MINI MEDICAL SCHOOL
WHAT'S THE DIFFERENCE BETWEEN SPASTICITY AND DYSTONIA?
PART 1: SPASTICITY
MATTHEW GWYNN, MD
ATLANTA NEUROLOGY
Another name for spasmodic torticollis is cervical dystonia. These two terms often produce confusion for physicians and laymen alike, especially when used interchangeably, because the words "spasmodic" and "dystonia" have different connotations to many people. Although "spasmodic" in this use is meant to convey the quality of spasms in ST, some may mistake it for words with quite a different meaning, "spastic" or "spasticity". They are very dissimilar. Although both dystonia and spasticity are hypertonic conditions associated with pulling and stiffness of muscles, their physiological mechanisms and the disorders that cause them are not the same.
To understand the difference between dystonia and spasticity we might do well with a Mini Medical School lesson. I hope that as I write these occasional articles, I might ask your favor of allowing me to share some of the same lessons I once paid handsomely to learn in medical school. Because the subject is identical to that which all medical students are taught, I thought I might call these essays "Mini Medical School". The first of these discussions will focus on spasticity. In a later discussion we will contrast spasticity with dystonia.
The study of the nervous system is - next to cosmology and particle physics - the most complex and enigmatic discipline I know of, so I preface my remarks by stating that he who eventually fully understands the brain and spinal cord will surely live not in our day but in future centuries or millennia. What we do not know is that certain parts of our brain and spinal cord (together collectively called the "central nervous system" or CNS) contain nerves that, in turn, control other nerves that begin in the spine and attach to the muscles (the peripheral nervous system" or PNS). All of our movements are controlled exquisitely and continuously through the constant, unending vigilance of the interaction between CNS and PNS.
In our brains there are particular regions which specifically attend to the control of muscle tone and movement. One of these areas is called the "corticospinal tract" because it contains neurons (nerve cells) that live in the outside of the brain called the cortex and send long processes that course down through the brain and connect to other neurons in the spinal cord. Many of the neurons that they connect to are the ones that begin in the spinal cord and go to the muscles. This two-layered system is the basis of movement. For example, if I wish to type this comma, a few of the neurons in the left cerebral cortex - the area a little above my ear, to be exact - send impulses down their axons (the long processes) through the brain and into the spinal cord in my neck where tiny amounts of a chemical are released at the end of the neuron. The site where the chemical is released is called a "synapse" [sin'-aps] and is the place where the end of one of these neurons connect to the beginning of the peripheral nerve. When the chemical travels from the first (CNS) motor neuron to the second (PNS) motor neuron, the second neuron then sends electrical impulses down its own axon through my right shoulder and into my forearm, ending on the muscle that flexes my middle finger, which then depresses the comma key on my computer (after my first misstroke because of my poor typing skills!). Because the first (CNS) neuron is literally higher in the body than the second (PNS) neuron, the first one is called an "upper motor neuron" and the second is a "lower motor neuron". Thus, just as it takes two to tango, it also takes two sets of motor nerves to control our bodies' locomotion and movements.
Because our nervous system has this two-tiered level of complexity, we should not be surprised that there are many things that can go wrong and that different types of disability can result from problems or interruptions of these two systems. Let's look at how injury or disease can affect our muscle control using only the knowledge I've laid forth in this brief lesson. We can all appreciate that if we damage a nerve in our arm, then the muscles controlled by that nerve no longer can receive the signals to contract and become paralyzed. Everyone has awakened from a deep sleep to find a "dead" arm that is numb and limp from a temporary injury to the nerves because of too much pressure from the head or body. This kind of weakness is called "flaccid" [fla'-sid] paralysis and results when there is lower motor neuron injury.
A somewhat different kind of weakness occurs when the upper motor neurons are injured. If I suffer a stroke so that the corticospinal tract is damaged (a very common type of stroke) I, too, will be the victim of weakness to whatever part of the body that is controlled by the upper motor neurons injured by this stroke. At first I will experience the same type of flaccid paralysis as mentioned before; however, within a few days or weeks a strange thing happens: instead of remaining limp, the weak limb develops an increase in muscle tone, often causing flexion at a joint as the muscle bellies shorten from this increased tone, so that any attempt to stretch the muscle is met with sudden resistance. Not only will I remain unable to voluntarily activate it normally, but, also, if someone tries to straighten it, they will find that for the first few degrees of movement, it is easy to pull, but, suddenly, they will meet a tug so that further lengthening is difficult. This is called spasticity, and an affected limb is said to be spastic. Thus, injuries to the brain or spinal cord (such as strokes, head or spine trauma, or tumors) often result in spastic paralysis. The exact underlying mechanism is not clearly understood, but the important thing to remember is this: injuries to the upper motor neurons result in spastic paralysis and injuries to the lower motor neurons result in flaccid paralysis.
Although dystonia and spasticity share some common features such as stiffness, in the next lesson we will discuss the nature of dystonia and the differences between this hypertonic disorder and spasticity. I think you will recognize quite clearly how dissimilar these two conditions are from each other. I'll see you in the next class!
