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High frequency of tetrahydrobiopterin-responsiveness among hyperphenylalaninemias: a study of 1,919 patients observed from 1988 to 2002.
[classical phenylketonuria]
Tetrahydrobiopterin
(
BH
(
4
)
)
-
responsive
hyperphenylalaninemia
(
HPA
)
is
a
recently
described
variant
of
phenylalanine
hydroxylase
deficiency
.
In
contrast
to
patients
with
classical
phenylketonuria
,
these
patients
respond
to
BH
(
4
)
loading
tests
(
20
mg
/
kg
)
with
decrease
of
plasma
phenylalanine
levels
4
and
8
h
after
administration
and
they
can
be
treated
with
BH
(
4
)
monotherapy
.
We
retrospectively
evaluated
1
,
919
loading
tests
from
33
different
countries
performed
in
our
laboratory
between
1988
and
2002
of
which
278
loading
tests
were
performed
with
6
R-BH
(
4
)
,
which
is
about
33
%
more
active
than
the
formerly
used
6
R
,
S-BH
(
4
)
.
The
loading
tests
were
performed
between
the
ages
of
one
week
and
4
.
6
years
,
using
2
.
6
-
30
.
0
mg
6
R
,
S-
or
6
R-BH
(
4
)
/
kg
.
Plasma
phenylalanine
levels
before
the
test
ranged
from
121
to
4
,
705
micromol
/
L
.
We
calculated
the
phenylalanine
"
hydroxylation
rate
"
4
and
8
h
after
BH
(
4
)
administration
and
plotted
the
slope
of
the
hydroxylation
rate
against
the
phenylalanine
levels
at
time
0
.
The
slope
was
greater
than
3
.
75
in
65
,
74
,
33
,
17
,
0
,
and
10
%
of
patients
with
basal
phenylalanine
levels
of
120
-
400
,
400
-
800
,
800
-
1
,
200
,
1
,
200
-
1
,
600
,
1
,
600
-
2
,
200
,
and
>
2
,
200
micromol
/
L
,
respectively
,
when
loaded
with
20
mg
6
R-BH
(
4
)
/
kg
(
p
>
0
.
0001
)
.
This
is
5
-
20
times
higher
compared
with
tests
using
6
R
,
S-BH
(
4
)
or
lower
doses
of
BH
(
4
)
.
More
than
70
%
of
patients
with
mild
HPA
(
<
800
micromol
/
L
)
are
found
to
be
BH
(
4
)
responders
.
Therapy
with
BH
(
4
)
(
approximately
10
mg
/
kg
/
day
)
was
initiated
in
several
patients
instead
of
a
low
-phenylalanine
diet
,
resulting
in
much
better
treatment
compliance
.
Our
data
further
demonstrate
that
BH
(
4
)
loading
tests
can
only
distinguish
between
BH
(
4
)
responders
and
non-responders
.
To
differentiate
between
BH
(
4
)
and
phenylalanine
hydroxylase
deficiencies
additional
tests
are
essential
.
Diseases
Validation
Diseases presenting
"resulting in much better treatment compliance"
symptom
classical phenylketonuria
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