Understanding Paralysis
The first step in treating a patient with paralysis is to accurately diagnose the source of the paralysis and determine what type of nerve injury they suffer from.
Paralysis can result from a disease of the muscle, or from an injury to the nerves, spinal cord, or brain. It can affect a single muscle, an entire limb, one side of the body, or even all four limbs.
Paralysis can be permanent (meaning the body cannot repair itself without intervention), or temporary.
Upper Or Lower Motor Neuron Injury?
Paralysis comes in two main categories: upper motor neuron and lower motor neuron. The upper motor neuron refers to the primary neuron which lives in the brain that initiates the command to move. This neuron travels down the spinal cord and synapses with the lower motor neuron in order to communicate with the muscles. The lower motor neuron lives in the spinal cord but extends its process (the axon) out of the spinal cord and through the peripheral nerves to eventually connect to a set of muscle fibers.
Lower Motor Neuron Injury
When a peripheral nerve in the arm or leg is injured, that is an injury to the axon of the lower motor neuron. The axon that is cut and separated from the cell body within the spinal cord disintegrates by a process called Wallerian degeneration. The muscle fibers therefore lose their axon connection and can no longer receive a signal to contract. That muscle will become flaccid and no longer contract in any way. Over time the muscle will atrophy and lose its potential for recovery. After a year or two is it irretrievably lost. This is the case with a cauda equina injury. Many people are told that they have suffered a spinal cord injury, but these are actually peripheral nerves that live within the spinal canal and result in a very different injury than a typical spinal cord injury with different solutions and strategies for recovery. The same sequence of events occurs if the cell body of the lower motor neuron that lives within the spinal cord is destroyed. This can happen when a spinal cord is crushed. The site where it is crushed contains those lower motor neurons. When they are destroyed, their axon will disintegrate in the same fashion as when the peripheral nerve was cut, but there is no longer any axon source to regrow because the actual cell body is now destroyed. In these cases, the upper motor neuron (the one that lives in the brain) is still alive and well, but it can no longer talk to the muscles because it doesn’t have a lower motor neuron with its axon by which to get that information to the muscle.
Upper Motor Neuron Injury
When the central nervous system is injured, we primarily think about an upper motor neuron injury as the reason for paralysis. Whether a stroke has resulted in destruction of the cell body and thus loss of its axon that was previously connected to the lower motor neuron or injury to the spinal cord has simply transected that descending axon, the lower motor neuron is left without a message from above to tell the muscle when and how to move. In this case the muscle remains connected to the spinal cord and can contract. It may undergo some atrophy due to lack of use, but not to the degree of a lower motor neuron injury. Reflexes remain present and spasms are common. This is because the lower motor neuron works hard to compensate for the missing information from above. This neuron will become sensitive to any input that it can receive so that if there are any signals from above at all, it would do its best to produce an appropriate movement. When this lower motor neuron becomes this sensitive, it now will overreact to the “noise” it hears from the sensory system. Whether the limb is bumped, there is a source of pain, or just a repositioning of the limb, the sensitized lower motor neuron will often respond to this input by eliciting a spasm. This can be a dramatic contracting, jerking or simply increased tone. In fact, in some cases, increased tone may be a constant experience in this condition
Combination Injuries
In some injuries of the central nervous system, both the descending axons of the upper motor neuron and the cell bodies of the lower motor neuron are affected by the same trauma. In spinal cord injury, for example, the axons descending from the brain are disrupted affecting the entire lower body with an upper motor neuron injury. The legs become hypertonic and respond to any touch with jerking. At the same time the lower motor neurons, which are at the same location as those descending axons, are destroyed. In this case the hands and forearms may be flaccid and develop atrophy with time. These muscles do not respond to stimulation and it is difficult to develop an effective tenodesis grasp. This can also happen with strokes of the brainstem where the face can have a lower motor neuron injury and hang flaccid, while the body below this becomes rigid and spastic.
Treating These Injuries
It is important to distinguish these injuries because they are managed very differently. Lower motor neuron injuries must be addressed within months after they occur. That is because of the degeneration of the muscles. Once those muscles are lost, they cannot be recovered. Research is ongoing to lengthen this window and to find out if there are ways to recover muscles in the chronic period, but this is not yet available. Today, nerve grafting and nerve transfers are the primary treatment if we have the opportunity to treat these within the allotted time window. When this time window expires, we can still consider tendon transfers and muscle transplants as an option in many cases.
In an upper motor neuron injury, the same time window does not exist. The lower motor neuron is alive and well and maintains that muscle so that if it ever receives communication again from above, it can once again function. Because of this, we prefer to wait before undertaking surgery. Therapy should be pursued aggressively. Plasticity can result in quite a bit of recovery. Plasticity refers to the ability of the central nervous system to rewire itself so as to better communicate with the lower motor neuron in a useful way. Plasticity is driven by demand – that is, if you are not actively trying to perform better, then plasticity is probably not being effectively promoted. Practice, practice, practice – that leads to more plasticity and better recovery. That being said, there is often a plateau. After a year or two of hard work you may find that you simply are not making more gains. This is when it is time to consider functional interventions. This may begin with Botox injections to temporarily remove the hindrance of uncooperative spastic muscle. Some people will make a lot of progress with this and no longer need these treatments. Others return every three months for another treatment and the effects become less robust with time.
These patients should consider selective peripheral neurotomies. This is a procedure where the nerves are trimmed to eliminate the problematic spasticity but in a way that is very specific and maintains the function of the muscle. Some muscles may never recovery their ability to activate effectively. These muscles can often be recovered with a nerve transfer or a tendon transfer (if a local less important nerve or muscle is found to have good control). In the upper motor neuron conditions, there is typically no time limit on when nerve transfers can be done. We have had success with these even 15 years after a spinal cord injury. Some patients with very high spinal cord injuries or extensive stroke may have fewer options for nerve or tendon transfers. In some of these patients, spinal cord stimulation and Neuroprosthetics are showing a lot of promise. We have an active research group working on these options and spinal cord stimulation is quickly becoming a clinical practice at Spaulding. Stem cell research is also underway to try to regrow those upper motor neuron connections within the spinal cord and wake up those patiently waiting healthy lower motor neurons.
Did you know?
Lower motor neuron injuries should be treated early for the best results.
Patients with spinal cord injury typically have a portion of their paralysis that is a result of a lower motor neuron injury and thus should have those muscle groups addressed early
Because hidden lower motor neuron injuries are often left undetected, we recommend an early evaluation to determine whether this type of injury is playing a part in your functional loss. If not, we will help you develop an effective rehabilitation plan and plan to see you back if you don’t make as full a recovery as desired over the next year.
Schedule a Consult with the Paralysis Center today (844) 930-1001.