00:01
It's important to recognize
that neurally mediated syncope
comes from two things,
the development of
arterial vasodilation
in the setting of relative
or absolute bradycardia.
00:13
It's an important sentence,
so let's break it down a little bit.
00:16
Typically, when the vessels dilate,
the heart will speed up.
00:20
And that's a normal reflexive action
in the body.
00:22
When the vessels dilate,
blood flow goes down,
blood pressure goes down,
the heart recognizes this
through the carotid bulb,
and the heart rate will speed up
or there's reflexive tachycardia.
00:35
Similarly,
if the heart rate slows down,
the blood vessels
will recognize this
through neural innervation
and vasoconstrict
and that will increase
blood pressure
in the setting of
relative bradycardia.
00:49
With neurally mediated syncope,
that process goes awry
for some reason
there is paradoxical
arterial vasodilation
and cardiac bradycardia
which is always abnormal.
01:01
In the literature,
this has been described
through many words
and is known as
vasovagal reaction
or vasovagal syncope,
neurocardiogenic syncope
emotional fainting,
reflex of syncope.
01:14
I think the best categorization
the best word to use
is neurally mediated syncope,
and we'll walk through
what that is
and how to think about it
in the next few slides.
01:26
So what's the pathophysiology?
How does neurally mediated
syncope work?
Well, it ends with a patient
passing out or fainting
and that's syncope.
01:36
One of the inputs one of the causes
of that is hypotension.
01:40
There is relative
hypotension too little blood
in the vascular compartment.
01:45
At the same time,
we see bradycardia
again that paradoxical vasodilation,
hypotension and bradycardia.
01:53
An abnormal paradoxical finding
for some reason.
01:57
Hypotension occurs
through vasodilation,
which typically develops as a
result of sympathetic withdrawal.
02:04
We know that the
autonomic nervous system
is composed of the sympathetic and
parasympathetic nervous system.
02:10
The sympathetic nervous system
speeds us up, it vasoconstricts
it increases blood pressure,
it's that fight or flight system,
The parasympathetic
is equal and opposite.
02:20
And those should be in balance.
02:22
In the setting of syncope,
which is abnormal,
the system is going awry.
02:27
And typically we see
sympathetic withdrawal.
02:30
There's not that
fight or flight response,
and the blood vessels
will vasodilate,
causing hypotension.
02:37
At the same time, we often see
increased parasympathetic tone.
02:41
So again, there's a
paradoxical response going on.
02:44
There's no balance between
the sympathetic and
parasympathetic systems.
02:48
Sympathetic withdraw and
parasympathetic activation.
02:52
The major nervous mediator
of the parasympathetic system
is the vagus nerve.
02:56
And so we see increased vagal tone,
which slows the heart down
and causes bradycardia.
03:02
So how is neurally mediated
syncope working?
What's happening is sympathetic
withdrawal and increased basal tone.
03:09
There are a number
of causes of that
and we'll walk through those.
03:13
At the end of the day, the
brainstem is the critical regulator.
03:17
It's what controls that
autonomic nervous system.
03:20
And sometimes the brain
is telling the nervous system
to work arwy for some reason,
and that's called
central neurally mediated syncope.
03:29
And we'll walk through that.
03:31
Sometimes the input
is coming from the heart
from activation of
ventricular afferents
as a result of an exaggerated
sympathetic activation
or circulating hypovolemia.
03:42
So, neurally mediated syncope
at the end result of syncope.
03:46
It is developed
because of vasodilation
in the setting of bradycardia,
which is paradoxical.
03:52
Typically, that's from
sympathetic withdrawal,
and parasympathetic
over activation.
03:56
And there are three inputs:
the central nervous system,
the brain,
and exaggerated
sympathetic activation,
or circulating hypovolemia.
04:06
This is an important way
to understand
what's going on
for patients presenting
with neurally mediated syncope,
and how to diagnose
and treat those patients?
So let's talk about those
three inputs:
the central, postural,
and situational causes
of neurally mediated syncope.
04:23
At the end of the day,
there is sympathetic withdrawal,
and an increased vagal tone or
increased parasympathetic tone
that results in
hypotension, bradycardia,
reduced blood flow to the brain,
cerebral hypoperfusion
and the patient passes out.
04:39
So what's going on in the body
is the same,
the input is a little different.
04:44
So let's start by talking about
central syncope.
04:48
Central syncope is induced
by a strong emotional stimulus.
04:51
You can think about a sudden
scare, anxiety, or panic.
04:55
Like when someone
scares you
when they jump out
from behind a wall.
04:59
The end result is
activation of vagal tone
and sympathetic withdrawal.
05:03
That's what's happening in all
of the neurally mediated syncope.
05:07
Here, the driver for that
starts within an exaggerated
sympathetic activation.
05:13
That initial scare drives
a sympathetic response
and then overcompensation of
the parasympathetic response.
05:22
So let's see what that looks like.
05:24
Again, with an initial scare,
there's an exaggerated
sympathetic activation.
05:29
And in the case of central syncope,
there is not an equal compensation
of parasympathetic tone,
but an overcompensation
of parasympathetic tone.
05:40
As a result,
their sympathetic withdrawal
relative sympathetic withdrawal,
leading to vasodilation
and hypotension.
05:46
An increase in vagal tone
increase, or exaggerated
overcompensating
parasympathetic tone
with bradycardia.
05:54
And the final common pathway
and result is syncope.
05:58
So the first clinical scenario
for neurally mediated syncope
is central syncope,
which is often induced by
a strong emotional stimulus.
06:06
And the treatment then is to avoid
that emotional stimulus.
06:12
Next, let's turn to
the second situation.
06:14
Postural Syncope.
06:16
Postural syncope results
from pooling of blood
in the venous system.
06:21
There's reduced venous return
of that blood to the heart.
06:25
There's a lack of
reflexive tachycardia
speeding of the heart,
resulting in inability to compensate
for this situation.
06:34
And some of the drivers of
postural syncope include
orthostasis, dehydration,
antihypertensive medications,
and autonomic neuropathy.
06:46
So let's see what's happening
in this situation.
06:49
Well, the inciting event
is circulating hypovolemia.
06:52
The most common situation
is when a patient stands.
06:55
When we stand
the blood rushes to our feet
as a result of gravity.
07:00
The normal process is for the nerves
around the vessels
around the veins
to recognize this
and for there to be
venoconstriction
Vein constriction pushes blood
back to the heart.
07:12
The heart recognizes
this relative hypovolemia
and speeds up,
there's reflexive tachycardia.
07:19
And as a result,
we maintain our blood pressure
and maintain
cerebral perfusion pressure.
07:26
In postural syncope, something
goes awry with that situation.
07:29
And most commonly,
this is as a result of
an abnormality
in nervous system tone around
the blood vessels in the leg.
07:37
Those nerves don't sense
the relative venodilation
and venoconstrict.
07:43
There's not the relative
reflex tachycardia
to speed the heart rate up,
and there's a reduction in
cerebral perfusion pressure.
07:51
So we don't activate the
ventricular afferents in the heart.
07:54
The brainstem doesn't recognize
this problem
with relative hypotension.
07:59
And in reflex tachycardia,
there's sympathetic withdrawal,
leading to vasodilation
and hypotension.
08:05
There's increased
parasympathetic tone,
resulting in relative bradycardia.
08:10
And the final result is syncope.
08:12
This is the most common cause
of neurally mediated syncope
and that is orthostasis.
08:17
When the patient stands,
the blood pressure drops.
08:21
The vascular tone doesn't increase
to drive that blood
back to the heart,
and patients have
decrease perfusion pressure,
decrease blood flow to the brain.
08:30
The interventions then become
to manage that orthostasis.
08:34
To take patients off or reduce
antihypertensive medications
to evaluate
autonomic neuropathy,
to avoid dehydration,
and to counsel on
lifestyle modifications
for orthostasis.
08:49
And then the third clinical scenario
for neurally mediated syncope
is situational syncope.
08:55
This is syncope that is induced by
micturition, defection,
there can be post-tussive of syncope
or valsalva induced syncope.
09:04
This specific stimulation is of
sensory and visceral afferents
and that's the driver
of this phenomenon.
09:10
There's activation of
strong parasympathetic tone
without compensatory
sympathetic counterbalance.
09:18
And we can see this in patients
with autonomic neuropathy.
09:22
So what's going on there?
Well, in this situation,
we have an exaggerated
parasympathetic activation
with micturition or
defecation or valsalva,
the driver of those,
the stimulus
is strong
parasympathetic activation
without sympathetic
counterbalance.
09:39
As a result, we see relative
sympathetic withdrawal,
vasodilation, and hypotension.
09:45
We see an exaggerated
parasympathetic response
or increased vagal tone
and bradycardia,
and the final result is syncope.
09:53
And so,
for the situational syncope,
we counsel patients
to avoid situations
that may induce
the syncopal event.