Magnetic resonance imaging for detecting root avulsions in traumatic adult brachial plexus injuries: protocol for a systematic review of diagnostic accuracy

Traumatic brachial plexus injuries (BPI) in adults are common following road traffic collisions. In England, there are 48,000 cases of major trauma per annum [1] and 1% have a brachial plexus injury [2]. Such injuries can result in permanent disability [3‐7], pain [8, 9], psychological morbidity [10, 11] and reduced quality of life [3, 5, 12]. With optimal reconstructive surgery, patients can recover useful function [3, 4, 12, 13] which is associated with improved quality of life [5].

To date, magnetic resonance imaging (MRI) is the best indicator of various pathologies affecting the brachial plexus [14], and in the context of trauma it is superior to pre-operative nerve conduction studies [15], high-resolution ultrasonography [16‐18] and intraoperative somatosensory-evoked potentials [19]. However, MRI is still unable to differentiate nerve injuries which need reconstructive surgery and those which will recover spontaneously. Further, MRI is unable to differentiate post-ganglionic (Fig. 1; green, blue and black arrows) and pre-ganglionic (Fig. 1; red arrow) nerve injuries which is of paramount importance because their reconstruction and prognosis is different. Post-ganglionic nerve injuries (ruptures or attenuations) have a more favourable prognosis because the anterior horn cells in the spinal cord persist [20, 21]; therefore, if continuity can be re-established in a timely fashion, then motor recovery can be expected [22‐30]. Conversely, reconstructing pre-ganglionic nerve injuries (known as root avulsions) requires nerve transfers as the cell bodies of the native motor neurones recede [5, 6, 13, 20] and re-implantation of roots remains of uncertain value [31‐33]. Therefore, the identification of root avulsion(s) is critical as it alters the operative plan and prognosis.

Delay to surgical reconstruction is the leading cause of poor outcome [28, 29], and so, most surgeons use pre-operative MRI and neurophysiological tests [15] in an effort to identify those with avulsion injuries. However, the reliability of pre-operative MRI is uncertain, and therefore, most surgeons still undertake early surgical exploration to identify root avulsion(s). Numerous studies have examined the diagnostic accuracy of MRI for traumatic BPI, but few have specifically considered root avulsion injuries and fewer still had an adequate reference standard, as many used clinical follow-up (i.e. the reanimation of the limb) [34] or electrophysiological studies [35, 36] as surrogate markers. Therefore, it is unsurprising that the literature is conflicting on the ‘overall accuracy’ of MRI for traumatic root avulsion.

Our rationale for conducting this review is to summarise the diagnostic accuracy of MRI for the identification of root avulsion in adult traumatic BPI. Radiologists and surgeons may use this information to rationalise such imaging, aid in its interpretation and guide future research focused on improving imaging of BPI.

The two objectives are:

This protocol was written in accordance with guidance in the Cochrane Handbook for Reviews of Diagnostic Test Accuracy [37] and PRISMA checklist (Additional file 1) and was registered on the PROSPERO database (CRD42016049702).

This review will summarise the diagnostic accuracy of MRI for the identification of root avulsion in traumatic BPI in adults. The outcome data may inform future research (focussed on improving shortfalls in the imaging), explain discrepancies between studies and ultimately help to improve the imaging of major nerve injuries.

There are many potential sources of bias in the studies of diagnostic accuracy [49]. We expect a minority of patients with BPI will not have undergone operative exploration for various reasons, e.g. they did not consent to surgery, anaesthesia was unsafe or the treatment of other injuries took precedence. This would upwardly bias the sensitivity of MRI if the proportion of false negatives is underestimated. This problem cannot be reliably mitigated by replacing or supplementing exploratory surgery with another reference standard, e.g. clinical observation because (a) the reanimation of the limb may evolve over several years and concurrent stiffness and contractures may prevent reliable and repeatable inferences about function; (b) testing a given muscle in isolation (and thus the supplying root) may be impossible as many movements involve several muscles working in synergy, each receiving input from different nerves and thus, different cervical roots; (c) cerebral cortical reorganisation may undermine clinical observations of muscle power and sensation, e.g. where two roots supply one muscle and one is injured, the cortex may reroute more input to the remaining fibres, confounding the assessment of function; and (d) the end organs of the limb (muscles, skin, joints, tendons, etc.) may receive input from several roots and equally, one root may innervate several structures, so the constellation of clinical abnormalities cannot be confidently attributed to a specific site of injury. In contrast, the diagnostic accuracy of MRI may be downwardly biased because patients may have been referred based on MRI findings rather than the presence of symptoms alone. We also expect most studies to be retrospective, and some studies may have recruited an unrepresentative sample of patients which may bias diagnostic accuracy and raise concerns about applicability. These and other issues will be addressed in the quality assessment, and we will interpret the findings of our review with respect to these potential limitations.