Brachial Plexus MRI for Inflammatory or Infectious Plexitis

Version 1.0 — August 2026

1. Executive Summary

1.1 Added Value over the Generic Protocol

Parsonage-Turner syndrome (neuralgic amyotrophy) is the dominant inflammatory brachial plexopathy and, together with the less common truly infectious plexitis, presents a distinct diagnostic pattern that the generic protocol’s coronal T1/STIR backbone detects reliably but does not, by itself, structure the interpretation around. This child page documents the specific semiotic pattern (nerve trunk T2 signal increase with a characteristic, often patchy denervation pattern in the affected muscle groups that frequently does not respect a single nerve root or peripheral nerve territory), the deliberate distinction from cervical radiculopathy and other mimickers, and the serial-imaging follow-up strategy the generic protocol does not specify.

1.2 Limits of the Dedicated Protocol

Parsonage-Turner syndrome remains fundamentally a clinical diagnosis, supported by a compatible imaging pattern; MRI findings, while genuinely useful, are not perfectly specific, and a normal or near-normal MRI does not exclude the diagnosis, particularly early in the disease course before denervation change has had time to develop. Distinguishing inflammatory from truly infectious plexitis on imaging alone is often not possible, and this distinction ultimately depends on clinical context (systemic infective symptoms, relevant exposure history) and, where infection is genuinely suspected, appropriate microbiological work-up rather than MRI signal characteristics alone.

2. Clinical Context

2.1 Clinical Presentation

The classic Parsonage-Turner presentation is acute-onset, severe, often nocturnal shoulder and upper arm pain, which — after a period ranging from days to a few weeks — is followed by the pain subsiding as motor weakness and, over subsequent weeks to months, muscle atrophy develop, typically in a patchy, multi-nerve distribution rather than a single classic nerve or root pattern. A preceding trigger (viral illness, vaccination, surgery, strenuous exercise) is identified in a substantial proportion of cases, though the syndrome also occurs without any identifiable antecedent event.

2.2 Inflammatory vs Infectious Distinction

The great majority of clinically encountered “brachial plexitis” is inflammatory/immune-mediated (Parsonage-Turner syndrome/neuralgic amyotrophy) rather than due to direct infectious involvement of the plexus itself; genuine infectious plexitis (direct bacterial, viral, or other pathogen involvement of the plexus, as opposed to a post-infectious immune-mediated trigger for Parsonage-Turner) is comparatively rare and typically occurs in the context of a broader, clinically apparent regional or systemic infective process (adjacent abscess, osteomyelitis, or severe systemic infection) rather than as an isolated presentation.

2.3 Differential Diagnosis (Clinical)

The acute pain phase of Parsonage-Turner syndrome is frequently, initially misattributed to a musculoskeletal shoulder problem (rotator cuff pathology, cervical radiculopathy) given the severity and location of the pain relative to the comparatively delayed onset of the more diagnostically specific motor weakness and atrophy — a diagnostic delay explicitly noted as clinically significant in the literature, underscoring the value of a compatible MRI pattern in supporting earlier recognition.

3. Indications, Timing, and Patient Selection

3.1 When the Dedicated Protocol Is Indicated

Acute severe shoulder/upper arm pain followed by motor weakness, particularly with a patchy, multi-nerve distribution not conforming to a single cervical root or peripheral nerve territory, and especially where a preceding trigger is identified, should prompt this dedicated protocol to support the clinical diagnosis and to exclude structural mimickers (mass, disc herniation with radiculopathy).

3.2 Optimal Timing

Because denervation-pattern muscle signal change (the most specific direct imaging finding) takes time to develop after nerve injury, MRI performed very early in the acute pain phase may show a normal or near-normal study despite genuine Parsonage-Turner syndrome; imaging performed once some weeks have passed, or repeated serially, substantially improves diagnostic yield for the denervation pattern central to this protocol.

3.3 Serial Imaging Strategy

Follow-up MRI at approximately 3-6 months is of genuine value both to confirm the evolving denervation-atrophy pattern supporting the diagnosis and to document the trajectory of recovery, since Parsonage-Turner syndrome’s natural history is one of gradual, though sometimes incomplete, recovery over months to years — serial imaging is not routinely required in every case but is a recognised, useful strategy when the initial presentation or imaging is ambiguous.

3.4 Red Flags Modifying Urgency

Fever, systemic signs of sepsis, or imaging evidence of an adjacent abscess or bone involvement should prompt urgent reassessment for genuine infectious plexitis or a related deep space infection, rather than the elective-timed inflammatory work-up described in the remainder of this child page.

4. Dedicated Protocol Design

4.1 Mandatory Core Sequences

The table below lists the complete mandatory protocol for inflammatory/infectious brachial plexitis — the five generic-protocol core sequences (1-5) plus the one deliberate coverage extension (6) detailed in Section 4.3. Unlike several other child protocols in this cluster, this indication does not require a genuinely new sequence type, only wider anatomical coverage of the existing core STIR sequence. See Section 4.2 for the full comparison against the generic protocol.

4.2 Protocol Delta vs the Generic Brachial Plexus Protocol

4.3 Mandatory Dedicated Sequences

• Coronal and axial STIR, per the generic protocol, specifically reviewed across the full shoulder girdle and proximal arm musculature — not confined to the immediate perineural region — since the denervation pattern in Parsonage-Turner syndrome frequently involves multiple, sometimes non-contiguous muscle groups.
• Coronal T1, per the generic protocol, for baseline anatomy and, on follow-up studies, assessment of chronic denervation change (fatty muscle atrophy).

4.4 Conditional and Advanced Sequences

• Axial sequences extending further distally than the standard supraclavicular/axillary field of view may be warranted where the clinical distribution suggests involvement of individual peripheral nerves distal to the plexus itself (a recognised feature of the broader neuralgic amyotrophy spectrum, which can affect isolated peripheral nerves as well as the plexus proper).
• Post-contrast T1 fat-suppressed sequences are not routinely required for a classic presentation but should be added where a mass cannot be confidently excluded, or where genuine infectious plexitis with possible abscess or adjacent osteomyelitis is a clinical concern.
• Follow-up (3-6 month) repeat STIR/T1 to document evolving denervation pattern and support the diagnosis where the initial study was ambiguous.

4.5 Rationale per Disease-Specific Sequence

Wide-field STIR muscle assessment The rationale for deliberately extending muscle signal review beyond the immediate perineural region is that Parsonage-Turner syndrome characteristically produces a patchy, multi-nerve denervation pattern that does not respect a single root or peripheral nerve territory — a pattern only appreciated by systematically reviewing the full shoulder girdle and proximal arm musculature, not simply screening the muscles immediately adjacent to the visibly abnormal nerve trunk.

Deliberate nerve trunk T2 signal assessment without an expected structural correlate The rationale is that, unlike traumatic or neoplastic plexopathy, Parsonage-Turner syndrome’s direct nerve finding — T2 signal increase within the affected trunks/divisions — occurs without an accompanying mass, discontinuity, or other structural abnormality; recognising this “signal abnormality alone” pattern as itself diagnostically meaningful, rather than searching for a structural cause, is the specific interpretive shift this protocol requires relative to the generic protocol’s more structurally-oriented default framing.

Serial follow-up imaging The rationale for building serial imaging into the protocol design, not only as an incidental option, is that the evolving denervation-to-atrophy pattern over 3-6 months both supports the diagnosis retrospectively and provides genuinely useful prognostic information on recovery trajectory, distinct from what a single time-point study can offer.

4.6 Dedicated Planes and Field of View

The generic protocol’s coronal oblique plexus plane and axial supraclavicular/axillary coverage remain appropriate, but should be considered for extension further distally along the course of individual, clinically affected peripheral nerves when the clinical distribution suggests involvement beyond the plexus proper — a recognised feature of the broader neuralgic amyotrophy spectrum rather than an exception to it.

4.7 Contrast Strategy

Gadolinium contrast is not routinely required for a classic Parsonage-Turner presentation, since the key diagnostic findings (nerve trunk T2 signal, denervation-pattern muscle change) are non-contrast findings. Contrast should be added where a mass cannot be confidently excluded on non-contrast sequences, or where genuine infectious plexitis with possible abscess, phlegmon, or adjacent osteomyelitis is a clinical concern, using standard macrocyclic GBCA dosing and Dixon/spectral fat suppression for the post-contrast series per site-wide policy.

4.8 Sequence Matching to Clinical Question

5. MRI Semiotics of Inflammatory/Infectious Plexitis

5.1 Direct Signs

T2 signal increase within the affected nerve trunks/divisions on fat-suppressed T2 or STIR sequences, without an accompanying mass, discontinuity, or other structural correlate, is the principal direct sign — a pattern of “signal change without structural cause” that itself carries diagnostic weight in the appropriate clinical context, distinct from the mass- or injury-associated signal change described in the tumour and trauma child pages.

5.2 Indirect and Secondary Signs

Denervation-pattern muscle oedema (STIR-bright, becoming fatty-atrophic on later/chronic studies) in the muscles supplied by the affected nerve trunks is the most consistently emphasised MRI finding in the reviewed literature — typically patchy and multi-focal rather than confined to a single myotome, a pattern that itself helps distinguish this condition from a single-root radiculopathy.

5.3 Severity, Extent, and Distribution Pattern

Reporting should specify which individual nerve trunks/divisions and which specific muscle groups show signal change, explicitly noting whether the distribution is patchy/multi-nerve (typical of Parsonage-Turner syndrome) or conforms instead to a single root or single peripheral nerve territory (favouring an alternative diagnosis).

5.4 Relevant Classification Frameworks

No formal MRI-specific severity or staging classification is in widespread use for Parsonage-Turner syndrome; reporting instead focuses on documenting the specific nerves/muscle groups involved and, on follow-up imaging, whether denervation change is stable, progressing, or resolving — a descriptive rather than a formally graded framework, consistent with the original clinical descriptions of the syndrome.

5.5 Differential Diagnosis on Imaging

The principal imaging differential is single-level cervical radiculopathy (a mono-radicular denervation pattern rather than the patchy, multi-nerve pattern typical of Parsonage-Turner syndrome), a discrete mass or structural lesion (excluded by the absence of a mass on the dedicated sequences in this protocol), and — where relevant clinical features are present — genuine infectious plexitis with an identifiable adjacent infective source.

5.6 Mimickers and Pitfalls

Early imaging, performed before denervation change has had time to develop, can appear normal or near-normal despite genuine Parsonage-Turner syndrome — the single most important interpretive pitfall in this protocol, and the specific reason optimal timing (Section 3.2) and, where the initial study is ambiguous, serial follow-up imaging (Section 3.3) are built into the protocol design rather than treated as optional afterthoughts.

6. Reporting Framework

6.1 Structured Reporting Template

Nerve trunks/divisions with T2 signal change: specify level(s). Distribution pattern: patchy/multi-nerve vs single-root/single-nerve. Muscle groups with denervation change: specify, and whether oedematous (acute/subacute) or fatty-atrophic (chronic). Mass excluded: explicitly stated. Comparison with prior imaging (if follow-up study): stable / progressing / improving.

6.2 Mandatory Reporting Elements

Every report should explicitly state whether the distribution pattern observed is more consistent with a diffuse, patchy, multi-nerve process (supporting Parsonage-Turner syndrome) or a single-root/single-nerve pattern (favouring an alternative diagnosis) — this distributional statement is the single most clinically useful piece of interpretive framing this protocol provides beyond a simple list of abnormal findings.

6.3 Critical/Actionable Findings

Any imaging finding suggesting a genuine mass, abscess, or other structural lesion — rather than the expected signal-only pattern of inflammatory plexitis — is a critical, actionable finding that should prompt reconsideration of the working diagnosis and appropriate further work-up, since it falls outside this child page’s expected pattern and into the tumour or infectious-complication differential instead.

6.4 Common Reporting Errors

Describing nerve trunk signal change without commenting on distribution pattern (patchy/multi-nerve vs single-root); failing to systematically review the full shoulder girdle and proximal arm musculature for denervation change beyond the immediately visualised nerve trunks; and providing a falsely reassuring “normal” report on a study performed too early in the disease course without noting this specific timing limitation.

7. Technical Pitfalls

7.1 Timing-Related False Negatives

As emphasised in Section 3.2 and Section 5.6, imaging performed very early after symptom onset carries a genuine risk of a false-negative or equivocal study, since the denervation-pattern muscle change central to this protocol’s diagnostic value has not yet had time to develop.

7.2 Sequence-Specific Technical Considerations

Because the denervation pattern in this condition is frequently patchy and can involve muscle groups outside the immediately obvious perineural field of view, technologists should ensure the standard generic-protocol coverage genuinely extends across the full shoulder girdle and proximal arm rather than being inadvertently narrowed to the plexus itself.

7.3 When the Generic Protocol Alone Is Insufficient

A brachial plexus MRI performed for suspected Parsonage-Turner syndrome using only the generic protocol’s standard, plexus-centred muscle screening, without the deliberate wide-field, multi-muscle-group review described in Section 4.3, risks under-appreciating the distributional pattern that is this protocol’s key diagnostic contribution.

8. MRI Technologist Pearls

8.1 Sequence Order Logic

No specific sequence-ordering modification beyond the generic protocol is typically required for this indication, since the diagnostic findings are read from standard coronal/axial STIR and T1 sequences rather than depending on a specialised, protocol-order-sensitive acquisition.

8.2 Positioning Tricks

Ensure coil coverage genuinely extends across the full shoulder girdle and proximal arm, not only the immediate supraclavicular plexus region, given the wide, patchy distribution of denervation change this condition can produce.

8.3 Fast Salvage Protocol

If examination time is constrained, prioritise wide-field coronal and axial STIR (the sequences carrying the denervation-pattern information central to this protocol) over additional anatomical detail sequences that add less specific diagnostic value for this particular clinical question.

8.4 Disease-Specific Common Avoidable Errors

Performing imaging too early relative to symptom onset without documenting this limitation; narrowing coverage to the plexus alone rather than the full shoulder girdle/proximal arm; and omitting an explicit statement about distribution pattern (patchy/multi-nerve vs single-root) in the report.

9. Quality Control Checklist

• Coverage confirmed to extend across the full shoulder girdle and proximal arm musculature, not only the immediate plexus region.
• Timing relative to symptom onset documented, with early-study limitations explicitly noted where relevant.
• Distribution pattern (patchy/multi-nerve vs single-root/single-nerve) explicitly assessed and stated.
• Mass explicitly excluded or, where doubt remains, addressed with post-contrast sequences.
• For follow-up studies, direct comparison with the baseline study performed and documented.

11. Evidence Gaps and Ongoing Debate

• Optimal imaging timing relative to symptom onset is not fully standardised. As emphasised throughout this page, early imaging carries a genuine false-negative risk, but no single, universally agreed optimal imaging window has been established across the reviewed literature.
• Distinguishing inflammatory from truly infectious plexitis on imaging alone remains genuinely difficult, and the reviewed literature does not describe reliable imaging-based discriminating features distinct from clinical and microbiological context — Section 2.2’s distinction remains substantially a clinical, not an imaging, determination.
• The relationship between imaging severity/extent and functional recovery prognosis is incompletely characterised. While serial imaging (Section 3.3) is a recognised useful strategy, quantitative imaging-based prognostic criteria for predicting the degree or timeline of recovery remain less developed than the diagnostic semiotics themselves.

12. Evidence-Based References

A. Guidelines / Consensus / Society Recommendations

No dedicated society guideline specific to inflammatory/infectious brachial plexitis MRI protocol design was identified as warranting a separate citation for this child page, beyond the general ACR Appropriateness Criteria for brachial plexus imaging referenced on the parent master page. Category A is therefore not separately populated here.

C. Important Prospective / Original Studies

D. Technical MRI Papers

E. Landmark Historical References

End of document — Brachial Plexus MRI for Inflammatory or Infectious Plexitis — Child Protocol under the MRIninja Neck / Brachial Plexus master page — v1.0 — August 2026 Parent page: Brachial Plexus MRI — Generic Standard Protocol