Reasons for performing study: The multifidus muscle plays a key role in spinal stabilisation. Multifidus atrophy ipsilateral to the side of osseous pathology has been demonstrated in man and pigs but has not been investigated in horses.
Objectives: To measure cross-sectional area (CSA) left/ right symmetry of equine multifidus ultrasonographically and relate asymmetry of multifidus with osseous spinal pathology in Thoroughbred racehorses. We hypothesised that ipsilateral multifidus CSA would be reduced when osseous pathological changes are present leading to left/right asymmetry in CSA.
Materials and methods: Twenty-two racehorses presented for euthanasia for primary reasons other than back pain were examined clinically. Ultrasonographic images on left/right sides were acquired at 5 thoracolumbosacral levels and CSA's of multifidus or sacrocaudalis dorsalis calculated. At necropsy, osseous pathological of the TL spine and pelvis were recorded by spinal level, anatomical site, and graded (0–3) according to severity. The mean typical measurement error in estimating multifidus/sacrocaudalis dorsalis CSA was used to determine the significance of left/right asymmetries. An association between multifidus CSA asymmetry and asymmetrical grading of pathological lesions was sought using Pearson's χ2 analysis.
Results: All horses had significant left/right asymmetry of multifidus CSA at >2 spinal levels, most commonly at L5 with total of 74 sites affected (22 horses). Seventeen horses had severe (grade 3) pathology, 16 of these had ipsilateral atrophy of multifidus/sacrocaudalis dorsalis. There was a significant association between pathological grade and degree of multifidus asymmetry.
Conclusions: Severe osseous pathological changes were common in this population of Thoroughbred racehorses and were associated with measurable left/right asymmetry in multifidus at or close to the level of pathology.
Potential relevance: Ultrasonography of multifidus may be a useful clinical tool in diagnosis of back problems in horses.