INTRODUCTION

Although the pathophysiology of adolescent idiopathic scoliosis (AIS) is not fully understood, the paraspinal muscles have been implicated as an important point in the cycle of events in the aetiology and progression of AIS. Histologic, and ultrastructural studies demonstrated the presence of paraspinal muscle asymmetry and morphologic changes (217, 242, 457). Real-time ultrasound imaging has shown a smaller cross-sectional area on the opposite side to the convexity of a primary thoracic curve in AIS (913). It is well established that MRI can detect evidence of acute and chronic muscle injury. (book) and changes in signal intensity on MRI have been demonstrated in paraspinal muscles after exercise (582). Increased resting signal intensity of the multifidus muscle was also described in patients with chronic low back pain. The objective of this study is to study the signal intensity of the multifidus muscle in AIS.

MATERIALS AND METHODS

51 patients with documented AIS were recruited. These were from three distinct groups. The first group consisted of 18 patients with severe or rapidly progressive curve planned for operative treatment. The second group consisted of 10 patients with abnormal results on somato-sensory evoked potential (SSEP) examinations. The third group consisted of 23 patients at their first presentation, selected for different degree of curve varying from mild to severe.

MRI examination was performed in a 1.5 T MR scanner, with a synergy spine coil. After whole spine sagittal T2 weighted sequence, and coronal T2 weighted sequence of the thoracic spine, axial STIR (short tau inversion recovery) sequence was obtained at the apex of the primary curve with the following parameters: TR 1400 msec, TE 15 msec, TI 140 msec, 4mm slice thickness with 0.8 mm gap, 20 slices, NEX 2, FOV 230 mm, 256x256 matrix. The axial images were evaluated for the relative signal intensities of the multifidus muscles on both sides.

RESULTS

16 of the 18 patients in group 1 with severe curve planned planned for operative treatment showed increase in signal intensity in the multifidus muscle on the concave side of the apex of the curve.

8 of 10 patients in group 2 with abnormal SSEP also demonstrated increased signal intensity in the concave side multifidus muscle at the apex. In group 2, of the 11 patients with mild curve (10-30 degrees), two had increased signal intensity in the multifidus muscle in the concave side; of the 12 with more severe curve, 9 had increase in multifidus signal intensity on the concave side.

DISCUSSION

Hyperintense signal could be demonstrated on STIR sequence on MRI in the concave side multifidus muscles in a significant proportion of patients suffering from AIS, with increasing prevalence with severity.

Hyperintense signal change in muscles on STIR sequence is a non-specific finding and may be seen in muscle injury, post-exercise-state, oedema, inflammation, and chronic muscle overuse syndrome.

The cause of hyperintense signal in exertional muscle injuries, as can be seen on STIR sequence, is the accumulation of lactate and the increased osmolarity which resulted in an increase in the water content of the muscle, causing a prolongation of the T2 relaxation time as well as increase in proton density (113). Increase in water content has not been documented in paraspinal muscles in AIS.

In muscle oedema and inflammation, the increase in signal intensity is due to increase in the amount of bulk water. Histological studies has revealed no evidence of inflammatory cellular infiltration in paraspinal muscles in AIS (457).

Prolongation of T2 and T1 relaxation time was also reported in Type I slow-twitch fibers, probably due to a greater extracellular water content of the slow-twitch fibers (113). It has been shown that at the apex of the curve in idiopathic scoliosis, there was preferential type II atrophy on the concave side and the type II fibers might be smaller on the concave side (242). More type II fibers was noted on the convex side (242). However, the effect of muscle fiber types on relaxation time is still controversial as it has also been shown that there is no correlation between the relative mass of type 1 or 2 fibers or nonmyofiber space and the relaxation times (16). To add further confusion, some histological studies in paravertebral muscles especially the multifidus muscle have shown a greater proportion of type I fibers in the erector spinae muscles on the convex side of a scoliotic curve (373, 457). The fiber type distribution, capillary count, and metabolic enzyme activity on the convex side resembles that seen after endurance training, suggesting a secondary adaptive origin of these changes. A higher EMG activity as noted on the convex side paraspinal muscle also suggest a compensatory mechanism.

On the other hand, pathologic changes including variation in fiber size, internal nuclei, vacuoles, target fibers, fiber splitting and type grouping have been found mainly on the concave side in idiopathic scoliosis (457.42). Khosla et al found ultrastructural changes in the myotendinous junction on the concave side and speculated that this can be of primary etiologic importance (457.22). In recurrent exertional or chronic muscle overuse syndrome, mild edema-like change is evident in muscle-tendon units on MRI (113).

This may bear resembalance to the mild oedema-like change evident in muscle-tendon units on MRI in chronic muscle overuse syndrome (113) and suggests a possible element of chronic muscle overuse in the concave side multifidus muscle in AIS. It is of interest that although the T2 relaxation time of multifidus muscle is longer that of the psoas muscles, the relaxation times of right- and left-sided muscles were similar (16) in cadavers not suffering from scoliosis.

Smaller cross-sectional of the multifidus muscle at L4 is noted on the convex side of a lumbar or thoraco-lumbar scoliotic curve (913). Joint pathology was noted on the convex side of curve and this may reduce activation of the muscle on the convex side, leading to wasting.

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