>To the Editor: We read with great interest the invited review by Drs Mora & Barera (1) on the assessment of bone mass and bone metabolism in pediatric gastrointestinal disorders.
Dual energy x-ray absorptiometry (DXA) is the commonest method used to screen for osteoporosis. About 45% of respondents in a recent international survey of physicians looking after children with Crohn disease routinely evaluate bone mineral density (BMD) by DXA (2). In the absence of end points such as fractures, bone mass is assessed in comparison with age and sex matched reference ranges that are now available from manufacturers and other population-based studies. However, it is increasingly recognized that BMD, as reported by DXA, is subject to growth-related artefacts and it inherently underestimates the bone density of short individuals and overestimates the density of tall individuals of the same age and sex (3). Growth failure is a frequently recognized complication of chronic inflammatory disease in childhood and it is, therefore, possible that the growth failure, itself, may be an additional, albeit artefactual, explanation for the low bone mass as observed by DXA in children with IBD and other similar inflammatory conditions.
We are surprised that the authors of this comprehensive review have not addressed this artefact in much detail. In fact, their summary of studies in Table 4 has not differentiated between those that corrected for the size artefact and those that did not. Although the authors have stated that the study by Cowan et al reported 'TB and FN BMC reduced', it is debatable whether 97% and 93% of predicted BMC for bone area as reported by these authors is truly reduced (4). Furthermore, the 44% prevalence rate of lumbar spine BMD SDS less than −2SD as reported by Herzog et al (also in Table 4) was reduced to 14% when the data were corrected for height rather than just age and sex (5). Our own experience, published in this journal earlier this year, clearly showed that children with IBD are shorter and have a smaller bone area when assessed by DXA. Failure to account for bone area led to a label of moderate or severe osteopenia in 65% of cases. After adjustment for bone area, the proportion of children with osteopenia fell to 22% (6).
It is important that the assessment of bone mass is performed by clinicians who have a detailed knowledge of the strengths as well as the pitfalls of bone densitometry. While there is no doubt that vertebral fractures and osteopenia occur in children and adults with IBD, correct interpretation of DXA may avoid the mislabeling of children as suffering from osteopenia, prevent many children from receiving aggressive bone mass enhancing treatment and improve the identification of the smaller subset of children with osteopenia, who may be at a real risk of fractures.
S. Faisal Ahmed
Bone & Endocrine Research Group; University of Glasgow; Royal Hospital For Sick Children; Yorkhill, Glasgow G3 8SJ, UK
Lawrence T. Weaver
Dept of Child Health; University of Glasgow; Division of Developmental Medicine; Yorkhill, Glasgow G3 8SJ, UK
1. Mora S, Barera G. Bone mass and bone metabolism in pediatric gastrointestinal disorders. J Pediatr Gastroenterol Nutr
2. Levine A, Milo T, Buller H, et al. Consensus and controversy in the management of pediatric Crohn's disease: an international survey. J Pediatr Gastroenterol Nutr
3. Fewtrell M, Ahmed SF, Allgrove J, et al. Bone densitometry in children assessed by Dual x-ray absorptiometry (DXA): uses and pitfalls. Arch Dis Child
4. Cowan FJ, Warner JT, Dunstan FD, et al. Inflammatory bowel disease and predisposition to osteopenia. Arch Dis Child
5. Herzog D, Bishop N, Glorieux F, Seidman EG. Interpretation of bone mineral density values in pediatric Crohn's disease. Inflamm Bowel Dis
6. Ahmed SF, Horrocks IA, Zaidi S, Patterson T, Ling S, McGrogan P, Weaver LT. Bone mineral assessment by DXA in Children with IBD: Evaluation by age or bone area. J Pediatr Gastroenterol Nutr