Nontraumatic “isolated” posterior interosseous nerve palsy: Reinterpretation of electrodiagnostic studies and MRIs

Introduction Different hypotheses have been proposed for the pathophysiology of posterior interosseous nerve (PIN) palsy, namely compression, nerve inflammation, and fascicular constriction. We hypothesized that critical reinterpretation of electrodiagnostic (EDX) studies and MRIs of patients with a diagnosis of PIN palsy could provide insight into the pathophysiology and treatment. Materials and methods We retrospectively reviewed patients with a diagnosis of nontraumatic PIN palsy and an upper extremity EDX and MRI. The original EDX studies and MRIs were reinterpreted by a neuromuscular neurologist and musculoskeletal radiologist, respectively, both blinded to our hypothesis. Results Fifteen patients met the inclusion criteria, i.e., having an “isolated” PIN palsy. Four patients (27%) had a defined mass compressing the PIN. The remaining 11 patients (73%) presented with at least one finding incompatible with the compression hypothesis: physical examination revealed that weakness in muscles was not innervated by the PIN in 4 patients (36%); EDX abnormalities not related to the PIN were found in 4 patients (36%); and reinterpretation of the MRIs showed muscle atrophy or nerve enlargement beyond the territory of the PIN in 9 patients (82%), without any evidence of compression of the PIN in the proximal forearm. Conclusion The eleven patients in our series with presumed isolated and idiopathic PIN palsy had evidence of a more diffuse nerve–muscle involvement pattern, without any radiologic signs of nerve compression of the PIN itself. These data would favor an inflammatory pathophysiology when a structural lesion compressing the nerve is ruled out with imaging.