Bone microarchitecture and bone mineral density in multiple sclerosis

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Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease of the central nervous system leading to demyelination and axonal loss. The aetiology is still not fully understood, but both genetic and environmental factors seem to trigger MS. Patients with MS are at increased risk of reduced bone mineral density (BMD) and fractures.1 BMD measured by dual‐energy X‐ray absorptiometry (DXA) is considered the standard technique for diagnosing osteoporosis. However, low‐energy fractures also occur in patients who would not be considered osteoporotic according to BMD measurements, and studies have revealed that BMD alone cannot predict fractures.2 This shortcoming of DXA has led to the development of trabecular bone score (TBS), which has shown to predict fracture risk independent of BMD without being too time‐consuming or expensive. TBS is an analytical tool that measures grey‐level variations in the lumbar spine from DXA images and provides an index of the microarchitecture of the trabecular bone of the lumbar spine. TBS has shown to be able to differentiate between individuals with the same BMD but otherwise a different microarchitecture and thereby to predict fracture risk independent of BMD. Particularly, this has been shown in patients with secondary osteoporosis due to rheumatoid arthritis (RA),3 diabetes mellitus,4 primary hyperparathyroidism6 and glucocorticoid (GC) treatment.8 These findings suggest that TBS can help in the assessment of bone health and fracture risk in many different conditions.10
Risk factors such as age, previous fragility fracture, parental history of hip fracture, smoking, excessive alcohol intake and prolonged GC treatment are all incorporated into the FRAX™ tool, which is able to assess the 10‐year probability of having a hip fracture and/or a major osteoporotic fracture (a clinical spine, hip, forearm or humerus fracture).12 An analysis from Manitoba showed that TBS could provide an incremental improvement in fracture prediction when combined with FRAX™ variables.13 However, at present the enhanced risk of reduced BMD and fractures due to MS is not incorporated in the FRAX™ tool. Patients with MS are not only at increased risk of fractures due to osteoporosis. Several other factors such as poor balance, impaired vision, decreased coordination, numbness and muscle weakness may all lead to an increased risk of falling. The combination of these factors makes MS patients more susceptible to fractures compared with the background population. Therefore, reduced bone health in MS patients implies even more devastating consequences than for healthy people. To date, the aetiology of bone loss in MS is unknown. As in the background population, osteoporosis in MS indeed affects elderly and disabled patients. However, Moen et al15 found that also young, newly diagnosed MS patients and patients with their very first relapse have lower BMD than healthy controls and therefore suggested that there might be shared aetiological factors between osteoporosis and MS. The aetiology of both MS and osteoporosis is complex and involves both genetic and environmental risk factors.
We have previously demonstrated in a cohort of Danish MS patients that the BMD was reduced compared with a healthy age‐ and sex‐matched reference population.16 To date, no studies have used TBS to evaluate bone health among MS patients. Therefore, the primary aim of this cross‐sectional study was to asses TBS in our MS cohort. Lifelong bone health is dependent on maximizing the peak bone mass in adolescence. Environmental factors play an important role in this critical period of life for both bone health and the risk of developing MS later in life. Therefore, we also aimed to evaluate the influence of environmental risk factors on both TBS and BMD during this vulnerable period in adolescence.

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