Ipsilateral atrophy of paraspinal and psoas muscle in unilateral back pain patients with monosegmental degenerative disc disease

Objectives

The aim of this study was to assess the cross-sectional area (CSA) of both paraspinal and psoas muscles in patients with unilateral back pain using MRI and to correlate it with outcome measures.

Methods

40 patients, all with informed consent, with a minimum of 3 months of unilateral back pain with or without sciatica and one-level disc disease on MRI of the lumbosacral spine were included. Patients were evaluated with self-report measures regarding pain (visual analogue score) and disability (Oswestry disability index). The CSA of multifidus, erector spinae, quadratus lumborum and psoas was measured at the disc level of pathology and the two adjacent disc levels, bilaterally. Comparison of CSAs of muscles between the affected vs symptomless side was carried out with Student''s t-test and correlations were conducted with Spearman''s test.

Results

The maximum relative muscle atrophy (% decrease in CSA on symptomatic side) independent of the level was 13.1% for multifidus, 21.8% for erector spinae, 24.8% for quadratus lumborum and 17.1% for psoas. There was significant difference (p<0.05) between sides (symptomatic and asymptomatic) in CSA of multifidus, erector spinae, quadratus lumborum and psoas. However, no statistically significant correlation was found between the duration of symptoms (average 15.5 months), patient''s pain (average VAS 5.3) or disability (average ODI 25.2) and the relative muscle atrophy.

Conclusion

In patients with long-standing unilateral back pain due to monosegmental degenerative disc disease, selective multifidus, erector spinae, quadratus lumborum and psoas atrophy develops on the symptomatic side. Radiologists and clinicians should evaluate spinal muscle atrophy of patients with persistent unilateral back pain.Paraspinal and trunk muscles play an important role in the kinetics and balance of the lumbar spine. They are considered as dynamic stabilisers applying their working force by providing stability to the spine–pelvis complex and motion to the spinal units. In addition, psoas is a significant hip flexor. Any decrease in the cross-sectional diameter (CSA) of these muscles could lead to loss of proper biomechanics and may be accompanied by the appearance of back pain [1-5]. Some authors have proposed that pain leads to a sedentary lifestyle and, furthermore, this creates extra muscle atrophy and pain, thus beginning a vicious cycle [6,7].In athletes with regular physical training, an increase in CSA of the paraspinal and trunk muscles has been demonstrated that reflects the improvement of muscle force and endurance [8]. In contrast, prolonged bed rest results in selective atrophy of the multifidus muscle whereas trunk muscles increase their CSA. The latter is probably the effect of shortening of muscle fibres or overactivity during bed rest [9].Many studies on paraspinal musculature have focused on the multifidus muscle because of its unique and segmental innervation [10]. A multifidus bundle''s unisegmental innervation always arises purely from the root exiting below the spinous process from which the fascicles originate, whereas in the other paraspinal muscles innervation is multisegmental. Several studies have demonstrated atrophy of multifidus following trauma, disc herniation or spinal nerve lesion by electromyographic, histological or radiographic measurements [1,11,12]. None of these studies focused on monosegmental degenerative disc disease.This study aims to examine the CSA of all muscles around the lumbar spine in patients with persistent unilateral back pain caused by monosegmental degenerative disc disease and correlate this with their symptoms and period of pain.