Researchers are starting to pay more attention to the effect of XLH on muscle function. Getting some data on this aspect of XLH is absolutely critical, both in pure-science terms (understanding the disorder better) and practical terms (understanding treatment needs).
If XLH is purely a bone disorder (which it’s not), then it makes sense to say that we only need treatment while our bones are growing and reaching peak mineralization. That is basically the position that some insurers in the U.S., and some government entities in other countries with socialized medicine have taken — they’re willing to pay for a highly effective but also extremely expensive treatment (burosumab) during childhood to grow/mineralize bones, but not during adulthood, after growth and peak mineralization are achieved.
If, on the other hand, XLH affects more than bones (which it does), and those effects are disabling (which they are), then it doesn’t make sense to limit treatment to childhood (or, worse, only to kids who have severe bone symptoms). That’s why I (and many other patient advocates) believe that since XLH is not limited to the obvious bone symptoms, and the less obvious symptoms are life-long and significantly disabling, then life-long, preventive treatment of all patients, regardless of age, is justified, despite the cost.
Researchers in the UK looked at “Physical Function and Physical Activity in Adults with XLH” and found that while we have fairly normal upper body strength, we have significantly reduced lower body strength and reduced physical activity in general, when compared to non-XLHers. They suggest that the reduced lower body strength could be related to either the underlying metabolic issues (inadequate phosphorus available for muscle function) or the reduced physical activity (presumably caused by pain/fatigue), but couldn’t provide definitive answers without more research. (If you want to read the abstract, click on the link above, download it as a PDF if it’s not already in that format, and scroll to page 33.)
This research doesn’t offer any answers or even tell us anything that patients don’t already know. (During the physical therapy study last year, the difference in the upper-body strength versus lower-body strength was particularly noticeable when the therapists kept over-estimating my lower-body strength and under-estimating my upper body strength for assigning exercises, so the upper-body ones were all too easy to start with, and lower-body ones were too hard, and both had to be adjusted in opposite directions). Still, this article is important, since it’s the first time, as far as I know, that our muscle issues (as opposed to bone issues) have been documented.
We should have some additional information soon, since there’s more research under way! One study is in France, where MRIs will be used to compare muscle function in XLH kids and non-XLH kids. It looks like they may also, incidentally, be able to compare muscle function in kids on burosumab versus kids on phosphorus/calcitriol treatment, although the number of patients (just ten total in the XLH group, and ten in the non-XLH group) may be too small for statistically significant results for comparing the treatment effects.
That study isn’t recruiting yet, but there’s another one that has finished recruiting, so it shouldn’t be too long to wait for results! It is intended to compare muscle function of XLH adults before treatment with burosumab and then after treatment with burosumab. It feels like I’ve been anxious to see these results forever, since the pandemic delayed recruiting. I’ll definitely be watching for the results and will share them with you when I find them!
***
Please note that the author is a well-read patient, not a doctor, and is not offering medical or legal advice.
If you’d prefer to read this blog as a newsletter, sent straight to your inbox, please sign up here.