Upper leg bones are compromised in SMA children who can’t walk: Study
New data show weaker bones in nonambulatory SMA vs. CP kids
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- SMA children who can't walk have significantly weaker upper leg bones than do nonambulatory kids with cerebral palsy, a comparative analysis found.
- This bone weakness in children with SMA correlates with a high fracture rate.
- Routine bone health assessments for SMA children should include bone density measurements of the upper leg, the researchers say.
Children with spinal muscular atrophy (SMA) who are nonambulatory, or unable to walk, have weaker upper leg bones — as assessed by tests of bone mineral density, or BMD — compared with nonambulatory children with cerebral palsy, a group of movement disorders.
Those are the results of a retrospective study showing that youngsters with SMA have lower BMD scores in testing of the distal femur, which is the leg bone above the knee, than children with cerebral palsy (CP). Lower bone density rates were tied to a higher rate of fractures in the SMA children, the data showed.
Despite these findings, however, the BMD of the lower spine was similar between the two groups, and fell within the normal range for the children’s age and height, according to the researchers.
Given these data, the scientists called for BMD testing of the upper leg bone to be incorporated into clinical care for children with SMA.
“We recommend including the distal femur bone density in routine bone health assessments,” the team wrote.
Their study, “A comparative analysis of bone mineral density in cerebral palsy and spinal muscular atrophy,” was published in the journal Bone Reports.
A rare inherited disorder, SMA is characterized by the degeneration of specific nerve cells in the spinal cord, resulting in progressive muscle weakness and wasting. CP, meanwhile, is a group of conditions that appear in infancy or early childhood and that permanently affect body movement and muscle coordination.
Over one-third of SMA children have experienced bone fractures
In children with SMA, type 1 is the most severe and common form, with onset before 6 months of age. It’s marked by profound weakness and an inability to sit, stand, or walk. SMA type 2 is typically first seen between 6 and 18 months of age, with patients able to sit but not walk. With a later onset, SMA types 3 and 4 are also milder forms of the disease, usually allowing for independent walking.
CP, which also varies by type, is thought to be caused by changes in the developing brain that disrupt its ability to control movement, posture, and balance. Severity can range from near-normal motor skills and minor limitations to severe limitations in head and trunk control that require wheelchair use.
Due to limited mobility, bone fractures are common in both of these conditions. According to the researchers, fractures occur in as many as 9% of cerebral palsy patients who are nonambulatory, meaning they are unable to walk or move around independently. Among SMA patients, at least 38% have sustained a fragility fracture, the data showed.
Additional risk factors for bone weakness include inadequate nutrition and certain medications.
In this report, researchers at The Ohio State University sought to compare BMD — a measure of the amount of minerals contained in a certain volume of bone — in nonambulatory children with SMA and cerebral palsy. To do so, the team used dual-energy X-ray absorptiometry, known as DXA.
“Clinical observations from the authors suggest that children with SMA may experience more severe reductions in bone mineral density (BMD) compared to those with [cerebral palsy]. We sought to determine whether this difference truly exists,” the researchers wrote.
Significantly lower bone density seen in upper leg bone in SMA kids
Altogether, the team examined medical records from 16 children with SMA and 274 with cerebral palsy, all of whom had undergone DXA scanning. The SMA children had a median age of 9, while the CP children had a median age of 14.
Some of the youngsters underwent DXA scans at both the lumbar (lower) spine and the distal femur, while others had scans at one site due to obstructions, such as metallic implants and contractures. The researchers used BMD Z-scores, which represent the difference in BMD relative to that of healthy children of the same age.
According to the analysis, the median lumbar spine Z-scores for BMD were below the normal range and comparable between the cerebral palsy and SMA children (-2.3 vs.- 2.6). However, after adjusting for height, the median lumbar spine BMD Z-scores for both groups fell within the normal range, with no significant difference between them (-0.9 vs. -1.6).
There were, however, differences in distal femoral BMD, with Z-scores below the normal range in both groups, but significantly lower in the SMA group than in the cerebral palsy group (-3.9 vs. -2.9), the data showed.
We found that nonambulatory children with [cerebral palsy] and SMA had low [bone mineral density, an indicator of bone strength] when measured in the distal femur [upper leg bone], and it was significantly lower for the children with SMA.
“In the case of SMA patients who have severe [low muscle tone], the reduced muscular force on the bones may explain the lower BMD than the patients with CP,” the team wrote. “In contrast, [high muscle tone], commonly seen in CP patients, may lead to higher levels of muscular resistance, resulting in better bone preservation.”
Because children with cerebral palsy were generally older than those with SMA, the team then directly compared children of similar age (median age 9.5 vs. 9.4). Despite this, the SMA group still had significantly lower femoral BMD Z-scores than the cerebral palsy group (-3.9 vs. -2.5).
“We found that nonambulatory children with CP and SMA had low BMD when measured in the distal femur, and it was significantly lower for the children with SMA,” the researchers wrote. The team noted that “this correlates with the high fracture rate in both groups.”
Given the findings, the researchers concluded that “distal femur bone density should be part of the routine assessment of bone health in all nonambulatory patients.” Further, the team recommended that “BMD should be adjusted for height when able.”
