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ANALGESIA

Dermatome Variation of Lumbosacral Nerve Roots in Patients with Transitional Lumbosacral Vertebrae

Yang Hyun Kim, MD^*, Pyung Bok Lee, MD^*, Chul Joong Lee, MD, Sang Chul Lee, MD, Yong Chul Kim, MD, and Jin Huh, MD

From the *Department of Anesthesiology and Pain Medicine, Seoul National University Bundang Hospital, Seoul, Department of Anesthesiology and Pain Medicine, Seoul National University Hospital, Seoul, and Department of Anesthesiology and Pain Medicine, Seoul Municipal Boramae Hospital, Seoul, Korea.

Address correspondence and reprint requests to Jin Huh, MD, Seoul Muncipal Boramae Hospital, Seoul, Korea. Address e-mail to huhjin510{at}yahoo.co.kr or amandla{at}empal.com.

Abstract

BACKGROUND: The presence of a transitional vertebra can create difficulty in identifying the lumbar level corresponding to an exiting nerve root at the time of a spinal nerve block. We investigated the possibility that the muscle innervation pattern and sensory dermatomes of the lumbar nerve roots are altered when a lumbosacral transitional vertebra is present using electrical stimulation.

METHODS: We determined the existence of transitional vertebrae using Castellvi’s criteria. Patients having transitional vertebrae with lumbosacral radiculopathy were recruited for the study. Selective nerve root blocks using electrical stimulation were performed. Neurologic symptoms caused by S1 or L5 nerve root compression in the patients with a lumbarized S1 or sacralized L5, respectively, were compared with those caused by either L5 or S1 nerve root compression in patients with a normal configuration.

RESULTS: Thirty-two patients had transitional vertebrae, of whom 12 had a lumbarized S1 and 20 had a sacralized L5. The distribution of motor and sensory symptoms caused by the lumbarized S1 (L6) nerve root stimulation was similar to that of the S1 nerve root stimulation in the normal configuration. In 17 patients, the distribution of motor and sensory symptoms caused by the sacralized L4 nerve root stimulation was similar to that of L5 nerve root stimulation in the normal configuration.

CONCLUSIONS: Our findings suggest that the function of the lumbosacral nerve roots is altered in patients with a sacralized L5, and that the L4 nerve root serves the usual function of the L5 nerve root.

Lumbosacral transitional vertebrae occur when the last lumbar vertebra has an elongated transverse process with varying degrees of fusion to the first sacral segment.1 Several studies have reported that the functions of the lumbar nerve roots can be altered in the presence of this anomaly.2,3 This is clinically important because the presence of a transitional vertebra can create confusion as to the lumbar level corresponding to a particular exiting nerve root at the time of an injection to treat radiculopathy.

Woff et al.4 reported that selective nerve root blocks failed to demonstrate uniform or distinct effects on sensory function, and the accuracy, specificity, and sensitivity of these blocks has been questioned. Current selective nerve root block techniques rely primarily on the relief of pain when a local anesthetic is injected. However, patient responses are often nonspecific, and the pain relief after injecting a local anesthetic is often difficult to interpret. Robert5 reported that stimulation with an electrical current can reproduce radicular pain, rather than relying on the response to the injection of a local anesthetic.

Therefore, using electrical stimulation, we examined the possibility that the muscle innervation pattern and sensory dermatomes of lumbar nerve roots are altered when a lumbosacral transitional vertebra is present. The result was compared with the standard table of segmental innervation via L4, L5, and S1 proposed by Marinacci and shown in Table 1.2

METHODS

Patients having transitional vertebrae with lumbosacral radiculopathy were recruited for this study. Selective nerve root blocks using electrical stimulation were performed. The study was approved by the ethics committee of the participating hospital and all patients gave informed consent.

We determined the existence of transitional vertebrae using Castellvi’s criteria (Table 2).6 The level of the T12 vertebra was determined by counting down the thoracic vertebrae on either the thoracic spine or chest radiographs. Needles were inserted into the foramina at the level that was suspected of causing pain, and at the levels above and below that level so that the patients could compare adjacent nerve roots.

For L4 stimulation, an electrode was placed adjacent to the nerve root in the intervertebral foramen. During fluoroscopy, anatomical placement of the electrode was controlled in the anteroposterior view so that the tip projected behind the upper lateral part of the facet column. The electrode tip was positioned in the dorsocranial quadrant of the intervertebral foramen using lateral fluoroscopic imaging. A thermocouple with a 10-cm electrode with a 5-mm active tip (22-guage, Radionics SMK-C10) was inserted. Once the needles were placed, 0.1–0.2 mL of contrast was injected until each nerve root was outlined.

To apply stimulation via the needles, a stimulator with an adjustable output regulator with the ability to generate pulses of at least 50 Hz (to stimulate sensory fibers) and 2 Hz (to stimulate motor fibers) was required. A tingling sensation in the corresponding dermatome had to be obtained at between 0.4 and 0.6 V.

Patients were asked whether the sensations were at the same location as their usual pain and were asked to point to the spot during the procedure. For example, a tingling sensation in the tibialis anterior and extensor hallucis suggests that the L5 root is involved, whereas a tingling sensation in the medial gastrocnemius suggests that the S1 root is involved. If fibrillation occurred in the tibialis anterior and extensor hallucis longus, this suggested that the L5 root was involved. Since the proximity of needles to the nerve root may differ, the current needed to produce the stimulation may vary. The stimulus must be applied evenly to each nerve root. Although the leg has many muscles, we found that the muscles listed in Table 1 were the easiest to examine electrically and the most likely to have a standard root supply. After selective L4 nerve root stimulation, L5 and S1 nerve root stimulations were performed as above. Once the procedure was finished, a mixture of local anesthetic and corticosteroid (triamcinolone 10 mg) was injected.

RESULTS

Thirty-two patients had transitional vertebrae, of whom 12 had a lumbarized S1 and 20 had a sacralized L5. They were type IIb and type IIIb in Castellvi’s classification (Table 2). In this report, we call a lumbarized S1 vertebra an L6 vertebra.

In 17 patients, the sacralized L4 nerve was similar to the L5 nerve root in the normal configuration. For example, when the L4 nerve root was stimulated, the patients pointed to where they felt a tingling sensation and fibrillation in the tibialis anterior, gastrocnemius lateral head, and extensor hallucis. The remaining three patients had an L4 similar to the L4 nerve root in the normal configuration. When the sacralized L5 nerve root was stimulated, we found a typically dominant S1 root in 13 patients. Five patients had a sacralized L5 similar to the L5. The remaining two patients overlapped between L5 and S1 (Table 3).

The distribution of the motor and sensory symptoms caused by the sacralized L5 nerve root stimulation involved either the typical dominant S1 dermatome or less frequently the L5 and S1 dermatomes in the normal configuration.

When the lumbarized L6 nerve root was stimulated, we found a typically dominant S1 root in seven patients. Five patients overlapped between L5 and S1. The distribution of the motor and sensory symptoms caused by stimulation of a lumbarized S1 (L6) nerve root was usually either the S1 dermatome or less frequently the L5 and S1 dermatomes in the normal configuration. The motor and sensory symptoms caused by L6 electrical stimulation were usually distributed to the gastrocnemius medial head, soleus, and abductor hallucis and less often to the extensor hallucis longus and tibialis anterior (Table 4).

DISCUSSION

The morphology of transitional lumbosacral vertebrae may vary. The common feature is an atypical lumbosacral articulation between the transverse processes of the inferior-most lumbar vertebrae and the sacrum.6–10 Estimates for the general population also vary greatly, but they are similar. The relationship between low back pain and lumbosacral transitional vertebrae is not clear, although several studies have ascribed low back pain to alterations at the aberrant articulations that are between the transitional vertebrae and sacrum.7,9,11 Consequently, physicians who treat lumbosacral symptoms should be aware of the high rate of transitional vertebrae. The alteration in nerve root innervation caused by a transitional vertebra makes it difficult to find the pain generator. Therefore, in patients receiving epidurals or selective nerve root blocks, one should be aware of a transitional segment before initiating the injection.

The common misleading features are short 12th ribs, giving the appearance of 6 lumbar vertebrae and long transverse processes of the first lumbar vertebra, simulating four lumbar vertebrae.12

Therefore, the presence of a lumbosacral transitional vertebra is best identified on a true 30° angled anteroposterior radiograph of the lumbosacral junction (Ferguson view), together with an anteroposterior view that includes the thoracolumbar junction to enable assessment of the vertebral level.13 Patients with type II or III transitional vertebrae according to Castellvi’s classification6 with lumbosacral radiculopathy were recruited for this study.

Seyfert3 examined the dermatome gap for transitional vertebrae using the cremasteric reflex and described the idea of a variant position of the lumbosacral dermatomes in the presence of transitional vertebrae in males. Seyfert found a variant position of the segmental sensory supply to the skin in male patients with a variant transitional vertebra. McCulloch and Waddell14 concluded that the functional L5 nerve root exits from the "last mobile level" of the lumbosacral segments in their study of 11 cadavers and 20 intraoperative recordings. They defined the last mobile level as the caudal-most lumbosacral segment that retains mobility. Therefore, according to their conclusion, the last mobile level in the normal configuration is usually L5–S1, and the functional L5 nerve root corresponds to the anatomic L5 nerve root. Conversely, in patients with a lumbarized S1, the last mobile level is usually L6–S2, and the functional L5 nerve root corresponds to the L6 nerve root. We did not detect this tendency in our intraprocedure stimulation study. We found that the neurologic symptoms caused by L6 nerve root compression resembled those of S1 nerve root compression, and a sacralized L5 root lesion presented with the features of S1. This means that when L5 is sacralized, the function of the L5 nerve root becomes more like that of the S1 nerve root. The most plausible explanation for this condition is that it occurs because the lumbosacral plexus is prefixed in these patients.15 A sacralized L5 nerve root contributes to the more caudal portion of the lumbosacral plexus. Other possible explanations include variation in root size.16 To our knowledge, no prospective study has estimated the pattern and frequency of the variation in the innervation of the L5 and S1 nerve roots with transitional vertebrae using electrical stimulation.

Our study had two limitations. First, the number of patients in our study was too small for us to draw a solid conclusion. Second, we did not consider significant segmental overlap of the innervation to a muscle and dermatome variation.

In conclusion, our findings suggest that the function of the lumbosacral nerve roots is altered in patients with a sacralized L5, and that the L4 nerve root serves the usual function of the L5 nerve root. Therefore, a full radiologic examination must be performed when transitional vertebrae are suspected before a lumbar epidural injection or selective nerve root block to prevent any intraprocedure confusion over level identification, and the likelihood of variable nerve root innervation should be considered. After identifying the nerve roots using electrical stimulation, injecting can be a useful diagnostic aid, as well as a therapeutic tool, in the patient with a transitional vertebra.

Footnotes

Accepted for publication October 8, 2007.

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