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Hello, this is Tom Carpenter.
I'm Professor of Pediatrics
at the Yale University
School of Medicine.
I've had a career
long interest in
metabolic bone
diseases in children,
and I'm grateful today to
the Henry Stewart group for
asking me to talk
about one of my favorite
topics in this area,
and that is the genetic disorder
of X-linked type
of phosphatemia,
the most common form of
inherited rickets that
clinicians encounter.
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X-linked hypophosphatemia is
the most common cause
of inherited rickets.
It's predominantly explained by
excessive renal
phosphate losses.
It presents usually in
young childhood with rickets
and leg deformities.
Osteomalacia also occurs,
but usually not visible
to the clinician.
The skeletal deformity
can be progressive,
and short stature is also
a common manifestation
of this disorder.
Other features that
occur that are less well
recognized as features of
this disorder are
dental abscesses,
which can be frequent and
devastating to overall
lifelong dentition.
And craniosynostosis may occur
to variable extend so
that the head shape may
be abnormal and that
rarely surgical intervention
may be required.
This photograph shows sisters,
actually twins, one of
which has this disorder.
And you can see
despite the same age,
the affected girl has both legs,
and it's much shorter.
Biochemical findings seen
in the disorder include
low blood phosphate levels and
decreased indices of renal
phosphate retention.
That is a decreased
tubular reabsorption
of phosphate and a reduced
threshold maximum for
phosphate as corrected
by glomerular filtration
rate or TMP/GFR.
These low indices indicate
that for a given serum
phosphorus level,
excessive amounts of phosphate
are showing up in the urine.
Also interestingly, there is
a secondary biochemical
abnormality
that for many years
has been unexplained,
that is until recently.
And that is that the
circulating 1,25 dehydroxy
vitamin D level,
the activated metabolite
of vitamin D tends to be
low or normal but
inappropriately
so given the low blood
phosphate level,
which is a general stimulus for
the production of this
active metabolite.