400 Section - Anterior Interosseous Nerve Involvement in Neuralgic Amyotrophy: Clinical, Imaging and Electrophysiological Insights

Objectives : To evaluate whether isolated AIN-pattern weakness should raise concern for Neuralgic Amyotrophy (NA) and how this distinction informs diagnosis and management.

Design:

Scoping Review

Results:

Across included studies, many patients with AIN Syndrome reported a severe, painful prodrome preceding weakness, a pattern characteristic of NA. Imaging was often performed to investigate persistent deficits or exclude neoplastic, infiltrative, or compressive causes; nevertheless, MRI and high-resolution ultrasound consistently demonstrated multifocal hourglass-like constrictions or fascicular torsions within the posteromedial fascicles of the median nerve, typically 2–5 cm above the medial epicondyle. The AIN remained structurally normal, indicating a more proximal lesion.

Electrodiagnostic studies showed a pure motor axonopathy in AIN-innervated muscles, with variable pronator teres or FCR involvement, reflecting selective fascicular injury—features incompatible with distal entrapment and highly consistent with NA.

Surgical series corroborated these findings: forearm explorations rarely revealed compression, while proximal exposures consistently identified hourglass constrictions (often multiple). Targeted microneurolysis yielded functional improvement in ≥80% of cases.

Conclusion:

The accumulated clinical, imaging, and electrodiagnostic evidence supports redefining AINS as a mononeuritic manifestation of NA. Crucially, isolated AIN-pattern weakness—especially when preceded by severe pain and without sensory loss—should immediately raise concern for NA, rather than distal entrapment or C8 radiculopathy. This presentation reflects focal involvement of AIN-related fascicles within the proximal median nerve, a hallmark of NA’s multifocal fascicular neuropathy.

Recognizing this mechanism has direct clinical implications. Early proximal nerve imaging is essential both to identify fascicular constrictions and to rule out alternative structural causes. Surgical microneurolysis may be appropriate when persistent paralysis corresponds to fixed constrictions on imaging. Overall, AINS illustrates the broader concept of NA as a multifocal, immune-mediated fascicular disorder, and isolated AIN involvement should be viewed as an important diagnostic clue. Further research is needed to better characterize this relationship, clarify diagnostic criteria and optimal surgical or non-surgical treatment strategies.