Exercise-related signal abnormality as an imaging entity
A significant proportion (10.8%) of MRI studies performed for suspected muscle injury in our cohort exhibited either a peritendinous ovoid region and/or a subfascial ring of faint increased signal on fluid-sensitive images in at least one muscle. Despite the relative frequency of these findings, these appearances have never previously been recognized as a distinct imaging pattern or entity in the literature, to the authors’ knowledge. To facilitate recognition and discussion of this entity, the authors propose the terminology exercise-related signal abnormality (ERSA lesion) for these MRI signal changes.
The terminology ERSA lesion is proposed for the presence of either the peritendinous ovoid ring alone (type A), the subfascial ring alone (type B), or both together (type C). Given the statistical association between the peritendinous ovoid ring and subfascial ring of altered signal change, and the co-occurrence in the same muscle in 9 instances in our cohort, it appears reasonable to consider both findings as different MRI manifestations of a single imaging entity.
Distinction of ERSA lesion from acute muscle injury
Muscle injuries can be divided into injuries resulting from direct trauma applied to the muscle, and indirect injuries resulting from excessive tension during muscle contraction, particularly during eccentric contraction . Direct injuries such as muscle contusions and lacerations will not be discussed further as the clinical context and imaging appearances are different, and should not pose diagnostic confusion from ERSA lesions. Indirect muscle injuries comprise muscle strains and tears that demonstrate a continuum of appearances depending on the extent of fiber and tendon involvement.
ERSA lesions are characterized by a faint increase in signal intensity on fluid-sensitive images that is less pronounced than indirect muscle injuries and lacks the feathery edema pattern that is typically seen with an acute muscle strain. (Fig. 5) Unlike muscle tears, there is no focal disruption in muscle fibers, nor architectural distortion of the muscles. Furthermore, there is no tendon involvement. Despite the relatively subtle findings on axial imaging, ERSA lesions demonstrate a prolonged cranial-caudal extension, with a mean craniocaudal length of 15.8 cm in this study. Lastly, rapid resolution of ERSA lesions, within 19 days after the initial MRI in one case, would be unusual for an acute muscle injury where the abnormality was seen over such a long longitudinal length.
It is critical to appreciate the difference in imaging appearances of ERSA lesions versus an acute muscle strain to avoid overestimating the significance of the imaging abnormality, particularly if classifying according to the BAMIC classification system. When utilizing the BAMIC classification system, grade 3 lesions have “high signal change of craniocaudal length of greater than 15 cm”. Whilst other criteria are used when using the BAMIC classification, the overall grade in BAMIC is assigned based on the finding that produces the highest grade. By definition as the mean length of signal change with ERSA lesions is 15.8 cm; these could be erroneously diagnosed as at least grade 3 lesions according to the BAMIC classification system . This would overestimate the significance of this imaging finding and potentially lead to a delay in return to play in up to 10% of professional soccer players referred with an acute thigh injury. This could also have negative implications for management/rehabilitation.
It is noted that the BAMIC classification system includes a grade 0b, which describes, “generalized pain following unaccustomed exercise” and may show either a normal MRI appearance or patchy high signal change throughout one or more muscles . The MRI appearances of grade 0b injuries have not been recorded in the literature and by definition, the professional soccer players included in this study were not partaking in unaccustomed exercise. For these reasons, grade 0b does not appear to apply to this patient cohort.
Distinction from other clinical and imaging entities
Delayed-onset muscle soreness (DOMS) is a clinical entity characterized by tenderness, pain, and tightness of a muscle with an onset of 24 h after activity, and lasting 5–7 days [12, 13]. DOMS is typically associated with unaccustomed exercise, being less pronounced even after a single episode of activity 6 weeks prior . As DOMS is a clinical entity, the MRI can be normal and there are no specific MRI features of DOMS. Supporting this, a study of Australian soccer players demonstrated poor MRI correlation to the clinical diagnosis of DOMS in the calf muscles . Nonetheless, reported MRI appearances of DOMS in the literature are of feathery edema similar to a muscle strain, diffuse signal change and patchy high signal change, all of which are distinct in appearances from ERSA lesions [13,14,15]. The authors consider that it is improbable that the imaging findings of ERSA lesions represent a previously unrecognized imaging manifestation of DOMS as the current cohort comprises only professional soccer players.
ERSA lesions should also be distinguished from signal intensity changes in muscles on MRI immediately after muscle activity. Diffuse increases in signal intensity on T2 and STIR have been demonstrated in muscles immediately after activity, which are thought to reflect transient increases in intracellular and extracellular water content during muscle activity [16, 17]. More recently, changes in diffusion tensor imaging parameters such as fractional anisotropy have been demonstrated in muscles following muscle activity . These post-activity changes in muscles are diffuse, in contrast to the findings in ERSA lesions which are of a discrete regional abnormality of the peritendinous or subfascial region.
Limitations and future studies
The symptom profile of ERSA lesions and the impact of ERSA lesions on clinical management are difficult to assess in this retrospective cohort. The majority (67.7%) of studies exhibiting ERSA lesions in our cohort had a more significant concurrent BAMIC grade 1–4 muscle injury which is likely to account for the clinical presentation of acute thigh muscle injury. These BAMIC grade 1–4 muscle injuries were largely (81.4%) in a separate anatomic compartment to, or on the contralateral side to the ERSA lesions. Clinical management including rehabilitation in our cohort was guided by these muscle injuries rather than the ERSA lesion. Directed clinical assessment and examination of muscles with ERSA lesions, performed contemporaneous to the MRI study, could be performed in future prospective studies to characterize the symptom profile of ERSA lesions. Nonetheless, the intended purpose of this study is to provide an imaging description of ERSA as an MRI entity, recognizing that subsequent studies will be required to fully understand the clinical correlate of these lesions. ERSA lesions can occur in isolation as was seen in 10 cases in this study. It would be interesting to examine this smaller subgroup with regards to return to sport in order to better understand the significance of this MRI abnormality.
ERSA lesions are described in this study as MRI entities in soccer players with suspected acute thigh muscle injury. This specific cohort of soccer players with suspected acute thigh injuries has been used for this study as they represent the largest proportion of MRI scans in professional athletes for the study centers, allowing for a more robust analysis. The prevalence of ERSA lesions in asymptomatic athletes, in other regions of the body and on other imaging modalities such as ultrasound remains unknown. Anecdotally, the senior author has identified similar lesions in MRI studies of track and field athletes in the build-up to major competitions. It is hoped that recognition of ERSA lesions as an imaging entity will spur future studies into the clinical and pathophysiological basis of this imaging entity.
Despite increasing participation of female athletes in soccer, soccer remains a male-dominated sport at a professional level. The male bias in the cohort of patients included in this study reflects this.