Cortical Representations of Transversus Abdominis and Multifidus Muscles Were Discrete in Patients with Chronic Low Back Pain: Evidence Elicited by TMS

Abstract

Introduction: The transversus abdominis (TVA) and multifidus (MF) muscles are the main segmental spinal stabilizers that are controlled by the primary motor cortex of the brain. However, relocations of the muscle representation in the motor cortex may occur after chronic lower back pain (cLBP); it still needs more evidence to be proven. The current study was aimed at applying transcranial magnetic stimulation (TMS) to investigate the changes of representation of TVA and MF muscles at the cortical network in individuals with cLBP.

Methods: Twenty-four patients with cLBP and 12 age-matched healthy individuals were recruited. Responses of TVA and MF to TMS during muscle contraction were monitored and mapped over the contralateral cortex using a standardized grid cap. Maps of the center of gravity (CoG), area, volume, and latency were analyzed, and the asymmetry index was also computed and compared.

Results: The locations of MF CoG in cLBP individuals were posterior and lateral to the CoG locations in healthy individuals. In the healthy group, the locations of TVA and MF CoG were closed to each other in both the left and right hemispheres. In the cLBP group, these two locations were next to each other in the right hemisphere but discrete in the left hemisphere. In the cLBP group, the cortical motor map of TVA and MF were mutually symmetric in five out of eleven (45.5%) subjects and leftward asymmetric in four out of ten (40.0%) subjects.

Conclusions: Neural representations of TVA and MF muscles were closely organized in both the right and left motor cortices in the healthy group but were discretely organized in the left motor cortex in the cLBP group. This provides strong support for the neural basis of pathokinesiology and clinical treatment of cLBP.

Figure 1

The standard 1 × 1 cm grid cap. (a) Schematic diagram. L: 6 × 7 cm grid; R: 6 × 7 cm grid. (b) Rendering figure. The red band is the primary motor cortex. The two black dots on the ventral side are the left and right dorsolateral frontal lobes. The black dot on the dorsal side is at 1 cm behind the central zero point.

Figure 2

Average representative location of TVA and MF responses to TMS for the healthy and cLBP groups on the left and right hemispheres. cLBP: chronic low back pain; L: left; R: right; TVA: transversus abdominis; MF: multifidus; CoG: center of gravity. Note: this figure was generated from the average data of all the patients. The posterior aspect of the map appeared to be not recorded fully over the entire cortical representation of the muscle. This was due to two cortical motor maps of cLBP patients located more posteriorly. The CoG of TVA and MF from all other participants' muscles in cLBP patients were within the region of the grid cap.

Figure 3

(a–c) Map area, map volume, and latency of TVA and MF MEP responses to TMS for the healthy and cLBP groups on the left and right hemispheres. (d) The relationship between TVA and MF in the left and right hemispheres of healthy and cLBP groups. The blue bubbles denote the CoGs of the muscles in the cLBP group, and the orange bubbles denote the CoGs of the muscles in the healthy group. cLBP: chronic low back pain; L: left; R: right; TVA: transversus abdominis; MF: multifidus; CoG: center of gravity.