What is the difference between MND and ALS?

The term Motor Neuron Disease (MND) encompasses several different conditions whose common feature is the premature degeneration of the nerves which control the body’s voluntary muscles. These nerves are known as motor neurons (sometimes spelled neurones).

There are two groups of motor neurons: upper motor neurons which travel from the brain down the spinal cord, and lower motor neurons which branch 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.

MND was first described in the mid-late 1800s, and the French Neurologist Jean-Martin Charcot is widely credited with the first detailed descriptions. He noticed that a common feature of most cases was muscle wasting, the medical term for which is amyotrophy (literally ‘lack of muscle growth’).

This is a feature of degeneration of lower motor neurons. The resulting muscle wasting can be thought of as a wilting leaf when the water supply from a damaged branch fails – there is nothing intrinsically wrong with the muscle but it wastes when there is no electrical or ‘nutritional’ input from the lower motor neuron.

When Charcot went on to look at the spinal cords of patients with MND, he also noticed scarring of the descending upper motor neuron pathways from the brain. The medical term for this is lateral sclerosis (literally scarring of the outermost pathways of the cord).

Damage to these pathways produces the stiffness that some patients notice (this is called spasticity), and results in exaggerated reflexes that the neurologist can detect on examination.

On their own, both amyotrophy and lateral sclerosis are processes that can occur as part of several other neurological conditions. Charcot’s genius was to recognise that in MND both processes were occurring simultaneously. He called it Amyotrophic Lateral Sclerosis (ALS). There are virtually no other neurological conditions in which both these processes occur simultaneously.

Nearly 90% of patients with MND have the mixed ALS form of the disease, so 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, regardless of where it first begins.

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 are termed Progressive Muscular Atrophy (PMA) and the latter, very rare, 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 progression.

There are other ways that neurologists sometimes categorise MND cases. One method is by the site where the disease symptoms begin – 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 shoulder regions 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.