vigilant and report back to our respective health agenciesabout any of these serious adverse events. This study may not

be the final word on the subject but is an important additionto the literature.

REFERENCES

1. Petition to the FDA requesting regulatory action concerning

the possible spread of botulinum toxin from the site of injec-

tion to other parts of the body (HRG Publication #1834).

http://www.citizen.org/publications/publicationredirect.cfm?

ID=7559 (accessed 1 December 2010).

2. US Food and Drug Administration. Early Communica-

tion about an ongoing safety review of botox and

botox cosmetic (Botulinum toxin Type A) and Myobloc

(Botulinum toxin Type B). February 2008. http://www.

fda.gov/Drugs/DrugSafety/PostmarketDrugSafetyInformation

forPatientsandProviders/DrugSafetyInformationforHeathcare

Professionals/ucm070366.htm (accessed 12 October

2010).

3. US Food and Drug Administration. Response to Citizen’s

Petition. April 2009. http://www.fda.gov/downloads/Drugs/

DrugSafety/PostmarketDrugSafetyInformationforPatientsand

Providers/DrugSafetyInformationforHeathcareProfessionals/

UCM143989.pdf (accessed 1 December 2010).

4. US Food and Drug Administration. Early Communica-

tion about an Ongoing Safety Review of Botox and

Botox Cosmetic (Botulinum toxin Type A) and Myobloc

(Botulinum toxin Type B). April 2009. http://www.fda.

gov/Drugs/DrugSafety/PostmarketDrugSafetyInformationfor

PatientsandProviders/DrugSafetyInformationforHeathcare

Professionals/ucm070366.htm (accessed 1 December

2010).

5. US Food and Drug Administration. Information for

Healthcare Professionals: OnabotulinumtoxinA (marketed

as Botox ⁄ Botox Cosmetic), AbobotulinumtoxinA (marketed

as Dysport) and RimabotulinumtoxinB (marketed as

Myobloc). August 2009. http://www.fda.gov/Drugs/Drug

Safety/PostmarketDrugSafetyInformationforPatientsand

Providers/DrugSafetyInformationforHeathcareProfessionals/

ucm174949.htm (accessed 1 December 2010).

6. O’Flaherty S, Janakan V, Morrow A, Scheinberg A, Waugh

M. Botulinum toxin A adverse events and health status in

children with cerebral palsy in all GMFCS levels. Dev Med

Child Neurol 2011; 53: 125–30. DOI: 10.1111/j.1469-

8749.2010.03814.x.

Bone density and size in ambulatory children with cerebral palsyRICHARD HENDERSONUniversity of North Carolina, Department of Orthopaedics, Chapel Hill, NC, USA.

doi: 10.1111/j.1469-8749.2010.03851.x

This commentary is on the original article by Wren et al. on pages 137–141of this issue.

Over the past 15 to 20 years there has been an increasingamount of attention focused on the clinical problem of skeletalfragility in persons with disabilities. The clinical subgroup thathas received the most attention is children and adolescentswith moderate to severe cerebral palsy (CP), and work in thisarea has been recently reviewed.1 A clear deficiency in thisbody of work is an assessment of children and adolescents withlesser degrees of impairment. Severe CP is associated withsignificant skeletal fragility, but what if any are the conse-quences of mild impairments?

Wren et al. use quantitative computed tomography (QCT)to assess various bone parameters in a small series of 37 chil-dren with CP, roughly half of whom were at Gross MotorFunction Classification System (GMFCS) levels I or II.2 Volu-metric bone density in the lumbar spine did not differ betweentypically developing controls and the mildly involved subset.But optimism over this finding is not justified. Volumetricbone density also did not differ between typical controls andthe more severely involved GMFCS levels III or IV subset –children who are known to have increased skeletal fragility.

In the children with CP, but not the controls, Wren et al.also measured volumetric bone density and size parameters inthe tibia.2 Here volumetric bone density, bone size (cross-sec-tional area), and cortical bone area all decreased with increas-ing severity, and all are measures of bone directly related to itsmechanical strength. In the absence of typical controls we can-not know if these findings reflect a potential problem in mildly

involved children, but extrapolation of the data is not reassur-ing for those with lesser impairment.

The clinically relevant outcome of skeletal fragility is frac-ture, so one could reasonably argue that this is the criterionstandard measure of skeletal fragility. There are, however,significant limitations to using fractures as the measure of skel-etal fragility. One limitation is the sensitivity of the measure.An individual who has sustained an atraumatic fracture doeshave skeletal fragility, but a negative fracture history does notrule out skeletal fragility or a high risk of fracture in the future.Another limitation when looking at groups rather than indi-viduals is that fracture events are not particularly common.As a result, finding clinically and statistically significantdifferences can require large numbers.

It follows that indirect measures of skeletal fragility andfracture risk are critical to work in this area, and the boneparameters measured by Wren et al. were obtained withQCT. QCT is a different and much less widely utilized assess-ment tool than dual energy X-ray absorptiometry (DXA). Animportant contribution to the field is that Wren et al. andBinkley et al. have both shown in small series that QCT mea-sures in the tibia are a very promising means of assessingparameters that are likely to reflect fracture risk in childrenwith CP.2,3 Conversely, Wren et al. have shown that volumet-ric bone density measures in the lumbar spine bear no relationto skeletal fragility in this population. It has already beenshown that the widely available DXA measures of areal bonedensity in the lumbar spine is of questionable value for assess-ing fracture risk in children with CP.4 The findings of Wrenet al. suggest that the widespread practice of ‘adjusting’ or‘correcting’ these DXA measures for the size of the individualwould in fact probably diminish the already uncertain clinicalrelevance of the measure. Interpreting a bone density evalua-tion in a child with disability requires very careful consider-ation; so too should the decision to obtain one.

102 Developmental Medicine & Child Neurology 2011, 53: 101–107

REFERENCES

1. Mergler S, Evenhuis HM, Boot AM, et al. Epidemiology of

low bone mineral density and fractures in children with

severe cerebal palsy: a systematic review. Dev Med Child

Neurol 2009; 51: 773–8.

2. Wren TA, Lee DC, Kay RM, Dorey FJ, Gilsanz V. Bone

density and size in ambulatory children with cerebral palsy.

Dev Med Child Neurol 2011; 53: 137–41. DOI: 10.1111 ⁄

1469-8749.2010. 03852.x.

3. Binkley T, Johnson J, Vogel L, Kecskemethy H, Henderson

RC, Specker B. Bone measurements by peripheral quantita-

tive computed tomography (pQCT) in children with cerebral

palsy. J Pediatr 2005; 147: 791–6.

4. Henderson RC, Berglund LM, May R, et al. The relation-

ship between fractures and DXA measures of BMD in the

distal femur of children and adolescents with cerebral palsy

or muscular dystrophy. J Bone Miner Res 2010; 25: 520–6.

The role of motor proficiency in bone health in genetic syndromesDAVID A STEVENSONDivision of Medical Genetics, Department of Pediatrics, University of Utah, Salt LakeCity, Utah, USA.

doi: 10.1111/j.1469-8749.2010.03799.x

This commentary is on the article by Chen et al. on pages 131–136 of this

issue.

Bone is dynamic and is impacted by a variety of factors. Bonemineral density (BMD) has traditionally been utilized as ameasure of bone health, primarily in adults. However, BMD isonly one measure to assess a very complex structure such asbone. A number of studies on genetic syndromes have founddifferences in BMD and other bone health variables1–3 raisingthe question of direct effects of genes on bone cellular pro-cesses or secondary effects due to neurological or structuralimpairment that decrease forces placed upon bones.

The new study by Chen et al.4 demonstrated that the cate-gorization level of the Gross Motor Functional ClassificationSystem correlated with bone density in the lower limbs, pro-viding further evidence that motor function has a significanteffect on bone density in children with conditions resulting inmotor dysfunction. Therefore, when one assesses genetic syn-dromes, both motor function and clinical features that impactforces placed upon bone deserve particular attention. Childrenwith neurofibromatosis type 1 (NF1) have decreased BMD,1

and in vitro studies of osteoprogenitor cells suggest that NF1haploinsufficiency directly increases various osteoclastic activi-ties.2 However, developmental motor delays, decreasedstrength, and decreased activity due to focal skeletal defects ofsome individuals with NF1 could lead to even furtherdecreases in bone density of certain skeletal regions. Individu-als with Prader-Willi syndrome are reported to have anincreased incidence of osteoporosis,3 but it is not well knownif this is primarily due to abnormal expression of genes withinthe Prader-Willi critical region on bone cellular functions orsecondary to associated findings (e.g. hypotonia, obesity,

hormonal imbalances, and ⁄ or inactivity) or a combinationof both.

A broad list of other variables such as, but not limited to,sex steroid and growth hormone production, age, height,weight, ethnicity, and calcium intake and absorption impactbone homeostasis and the interpretation of BMD values.5 Thismakes analysis of bone health in children with rare conditionsdifficult, particularly when trying to identify causative factorsthat can be targeted for intervention. Although genetic factorsmay contribute to cerebral palsy, it is unlikely that a singlegene is causative, which provides support that the degree ofmotor dysfunction rather than a specific genetic contributionplays a primary role in bone density. Physiotherapy has beenreported to improve BMD, muscle force, and gross motorfunction in children with cerebral palsy.6 These observationsprovide support that genetic disorders associated withdecreased bone density could potentially improve bone healthwith similar techniques that increase motor function and over-all activity levels. In addition, characterization of bone densityin different skeletal regions may help determine specific typesof interventions that improve a focal region.

Despite the mounting reports of differences in BMD andother bone health parameters as measured by a variety ofmodalities such as peripheral quantitative computed tomo-graphy, dual energy x-ray absorptiometry, and quantitativeultrasound in various conditions presenting in childhood, onemust consider whether or not these differences result in clini-cal consequences necessitating interventions that may becostly and time consuming. For example, although Chenet al.4 demonstrate that distal femur areal BMD was decreasedin children with cerebral palsy, only one child had a history ofa femur fracture. Prospective studies into adulthood, and moredetailed clinical assessments in larger cohorts will be neededin order to tease out the clinical impact of decreased bone den-sity in specific pediatric conditions.

REFERENCES

1. Stevenson DA, Moyer-Mileur LJ, Murray M, et al. Bone

mineral density in children and adolescents with neurofibro-

matosis type 1. J Pediatr 2007; 150: 83–8.

2. Heerva E, Alanne MH, Peltonen S, et al. Osteoclasts in neu-

rofibromatosis type 1 display enhanced resorption capacity,

aberrant morphology, and resistance to serum deprivation.

Bone 2010 47:583–90.

3. Kroonen LT, Herman M, Pizzutillo PD, Macewen GD.

Prader-Willi Syndrome: clinical concerns for the orthopaedic

surgeon. J Pediatr Orthop 2006; 26: 673–9.

4. Chen C, Ke J, Wang C, Wu KP, Wu C, Wong AM. Factors

associated with bone density of different skeletal regions in

children with cerebral palsy of various motor severities. Dev

Med Child Neurol 2011; 53: 131–36. DOI: 10.1111/j.1469-

8749.2010.03809.x.

Commentaries 103