Can MRI identify discontinuity of the nerve or nerve edema due to injury of the motor branch of the radial nerve?

MRI Capability in Radial Nerve Motor Branch Injury

Yes, MRI can identify both nerve discontinuity and nerve edema in motor branch radial nerve injuries, though specialized MR neurography protocols with high-resolution sequences are required for optimal visualization. 1

Direct Nerve Visualization

MRI can directly demonstrate nerve discontinuities, neuromas, and perineural musculofascial edema in peripheral nerve injuries. 1 However, standard MRI protocols have significant limitations:

• Routine noncontrast MRI lacks adequate imaging planes and resolution for confident assessment of peripheral nerves around the shoulder and upper extremity 1
• 3-Tesla MRI with high-resolution sequences provides superior soft-tissue contrast and can delineate focal nerve discontinuities, neuromas, and musculofascial edema 1
• MR neurography is gaining acceptance for peripheral nerve injury assessment, though systematic studies specifically for radial nerve injuries in the post-traumatic setting are limited 1

Nerve Signal Abnormalities

T2-weighted sequences are particularly valuable for detecting nerve pathology:

• T2-weighted signal alterations in nerves are more frequently detected than muscle denervation changes in peripheral motor neuropathies 2
• Nerve lesions on MRI are encountered more frequently than target muscle group denervation in motor neuropathies 2
• Increased T2 signal without concomitant nerve lesions should raise suspicion of etiologies other than peripheral neuropathy 2

Muscle Denervation as Secondary Sign

MRI can detect denervation changes in muscles innervated by the injured radial nerve motor branch:

• Short-tau inversion recovery (STIR) sequences show increased signal in denervated muscles as early as 4 days after injury, which is significantly earlier than electrodiagnostic changes 3
• Muscle edema and atrophy can be visualized even when no compressive lesion is identified 4
• Denervation almost never occurs without detectable lesions in corresponding nerves, making isolated muscle signal changes without nerve abnormalities suspicious for alternative diagnoses 2

Timing Considerations

For traumatic injuries, imaging timing is critical:

• Imaging should ideally be delayed approximately 1 month after trauma to allow resolution of hemorrhage and soft-tissue edema that can obscure nerve visualization 1, 5, 6
• Blood and regional soft-tissue edema take 3-4 weeks to resolve sufficiently for optimal nerve assessment 1

Technical Requirements

Optimal MRI protocols require specific technical parameters:

• Dedicated peripheral nerve protocols differ significantly from routine extremity MRI and should include thin-cut high-resolution sequences 6, 7
• Fat-saturated T2-weighted or STIR sequences are essential for detecting nerve and muscle edema 6, 3
• 3-Tesla imaging allows higher resolution compared to 1.5-Tesla systems for peripheral nerve evaluation 1

Clinical Impact

MRI substantially influences patient management in peripheral neuropathies:

• MRI had "major" impact on management in 47% and "moderate" impact in 37% of patients with upper extremity peripheral neuropathies and unclear clinical findings 8
• The correlation between MRI and clinical findings is moderate for radial nerve assessment (Spearman's ρ = 0.51) 8

Common Pitfalls

Several important caveats exist:

• Standard extremity MRI protocols are inadequate; specialized nerve imaging protocols must be specifically requested 6, 7
• Even with optimal protocols, some nerve injuries causing clinical dysfunction may not demonstrate visible MRI abnormalities 6
• Imaging in the acute post-traumatic period may be obscured by hemorrhage and edema 1, 5
• CT is not the modality of choice for nerve assessment due to limited soft-tissue contrast 1