Chapter 38: Injectables

INTRODUCTION

Botulinum toxin (BoNT) has long been recognized as a potent neurotoxin, which causes symmetric flaccid paralysis upon ingestion of food contaminated with bacterial spores. Clostridium botulinum, a spore-forming, gram-positive, anaerobic bacterium, produces the toxin along with its associated proteins.¹

The use of BoNT in medicine dates back to the 1970s, with its initial use in ophthalmology to treat strabismus via its paralyzing effects on the extraocular muscles. The first cosmetic use of BoNT in facial aesthetics occurred in the late 1980s.² Use of BoNT for facial rejuvenation now ranks as the number one cosmetic treatment in the United States.³ The only current FDA-approved cosmetic indication for botulinum toxin type A (BoNT-A) in the United States is treatment of moderate-to-severe glabellar lines in adults, with three different BoNT-A agents: Ona- (Botox®. Allergan. Irvine, CA. USA), Abo- (Dysport®. Ipsen.), and Inco- (Xeomin®. Merz. Germany). However, the BoNT-A agents are commonly used off-label for other aesthetic purposes on the face, neck, and sometimes the upper chest.

In the United States, AbobotulinumtoxinA, IncobotulinumtoxinA, and OnabotulinumtoxinA are also approved by the FDA for the treatment of cervical dystonia. OnabotulinumtoxinA is approved for the treatment of blepharospasm, strabismus, and severe primary axillary hyperhidrosis that is inadequately managed by topical agents. IncobotulinumtoxinA is also approved for treatment of blepharospasm.

AVAILABLE TOXINS

Currently, seven serotypes of BoNTs exist (A–G), all resulting in temporary muscle paralysis. Only serotypes A and B are currently commercially available.¹ For the three preparations of BoNT-A licensed in the United States for cosmetic use, OnabotulinumtoxinA, AbobotulinumtoxinA, and IncobotulinumtoxinA, there is no standard equivalency ratio. Several clinical studies have attempted to determine a ratio specifically for OnabotulinumtoxinA and AbobotulinumtoxinA. A single unit of OnabotulinumtoxinA is approximately equivalent to 2 to 4 units of AbobotulinumtoxinA but this estimation varies between studies, physicians, countries, and by indication.^4–6 Although there are current investigations underway comparing the potency of IncobotulinumtoxinA and OnabotulinumtoxinA, a recent study suggests close to a 1:1 ratio between the products (Table 38-1).⁷