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.