models
Model | Identifying techniques |
|---|---|
Systemic (Milan and post-Milan) | Circular questioning, therapist neutrality, hypothesising, an observing or reflecting team |
Structural | Enactment, boundaries, unbalancing |
Strategic | Directives, paradoxical tasks |
Consent
In England and Wales, clinicians assess Gillick competence under 16 and decision-specific capacity at 16 to 17.
Low weight or refusal does not by itself prove incapacity.
Section 131 stops parents consenting to informal admission for a capacitous 16- or 17-year-old who refuses.
Under the Mental Health Act, section 63 can authorise feeding as treatment for the mental disorder in a detained patient.
Pica
Pica subtypes are named from the Greek root for the substance eaten.
Subtype | Substance eaten |
|---|---|
Amylophagia | Starch |
Pagophagia | Ice |
Geophagia | Earth or clay |
Trichophagia | Hair |
Lithophagia | Stones |
Iron deficiency, with or without anaemia, is the nutritional deficiency most consistently linked with pica, so screen for it when pica presents.
Zinc deficiency is the other trace element deficiency consistently found in pica.