Video: Dermatomes

Imagine this scenario. You wake up in the morning, it's dark outside and you're still half-asleep. You stick your legs out from your bed and lower them onto the floor. Brrrr! It's icy cold on the soles of your feet. You feel something soft brush up against your shin. Oh, it's your cat, asking for his breakfast. You start towards the stove to make your early morning coffee. A flick of the match, turn of a knob, and your coffee is bubbling away. You can feel the pleasant warmth radiating from the kettle in your hands. Suddenly, you drift into a daydream and stick your finger right into the flame! Okay, maybe you've had better mornings, but take a moment to think just how amazing it is that you manage to experience so many sensations within a couple of minutes. It's all thanks to the largest organ in your body – the skin.

But how do you wire such a huge surface area with so many different types of sensations? The solution the body has come up with is really rather ingenious. The surface of the body, or the skin, is divided into small sections that each have a guardian spinal nerve which is responsible for the innervation to that area. So, what do you say? Shall we learn more about this clever arrangement? Well, then stay with me while we explore the dermatomes.

All right folks, I know you're itching to learn all about dermatomes. but let's quickly run through what we'll be looking at in this tutorial. First, I want to start with the basics and have a little recap on an important group of structures known as the spinal nerves, which play an essential part in mapping out the innervation of the skin. We'll then move on to learn more about what dermatomes are, how they work, or perhaps why they don't always work, and we'll introduce you to a couple of different dermatome maps. We’ll then work our way through the dermatomes of the axial structures, so the head and the torso, then move on to the limbs.

We’ll then whiz through a couple of important landmarks associated with each dermatome when the dermatome map is unavailable and we'll finish up by having a look at some clinical notes, which will hopefully prove to you just how you saw the system is in diagnostic medicine. What do you say? Shall we get cracking?

What we're looking at here is the posterior aspect of the spinal cord as it sits within the vertebral canal with spinal nerves branching away from it. These spinal nerves course through all the peripheral regions of the body providing both sensory and motor innervation to the trunk wall, head, and neck, and, of course, our limbs. Most relevant to us today, of course, is their role in the innervation of the majority of the skin of the body with the exception being certain areas of the face which are innervated by branches of the fifth cranial nerve – the trigeminal nerve. To refresh your memory, we have eight cervical, twelve thoracic, five lumbar, and five sacral spinal nerves, with just one pair of coccygeal nerves at the end. So, thirty-one pairs in total.

Okay, so what exactly are these dermatomes we keep hearing about? The term dermatome is derived from Greek and translates to a cutting of the skin. Each of the spinal nerves which we looked at just now with the exception of C1 are said to innervate a roughly defined segmental field of the skin of the body. This is what's known as a dermatome. The reason the C1 spinal nerve does not have a related dermatome is due to the fact that its posterior root is often absent, underdeveloped, or does not present a dorsal root ganglion. This means that it generally does not carry a sensory component.

The coccygeal nerve is also a bit of an exception as it too has no official assigned dermatome; however, some sources mentioned that it is thought to innervate a small area of skin over the coccyx. So, for argument's sake, we'll say we have a dermatome corresponding to each of the spinal nerves between C2 and S5.

All the dermatomes drawn onto an illustration of the body in a single image is called the dermatome map, and if you've managed to work your way through a few textbooks, you may have noticed that they don't always look the same. This is due to the fact that several different dermatome maps have been proposed over the years by different researchers, each established using different methods. And let me give you a word of warning, none of them are perfect. Each have their own flaws which we'll discuss a little more as we work through the tutorial.

In this video, we're going to look at the two most popular variations of the dermatome map.

The first dermatome map we're using here is based on the Keegan and Garrett map proposed in 1948. It's probably the most commonly used map in major textbooks and is preferred by some because of its correlations to developmental concepts and continuous dermatomes, which are easy to follow. That being said, although it is the most popular of the dermatome maps from a clinical perspective, it's arguably the most flawed. Funnily enough, researchers have not been able to replicate the findings of this map. Nevertheless, it lives on in its popularity, so it's only right that we're familiar with it.

The other map that we're going to look at in this tutorial is known as the Foerster map, which looks a little something like this. It was produced in 1933 by Otfrid Foerster, and is the next most widespread in major anatomical textbooks. It shows a more segmental distribution of dermatomes, especially on the limbs, but we'll talk more about this shortly. One of the main issues with the reliability of this map is due to the fact that Foerster did not consistently record his methodology when addressing and recording the dermatomes in his map. From a clinical perspective, it is generally considered to be somewhat more reliable than the Keegan and Garrett map, but that being said, no dermatome map is one hundred percent accurate for reasons I'm going to share in just a moment.

Now that we've introduced two of the major dermatome maps, let's take a closer look to see where exactly their main differences lie. All skin sensations are carried by cutaneous branches of the peripheral nerves. In the trunk, each spinal nerve innervates a strip of skin, so the area supplied by each nerve is identical to its dermatome. In the limbs, however, things get a lot more complicated due to these pesky brachial, lumbar, and sacral plexuses which makes up spinal nerve fibers, meaning the peripheral nerves contain fibers from multiple spinal nerves.

So, let's look at dermatome areas across the body using both the Keegan and Garrett and Foerster maps for comparison. I like to work my way through craniocaudally, so we'll start with the dermatomes of the face. It's important to remember that the dermatomes of the face are not actually innervated by spinal nerves. There are three dermatomes in the region of the face, all of which are innervated by branches of the trigeminal nerve, which we know is a cranial nerve. We know that there are three divisions of the trigeminal nerve, which means – yep, you guessed it – each division of this nerve innervates one of the dermatomes of the face.

The forehead and the nose are innervated by cutaneous branches of the ophthalmic nerve. On the maxillae or cheekbones, we find the dermatome referred to as V2 innervated by the maxillary nerve. Finally, the mandible and the area in front of the ear is innervated by the mandibular nerve, and forms the V3 dermatome. In reality, however, studies have shown there's a significant overlap in these particular dermatomes, which should be taken into account in clinical practice.

Right, so now that we've covered all there is to know about the dermatomes of the face, let's move on to looking at the rest of the dermatomes – you know, the ones that are innervated by the spinal nerves.

Once again, we're looking at comparative images of the Keegan and Garrett dermatome map – seen here – and the Foerster dermatome on the opposite side – right here. We'll also be using images of the dermatome maps depicting both the anterior and posterior aspects of the body. That is, of course, because the dermatomes stretch across the whole body, but they may not necessarily look the same on the anterior aspect as they do on the posterior aspect. Finally, let's have a look at the C2 and C3 dermatomes.

As you can see from both maps, it is generally agreed that this dermatome covers the superoposterior aspect of the head. How far it extends down the back at the head is another question. As you can see here, the Keegan and Garrett map shows the area around the ear and lateral cheek in addition to the region immediately below the jawline and chin as belonging to the C2 dermatome. However, you'll notice that in the Foerster map, this area is attributed to the C3 dermatome instead, which has now been confirmed by a clinical study. You'll also notice that the C3 dermatome of the Foerster map extends onto the supraclavicular region on the anterior aspect, which is not demonstrated on the Keegan and Garrett map.

The C4 dermatome also carries a significant variation between the two maps, and this one is worth noting as it is a common dermatome tested in neurological examinations. Looking first at the Keegan and Garrett map, you'll notice it consists of just a thin band on the posterior perspective and is limited mainly to the base of the neck and regions above the clavicles on the anterior view. Notably, it does not extend laterally towards the shoulders. I want you to also note that the dermatome on this map immediately below C4 is the C5 dermatome, which you might expect, but more on this later.

Looking at the Foerster map, however, we can see from the posterior perspective that the C4 dermatome extends much further laterally and expands over the posterior aspect of the shoulder continuing on to the anterior aspect of the shoulder and medially over the pectoral regions. Note here, however, that the dermatome immediately below the C4 here is the T2 dermatome.

So, there are significant differences here, but who's right? Well, we found sources which support both arguments which more than likely comes down to individual variation. For example, the superior pectoral area as well as the deltoid areas are innervated by the supraclavicular nerves, which happens to show variably in at least fifty percent of the population. For example, this group of nerves are usually composed of nerve fibers from only the C3 and C4 spinal nerves, which would support the Foerster map. It may also variably contain fibers from the C5 spinal nerve, which in such cases, would support the Keegan and Garrett map.

Before we learn about the dermatomes of the upper limb, I want to quickly show you the dermatomes of the trunk because they're super easy. Although our two maps largely agree with one another, there are one or two major differences, the most obvious of these is the presence or absence of the T1 dermatome on the anterior chest wall. Once again, anatomical variation may, at least to some extent, account for the difference here.

In general, all intercostal nerves which are branches of the thoracic spinal nerves have anterior and lateral cutaneous branches which innervate the thoracic wall. The first intercostal nerve, which is a branch of the T1 spinal nerve, usually lacks these cutaneous branches, which would innervate the overlying skin. This is why the T1 dermatome is omitted on the chest wall on the Foerster map.

As we move down through the remainder of the thoracic dermatomes, you'll be happy to know this section of our dermatome map is pretty straightforward. From around T2, or the level of the axilla, the dermatomes are basically just longitudinal strips stacked on top of each other all the way down to the T12 dermatome at the pelvic girdle anteriorly. Got it? Great! Moving on.

Moving on to the upper limb, things can get a little tricky, and yet again, our two variations of the dermatome maps present significant differences. The Keegan and Garrett map presents the C5 to T1 dermatomes as running continuous from the spinal cord along the length of the upper limb whereas the Foerster maps showed C5 to T1 dermatomes as being discontinuous in appearance. But how could that be?

Well in order to understand the organization of the dermatomes of the upper limb, we have to go back – way back – to embryonic development.

In the early stages of fetal development, we see segmentation with each segment representing a different spinal level and, therefore, a different dermatome. From here on, the limb buds start developing laterally and forward, away from the body. So far, so good, because the dermatomes are still just in sequence with the trunk. The upper bud is innervated by spinal nerves C5 to T2. As it grows and elongates, it pulls the anterior parts of the C6 to C8 dermatomes into the limb. So, on the anterior aspect of the body, C6 mainly innervates the area over the thumb, C7 the middle finger, and C8 the little finger. The index and ring fingers are too variable to ascribe to any one dermatome.

You'll see the same arrangement on our dermatome map. From an anterior perspective, the C6 to C8 spinal nerves provide cutaneous innervation for the hand. As a result, in the pectoral region, the C5 dermatome lies immediately adjacent to T1. This is the only area of the body where there's no overlap of dermatomes and, therefore, the bands are very clearly defined. However, the dermatomes are continuous on the back.

Right, so let's move on to the lower limbs where, unfortunately, the disagreement continues in our two primary dermatome maps. Similar to the upper limb, the Keegan and Garrett map presents swirling unbroken dermatomes which extend from the midline until their termination in the lower limb. The Foerster map, on the other hand, presents more segmented dermatomes.

The L2 and L3 dermatomes appear reasonably similar in both maps as they are mostly contained to the area above the knee. The differences become especially apparent in the L4, L5, and first sacral dermatomes, which extend from the lower trunk all the way down to the leg and foot in the Keegan and Garrett map, but are discontinuous and mostly distal to the knee in the Foerster map. This obviously can create a lot of confusion. For instance, if you had a patient who presented with altered sensation on the anterolateral thigh, it's possible to localize the injury to the L2, L3, L4, or even L5 depending on the map you use.

Well, in order to understand the organization of the dermatomes of the lower limb, let's look again to some embryonic development. In the early stages of fetal development, we see segmentation with each segment representing a different spinal level and, therefore, a different dermatome. From here on, the limb buds start developing laterally and forward away from the body. So far, so good, because the dermatomes are still just in sequence with the trunk. The lower limb is innervated by spinal nerves L2 to S3. As it grows and elongates, it pulls the anterior parts of the L3 to S2 dermatomes into the limb creating discontinuous dermatomes. This explains why L2 and S3 are bordering each other in the inguinal region.

As I've already mentioned, there's a huge overlap in dermatomes and sometimes you may not have a dermatome map at hand. something that we also need to keep in mind is that there's a huge individual variation, which can affect dermatome identification. That's why, it's super useful to have some landmarks, which are mostly uniform despite the overlap and individual variation. We'll start with the most superior dermatome, C2, and work our way caudally.

So, let's get started with the landmarks you need to know. For C2, we have the occipital protuberance and, for C3, we have the supraclavicular fossa. Jumping over, the shoulder usually falls under the C4 dermatome, and skipping down to the middle finger, we have C7. Now if we know C4 is at the shoulder and C7 is at the middle finger, we can guess where C5 and C6 will lie. C5 can be tested along the lateral aspect of the arm with C6 more distally at the thumb. Moving back up the arm, we start again at the middle finger at C7, move over to the little finger, and you have C8, and then up along the medial aspect of the forearm, you'll have T1. The next is T2, which leads right up to the axilla.

The trunk is a little bit more difficult because there's so much overlap and the dermatomes are so narrow. However, we have a couple of obvious landmarks. T4 is marked by the nipple and T10 is marked by the umbilicus, commonly known, of course, as the bellybutton. That makes it quite easy to work out that T3 must be between the level of the nipple and the axilla, and dermatomes T5 to T9 are distributed roughly evenly between the nipple and the umbilicus.

For the lower limb, there are two particular useful landmarks to remember. The first is the L2 dermatome with the hip as its landmark. The other is L5 associated with the big toe. So, from L2, we can continue down the anterior aspect at the lower limb to find the L3 dermatome at the knee and L4 at the ankle. From the big toe at L5, we move across to the little toe to find S1 then turning onto the posterior aspect at the lower limb, we can follow up to find the S2 dermatome at the popliteal fossa extending up onto the posterior thigh and S3 on the buttocks, with S4 and S5 continuing into the anal region.

Anteriorly, the landmarks for S2 and S3 are mainly to be found around the external genitalia. Extremely important to note is that when testing sensation using dermatome landmarks, you should always make sure to check the same spots on both sides of the body.

So, you might ask why bother with all this dermatome palava, if there's so much overlap and disagreement over it? Well, sensory testing of the skin using dermatomes is a common noninvasive method of examining the function of central and peripheral components of the nervous system. Of course, dermatomes are not used for making diagnoses on their own, but nevertheless, they still serve as an important tool for localizing neurological injury.

For example, if an area of paresthesia or numbness coincides with the territory of a cutaneous nerve then we're probably looking at a problem with the nerve supplying that area. However, if the paresthesia is more similar to the dermatome of a spinal nerve, that means the issue lies in the central nervous system or spinal nerve before it enters any plexus, and using the dermatome map, we can localize to a specific spinal level. Two reasons as to why existing dermatomes cannot be exclusively relied upon in clinical practice are that they do not account for general anatomic variation within the population, and secondly, they do not account for overlap between adjacent dermatomes.

Actually, let's make it three reasons. It has also recently been acknowledged that the cutaneous innervation of the skin has a somewhat dynamic nature. This means that in reality, dermatomes can expand and shrink depending on the anatomical and physiological condition of the adjacent spinal cord segments. Who knew?

Overall, the take-home message for today is that it's best not to religiously stick to the classical dermatome maps we've shown you today, remembering that none of them are perfect despite their popularity. It's best rather to use them only as a guide to get you started, remembering to take into account individual variation and overlap between adjacent dermatomes.

Alright, that's us done. We've made it to the end.

Before we say our final goodbyes, let's quickly remind ourselves of the major dermatome landmarks which are good to keep in mind in clinical practice. We started with the dermatomes of the face and saw that the forehead and the nose were innervated by the ophthalmic nerve, the maxilla by the maxillary nerve, and the mandible by the mandibular nerve, all of which are branches of the trigeminal cranial nerve. As we moved caudally, we saw dermatomes C2 to C4 on the back of the head and the neck, and T2 to T12 creating a simple stacked pattern on the torso. Noticeable landmarks here were T4 marked by the nipple and T10 by the umbilicus.

We then went on to look at the dermatomes of the limbs. We saw that the upper limb dermatomes receive innervation from C4 to T2 spinal segments. It is useful to remember that the shoulder is usually innervated by C4, the thumb by C6, the middle finger by C7, and the little finger by C8. The lower limb dermatomes are innervated by L2 to S3 to spinal segments with L2 at the hip, L5 at the big toe, and S1 at the little toe. The dermatomes S3 to S5 converge around the perianal area.

Alright, that's it peeps! Thanks for watching and see you next time!