Bone Mineral Density in Pediatric Liver Transplant Recipients

Abstract

Drs. Arikoski and Holmberg commented on a study, performed by our group, in which we found that bone mineral density in pediatric liver transplant recipients is normal after about a decade from surgery (1). From their statements one should gather that we currently lack a noninvasive test to reliably assess bone mineralization in growing children. We entirely agree with that. A previously published longitudinal study performed in children in the first year after liver transplantation showed clearly that remarkable bone mineral density (BMD) gain was proportional to height gain. Catch-up growth in height, and very likely increase in bone thickness, are responsible at least partially for the (fictitious?) increase in bone mineral density assessed by DXA scan in these patients (2). BMD by DXA should be corrected by age, sex, pubertal status, and height (3–4). Unfortunately such corrected normal values are not available. For this reason the authors talk about increased or normal BMD, not bone mineralization, in these two papers and the suggestion that BMD in the long-term after liver transplantation is normal should be accepted as evidence-based. Several studies have shown that even bone mineral apparent density (BMAD) is not satisfactorily reliable, being in disagreement with other noninvasive tests calculating bone volume (5). Moreover, age-matched normal values for BMAD, performed with a modern DXA device, are scant. For these reasons, the authors decided not to include such measurements in their submitted draft; however, in the review process we were requested to provide these data and we enclosed them. In the experience of Drs. Arikoski and Holmberg, some transplanted children had declining DXA values in the longitudinal follow-up. We know that several factors can be implicated in bone loss, including complications related to the graft and immunosuppression. As stated in the title of our study, we selected patients who had a successful liver transplantation to minimize the effect of external factors on our assessment. Nevertheless, as shown in the unpublished Figure 1 provided below, we too observed a slight decrease of BMD in some of our children studied longitudinally.FIGURE 1.: L1-L4 spinal bone mineral density at serial DXA scans according to follow up after liver transplantation (OLT). Subjects numbers refer to the patients described in the original article (1).Different from Drs. Arikoski and Holmberg, although keeping in mind the limits of DXA scan, we decided to rely preferably on BMD as a widely adopted test used in children with osteodystrophy, even in other settings and for which larger experience and normal controls are warranted. Instead of arguing on which noninvasive test we should rely on to assess bone status in growing children, we prefer to agree that we should probably move to perform bone biopsies, which will likely provide consistent information on this topic. This, though, is far from being noninvasive. Lorenzo D’Antiga Lucia Zancan Pediatric Department University of Padova, Padova, Italy