The Neurotransmitters: Intro to Spinal Cord Anatomy

Transcript

Hello, and welcome back to The Neurotransmitters, a podcast about everything related to clinical neurology with the goal of reducing your neurophobia.

I'm your host, Dr. Michael Kentris, and thank you again for joining me today.

We are continuing our march through the neuroaxis, starting at the brain and heading south.

And today we are in the spinal cord.

So when does one consider a myelopathic or a spinal cord localization?

There are some key clinical features that can guide you in that direction.

A myelopathy shouldn't be causing any abnormalities of cognition, cranial neuropathy, etc.

So if there's something going on in the head region, we should be revising our opinion.

Now there are etiologies that can cause myelopathy, as well as cognitive problems and so on and so forth, but we're going to be focusing on primary myelopathic localizations at this particular junction.

So once we've excluded an intracranial localization, we are left with a few key signs that would point us towards a myelopathy versus something more distal down into the peripheral nervous system.

So some of the things that really act my eye are signs of upper motor neuron involvement.

In particular, I'm looking for things like increased tone, hyperreflexia, abnormal motor reflexes like of a Babinski being present.

And the second piece is some sort of crossed sign.

In particular, if we see motor and vibratory sense being affected on one side and pain or temperature sense being affected on the contralateral side, that is very suspicious for something going on in the spinal cord.

Now let's talk a little bit about spinal cord anatomy and how its structure leads us to these clinical signs and points us in this direction to come to a confident localization.

So the spinal cord essentially runs from the lower edge of the brainstem down into the lumbar spine, the L1-2 region.

Like many parts of the nervous system, there are a lot of things happening in the spinal cord.

But from a clinical assessment perspective, I think there are just a few aspects that will get you 90% of the way there.

And that is looking at the tracts.

So there are two components to this.

One, which tract is involved, and we'll go through those in a minute.

And two, where in the spinal cord are those tracts located?

So the three main tracts that we're really looking at are the spinal thalamic tract that is responsible for pain, temperature, crud touch sensations, the dorsal columns, which are responsible for proprioception and vibratory sense, and last but not least is the corticospinal tract responsible for motor signals from the brain getting to the rest of the body.

A lot of the clinical interpretation of the signs related to amylompathy are related to where in the spinal cord these different tracts run relative to one another.

So I think one of the first things to think about is the decalation or where do the fibers cross.

So for the corticospinal and the dorsal columns, these are going to decacinate up in the brain stem in the medulla.

And this is why weakness, vibratory sense, and proprioception sense will often be ipsilateral to the lesion in myelopathy.

The spinal thalamic tract is the odd one out as it decacitates in the cord, usually a couple levels above its point of origin into the spinal cord.

So this means that with unilateral spinal cord lesions, the pain and temperature loss will usually be contralateral to the side that is affected.

The classic name that you may have heard for this pattern of symptoms is called Brown-Sequard Syndrome or Hemicord Syndrome, where you have ipsilateral weakness, loss of vibratory and proprioception sense on the same side as the lesion, and contralateral loss of pain and temperature sense.

So you may hear people in clinical practice say like, "Does the patient have a level or a spinal level?"

And what they're referring to is where is the lowest level down where we have the weakness, the numbness, the sensory disturbances, etc.

And ideally we should be able to put those all together into one place for a proper localization.

Another key thing to remember when we're considering localization in the spinal cord is that if the arms and the legs are involved, we're usually looking at the cervical spinal cord.

Particularly if weakness is involved, the lowest level in the arms and hands is usually going to be T1, and this is similar for sensory abnormalities as well.

So anything that localizes to the myotome or the dermatome above T1 should be involving the cervical spine if we're considering a myelopathy.

If we have a sensory level to reduce sensation to pinprick across the torso, say somewhere on the chest wall, then we're really looking more in the thoracic spine at this point in time.

And if there is weakness, it should not involve the arms theoretically if it is in the mid to lower thoracic spine.

I will say anecdotally that you will sometimes have an exam where you're like, "Oh, this very clearly localizes to C7 or T2," and you only get the MRI of the cervical or thoracic spine, and then you find, "I just missed the lesion," or you've only caught a piece of it, and so then you have to send the patient back for repeat imaging of the piece that you left off.

So things that are right on that cervical thoracic border, I would usually get pictures of both the cervical and thoracic spine, personally speaking.

Now we've been talking a little bit about the vertical in terms of the spinal cord, but let's talk a little bit about the cross-sectional anatomy for just a moment.

This will be important for conceptualizing the other patterns of myelopathy that we often see in clinical practice.

So the lateral corticospinal tracts, as the name would suggest, are mostly lateral, little posterior.

The dorsal columns are, as you might guess, dorsal or posterior, and medial, and the anterior lateral, or spinal thalamic tracts, are anterior and lateral.

Again, it is nice when things are named in a somewhat descriptive fashion that gives us useful information just from the name.

Now in addition to these tracts' locations within the spinal cord themselves, they also individually have somatotopic organization.

That is to say there is a pattern of, you know, cervical, thoracic, lumbar, sacral, from medial to lateral for each one of these tracts.

So for the corticospinal and spinal thalamic tracts, cervical is more medial, sacral is more lateral.

And just because nothing in life can ever be too easy, the dorsal columns are the opposite of that.

So the sacral tracts are more medial, and the cervical tracts are more lateral.

Got all that?

No?

Let's say it one more time.

Corticospinal and spinal thalamic tracts, cervical medial, sacral lateral, dorsal columns, sacral medial, cervical lateral.

Repetition is the mother of all learning, as they say.

So let's move on to some clinical syndromes with this knowledge in hand.

The spinal cord syndrome whose identification depends most on knowing this information is central cord syndrome.

Now central cord syndrome is most commonly going to be related to something that is causing dilation of the central canal.

The most common thing would likely be a serinks.

Because the central canal is right next door to where the spinal thalamic fibers cross, particularly the ones going into the cervical cord, this leads to abnormal pain and temperature sensation, primarily in the upper extremities and upper torso.

Another spinal cord syndrome to be aware of is anterior cord syndrome, and this is going to involve the tracts in the anterior part of the spinal cord, which are going to be the spinal thalamic and corticospinal tracts primarily.

Because this is often bilateral, below the level of the lesion we are going to see weakness and impaired pain and temperature sensation, but intact proprioception and vibratory sense.

Cerecord syndrome in particular is often ischemic in nature, and this has to do with the vascular supply to the anterior spinal cord.

The anterior spinal artery tends to receive feeders from different radicular arteries.

The most prominent of these is known as the artery of Adamkowicz, usually found in the lower thoracic area around T9 and T10.

So in the setting of big drops in blood pressure and someone who maybe has some atherosclerotic disease in that artery, or in people who have an abdominal aortic aneurysm around that level who have to go into a surgery, these are all reasons why that artery may become occluded, leading to an acute spinal cord infarction, presenting with that anterior cord syndrome potentially.

The last spinal cord syndrome that I wanted to mention today is subacute combined degeneration, and this one is a mixture of the dorsal columns and the corticospinal tracts.

This can be due to several different causes, the classic one being B12 deficiency, which could be from people who are malnourished from a history of gastric bypass who maybe had a recent gastrointestinal illness.

You can see similar things with copper deficiency as well, and you can also see similar patterns with things like HIV.

One specific pattern to bring up with these entities, particularly B12 and copper deficiency, is a myelotneuropathy, and this is what it sounds like.

It's a mixture of myelotathy and neuropathy, so you can't have mixed upper and lower motor neuron signs, and we're going to talk more about the approach to weakness in a later episode, but briefly, upper motor neuron signs are going to be spasticity, hyperreflexia, lower motor neuron signs would be more expected to be decreased tone, arreflexia, and potentially fasciculations and atrophy depending on the timeline.

So in summary, if you keep your three tracts in mind, your corticospinal, spinal thalamic, endosal columns, and their relative relation to one another in mind, you can localize pretty well in the spinal cord, and keep in mind the patterns of different spinal cord syndromes.

These localizations all have tendencies to be associated with different pathologies, some more or less likely than others in different clinical contexts.

So again, putting the physical exam findings that you see in the context of your patient's history is always of the utmost importance.

Something occurring over the span of minutes is going to have a much different differential diagnosis than something occurring over the span of months or years.

Thank you again for joining me today as we build up these fundamentals of neuroanatomy and clinical neurology and help reduce your neurophobia, make you into a better clinician, better able to diagnose and treat your patients.

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If you have any questions, show suggestions, or just want to say, "Hey, feel free to reach out to me on Twitter @drkentris."

That's Dr.

Kentris, K-E-N-T-R-I-S.

You can also reach me by email at theneurotransmitterspodcast@gmail.com.

I hope you have a great day and keep on localizing out there.

We'll see you next time. [music] [BLANK_AUDIO]