Botox foundation handbook: the three licensed areas

Foundation-level cosmetic use of botulinum toxin type A concerns three areas of the upper face covered by marketing authorisation: the glabellar complex, the lateral canthal lines and the frontalis. The anatomy of these three regions accounts for most of what a foundation reference needs to cover, because almost every predictable adverse outcome in the upper face is a consequence of a muscle being treated that was not the intended target.

Mechanism at the neuromuscular junction

Botulinum toxin type A is taken up at cholinergic nerve terminals and cleaves SNAP-25, a protein required for the fusion of acetylcholine-containing vesicles with the presynaptic membrane. Without SNAP-25 the vesicle cannot release its contents into the synaptic cleft, and the muscle fibre served by that terminal receives no signal.

The effect is presynaptic and chemical. The muscle is structurally intact and the nerve is not damaged; transmission is interrupted. This is why the effect is temporary and why recovery follows a biological timetable rather than a mechanical one.

The three areas

The glabellar complex is not a single muscle. It comprises the paired corrugator supercilii, the procerus, and depressor supercilii fibres, with variable contribution from the medial orbicularis oculi. These muscles depress and draw the brows medially, producing the vertical and horizontal lines between them. The corrugator’s course carries it from the medial supraorbital rim laterally and superiorly into the dermis of the mid-brow, and it lies deep at its origin and superficial at its insertion.

Lateral canthal lines arise from the lateral fibres of the orbicularis oculi, a superficial sphincter surrounding the orbit. The relevant point is that the same muscle contributes to blink, to lower lid tone and to the lacrimal pump, so its treatment is confined to the lateral segment outside the orbital rim.

The frontalis is the sole elevator of the brow. There is no second muscle capable of raising the brow if frontalis activity is reduced, which is the central fact of upper-face practice. Every other muscle acting on the brow — corrugator, procerus, depressor supercilii, orbicularis — is a depressor. Treatment of the frontalis therefore always trades horizontal line reduction against brow position, and the resting balance between frontalis and the depressor group determines how much of that trade a given patient can tolerate.

Onset and duration

Clinical effect begins at approximately two to three days and reaches its full extent at around two weeks. The delay reflects the intracellular steps required — binding, internalisation, translocation of the light chain, and cleavage of enough SNAP-25 to reduce transmission below threshold — rather than diffusion through tissue.

Duration in the upper face is conventionally described as three to four months, with wide individual variation. Recovery occurs in two phases: transient sprouting from the blocked motor nerve terminal restores some transmission, followed by return of function at the original terminal and elimination of the sprouts. Repeated non-response despite adequate technique raises the separate question of immunogenic resistance, which has been documented as a clinical entity in multi-site case series.

Ptosis and the value of its timing

Two distinct events are described as “ptosis”. Brow ptosis follows over-reduction of frontalis activity, particularly in a patient relying on compensatory frontalis tone. Eyelid ptosis follows involvement of the levator palpebrae superioris, most often explained by spread through the orbital septum from a medial glabellar injection placed too low or too deep.

The onset interval is diagnostically useful. A drooping lid appearing two to seven days after treatment is consistent with toxin reaching the levator, because that is the pharmacological onset window. A lid abnormality present immediately after injection is not, and should prompt consideration of oedema, haematoma or an unrelated cause. Both forms resolve as the toxin effect wears off; the timescale is that of the treatment itself, not of the complaint.