Neurotrauma Review Series Part 3: What’s in a Dermatome?

• Define the word dermatome.
• Describe the locations of the four dermatomes discussed in the article.
• Explain why cord transection or delayed damage from swelling to the regions of the C4 or T1 dermatomes may create an acute crisis for patients.

In this, the third in a series of articles on neurological injuries, we talk about using the dermatomes to help us to improve our assessment and diagnosis of neurotrauma patients.

Our skin contains lots and lots of touch, vibration, position, pain and temperature sensors that help us perceive the world around us. To do their job, those sensors must be connected to the spinal cord in much the same fashion as an electrical outlet is connected to a fuse box. In the body, the connecting "wires" are nerves, which are also called sensory neurons.

Once they enter the spinal cord, sensory neurons run together in a bundle of nerves called a tract, which goes to the brain.¹ Luckily for us sensory neurons don't grow randomly around the body. Instead they are arranged in a striped pattern which looks very much like the stripes of a zebra. The striping pattern is known as a dermatomal pattern, and each stripe is called a dermatome. The neat thing about dermatomes is that they are fairly discrete, meaning there is really only one sensory neuron per dermatome. Because of this, finding a defect in sensation at a dermatome can tell us about damage at a specific level of the spinal cord.

Preparing for 2 a.m.

The dermatomes and nerves they give information to are both named for the portion of spinal cord where the nerves connect. For instance, the nerve entering the spinal cord just below the T4 vertebra is called T4, and its dermatome is also called T4. In this article we will talk about four important dermatomes: C4, T4, T10 and L1 (Figure 1). I picked these four dermatomes for discussion because they are useful for estimating the spinal cord level of an injury. They are also easy to remember and find even at 2 a.m. (The 2 a.m. rule: Nobody is at their smartest at 2 a.m., but we still have to give good care.)

The first of the four dermatomes is C4. The C4 dermatome is at the level of the clavicles and makes a necklace around the neck, beginning and ending at the fourth cervical vertebral bone. Remember it by thinking, C may be 4 cookie, but when we're talking spinal cord injury, C is 4 clavicles.

The next is T4. The T4 dermatome is at the nipple line. Remember: T is 4 tips of the nipples.

The third is T10. The T10 dermatome is at the umbilicus. If you think of the 0 in T10 as looking kind of like a cute little lint-free umbilicus, this one is easy to remember too.

L1 is the last, and it is at the inguinal line (right where the top half of your orange Speedo tan line will be if you decide to be a lifeguard this summer). Remember: L1 is Lifeguard 1.

So, we have C4 clavicles, T4 tips, T10 at the umbilicus and L1 for Lifeguard 1. They're just four dermatomes. And they're easy. Even at 2 a.m.

Why is level important? In cases where trauma causes complete cord transection, all sensory and motor function (except for reflex arcs) below the level of the injury is lost. In these cases dermatomes are interesting, and the ability they give you to estimate where the spinal cord is injured can make you feel smart, but the damage has been done. You are going to treat your patient symptomatically. Take care of the ABCs. Intubate and ventilate if he is apneic. Give fluids and vasopressors if she is hypotensive.

As we well know, however, not every neurotrauma patient has a completely transected spinal cord. In the last article we noted there are different severities of spinal cord injury, and I discussed the common occurrence of spinal cord swelling following an injury. In cases where there is indirect injury or incomplete transection, swelling can cause a patient who originally presented to us as traumatized but stable to deteriorate swiftly and significantly right on our cot. In these cases using our knowledge of dermatomes to estimate the level of injury can help us anticipate what will happen next.

C is for Clavicles

We'll start our look at how to use the dermatomes with dermatome C4. When I find my trauma patient has sensory deficit in her legs, arms and thorax all the way up to her clavicles (the level of C4), I get goose bumps on my arms. This is because sensory nerves are not the only nerves located in this region of the cord. As in all regions of the spinal cord, there are motor nerves present here as well (Figure 2). Even if they haven't been damaged yet when we first assess our patient, swelling in the spinal canal might damage these motor nerves during the next minutes and hours. Although cervical spinal cord damage often causes quadriplegia, our patient's limb paralysis is not what should scare us the most. What should scare us is that patients with injury at C3, 4 and 5 very often become apneic.

Remember the famous mnemonic C3, 4 and 5 keep your diaphragm alive. It refers to your phrenic nerves. The phrenic nerves are two nerves (one for each side) that descend from the cervical spinal cord to innervate the diaphragm so it can move and help with breathing. The roots of the phrenic nerves are in the spinal cord just above vertebral bones C3, C4 and C5. Trauma at or near the C4 level may not only damage the phrenic nerves and so paralyze or partially paralyze the diaphragm, but because they lie farther down the spinal cord, the intercostal nerves that tell the rib cage to move will be damaged as well.

Patients with this kind of damage develop severe dyspnea or apnea, need emergent ventilation and ideally should be intubated. Any indication of trauma at this spinal level, whether it is spinal deformity or tenderness, decreased (or absent) sensation below the area of the C4 dermatome, or decreased (or absent) movement below the dermatome, should set off all our warning systems and cause us to anticipate the need for advanced airway control.

An aside about the intercostals and thoracic spine: The intercostal nerves, which innervate the intercostal muscles and allow the rib cage to assist with respiration, are associated with the T1–T12 vertebrae. (This makes sense, as we humans have 12 ribs and each rib is connected to a thoracic vertebra.) Patients with damage between the T1 and T12 vertebrae can lose the function of the intercostal muscles, but because they haven't damaged their phrenic nerves, they retain use of the diaphragm.

Unlike cervical spinal injury patients, thoracic spinal cord injury patients do not initially present with apnea. If the damage is at the upper parts of the thoracic spinal cord, however, these patients do eventually tire of using their diaphragm exclusively to breathe and will eventually need ventilatory assistance. CPAP is sometimes adequate to assist these diaphragm-weary patients, but often they will need support via BVM. In systems that allow it, RSI is a good option for these patients, particularly in the face of long transport or flight times. These patients will present with intact movement and sensation to most of the regions of their arms because the nerve roots for the brachial plexus, which innervates the arms, are in the cervical, not the thoracic spine. However, these patients will have decreased or absent movement and sensation below the level of the axilla.

T is for Tips of the Nipples

The next easy-to-pick-out dermatomal landmark is T4. T4 correlates with the nipple line. There are two reasons why spinal cord damage in the region of T4 worries me. The first is the risk of losing innervation to the intercostal nerves, discussed above. The second is the risk of losing innervation to the sympathetic nerves.

The sympathetic nervous system nerves exit the spinal cord between T1 and L2 (Figure 1). Because of this, any spinal cord damage between T1 and L2 can cause damage to the sympathetic nerves. Our sympathetic nerves function to increase our heart rate, constrict our blood vessels and dilate our bronchi. Sympathetic nerve damage keeps the body from performing these activities below the level of injury. Severe spinal cord damage at or above T1 will stop the signaling to all the sympathetic nerves and so can cause neurogenic shock (also known as spinal shock).

Because the sympathetic nerves are not working anymore in neurogenic shock, massive vasodilation occurs throughout the body. This causes hypotension, which can become quite severe in the setting of hemorrhage from other injuries. This hypotension is often refractory to even large fluid boluses. To maintain blood pressure in these patients, the use of vasopressors in addition to IV fluids is often needed.

Normally the heart rate rises in response to hypotension; however, in cases of neurogenic shock, because the sympathetic innervation for the heart comes from spinal nerves T4–5, the heart does not respond. Patients with this syndrome will often present with wheezing if the damage is at or above the region of the carina, because the sympathetic stimulation that keeps the bronchi open has been lost. Gradations of this syndrome happen if damage is at a lower level of the spinal cord, and are striking because upon exposing the patient, we can see an actual dermatomal line, above which skin still having intact sympathetic innervation will be pale and clammy and below which skin lacking sympathetic innervation will be warm, pink and dry.

For example, damage at T10 may cause cool, pale, diaphoretic skin above the umbilicus, where innervation is still intact and the body is compensating for shock by shunting blood away from the skin and toward the core. The same patient will have vasodilation and warm, dry skin below the level of the umbilicus even in situations where multitrauma has caused hemorrhage or hypotension.

The Umbilicus 0 and Lifeguard 1

When we find signs of spinal cord damage that is isolated at or below the T10 dermatome, we should breathe a relative sigh of relief. Our patient may still be quite ill; however, her spinal injury will probably not lead to the critical ventilatory compromise that occurs with higher spinal cord injury. Although the T10, T11 and T12 spinal nerves innervate intercostal muscles, spinal damage at this level leaves the majority of the nerves to the rib cage intact and working. T10-injured patients may develop hypotension that needs our attention, but this hypotension is not as severe as with higher cord injury. Damage to the cord at or below L1 has even fewer acute systemic effects. Although these injuries may still cause tragic paraplegia, their systemic effects do not normally cause patients (or paramedics) to become unstable.

Conclusion

Spinal cord injuries occur with infinite variations in cause, location and severity, and because we do not yet have MRI sunglasses, we can never be sure whether a spinal injury is still evolving or has reached its final stage of severity when we first assess our patient. For this reason, performing the best physical examination we can, and performing it serially (every 5 to 10 minutes for the duration of our care), is of paramount importance. Adding a knowledge of dermatomes to our standard neurologic exam (see February's Part 2: "What Should a Neurologic Exam Entail?") helps us give that great assessment. Not only does documentation of a great assessment of both motor and sensory systems help us to work as a team with ED physicians, neurologists and rehabilitation therapists downstream, but it helps us anticipate a patient who is becoming unstable and gives us time to intervene.

Footnote

1. In the last article we discussed two nerve tracts, the lateral corticospinal tract, a downward-going tract for motor nerves, and the anterolateral spinothalamic tract, an upward-going tract for sensory nerves, and reviewed the idea that neurological injury due to swelling takes place over time.

• Tintinalli JE, et al. Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 6^th ed. McGraw Hill, 2004.
• Noback CR, Strominger NL, Demarest RJ, Ruggiero DA. The Human Nervous System: Structure and Function, 6^th ed. Humana Press, 2005.
• Cherry R. Neurotrauma lecture, SUNY Upstate Medical University, 1997.
• Blumenfeld H. Neuroanatomy Through Clinical Cases. Sinauer Associates, 2002.

Tiffany Bombard, NREMT-P, MD, has been an EMS provider, firefighter and paramedic for many years in Vermont, Utah, New York, New Hampshire and Maine. She is currently a resident emergency physician at Albany Medical Center and a paramedic for the Albany County Sheriff's Office in New York. She loves mail. Write to her with questions or suggestions at bombieskifast@yahoo.com.

Remember:

C is for Clavicles,

T is for Tips,

T10 circles the umbilicus (sans lint), and

L1 is Lifeguard 1's Speedo line.

Trauma at or near the C4 level may not only damage the phrenic nerves and so paralyze or partially paralyze the diaphragm, but because they lie farther down the spinal cord, the intercostal nerves that tell the rib cage to move will be damaged as well.

When we find signs of spinal cord damage isolated at or below the T10 dermatome, we should breathe a relative sigh of relief.

To maintain blood pressure in these patients, the use of vasopressors in addition to IV fluids is often needed.