SATB2 plays an important role in bone formation by influencing bone cells - specifically the multiplication and differentiation of osteoblasts. Low bone density can occur in children in SAS and likely also in adults, though research data for ages 18+ are still missing. When low bone density is present, the bone tissue has less minerals (f.i. calcium) built-in and as a result can fracture more easily.
Diagnostics and terminology
To say something about prevalence, it’s necessary to explain about diagnostics and terminology.
Bone density can be measured by a DEXA scan of the hip and lumbar spine. In adults, scores are compared to bone mass in healthy young adults, which is called a T-score. This score is expressed as the difference to the mean score, in standard deviations (SD).
A T-score of 0 is average, a positive score is above average, between -1 and -2 SD is called osteopenia, and a T-score < -2 SD is called osteoporosis.
In children, scores are compared to other children matched for age, sex and size, which is called a Z-score. A Z-score above -2 SD is considered within the expected range for age. A Z-score < -2 SD is called low bone mineral density (BMD) for age, but this alone does not establish a diagnosis of osteoporosis. Only in the presence of clinically significant fractures (e.g. ≥2 long-bone fractures before age 10, or ≥3 before age 19), is a diagnosis of pediatric osteoporosis made.
Prevalence
The largest study done so far, showed that 5 out of 32 children with SAS (16%) had low BMD for age. Two of these had nontraumatic fractures and two had a fracture due to adequate trauma. In another study, 6 of 17 children (35%) had a history of fractures. It is important to note that these children were recruited through a centre for bone diseases, and that the ‘life time risk’ of a bone fracture in children aged 16 years is 27 - 64%. In several studies, high levels of markers for bone turnover (f.i. alkaline phosphatase) have been measured, but so far these do not seem to predict the presence of low BMD or osteoporosis.
Management:
When people with SAS have a bone fracture, it is recommendable to do a DEXA scan to assess bone density. In the absence of fractures, it can also be considered to do a DEXA scan at least once, even at ages as young as 3 to 5 years.
In the case of low bone density (for age) or osteoporosis:
- assess for modifiable risk factors (vitamin D deficiency, limited mobility, low calcium intake)
- encourage weight-bearing activity as tolerated
- have a specialist (paediatrician or paediatric endocrinologist) perform standard follow-up and/or treatment
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