Identifying upper vs lower motor neuron lesion

Upper vs Lower Motor Neuron Lesion

So, first for a bit of context and anatomy. Upper motor neurons (UMNs) originate in the brain, specifically in the motor cortex and brainstem, and send signals down to lower motor neurons (LMNs) via the corticospinal and corticobulbar tracts. Lower motor neurons are located in the spinal cord and brainstem, directly innervating muscles through peripheral nerves. They transmit motor commands from the central nervous system to the muscles, enabling movement. So basically, upper motor neurons are part of the central nervous system, and lower motor neurons are part of the peripheral nervous system.

A lot of people think that the spinal cord runs down along the whole spine. Of course, not you, because you are a super smart, good-looking, intelligent person who makes amazing choices. How do I know about your amazing choices? Well, you read the BCP, don’t you? And let’s just forget that night in the pub where you had one too many and attacked that ortho bro who said that strong quads make knee OA worse. But just in case you are now friends with that ignorant ortho bro and he is reading this with you—the spinal cord ends at about the level of L1 or L2 in conus medullaris and then transitions into the structure known as cauda equina. So basically, the CNS runs to the level of L1, and that is important because signs of CNS compromise are NEVER normal in musculoskeletal (MSK) patients. Again, lower motor neuron signs are normal in certain MSK patients, but upper motor neuron signs are not.

 So, what kind of pathology causes UMN lesions? Well, some of the more common ones are: cerebrovascular accidents, anoxic brain injury, malignancy, metabolic disorders, inflammatory disorders, neurodegenerative disorders, infections, traumatic brain injuries, and degenerative musculoskeletal disorders.

 And at this point, you are probably wondering why in the hell should I care what the difference is between UMN and LMN? You are thinking to yourself: I don’t work with neurological patients, and certainly, if one of my spinal stenosis patients is having a stroke, I’ll spot that. And I certainly hope you will and won’t just shout at him, stop making that face, pendejo (asshole in Spanish). I am currently binging Narcos: Mexico, and I love that word. Most of the British readers will also be familiar with it since you collectively, as a nation, have decided to just wreck the Spanish coastal cities. So much so that they are now protesting tourism altogether. They probably get PTSD anytime they see a Wizz Air jet. Now I love my British people so to appease you here are some phrases that will make you forget my insults: bangers and mash, god save the queen fancy a cuppa, let’s jet off to Mallorca, mate… But let’s not get too far off-topic.

 And I agree with you. Most of the pathologies that cause UMN lesions are quite easy to spot or at least won’t usually present in MSK patients. But there are two subgroups that are more common in your MSK population, especially if you work a lot with specific spinal pathology and older patients. The two subgroups are malignancy and degenerative musculoskeletal disorders. 

Let’s look at malignancy first. Cancer often metastasizes to the spine, especially lung, breast, thyroid, and prostate cancer. The most common place for spinal metastasis is the thoracic spine. So, if it affects the spinal cord at that level, you will get myelopathy, which is basically cord compression. Myelopathy, because it affects the spinal cord (which is a part of the CNS), causes upper motor neuron symptoms. So, along with the obvious red flags for cancer like weight loss, age, and a previous diagnosis of cancer, be on the lookout for symptoms of an upper motor neuron lesion like spasticity, problems with gait, hyperreflexia, clonus, and pathological reflexes. 

Now, with degenerative musculoskeletal disorders, the ability to differentiate UMN and LMN signs is even more important. Now, if you are familiar with tandem spinal stenosis (TSS), you know that it is a stenosis of two or more areas of the spine, the most common being lumbar and cervical. Now, if you are not familiar with it, I encourage you to read the blog about TSS. Tandem spinal stenosis is rare in the general population, but it is not rare in people with existing stenosis at one area of the spine. The diagnosis is often delayed because it is a mix of UMN and LMN symptoms. So, if you have a patient who has diagnosed lumbar spinal stenosis and, upon examination, you see neurogenic claudication (LMN sign), absent Achilles reflex (LMN sign), but they walk with a wide-based gait (UMN sign) and have hyperreflexia of the patellar reflex on the right side (UMN sign), you should probably refer them to a neurologist or to get a whole spine MRI done.

 Another quite common example is a person who already underwent surgery for lumbar spinal stenosis but still presents with symptoms. In such a patient, it is prudent to do a short neurological screen to see if there are any UMN signs. Now, I would argue that a short neurological screen is vital in all spinal stenosis patients and with all patients where you have a gut feeling something is wrong.

 In this table, we will look at some of the more major differences in symptoms between UMN lesions and LMN lesions. If you take away one thing from this blog, it should probably be this table or the short neurological screen. 

Now, from this table, we can make a short neurological screen that I use with ALL my SPINAL STENOSIS PATIENTS. Now, a caveat here—normal findings mean normal findings for patients with lumbar spinal stenosis, not normal findings for the general population!

This screen takes about 5 minutes and is very easy to do. If you don’t have a reflex hammer, just karate chop your patient’s tendon. For basic instructions, watch The Karate Kid. Now, if you find upper motor neuron symptoms, refer your patient to the appropriate specialist. And never diagnose anything specific related to UMN unless you are a SPECIALIST. If I find abnormal signs (so UMN signs), I say:

 "Well, you have this sign (insert whatever you find here), which is not typical for lumbar spinal stenosis (or other MSK pathology they might have) and may indicate a problem higher in the spine or in the brain. I am not an expert in this field, so I will refer you to a specialist (usually a GP or a neurologist, depending on the healthcare system in your country)."

Thank you for reading and I hope this helps you in your clinical practice.

 Overley, S. C., Kim, J. S., Gogel, B. A., Merrill, R. K., & Hecht, A. C. (2017). Tandem spinal stenosis: a systematic review. JBJS reviews, 5(9), e2.

Emos, M. C., & Agarwal, S. (2023). Neuroanatomy, upper motor neuron lesion. In StatPearls [Internet] StatPearls Publishing

Zayia, L. C., & Tadi, P. (2020). Neuroanatomy, motor neuron.