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Dermatomes: Tips for navigating the pain puzzle.
The nerves that come out of the spine "pin" into specific areas of the skin, and then allow (among other things) the perception of sensory stimuli: touch, pressure, vibrations, hot/cold, and of course pain.
An important part of any examination is testing those areas of the skin known as dermatomes to ensure they are functioning properly. Injury to a nerve root, for example, compression caused by a herniated disc, can cause our sensations to be somewhat altered. Specifically, you may experience hyperesthesia (amplification of touch sensations), hypoesthesia (lack of touch sensation or weakened sensation), or you may feel pain.
While MRI and EMG/NCV tests remain the gold standard in diagnosing disc-related lumbar radiculopathy caused by a herniated disc or spinal canal stenosis (sciatica, radicular pain, stenosis), conducting a thorough interview with the patient regarding the precise location of radiating pain to the lower extremity is good clinical practice. This is because studies have shown that the presence of radicular pain, which is isolated to one (or even two) dermatomes (skin area in the lower extremity mainly supplied by a single spinal nerve root), is more diagnostically accurate for radiculopathy than conclusions drawn from muscle strength testing, changes in sensation, changes in tendon reflexes, or a simple straight leg raise test (Lasegue's test). [1-3]
The drawing next to it is an example of the right L5 dermatome (green color) in a patient suffering from radicular pain from a large right paracentral disc herniation at the L4 level. The L5 spinal nerve roots reach the skin on the side of the lower leg (shin) and on the back of the foot. This will be discussed further below. The lateral side of the right thigh and right buttock are usually also affected, although this is not shown.
Therefore, it is extremely important for a doctor/therapist to know the dermatomes of the lower and upper limbs, as sooner or later a patient with radiculopathy will come into the office.
Studies on dermatomes
Although the earliest mapping of dermatomes was done in the 19th century, most dermatome charts hanging on the walls of many medical offices today use data collected in 1913 and 1968.
The first well-designed study on dermatomes was completed in 1948 by Keegan et al., who carefully recorded skin pain patterns in patients with surgically confirmed disc herniation. 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 disc herniation and it compresses the nerve root moving below the segment, so a disc herniation in the L4 area will compress the moving nerve root of L5 and usually affects the L5 dermatome.) Scientists found that 93% of the group actually had radicular pain, which isolatedly affected one (rarely two) dermatomes.
Using previously reported dermatome maps, they found that pain in the S1 dermatome was the result of expected L5 disc herniation in only 63% of cases. Thirty-four percent of cases were unexpectedly from L4 herniation. Pain in the L5 dermatome was the result of expected L4 disc herniation in 80% of patients. In the remaining 20%, it came from L5 disc herniation.
Disc herniations in the upper lumbar spine (L2 and L3) caused correct skin root pain in only 10% of patients! In 35% of cases, root 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 - herniation of upper level discs is 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 a nerve root (the one considered to be implicated in their radicular pain) blocked/paralyzed by 1.5 ml of Xylocaine, which was delivered through a transforaminal selective nerve root block. The appropriate distribution of numbness was drawn using a marker (mapped) on the entire lower limb.
These are the results of the study (as of 2013) that present the mapping of the dermatome 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 problem is occurring in the spine.
LUMBAR RADICULAR PAIN S1
If the L5 disc herniation enters the lateral recess (which applies to most L5 disc herniations) and compresses the passing S1 nerve root, the patient may suffer from S1 radicular 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 radicular pain. As seen, the majority of patients (75%) experience burning, stabbing, and nagging pain of sciatica in the lateral, outer part of the lower part of the foot, back-lateral part of the lower leg, thigh, and buttock.
S1 radicular pain results from 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 portion (part of the nerve root that connects to the muscle) of the S1 nerve root is damaged or irritated by the disc herniation, the patient may experience weakness or atrophy of the calf muscles (gastrocnemius muscle), peroneal muscles (foot inverters), or muscles that flex or extend the big toe.
The Achilles tendon reflex and Babinski sign (plantar reflex) may also be diminished or absent in cases of S1 radicular pain, however, in nearly half of the cases, information from testing tendon reflexes may be inaccurate.
L5 ROOT PAIN
If the L4 disc herniates into the lateral recess, which is by far the most common type and level of disc herniation, [2] and compresses/irritates the L5 nerve root, then the patient may suffer from L5 radiculopathy (other terms: L5 root pain, sciatica L5).
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 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 experience pain in the back-lateral thigh and buttock.
As mentioned above, root pain results from axon irritation as a result of compression and inflammation caused by disc herniation.
If the motor (movement; nerve root part that connects to the muscle) part of the L5 nerve root is damaged by a disc herniation, the patient will experience weakness of the extensor hallucis longus muscle or muscles that lift the foot upwards. If the loss of muscle strength is severe, the patient may experience a symptom called foot drop, which occurs during walking. Specifically due to weakness of the dorsiflexor muscles of the foot, the patient will not be able to lift the foot high enough to avoid scraping it on the ground while walking. Other times, the foot will drop to the ground when placing it on the ground, as the dorsal part of the foot muscles is not strong enough to slow down foot drop.
As for testing reflexes, there is no reflex associated with the L5 nerve root.
L4 ROOT PAIN
If the L3 disc slips into the lateral recess and compresses, causing inflammation of the descending L4 nerve root, the patient may suffer from L4 radiculopathy (also known as L4 radicular pain or sciatica).
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 of sciatica 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 anterior side.
If the motor component (the part of the nerve root that connects to the muscle) of the L4 nerve root is damaged or irritated by a herniated disc, the patient will experience weakness of the quadriceps muscle (the muscles that extend the leg at the knee). If the irritation is severe, the patient may not be able to squat or rise from a chair.
In severe cases, the patient may have reduced or absent knee jerk reflex (reflex straightening of the leg at the knee joint in response to a tap on the patellar tendon below the kneecap). However, the knee jerk reflex may not be reliable as confirmation of radiculopathy and its validity is often questioned.
L3 radicular pain usually occurs on the front part of the thigh and may descend in a circular manner, 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 is at risk of inaccuracy. On the other hand, patients' complaints of pain along a dermatome are surprisingly accurate in suspecting the presence of intervertebral disc herniation or stenosis causing radiculopathy. Therefore, it is worth mapping the pain pattern reported by the patient on a diagram during each consultation.
Literatura:
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.