In this lecture, we're going to learn about
the distal predominant muscular dystrophies
and specifically talk a lot
about myotonic dystrophies.
So what are the myotonic dystrophies
and what is the distribution that we see
in both myotonic dystrophies and distal
predominant muscular dystrophies?
Well, as the name implies,
the symptoms are distal.
So we see weakness in the
distal upper extremities,
wrist extensors and wrist flexors,
as well as the distal lower
extremity, difficulty with foot drop
and utilizing and
maneuvering the foot.
In some of these conditions,
and specifically the myotonic dystrophies
which is the most important of the
distal predominant muscular dystrophies.
We also see respiratory and
cardiac muscle involvement
and some symptoms in the face.
And that's an important piece to
understand about these conditions
because they can be
recognized on physical exam.
So let's talk about the
two myotonic dystrophies
which are the prototypical distal
predominant muscular dystrophies.
And we're going to learn about Myotonic
Dystrophy 1 and Myotonic Dystrophy 2.
DM1 which is not to be confused
with diabetes mellitus.
This is Myotonic Dystrophy Type I
is an inherited muscular dystrophy.
And here the inheritance pattern
is a trinucleotide repeat.
So it's not X-linked,
it's not autosomal dominant,
it's not autosomal recessive.
This is a trinucleotide repeat disorder
and results from this CTG repeat.
Patients present with a number of muscle
disorders, muscle weakness disorders
that can include
prominent facial weakness.
And this is important that facial weakness
is one of the first things we see on exam
and sometimes the thing that is
accused us in on a clinical vignette.
Patients may have ptosis, or open
mouth as a result of bifacial weakness.
This is a distal predominant muscular
dystrophy and so the symptoms are distal.
We see distal upper extremity and
lower extremity weakness and atrophy.
You can see here in the
picture to the far right,
this patient has a atrophic distal
muscles in the upper extremity
both arms symmetrically
as well as in the ankles with
prominent atrophy of the calves.
Importantly, we can see other
symptoms in these conditions.
Patients may have
reduced gray and white
matter within the brain
that contribute to that
frontal balding is
a prominent finding.
And you can see that a little bit in the
picture here with a prominent forehead.
Temporal wasting or loss of muscle
along the bilateral aspect of the face
can result in a thin appearing
face that often is pointed.
And that gives the face
a hatchet appearance
and that's been termed in the
literature, a hatchet face.
Entosis as a result of
the bifacial weakness.
So some characteristic findings that make
this condition very different on exam
and at presentation than some of
the other muscular dystrophies.
patients may have early cataracts
and that's important
to screen for.
This condition affects
and we can see cardiac
in patients who have those
abnormalities may need
screening and early
This is very important for managing
a potential life ending complication.
And that said through
We can see endocrine abnormalities
hypogonadism, pituitary dysfunction
as a result of this condition, which really
affects function across the whole body
and respiratory muscle
weakness and dysfunction.
Hypoventilation or obesity hypoventilation
can be a significant symptom
that also needs to be
screened and managed.
What about Myotonic
Dystrophy Type II?
How does that differ in
its clinical presentation
from Myotonic Dystrophy Type I?
Well, again, this is a repeat disorder
here not a trinucleotide repeat disorder.
This is a CCTG repeat condition.
It's prevalent in European predominant
families and inheritance patterns
so European predominant disease.
Here we see distal arm and proximal leg
weakness which makes it different from DM1.
we also see calf hypertrophy.
Again from fibrosis and scarring
within the distal muscles
as well as myaiglas and you
can see here in the picture
how different this
presentation is from DM1.
We saw thin distal upper extremities
and thin calf muscles in DM1.
Here we see thin, upper extremities
the forearms of the upper extremity
but calf pseudohypertrophy
and there is proximal lower extremity
weakness which differs from DM1.
Often without cognitive dysfunction,
which can be seen in DM1
and we do see associations with cataracts
and cardiomyopathy in patients with DM2
which is important for screening and
longitudinal management of these patients.
So DM1 and DM2 are the two most important
distal muscular dystrophies to consider
but we do see others.
And for these,
I just want you to know the names.
These result in dysfunction of
other proteins that are involved
in that anchoring of the of the
circle glycan complex in the muscle.
We see Welander's,
which is really an extensive weakness.
There's problems with
distal extensor muscles.
Dysferlinopathies are a common group of
distal predominant muscular dystrophies.
Myofibrillar myopathy, which we'll
talk about in a subsequent lecture.
And then there's some
non muscular dystrophies
which is the hereditary or
inherited inclusion body myositis
would be the most prototypical,
non muscular dystrophy
cause of inherited distal weakness and
that's hereditary inclusion body myositis.