Low bone mineral density and fractures are prevalent in children with spinal muscular atrophy

Halley Wasserman; Lindsey Hornung; Peggy Stenger; Meilan Rutter; Brenda Wong; Irina Rybalsky; Jane Khoury; Heidi Kalkwarf

doi:10.1530/boneabs.4.P192

Objectives: Spinal muscular atrophy (SMA) results in varying degrees of hypotonic immobility. Prior reports demonstrate an increased risk of fracture and a trend toward low bone mineral density (BMD) in this population. We aim to further characterize bone health in paediatric SMA patients by reporting the prevalence of fractures and low BMD (z-score≤−2.0) by SMA subtype, BMD of the lateral distal femur (LDF; an important fracture location in non-ambulatory children and young adults), and prevalence of osteoporosis according to 2013 ISCD criteria.

Methods: A retrospective chart review was conducted of 86 patients, ages 12 months to 25 years, with confirmed diagnosis of SMA seen from 2005 to 2015. Lumbar spine (LS), total body (TB) and LDF DXA scans were obtained for clinical care; initial DXA scans results are reported herein. Fracture history was recorded at annual clinic visits. Cumulative fracture frequencies from patients’ last encounters are reported.

Results: Median age at initial SMA visit was 1.8 years, but differed by SMA subtype. DXA data were available on 69% of patients: of these, 90% had a BMD z-score≤−2.0 at first DXA. BMD of all sites was lower with worsening SMA severity. Fractures occurred in 36% of patients, the femur being the most common location (25 of 53 fractures). 13% of patients had multiple fractures.

Table 1
	SMA 1 (most severe)	SMA 2 (moderate)	SMA 3 (mild)	P-value
Number (n)	24	44	18
Female (%)	15 (63%)	23 (52%)	8 (44%)	0.50
Age (y) at initial neuromuscular visit	0.6 (0.3, 1.1) (n=24)	2.0 (0.9, 4.4) (n=44)	3.9 (2.1, 8.7) (n=18)	0.0003
Age (y) at last encounter	7.6 (2.2, 12.4) (n=24)	6.2 (3.5, 12.2) (n=44)	12.9 (7.7, 17.9) (n=18)	0.01
Age (y) at 1^st reported fracture	3.0 (1.9, 6.0) (n=11)	6.6 (3.3, 11.1) (n=12)	10.4 (9.2, 11.5) (n=8)	0.004
Patients ≧1 fracture	11 (46%)	12 (27%)	8 (44%)	0.22
Fractures at femur	13/22 (59%)	7/19 (37%)	5/12 (42%)	0.33
Age (y) at 1^st DXA	3.9 (2.8, 4.8) (n=14)	5.4 (4.1, 6.6) (n=28)	8.1 (5.1, 11.3) (n=17)	0.007
LS BMD z-score	−4.7 (−5.7, −3.6) (n=14)	−2.5 (−3.3, −0.7) (n=22)	−0.2 (−1.8, 0.2) (n=13)	<0.0001
TB BMD z-score	−2.8 (−2.9, −2.2) (n=7)	−1.8 (−2.7, −0.5) (n=16)	−1.9 (−2.8, −1.6) (n=10)	0.25
LDF BMD z-scores Region 1	(n=6) −4.5 (−5.1, −3.6)	(n=22) −3.5 (−4.3, −2.9)	(n=13) −2.7 (−3.7, −1.1)	0.01
Region 2	−4.6 (−5.8, −3.6)	−3.8 (−4.2, −3.1)	−2.3 (−4.1, −1.4)	0.02
Region 3	−3.9 (−5.1, −3.1)	−2.9 (−3.5, −2.1)	−1.6 (−3.5, −1.3)	0.06
Prevalence of BMD z-score ≤−2.0 at 1^st DXA (any site)	14/14 (100%)	25/28 (89%)	14/17 (82%)	0.34
Osteoporosis by ISCD criteria at last encounter	2/14 (14%)	1/28 (4%)	2/17 (12%)	0.07
Data expressed as n (%) and median (25th, 75th percentile).

Conclusion: Low BMD is highly prevalent in SMA patients at time of first DXA. Fracture frequency is also high with predominance of femur fractures in all subtypes. However, few patients met ISCD diagnostic criteria for osteoporosis. Our data suggest poor bone health is a significant concern for SMA patients, but may be underestimated using the 2013 ISCD criteria for diagnosis of osteoporosis in children.

Disclosure: The authors declared no competing interests.

Bone Abstracts

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