Dermatome variation of lumbosacral nerve roots in patients with transitional lumbosacral vertebrae

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.

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.

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.

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.