Rationale: This is a report about a rare case of idiopathic neuralgic amyotrophy (INA) involving selective peripheral nerve branches of bilateral upper extremities, which exhibited a stepwise progression.
Patient concern: A 66-year-old woman presented with paresis of selective branches of bilateral median nerves, followed by paresis of bilateral posterior interosseous nerve (PIN) 8 weeks later.
Diagnoses: We diagnosed it as INA involving the selective motor branches of bilateral median nerves and bilateral PINs. Forearm magnetic resonance imaging combined with electrodiagnostic testing helped accurately identify the affected regions, and ultrasonography demonstrated a severe constriction of the left PIN.
Interventions: Intravenous methylprednisolone partially relieved the pain and paralysis. Surgical neurolysis of the constricted left PIN was done for persistent paralysis.
Outcomes: The muscle power of the bilateral median nerve territories was recovered to nearly normal, but the muscle power of the left PIN territories remained at grade 1.
Lessons: This case indicates that INA can manifest as a multiple mononeuropathy involving individual fascicular levels of peripheral nerve branches with focal constriction, and electrodiagnostic study combined with forearm MRI and ultrasonography can help in identifying affected lesion and predicting the prognosis.