Re: Is there a connection between myelin degeneration and heart arrythmias

Before I attempt to answer this question, let me back up and give some 
introductory information regarding the myelin sheath and cardiac arrhythmia 
so that other interested people can follow.

The nervous system is made up of cells called neurons that communicate with 
each other (and other cells like muscle cells) by sending electrical 
impulses (called action potentials) down long cellular extensions called 
axons.   Now, as it turns out (I won't go into the theory here), the rate 
at which an action potential can travel down an axon is inversely 
proportional to the resistance of the cytoplasm within the axon (usually 
referred to as the axoplasm) and also inversely proportional to the 
capacitance of the axonal membrane.  Since having high speed signaling is 
very important for survival, evolution has come up with two solutions to 
this problem.  In invertebrates, the solution has been to either become 
very small (so axons are very short and therefore the speed of the action 
potential is not so critical) or to have very large diameter axons (which 
reduces the resistance of the axoplasm).  In vertebrates, a very elegant 
solution was found.  Basically, a membrane gets wrapped around the axon 
many times to form an insolating covering called a myelin sheath.  The idea 
is that this covering greatly decreases the capacitance of the axonal 
membrane and thus speeds up action potentials without needing to increase 
the diameter of the axon.  This is very lucky for us because without 
myelin, it would be impossible to have a highly complex nervous system 
because there would be no room to fit all the large-diameter axons.

Myelin itself is actually not part of the neuron, but is in fact the result 
of an associated cell wrapping itself around the axon many many times to 
form the sheath.  These associated cells are called oligodendrocytes (if 
they are in the central nervous system - CNS) or Schwann cells (if they are 
in the peripheral nervous system - PNS).  Thus, as it turns out, there are 
two types of myelin diseases - those that affect the CNS myelin and those 
that affect the PNS myelin.  The most well-recognized of these diseases is 
multiple sclerosis (MS), which causes (for unknown reasons) a loss of parts 
of the CNS myelin and results in all kinds of problems due to slow (or 
sometimes absent) action potentials.  Also, over the life span of an 
individual, there is a very slow progressive loss of myelin - but this loss 
(when compared to MS for example) is still very small and therefore it is 
normally unnoticeable.  There is also a similar slow loss of the neurons 
themselves with age.

However, in the heart there is no myelin.  The heart is made of muscle 
cells called cardiac myocytes which are definitely NOT neurons.  The 
cardiac pacemaker is also made of modified cardiac myoctyes and is directly 
attached to the heart (the SA node).  This makes the heart a self-contained 
beating machine and can actually be removed from the body (don't try this 
at home) and still continue beating!  So there is nothing that a 
demyelination disease can do to affect this system.

Now an arrhythmia is caused when the timing of the heartbeat is not normal. 
 There are actually many types of arrhythmias, many of which are very 
common and not even dangerous.  The four most common causes of this 
condition are as follows:

1. underlying heart disease (such as coronary artery disease or valve 
problems, etc.)

2. abnormal electrolyte levels (such as potassium for example) that effect 
the electrophysiology of the heart.

3. congenital problems (very unusual)

4. alcohol, tobacco, or stimulants (like caffeine)


For an older patient, causes #1 and #4 are BY FAR the most common.

All of these causes affect the heart directly (as would be expected since 
the heart is self-contained).

However, this heart is actually not completely self-contained.  The heart 
rate is controlled by two different sets of neurons originating from the 
brain stem.  The vagus nerve causes the heart to slow down, while the 
sympathetic fiber causes it to speed up.  Thus, there is a push-pull system 
that allows the brain stem to control the heart rate.  Since these neurons 
(as well as the circuitry in the brain stem itself) are myelinated, they 
could in principle be affected by demyelination.  So, if this control 
system went haywire could it cause cardiac arrhythmia?    

The only indication I could find in the medical literature that this could 
happen are a few reports of MS patients that also have certain forms of 
cardiac arrhythmia.  I have included the references below.  But it must be 
stressed that you can find a few reports of practically anything in the 
medical literature and certainly most MS patients have no associated heart 
problems at all.  To show any real association between MS and arrhythmia 
would require a very expensive large scale statistical study that no one is 
likely to fund anytime soon.  Then, of course, if you DO happen to prove 
that an association exists, it still might have nothing to do with 
demyelination because certainly MS is a much more complex disease than just 
simply a selective demyelinator.  

So it is possible that an older patient with normal low-levels of 
age-induced demyelination could have cardiac arrhythmias as a result of 
this?  Yes - but it would be the first recognized case!  And of course it 
would be very difficult to establish that demyelination was in fact the 
cause.  If I were examining a patient, I would more easily believe that 
perhaps I missed some subtle form of heart disease, or that the patient has 
been exposed to some form of toxin or is using too much alcohol, drugs, or 
caffeine. 

I hope I've answered most of your questions! 

Take care,
Dave


REFERENCES:

Drouin E, et al. 
     Abnormalities of cardiac repolarization in multiple sclerosis: 
relationship with a model of allergic encephalomyelitis in rat. 
     Muscle Nerve. 1998 Jul;21(7):940-2. 


Schroth WS, et al. 
     Multiple sclerosis as a cause of atrial fibrillation and 
electrocardiographic changes. 
     Arch Neurol. 1992 Apr;49(4):422-4.


Chagnac Y, et al. 
     Paroxysmal atrial fibrillation associated with an attack of multiple 
sclerosis. 
     Postgrad Med J. 1986 May;62(727):385-7.