Reinterpretation of Electrodiagnostic Studies and MRIs in Patients with Non-Traumatic “Isolated” Anterior Interosseous Nerve Palsy

INTRODUCTION:: Different hypotheses have been proposed for the pathophysiology of anterior interosseous nerve (AIN) palsy: compression, fascicular constriction or nerve inflammation (Parsonage-Turner Syndrome). We hypothesized that critical reinterpretation of electrodiagnostic studies (EDX) and MRIs of patients with a diagnosis of AIN palsy could provide insight into the pathophysiology and treatment. MATERIALS AND METHODS:: A retrospective review was performed of all patients with a diagnosis of non-traumatic AIN palsy and an upper extremity MRI performed at our institution. The original EDX and MRI reports were re-interpreted by a neuromuscular neurologist and musculoskeletal radiologist respectively, both blinded to our hypothesis. RESULTS:: One hundred and twenty-three patients were identified with non-traumatic AIN palsy. Of these, 16 patients met the inclusion criteria as having “isolated” AIN palsy. Physical examination revealed weakness in muscles not innervated by the AIN in 5 cases (31%) and EDX abnormalities not related to the AIN were found in 9 cases (60%). The initial MRI report described atrophy in muscles not innervated by the AIN or nerve enlargement different from the AIN in 8 cases (50%). In all cases, reinterpretation of the MRIs demonstrated atrophy in at least one muscle not innervated by the AIN and did not reveal any evidence of compression of the AIN. CONCLUSION:: All patients in our series with presumed isolated AIN palsy had MRI evidence of a more diffuse muscle involvement pattern, without any radiologic signs of nerve compression of the AIN branch itself. These data strongly support an inflammatory pathophysiology.