In brachial neuritis, what is the MRI appearance of the brachial plexus and at what time point after symptom onset?

MRI Findings in Brachial Neuritis

In brachial neuritis (Parsonage-Turner syndrome), MRI typically shows thickening and T2 hyperintensity of affected nerves, but importantly, the brachial plexus proper often appears normal, with abnormalities localized to plexus branches and peripheral nerves rather than the plexus itself; optimal imaging timing is delayed until approximately 1 month after symptom onset to allow for resolution of acute edema and development of characteristic findings.

MRI Appearance

Location of Abnormalities

The distribution of MRI findings in brachial neuritis differs from what the name suggests:

• The brachial plexus proper appears normal in most cases (24 of 27 patients in one study) 1
• Abnormalities localize to plexus branches and peripheral nerves rather than the plexus itself 1
• When plexus changes are visible, signal hyperintensity appears immediately proximal to the take-off of abnormal branch nerves 1
• This suggests brachial neuritis represents one or more mononeuropathies rather than true plexopathy 1

Specific MRI Characteristics

The affected nerves demonstrate:

• Thickening of involved nerves 2, 3
• T2 hyperintensity (increased signal on T2-weighted sequences) 2, 3
• Edema within the nerve 3
• Focal intrinsic constrictions (hourglass-like constrictions or HGCs) detected in 84% of abnormal nerves (32 of 38) 1, 4

Muscle Changes

MRI of shoulder and upper arm musculature may reveal denervation changes within days of symptom onset, allowing earlier diagnosis than electrodiagnostic testing 5

Optimal Timing for MRI

Imaging should ideally be delayed until approximately 1 month after symptom onset 6

Rationale for Delayed Imaging

• Takes 3 to 4 weeks for characteristic findings to develop 6
• Allows time for:
  • Resolution of acute hemorrhage and soft-tissue edema that can obscure findings 6
  • Development of pseudomeningoceles (if nerve root avulsion present) 6
  • Evolution of nerve signal abnormalities to become more apparent 6

Exception: Early Muscle Imaging

While plexus imaging is optimally delayed, MRI of shoulder/upper arm musculature can show denervation within days, potentially allowing earlier diagnosis than the traditional 3-4 week wait for electrodiagnostic testing 5

Clinical Correlation

Diagnostic Value

• MRI is highly sensitive for detecting brachial plexus lesions, particularly in brachial neuritis where conventional MRI may be normal 3
• Magnetic resonance neurography (MRN) shows edema, thickening, and T2 hyperintensities localized to the brachial plexus region 3
• MRI can demonstrate involvement of the whole brachial plexus even when electrodiagnostic findings suggest only distal lesions 2

Key Differentiating Feature

The temporal profile is critical: brachial neuritis presents with severe pain preceding weakness by days to weeks 5, whereas cervical radiculopathy presents with simultaneous pain and neurologic deficits. This distinction helps avoid unnecessary cervical spine surgery in patients with plexitis 5.

Contrast Administration

MRI without IV contrast is generally sufficient for initial evaluation of brachial neuritis 6. Contrast may help differentiate vascular structures from nerves but does not provide significant additional diagnostic information in most cases 6.

Common Pitfalls

• Don't image too early: Acute soft-tissue edema and hemorrhage can obscure findings in the first few weeks 6
• Don't assume plexus proper involvement: Look carefully at branch nerves and peripheral nerves, as these are more commonly affected than the plexus itself 1
• Don't rely solely on electrodiagnostics: EMG requires 3-4 weeks to show abnormalities 5, while muscle MRI can show denervation within days 5
• Don't confuse with cervical radiculopathy: The pain-before-weakness temporal pattern is diagnostic 5