Dermatomes: tips for navigating the pain puzzle
Nerves that emerge from the spine "plug" into specific areas of the skin, allowing (among other things) the perception of sensory stimuli: touch, pressure, vibrations, heat/cold, and of course pain.
An important part of any examination is testing those areas of the skin that are called dermatomes to ensure they are functioning properly. Injury to a nerve root, such as compression caused by a herniated disc, can cause our sensations to be somewhat erratic. Specifically, you may experience hyperalgesia (increased sensitivity to touch), hypoalgesia (decreased sensitivity to touch or a dulled sensation), or you may feel pain.
Although MRI and EMG/NCV tests remain the gold standard in diagnosing discogenic lumbar radiculopathy caused by a herniated disc or spinal canal stenosis (sciatica, radicular pain, stenosis), a thorough interview with the patient regarding the precise location of radiating pain to the lower limb is good clinical practice. This is because studies have shown that the presence of radicular pain isolated to one (or even two) dermatomes (an area of skin in the lower limb mainly supplied by a single spinal nerve root) is diagnostically more accurate for radiculopathy than conclusions drawn from muscle strength testing, sensory changes, changes in tendon reflexes, or a simple straight leg raise test (Lasegue's test).
The drawing above is an example of the right L5 dermatome (green color) in a patient suffering from radicular pain from a large right paracentral herniation at the L4 disc level. The L5 spinal nerve roots supply the skin on the side of the lower leg (shin) and the top of the foot. This will be discussed further below. The lateral side of the right thigh and the right buttock are also typically affected, although this is not shown.
Therefore, it is extremely important for a doctor/therapist to be familiar with the dermatomes of the lower and upper limbs, as sooner or later a patient with radiculopathy will enter the clinic.
Studies on dermatomes
Although the earliest mapping of dermatomes was done in the 19th century, [6] most dermatome charts hanging on the walls of many medical offices today use data collected in 1913 [4] and 1968. [5]
The first well-designed study on dermatomes was completed in 1948 by Keegan et al. [7], who carefully noted the pain patterns of the skin in patients with surgically confirmed disc herniations. Unfortunately, these studies were essentially ignored by clinicians at that time.
In 1985, Kortelainen et al. [2] published the results of preoperative studies on 336 patients who underwent spine surgery for intervertebral disc herniation at all levels of the spine (but mainly at L4 and L5). (* Please note that the most common cause is a disc herniation pressing on a nerve root below the segment, so a disc herniation at the L4 level will compress the moving L5 nerve root and usually affect the L5 dermatome.) Researchers found that 93% of the group actually had radicular pain that isolatedly affected one (rarely two) dermatomes.
Using previously reported dermatome maps, they discovered that pain in the S1 dermatome was a result of an expected L5 disc herniation in only 63% of cases. Thirty-four percent of cases were unexpectedly from an L4 herniation. Pain in the L5 dermatome resulted from an expected L4 disc herniation in 80% of patients. In the remaining 20%, it originated from an L5 disc herniation.
Disc herniations in the upper lumbar spine (L2 and L3) caused proper radicular skin pain in only 10% of patients! In 35% of cases, radicular pain occurred in both the L5 and S1 dermatomes. * It should be noted that these results were based on a very small number of patients (N = 10), which is statistically meaningless - disc herniations at upper levels are quite rare.
In 1993, Nitta et al. [8] published the results of a very well-designed study in which fluoroscopic nerve root blocks were used to map the three lowest dermatomes (L4, L5, S1). Specifically, each patient had the nerve root (the one considered involved in their radicular pain) blocked/paralyzed by 1.5 ml of Xylocaine delivered through transforaminal selective nerve root block. The appropriate distribution of numbness in the skin was drawn with a marker (mapped) across the entire lower limb.
These are the results of the study (as of 2013), which illustrate dermatome mapping in the lower limb. Note the variability of these dermatomes. It is important to understand that not all patients will experience radicular pain in exactly the same part of the leg. In fact, dermatomes only give clinicians a rough idea of where the spine issue may be occurring.
S1 ROOT PAIN
If the L5 disc herniation enters the lateral recess (which applies to most L5 disc herniations) and compresses the passing S1 nerve root, then the patient may suffer from S1 root pain (also known as S1 radicular pain or sciatica S1).
Figure #4 shows the regions in the lower limb where the patient is most likely to experience symptoms of S1 root pain. As seen, the majority of patients (75%) suffer from a burning, stabbing, and throbbing pain of sciatica in the lateral, outer part of the lower part of the foot, posterior-lateral part of the lower leg, thigh, and buttock.
S1 root pain is the result of damage to the axon and death of small unmyelinated C fibers, which are contained in the transverse nerve root and manifest in the S1 dermatome.
If the motor part (the part of the nerve root that connects to the muscle) of the S1 nerve root is damaged or irritated by the disc herniation, then the patient may experience weakness or atrophy of the muscles in the lower leg (calf muscle), peroneal muscles (foot inverters), or the muscles that flex or curl the big toe.
The Achilles tendon reflex and Babinski sign (plantar reflex) may also be decreased or absent in cases of S1 root pain, however, in almost half of the cases, information from testing tendon reflexes will be inaccurate.
L5 ROOT PAIN
If the L4 disc protrudes into the lateral recess, which is the most common type and level of disc herniation [2], and compresses / inflames the transversely running L5 nerve root, then the patient may suffer from L5 radiculopathy (other terms: L5 root pain, L5 sciatica).
Figure #5 shows the regions in the lower limb where the patient is most likely to experience symptoms of L5 root pain. As seen, the majority of patients (75%) experience a burning, stabbing, or dull pain of sciatica in the upper and inner part (dorsum) of the foot and on the outer front part of the shin (lower leg). Only 25% of patients feel pain in the back-lateral thigh and buttock.
As mentioned above, root pain is a result of axon irritation as a result of compression and inflammation caused by disc herniation.
If the motor (movement; part of the nerve root that connects to the muscle) part of the L5 nerve root is damaged by disc herniation, the patient will experience weakness in the muscle that lifts the big toe (extensor hallucis longus) or the muscles that raise the foot upwards. If the loss of muscle strength is severe, the patient may experience a symptom called foot drop, which occurs while walking. Specifically, due to the weakness of the muscles that raise the foot (dorsiflexors of the foot), the patient will not be able to lift the foot high enough to avoid dragging it on the ground while walking. At other times, the foot will drop to the ground when placing the foot on the ground, because the dorsal part of the foot muscles is not strong enough to slow down the dropping of the foot.
Regarding tendon reflex examination, there is no reflex associated with the L5 nerve root.
L4 radicular pain
If the L3 disc protrudes into the lateral recess and compresses and inflames the descending root of the L4 nerve, the patient may suffer from L4 radiculopathy (also known as L4 radicular pain or sciatica L4).
Figure #6 shows the areas in the lower limb where the patient is most likely to experience symptoms of sciatica. As seen, the majority of patients (75%) experience a burning, stabbing, or dull pain in the anterior and inner part of the lower limb. Twenty-five percent of patients will feel pain in the thigh, calf, and foot on the front side.
If the motor part (the part of the nerve root that connects to the muscle) of the L4 nerve is damaged or irritated by a disc herniation, the patient will experience weakness in the quadriceps muscle (the muscles that straighten the leg at the knee). If the irritation is severe, the patient may not be able to squat or rise from a chair.
In cases of severe problems, the patient may have reduced or absent knee reflex (reflex extension of the leg at the knee joint due to a tap on the tendon of the quadriceps muscle below the kneecap). However, the knee reflex may be unreliable as confirmation of radiculopathy and its validity is often questioned.
L3 radicular pain typically occurs on the front part of the thigh and may descend circumferentially wrapping around from the back of the buttocks. Interpretation difficulties may arise here - pain occurring in the front or front-lateral, and even front-medial part of the thigh may originate from structures such as the hip joint (acetabulum), facet joints (in the spine), sacroiliac joints, and from damage within any of the lumbar discs.
SUMMARY:
One should not rely solely on dermatomes in predicting the exact level of disc herniation or the level of stenosis, as this approach carries the risk of inaccuracy. On the other hand, patient complaints of pain along a dermatome are surprisingly accurate in suspected presence of intervertebral disc herniation or stenosis resulting in radiculopathy. Therefore, it is worth mapping the pain scheme reported by the patient on a diagram during the consultation with each patient.
Reference:
1) Hancock MJ, Koes B, Ostelo R, Peul W. Diagnostic accuracy of the clinical examination in identifying the level of herniation in patients with sciatica. Spine 2011; 36:E712-E719.
2) Kortelainen P, Puranen J, Koivisto E, et al. Symptoms and signs of sciatica and their relation to the localization of the lumbar disc herniation. Spine 1985; 10:88-92.
3) Al Nezari NH, et al. Neurological examination of the peripheral nervous system to diagnose lumbar spinal disc herniation with suspected radiculopathy: a systematic review and meta-analysis. Spine J 2013;13:657-674.
4) Foerster O: "Zur kenntniss der spinalen segmentinnervation der muskelin." Neurol Zbl 32:1202-1214, 1913
5) Uihlein A, et al. "neurologic changes, surgical treatment, and post operation evaluation. Symposium: Low back and sciatic pain." J Bone Joint Surg 50A:1, 1968
6) Bolk L. "Die Segmentaldifferenzigrung des menschlichen Rumpfes und seiner Extremitaten." morphol Jahrb 1898 - 1899; 25:465-543; 26:91-211; 27:630-711;28:105-46
7) Konstantinou K, Dunn KM. "Sciatica: review of epidemiological studies and prevalence estimates." Spine 2008;33:2464-2472.
8) Nitta H, et al. "Study on dermatomes by means of selective lumbar spinal nerve root block." Spine 1993;18:1782-1786.
9) Taylor CS, Coxon AJ, Watson PC, et al. Do L5 and S1 nerve root compressions produce radicular pain and a dermatomal pattern? Spine 2013;38:995-998.
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