The “Ram's Horns Sign”: A Case Report of an Unusual Side Effect of OnabotulinumtoxinA in a Chronic Migraine Patient

OnabotulinumtoxinA (BoNT‐A) has been reported as an effective prophylactic treatment for chronic migraine to reduce disease severity improving health‐related quality of life. However, BoNT‐A, due to its activity on either the injected or adjacent muscles of the upper face, may induce well‐known side‐effects, such as the eyebrow or eyelid ptosis. However, unusual muscular side effects, related to the neurotoxic mechanism, may also arise. We describe the clinical case of a 55‐year‐old male patient who has been treated for chronic migraine by the injection of BoNT‐A, according to the PREEMPT protocol. Two weeks later, the patient developed two symmetrical bumps on the upper part of the forehead similar to the horns of a ram. We report, for the first time, this peculiar BoNT‐A side effect and suggest that the injection of additional BoNT‐A doses in the upper medial frontal fibers, for each side, can normalize the forehead shape over two weeks.