Are dual-energy X-ray (DEX) scans indicated in adolescent girls with anorexia?

DXA Scans in Adolescent Girls with Anorexia Nervosa

Yes, DXA scans are indicated in adolescent girls with anorexia nervosa to assess bone mineral density, as these patients are at high risk for significant bone loss during a critical period of bone mass accrual, and early identification of low BMD is essential for preventing long-term skeletal complications and fractures. 1

When to Obtain DXA Scanning

DXA should be performed in athletes and adolescents with anorexia nervosa who have low energy availability, disordered eating, eating disorders, or amenorrhea of over 6 months duration. 1 The scan should include:

• Whole body (head excluded) in addition to the lumbar spine for adolescents, as this provides comprehensive assessment of both trabecular and cortical bone 1
• Spine and total body less head BMC and areal BMD with appropriate adjustments for short stature or growth delay using either bone mineral apparent density or height Z-score 1

Interpreting Results in Adolescents

The interpretation of DXA results differs critically from adults:

• Use Z-scores (age- and sex-matched), not T-scores, as the WHO definition based on T-scores is not applicable in children and adolescents 1
• A Z-score <−1.0 SD warrants further attention in athletes and active adolescents, as they should have 5–15% higher BMD than non-athletes 1
• Low BMD is defined as a Z-score between −1.0 and −2.0 SD when accompanied by history of nutritional deficiencies, hypoestrogenism, stress fracture, or other secondary clinical risk factors for fracture 1
• Osteoporosis is defined as Z-score below −2.0 SD with the presence of secondary clinical risk factors 1

Importantly, diagnosis of osteoporosis in children and adolescents should not be made on densitometric criteria alone—an overall assessment of bone health including the presence of fractures due to low trauma must be considered. 1

Clinical Significance and Timing

The rationale for DXA screening is compelling:

• Osteopenia occurs as a frequent and often early complication of anorexia nervosa in adolescence, with deficits developing within the first year of illness 2
• 42% of adolescent patients have lumbar BMD more than 1 SD below the mean, and 63% have lateral spine BMD more than 1 SD below normal 3
• Adolescence is the critical period for bone mass accrual toward attainment of peak bone mass, an important determinant of lifelong bone health and fracture risk 4, 3
• Duration of illness is the most significant predictor of spinal BMD, with longer illness associated with greater bone loss 3

Follow-Up Scanning Intervals

The recommended interval to reassess BMD via DXA scan is:

• Minimum of 6 months in adolescents who are at risk or being treated for low BMD 1
• 12 months in adults 1

Pathophysiology Relevant to Screening

Understanding the mechanism of bone loss supports the screening indication:

• Bone formation is reduced and uncoupled from bone resorption in adolescents with anorexia nervosa 3
• Both cortical and trabecular microarchitecture are altered, with decreased cortical area and thickness, increased cortical porosity, and increased trabecular separation 5
• Finite element analysis shows decreased failure load, indicating reduced bone strength even when distal radius BMD appears normal 5
• IGF-I levels are reduced to 50% of normal and are the major correlate of bone formation markers in anorexia nervosa 3

Common Pitfalls to Avoid

• Do not assume normal laboratory results exclude serious bone disease—more than half of adolescents with eating disorders have normal test results despite being medically unstable 6
• Do not rely solely on radial bone density—midradius bone density may not be significantly reduced even when lumbar spine and whole body bone mass are markedly decreased 2
• Do not delay DXA scanning in patients with prolonged amenorrhea (>6 months), significant weight loss, or history of stress fractures 1
• Recognize that osteopenia may persist after recovery—deficits in bone mineral acquired during adolescence may not be completely reversible, making early detection and intervention critical 7

Integration with Comprehensive Assessment

DXA scanning should be part of a broader evaluation that includes:

• Menstrual history assessment to identify hypothalamic-pituitary-gonadal axis suppression 6
• Evaluation of calcium and vitamin D status 1
• Assessment of weight history, including maximum weight, minimum weight, and timeline of weight loss 6
• Documentation of current height, weight, and BMI percentile compared to previous growth charts 6