(A) Frontalis test: unilateral injection of the frontalis muscle with botulinum toxin (BoNT). (A) Frontalis test: unilateral injection of the frontalis muscle with botulinum toxin (BoNT). (B) Patient non-resistant to BoNT: paralysis of the frontalis muscle and incapacity to raise the eyebrow on the side of the injection. (C) Patient resistant to BoNT: absence of paralysis of the frontalis muscle on the side of the injection and symmetrical elevation of eyebrows. Marie-Helene Marion et al. Pract Neurol 2016;16:288-295 ©2016 by BMJ Publishing Group Ltd