Study design: Case series.
Background: A clinical prediction rule (CPR) has been developed and validated that accurately identifies a subgroup of patients with low back pain (LBP) likely to benefit from spinal manipulation; however, the mechanism of spinal manipulation remains unclear. The purpose of this case series was to describe changes in lateral abdominal muscle thickness using rehabilitative ultrasound imaging (RUSI) immediately following spinal manipulation in a subgroup of patients positive on the rule.
Case descriptions: Data from 9 patients (5 female, 4 male; 18-53 years of age) with a primary complaint of LBP are presented. All patients had symptoms for less than 16 days (range, 3-14 days) and did not have symptoms distal to the knee, satisfying the 2-factor rule for predicting successful outcome from spinal manipulation. The Oswestry Disability Index scores ranged from 8% to 52%. Lateral abdominal muscle thickness was assessed with the patient at-rest and while contracted during an abdominal drawing-in maneuver (ADIM) using RUSI. Measurements were taken before and immediately after spinal manipulation. Patients completed a 15-minute training session of the ADIM prior to assessment, to mitigate the potential for a learning effect to occur.
Outcomes: Based on changes that exceeded the threshold for measurement error, 6 of 9 patients demonstrated an improved ability (11.5%-27.9%) to increase transversus abdominis (TrA) muscle thickness during the ADIM postmanipulation. Additionally, TrA muscle thickness at-rest postmanipulation decreased for 5 patients (11.5%-25.9%), while at-rest internal oblique muscle thickness decreased for 4 patients (6.4%-12.2%).
Discussion: This case series describes short-term changes in lateral abdominal muscle thickness post spinal manipulation. Although case series have significant limitations, including the fact that no cause-and-effect claims can be made, the decrease in muscle thickness at rest and the greater increase in muscle thickness during the ADIM postmanipulation observed in some of the patients could suggest an improvement in muscular function. Future research is needed to determine if increased muscle thickness is associated with improvements in pain and disability and to further explore neurophysiologic mechanisms of spinal manipulation.