Botulinum toxin A (henceforth BTN-A) is a very potent neurotoxin produced by the bacterium Clostridium botulinum. Serious poisoning from food borne botulinum toxin is a medical emergencies resulting in widespread paralysis, usually starting inn the face and eye muscles. BTN-A exerts its effect by irreversibly binding to protein SNP25 on the presynaptic nerve ending and inhibiting the release of the neurotransmitter acetylcholine. Acetylcholine is the substance that transduces the electrical signal (nerve impulse) into mechanical action (muscle contraction) at the synapse, which works as an electromechanical coupler between nerve and muscle. The paralytic effect vanishing once the body builds new SNP25 or sprouts new nerve endings.
Centuries ago Paracelsus stated that the main difference between medication and poison is the dose. At a low dose and when used by direct local injection into muscles BTN-A has multiple useful therapeutic applications. In strabism it is used to selectively weaken some of the eye muscles to achieve better globe position and cure double vision in various directions of gaze. In cases of spasticity after brain injury it is used to selectively weaken the overactive muscles. This is useful e. g. in cases of cerebral palsy with upper extremity spasticity to achieve better hand function temporarily and simulate the effects of a surgical intervention. In migraine headaches it is used to counteract the compression of certain nerves by their surrounding muscles at the front and back of the head.
Its use in aesthetic medicine is based on the anatomical properties of the facial, mimetic musculature. The pull of these muscle on the skin not only enables the expression of emotions but also generates wrinkles. These dynamic wrinkles are the target of BTN-A treatments. Dynamic wrinkles appear when the corresponding mimetic muscles are activated during expression of emotions and disappear when the muscles are relaxed. Static wrinkles on the other hand are present permanently at sites such as the forehead and frown area even if the mimetic muscles are not activated. Static wrinkles are not a good indication for BTN-A treatments. BTN-A is also not well suited to treat fine wrinkles resulting from intrinsic skin changes, as its target really is muscle, not skin.
Best research and track record of safety is available for the frown area (glabella) between the eyebrows and the forehead. Much of the scientific evidence of the benefits and safety of BTN-A was produced by Drs. Alistair and Jean Carruthers of Vancouver. Their research helped define optimal doses and reinjection intervals.
The aesthetic objective is to produce a smoother look with reduced depth and occurrence of wrinkles. A frozen appearance has to be avoided. Malposition of eyebrows and eyelids are occasional, temporary side effects of BTN-A treatments. Understanding of the closure dynamics of the eyelids and the muscle balance necessary to set lid and eyebrow position by the injector as well as a conservative dosing and injection regimen contribute to the avoidance of these effects and the ultimate satisfaction of the patient.