Know your spinal cord – Motor neuron disease

Kultschitzky staining was used here and shows marked symmetrical demyelination (whitening regions) in the anteriolateral roots (especially pyramidal tract) of the spinal cord

It’s day twenty-six already! For those of you who are just finding us, you can find all of the posts in our super cool neuroanatomy category. We’ve already covered a lot of spinal disorders, but one of the larger diseases that we have yet to cover is motor neuron disease. Technically a family of diseases, we will look at the commonalities and causes, so with that introduction, let’s get going.

ALS (amyotrophic lateral sclerosis) is probably the more famous motor neuron disease, in part due to Stephen Hawking suffering from it. However, there are several other types of motor neuron disease that are classified based on how they affect the spinal cord grey matter. Motor neuron diseases are progressive and typically show up later in life. There are several (such as ALS) that will shorten a persons life expectancy, there are some that do not. That’s about as good as the news gets when we talk about motor neuron disease. Below shows a transverse spinal cord slice stained to show the loss of neurons in the anterior horn.

Technically speaking, motor neuron disease is a group of diseases that are characterized by progressive muscle weakness and degeneration of motor neurons which will typically lead to not being able to move the affected limbs. There is currently some discrepancy with how we classify motor neuron disease because like I mentioned, we classify based on the symptoms. There are two major defining characteristics of motor neuron disease that we use to classify the type:

1. Sporadic (acquired) or inherited, does the person have a family history of the disease?
2. Does the disease affect upper motor neurons, lower motor neurons, or both?

Based on these we can classify the type of motor neuron disease into the two main groups, sporadic or inherited and from the second we can then label the disease as one of the following.

• Sporadic amyotrophic lateral sclerosis (ALS), which effects both upper and lower motor neurons.
• Primary lateral sclerosis (PLS), which preferentially affects upper motor neurons
• Progressive muscular atrophy (PMA), which preferentially affects lower motor neurons
• Progressive bulbar palsy (PBP) which affects the upper motor neurons and the lower bulbar region (bulbar is a term we use for tracts connected to the medulla)
• Pseudobulbar palsy affects only the upper motor neurons in the bulbar region
• Monomelic amyotrophy (MMA), which preferentially affects lower motor neurons.

Below you can see the list of symptoms that occur in motor neuron disease. There is an inherited form of ALS, the only difference to sporadic ALS is the family history. You may notice MMA and PMA affect the lower motor neurons preferentially. The difference between the two, PMA is similar to ALS. It is progressive and is seperate from ALS because it affects the lower motor neuron pool preferentially. MMA on the other hand is unique in that it is slowly progressive, but unlike ALS and PMA eventually the disease will plateau and remain stable.

Now for the bad news, we’re not sure what causes it. As of now, we think there are some environmental or genetic component. Men are more affected than women, which somewhat suggests a genetic component and it affects children as well as adults. The prognosis isn’t great either, with the most common form (ALS) having a life expectancy after diagnosis of roughly 2-5 years. Pseudobulbar palsy causes no change in life expectancy, however it is an exceedingly rare form of motor neuron disease. Moreover, we have no way of treating it, so once diagnosed there is no medication or physical therapy to help prevent the progression.

While the outcome of the disease isn’t great, there are a lot of very smart people working on finding a cause and a cure. However, with the variability of the disease and our lack of understanding of the central nervous system, we are still (most likely) several decades from finding a treatment.

That is pretty much it for motor neuron disease. Since I’m sort of swamped for time, I don’t know what tomorrow’s post will be on, but I’m sure I will come up with something good. Hopefully it will be something slightly less depressing.

Until next time, don’t stop learning!