The term Motor Neuron Disease (MND) encompasses
several different conditions whose common feature is
the premature degeneration of motor nerves (known as
neurons or sometimes neurones).
There are two groups of motor neurons - upper motor
neurons travelling from the brain down the spinal
cord, and lower motor neurons branching outwards to
supply muscles in the face, throat, arms, chest and
legs. Both groups of neurons tend to be involved in
MND but to varying extents, which is one of the many
reasons why each patient's disease is subtly unique.
Nearly 90% of patients with MND have the mixed ALS
form of the disease, so that the terms MND and ALS
are commonly used to mean the same thing. Within
this large ALS group there is still a huge variation
in the way the disease presents and progresses. With
further study it is apparent that the other 10% of
patients tend to show either predominantly lower
motor neuron damage with prominent muscle wasting,
or solely upper motor neuron degeneration with
relatively little muscle wasting but prominent
stiffness. The former group is termed Progressive
Muscular Atrophy (PMA) and the latter group Primary
Lateral Sclerosis (PLS), to reflect each end of a
spectrum. Some of these cases, particularly those
with PLS, seem to have a much slower rate of
So the terms MND and ALS effectively mean the same
Finally, there are other ways that neurologists
sometimes categorise MND cases. One method is by the
site where the disease symptoms being - for example
if it is in the speech and swallowing motor nerves
(which arise from the 'bulb' of the brain stem) then
it is termed bulbar-onset MND. Another group of MND
patients have a disease that predominantly affects
the arm muscles, and is termed the 'flail arm'
variant. These so-called 'regional phenotypes'
follow some common trends in their patterns of
progression, but no system of categorisation to date
can predict with certainty the course of the disease
for an individual patient.