Without a family history of XLH, diagnosis (even when rickets symptoms are recognized) carries its own frustration. Without experience in this unusual disorder, it is not unheard of for a physician to not even diagnose a child of an XLH parent until much later than it could be, putting bowing off to "bulky diapers" or saying "all toddlers legs are bowed" rather than even x-raying the child for rickets, or comparing the child's blood phosphorus level to an adult reference range since not all labs report the higher ranges seen in normally growing children.
XLH occurs probably in only one birth out of 20,000, so a pediatrician or general practitioner may never see it in his or her career. Even if he does diagnose the child with rickets, he might not know about XLH and presume it to be due to lack of enough vitamin D and try that first. However, XLH resists typical Vitamin D therapy, so a patient might then be put through a complete series of tests, X-rays, and consultations with specialists and then might not be diagnosed with XLH until age 3 or 4.
Precious treatment time is lost, so it's important to find a specialist with experience in this area when symptoms are first observed. This type of specialist is generally an endocrinologist or nephrologist, but specifically one with an interest in metabolic bone disorders as opposed to more routine endocrine or nephrology issues. Very few have experience in rare metabolic bone disorders, so it's important to make sure the one you see does.
If there's a family history of XLH, it should be easier to diagnose before symptoms of rickets even develop, but it's extremely important to compare lab results for serum phosphorus levels to normal levels for children of the same age, which some labs do not report. In fact, the lab may not even indicate that the reference range is suitable only for adults, so a physician inexperienced with XLH may not realize the child's results are abnormal. This causes an unnecessary delay in diagnosis.
Specialists who have several XLH patients, however, might diagnose a patient without much difficulty and know what tests to run to differentiate between the different disorders that cause rickets and osteomalacia. The earlier the diagnosis, the earlier therapy can begin, and the better the results are likely to be.
There are several phosphate wasting disorders that cause similar symptoms to XLH, so it's important to know which one the patient has since treatment for one might not be correct for another.
People with Autosomal Dominant Hypophosphatemic Rickets can have symptoms that differ only in subtle ways from those experienced by people with XLH. ADHR, as indicated by its name, is another dominantly inherited hypophosphatemic disorder, but it is caused by a different gene found on a dfferent chromosome than XLH. Its symptoms can first occur in people of any age, and in some cases its symptoms have even been found to resolve on their own.
And, certain very rare tumors [known by the names Tumor-Induced Osteomalacia (TIO) and Oncogenic Hypophosphatemic Osteomalacia (OHO)] or rare cases of giant nevus sebaceous (certain very large birthmarks, which may be related to TIO) may also cause bone problems due to hypophosphatemia. These disorders are not considered to be hereditary.
Source: The XLH Network Board of Directors
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