Background: Different hypotheses have been proposed for the pathophysiology of anterior interosseous nerve palsy: compression, fascicular constriction, or nerve inflammation (Parsonage-Turner syndrome). The authors hypothesized that critical reinterpretation of electrodiagnostic studies and magnetic resonance imaging scans of patients with a diagnosis of anterior interosseous nerve palsy could provide insight into the pathophysiology and treatment. Methods: A retrospective review was performed of all patients with a diagnosis of nontraumatic anterior interosseous nerve palsy and an upper extremity magnetic resonance imaging scan. The original electrodiagnostic study and magnetic resonance imaging scan reports were reinterpreted by a neuromuscular neurologist and musculoskeletal radiologist, respectively, both blinded to the authors' hypothesis. Results: Sixteen patients met the inclusion criteria as having "isolated" anterior interosseous nerve palsy. Physical examination revealed weakness in muscles not innervated by the anterior interosseous nerve in five cases (31 percent), and electrodiagnostic studies showed abnormalities not related to the anterior interosseous nerve in nine of 15 cases (60 percent). In all cases, reinterpretation of the magnetic resonance imaging scans demonstrated atrophy in at least one muscle not innervated by the anterior interosseous nerve and did not reveal any evidence of compression of the anterior interosseous nerve. Conclusions: All patients in the authors' series with presumed isolated anterior interosseous nerve palsy had magnetic resonance imaging evidence of a more diffuse muscle involvement pattern, without any radiologic signs of nerve compression of the anterior interosseous nerve branch itself. These data strongly support an inflammatory pathophysiology.
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