Eating Disorders
A high-yield Step 2 CK walkthrough of the DSM-5 eating disorders—anorexia, bulimia, and binge-eating disorder (plus ARFID)—emphasizing the tested durations, buzzword presentations, and next-best-step management including first-line therapy and medications. Built around comparison tables and classic vignettes so the differentiators (weight, compensatory behavior, electrolytes) map directly to exam decisions. Clinical accuracy verified: DSM-5 criteria/durations, BMI severity bands, first-line drugs (fluoxetine 60 mg for BN, lisdexamfetamine for BED, olanzapine adjunct for AN), the bupropion contraindication, and refeeding-syndrome mechanism all confirmed correct.
Overview: the big three (+2)
Eating disorders are among the most lethal psychiatric illnesses—anorexia nervosa (AN) carries the highest mortality rate of any mental disorder (cardiac arrest + suicide). Boards test three core diagnoses plus two "atypical" ones.
AN = energy restriction → significantly low body weight, intense fear of weight gain (or behavior preventing it), and distorted body image. DSM-5 dropped the amenorrhea requirement and added BMI-based severity.
Bulimia nervosa (BN) = binge–purge cycles in a normal or slightly overweight patient.
Binge-eating disorder (BED) = binges without compensatory behavior—the most common eating disorder in the U.S.
Watch the durations: BN and BED both need episodes ≥1×/week for 3 months; AN has no minimum duration. The exam rewards recognizing buzzwords (lanugo, Russell sign, parotid swelling) and picking the correct next step—usually ECG/electrolytes and an admission decision before any medication.
- Criteria: restriction → low weight + fear of gaining (or behavior preventing gain) + body-image disturbance; no amenorrhea criterion in DSM-5
- Subtypes: restricting vs binge/purge—but still underweight (this is what separates it from BN)
- Severity by BMI: mild ≥17, moderate 16–16.99, severe 15–15.99, extreme <15
- Epidemiology: female:male ~10:1 (DSM-5), onset in adolescence
- Exam signs: bradycardia, hypotension, hypothermia, lanugo, amenorrhea, osteoporosis, prolonged QT
- Labs: ↓FSH/LH/estrogen, euthyroid-sick (↓T3), leukopenia, ↑cholesterol, ↑cortisol, ↑carotene
- Highest mortality rate of any psychiatric disorder (cardiac arrest + suicide)
- BN criteria: binge + compensatory behavior (vomiting, laxatives, fasting, exercise), ≥1×/week × 3 months; self-worth tied to shape/weight
- BN body habitus: normal or overweight (key AN-vs-BN discriminator)
- BN signs: Russell sign (knuckle calluses), dental erosion, parotid swelling (sialadenosis), Mallory-Weiss tear
- BN labs (vomiting): hypokalemic, hypochloremic metabolic alkalosis; laxative abuse → non-gap metabolic acidosis
- BED criteria: binges ≥1×/week × 3 months, marked distress, NO compensatory behavior; often obese
- BED = most common eating disorder in the U.S.
- ARFID: avoidant/restrictive intake with no body-image disturbance (low interest, sensory aversion, or choking fear)—often younger children
AN vs BN vs BED vs ARFID
| Feature | Anorexia (AN) | Bulimia (BN) | Binge-eating (BED) | ARFID |
|---|---|---|---|---|
| Body weight | Low (BMI-defined) | Normal / overweight | Overweight / obese | Low / faltering |
| Duration | No minimum | ≥1×/wk × 3 mo | ≥1×/wk × 3 mo | — |
| Compensatory behavior | ± (subtype) | Yes | No | No |
| Body-image disturbance | Yes | Yes | No | No |
| First-line therapy | FBT (adolescents) | CBT | CBT / IPT | Nutritional / behavioral |
| First-line med | none proven; olanzapine adjunct | Fluoxetine 60 mg | Lisdexamfetamine | — |
Vignette: A 16-year-old girl is brought in for amenorrhea and fatigue. BMI 15.5. She runs 10 miles daily, insists she is "fat," and eats <600 kcal/day. Exam: HR 42, BP 85/55, fine downy hair over her back. K⁺ 3.1.
- Diagnosis: Anorexia nervosa, restricting type (severe by BMI 15–15.99); excessive exercise is not purging for subtyping
- Next best step: ECG + electrolytes, then admit—bradycardia, hypotension, and hypokalemia meet hospitalization criteria
- Then: supervised nutritional rehabilitation / weight restoration; monitor for refeeding syndrome (↓phosphate, ↓K, ↓Mg) → advance calories slowly, give thiamine
- Do NOT start an SSRI expecting weight gain—ineffective in acute AN
Vignette: A 22-year-old woman of normal weight reports uncontrollable eating binges followed by self-induced vomiting ~4×/week for 6 months. Exam: eroded dental enamel, calluses over her knuckles, bilateral cheek fullness. Labs: K⁺ 2.9, low Cl⁻, HCO₃⁻ 34.
- Diagnosis: Bulimia nervosa (Russell sign + parotid swelling + hypokalemic, hypochloremic metabolic alkalosis)
- Next best step: replete potassium (and correct volume) first, then start CBT + fluoxetine 60 mg/day (first-line, FDA-approved)
- Avoid bupropion—lowers the seizure threshold and is contraindicated in eating disorders
- Bulimia Bans Bupropion — Bupropion is contraindicated (seizure risk amplified by purging/electrolyte shifts)
- Russell sign = knuckle calluses from teeth during self-induced vomiting → think BN
- Purge = alkalosis: you vomit acid, so you're left alkalotic (hypokalemic, hypochloremic)
- Refeeding = low PHOSPHATE: the hallmark killer—feed the starved patient, watch the phosphate
- "Sixty for the binge": BN fluoxetine dose is 60 mg (vs 20 mg for depression)
- AN first move = medical stabilization, not psych meds. Admit for HR <45–50, SBP <90 (or orthostasis), K⁺/PO₄ derangement, weight <75% expected, hypothermia, or arrhythmia
- AN therapy: family-based therapy (Maudsley) is first-line for adolescents; olanzapine may aid weight gain + obsessive thoughts (adjunct)
- AN meds: SSRIs do not restore weight and are avoided in acute malnutrition
- BN: CBT + fluoxetine 60 mg; correct hypokalemia first (arrhythmia risk)
- BED: CBT/IPT; lisdexamfetamine (FDA-approved) or an SSRI; topiramate as an alternative
- Refeeding syndrome: insulin surge drives phosphate/K/Mg intracellularly → arrhythmia; prevent with slow refeeding + thiamine + electrolyte monitoring
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