A broad and variable lumbosacral myotome map uncovered by foraminal nerve root stimulation

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  • 1 Department of Neurosurgery, NYU Langone Health, New York, New York;
  • | 2 Rutgers—New Jersey Medical School, Newark, New Jersey;
  • | 3 Neurological Surgery Professional Corporation, Rockville Centre, New York; and
  • | 4 Neurosurgical Associates of Lancaster, Lancaster, Pennsylvania
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OBJECTIVE

The human myotome is fundamental to the diagnosis and treatment of neurological disorders. However, this map was largely constructed decades ago, and its breadth, variability, and reliability remain poorly described, limiting its practical use.

METHODS

The authors used a novel method to reconstruct the myotome map in patients (n = 42) undergoing placement of dorsal root ganglion electrodes for the treatment of chronic pain. They electrically stimulated nerve roots (n = 79) in the intervertebral foramina at T12–S1 and measured triggered electromyography responses.

RESULTS

L4 and L5 stimulation resulted in quadriceps muscle (62% and 33% of stimulations, respectively) and tibialis anterior (TA) muscle (25% and 67%, respectively) activation, while S1 stimulation resulted in gastrocnemius muscle activation (46%). However, L5 and S1 both resulted in abductor hallucis (AH) muscle activation (17% and 31%), L5 stimulation resulted in gastrocnemius muscle stimulation (42%), and S1 stimulation in TA muscle activation (38%). The authors also mapped the breadth of the myotome in individual patients, finding coactivation of adductor and quadriceps, quadriceps and TA, and TA and gastrocnemius muscles under L3, L4, and both L5 and S1 stimulation, respectively. While the AH muscle was commonly activated by S1 stimulation, this rarely occurred together with TA or gastrocnemius muscle activation. Other less common coactivations were also observed throughout T12–S1 stimulation.

CONCLUSIONS

The muscular innervation of the lumbosacral nerve roots varies significantly from the classic myotome map and between patients. Furthermore, in individual patients, each nerve root may innervate a broader range of muscles than is commonly assumed. This finding is important to prevent misdiagnosis of radicular pathologies.

ABBREVIATIONS

AH = abductor hallucis; EMG = electromyography; SE = sphincter externus; TA = tibialis anterior; TOF = train-of-four neuromuscular junction test.

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