Neck and abdominal muscle activity in patients with severe thoracic scoliosis

Patients with severe chronic obstructive pulmonary disease (COPD) do not use the sternocleidomastoid muscles when breathing at rest, but have a greater than normal neural drive to the rib-cage inspiratory muscles, the abdominal muscles, and the diaphragm. Yet the increased activation of the abdominal muscles and diaphragm in such patients has only limited mechanical effects, and this has led to the suggestion that the overall increase in neural drive is simply an automatic response of the respiratory system to a greater than resting stimulation. To test this hypothesis, we examined the pattern of respiratory-muscle activation in eight patients with severe thoracic scoliosis (Cobb angle between 100 degrees and 136 degrees). We recorded electromyograms of the sternocleidomastoid, scalene, rectus abdominis, external oblique, and transversus abdominis muscles; esophageal (Pes) and gastric (Pga) pressures; and the anteroposterior (AP) diameter of the abdomen during resting breathing in the seated posture. All patients had invariable phasic inspiratory activity in the scalenes; and five patients had invariable phasic expiratory activity in the transversus; intermittent expiratory activity in the transversus was also recorded in three patients. In contrast, only one patient had invariable phasic inspiratory activity in the sternocleidomastoid, and only one patient had invariable phasic expiratory activity in the external oblique. The decrease in abdominal AP diameter during expiration was commonly associated with a rise in Pga. These observations therefore indicate that the pattern of respiratory-muscle activation in patients with severe thoracic scoliosis is essentially similar to that seen in patients with severe COPD. This supports the concept that the order of recruitment of the respiratory muscles during breathing is an automatic response of the central controller.