Advanced Assessment of Lower and Upper Motor Neuron

00:00 So next up, we're going to talk about the more detailed neuromuscular part of the exam.

00:05 We've already discussed how different skeletal problems and muscle problems may cause pain to the low back or around the neck, but now we want to tease apart whether or not there is any involvement of the central nervous system or peripheral nervous system.

00:18 Is there any evidence that a nerve root is being encroached upon in any way or even any evidence that the spinal cord itself is being pinched. I would like to use a little bit of etymology here to remember the terminology we're using. When we're talking about a lumbar radiculopathy, r-a-d-i-c opathy, I always remember that there's a vegetable called a radish which is a root that grows in the ground and this is coming from the same word root which means a root, a root growing in the ground. So a nerve root and radiculopathy are all part of the same terminology to help you remember what we're talking about when we refer to radiculopathy. In contrast, myelo, m-y-e-l-o refers to a cord in this case the spinal cord. So myelopathy means the spinal cord itself is being pinched and myopathy, m-y-o is muscle. So a problem with the actual muscles themselves. So, when we're trying to evaluate a patient to decide whether or not their back pain and potentially some paresthesias they are experiencing are related to radiculopathy or myelopathy. We have to always keep in mind that we're trying to distinguish between upper motor neuron problem and a lower motor neuron problem. One of the amazing things despite the complexity of the brain, the amazing thing about making a muscle fiber twitch is that only 2 neurons are involved. There is the upper motor neuron whose nuclease is up here in the cerebral cortex in the motor strip that's sending an axon down on the left hemisphere down to his medulla oblongata where it's going to decussate and cross over and then continue down in the lateral corticospinal tracts down to the level that's appropriate for the muscle that we're talking about and at that level it's going to synapse on to the second neuron, the lower motor neuron, in the anterior horn of the spinal cord and then the axon from that neuron will actually head out initially to a nerve root then to a peripheral nerve and ultimately to land on 1 fiber in Sean's bicep muscle. So, with such a simple circuit, we can really just tease apart on physical exam whether the problem is an upper motor neuron perhaps involving a stroke, perhaps involving a spinal cord injury or if it's a lower motor neuron problem namely involving encroachment on the nerve root radiculopathy or a peripheral nerve. So patients with upper motor neuron disease essentially are not sending inhibitory input down to the lower motor neuron, so the lower motor neuron is firing inappropriately. And so the types of manifestations that you'll see in that context are spasticity in the limb. Because essentially if the muscle is firing without any inhibitory input is just going to continue to fire and so patients end up getting contractures. Initially it's just some spasticity and a little bit of resistance. When I'm trying to pull their arm apart, he would be resisting me and then over time you can develop contractures if it goes untreated and this can occur in the setting of cerebral palsy for example or any hemispheric stroke would cause the same findings. The muscle bulk, while it will go down of course with an upper motor neuron problem, the reality is that since the muscle is continuously active the muscle bulk maybe relatively preserved. In contrast with a lower motor neuron lesion, there's no input going to the muscle whatsoever. So in that case, #1 the first thing that you may see is atrophy and in the setting of a peripheral neuropathy you should see fairly focal atrophy just on that side. So for example if somebody has median neuropathy, i.e. carpal tunnel syndrome, you may overtime see atrophy of the thenar eminence here. Since that median nerve innervates the flexor muscles of the thumb.

04:06 In contrast, if somebody has evidence of bitemporal wasting and you can really see their intercostal muscles etc., that may be evidence of a systemic process such as a cancer or HIV or things of that nature that are leading to a diffuse wasting of protein ultimately wasting of muscle mass. So, focal finding would be supportive of a lower motor neuron lesion. In addition, since those muscle fibers aren't getting any input, they start to fire spontaneously and those are called fasciculations. Fasciculations you can see in any muscle group, you can see them on large muscles and small muscles. You can even see them in the tongue sometimes for folks who are having bulbar issues with a lower motor neuron problem. And so those are the types of things that you will be looking for. In addition, patients, as I said, so that was looking at muscle bulk. With muscle tone, you'll have spasticity with upper motor neuron lesion. Whereas with the lower motor neuron lesion, you have complete flaccid paralysis so you pick up their arm and then let it drop, just like that. It looks like a dead arm. And while he is doing a good job of looking like his arm is dead, a person who has absolutely no muscle tone you'll be amazed how much the wrist can really fall all the way down, it really has quite a degree of collapse when there's no tone to it and that will be the case with any muscle group that we're looking at. So, those are some of the main features between a lower motor neuron and an upper motor neuron problem. The next important steps that we'll look for which helps us to distinguish those things is going to be the reflexes and I'm looking at strength as well.