Introduction

The mid-face is the region between the zygomatic arches superiorly and the intercommis ural line inferiorly. The mid-face has complex muscular anatomy demanding full anatomical knowledge and precise injection technique. The small muscles in this area are important in expressing emotion, elevating the upper lip, and smiling. These smaller muscles are more closely arranged than their upper facial counterparts, and functionally integrated through the superficial muscular aponeurotic system (SMAS). The upper lip elevators function to produce a smile and are essential for the accurate conveyance of positive emotion. Even minor dysfunction of these muscles can have dire social consequences. Injection in the mid-face area is not for the novice injector, but can deliver real and tangible benefit for the patient when applied correctly.

The smaller mid-facial muscles require smaller doses when compared with their upper facial counterparts, especially in initial treatments. The advent of multiple botulinum toxin formulations has added another layer of complexity to this already challenging application and it is important to be aware of proper conversions / dilutions for each toxin (Table 17.1). Small errors in absolute dosage make a large difference in the percentage of the intended doses in this area. The activity of onabotulinumtoxinA (Botox^®) and incobotulinumtoxinA (Xeomin^®) is measured in murine lethal units (U), with both agents available as 50 U or 100 U per bottle with unit equivalency. It is best to use a higher concentration (e.g. 100 U/mL for Botox^®) in the mid- and lower face to minimize toxin diffusion. Abobotulinum toxin A (Dysport^®) activity is measured in Speywood units (sU) with either 500 sU or 300 sU/bottle, with a typical dilution of 250 sU/mL. As such, onabotulinum / incobotulinum toxin units and abobotulinum units are not interchangeable, and the exact conversion factor is controversial. According to FDA-approved labeling for the treatment of the glabella, 1 U Botox^® is approximately equal to 2.5 sU Dysport^®. However, studies have found abobotulinumtoxinA : onabotulinumtoxinA unit ratios to be in the range of 2.5–5 : 1. Some studies such as that by Husseman & Mehta have found abobotulinumtoxinA (Dysport^®) to have greater diffusion than onabotulinumtoxinA, meriting even additional caution given the intimately associated nature of the muscles in this area.

Table 17.1 Mid-facial indications and dosages for botulinum toxin

Additional points to bear in mind when treating the mid-face are that it is critical to avoid initial overtreatment; a standard of low starting doses and upward dose titration as needed should be adopted. Exact placement of the neurotoxins is critical – and marking the injection site before inserting the needle is suggested. Photographic documentation of the marked sites can be a helpful part of the patient’s records. The expectation that botulinum toxin alone will achieve all of the aesthetic correction in the mid-face is fallacious. Fillers are often used in this area to correct age-associated volume loss. Botulinum toxins can improve the appearance of the mid-face and, with proper patient selection, rigorous knowledge of facial anatomy and precise injection technique, toxins can prove a useful tool in this area.

‘Bunny’ / nasal sidewall scrunch lines

‘Bunny’ lines or nasal sidewall ‘scrunch’ lines are angled, horizontal rhytides that traverse the nasal bridge. While not necessarily associated with aging, many patients find them distressing as they convey the impression of distaste. Bunny lines may appear in individuals who have had a number of BoNT-A treatments in the glabella. Bunny lines result primarily from contraction of superior nasalis (pars transversalis) fibers with accessory action from the levator labii superioris aleque nasii (LLSAN) and inferomedial orbicularis oculi.

Injection sites / dosage

Injection sites for bunny lines are highly individualizable, as are the patterns of the lines. In general, one should ‘inject the fold’ within the following parameters. Injection sites should be kept high on the lateral nasal sidewall (Fig. 17.1) and well medial to the nasofacial sulcus to avoid inadvertent LLSAN weakness. Injections need to be placed only superficially (i.e. intradermally, raising a bleb) to avoid increased diffusion in the low-resistance subnasalis plane. The typical dose ranges have been reported as 2–8 U for onabotulinumtoxinA on each side or 10–20 sU for abobotulinumtoxinA. Figure 17.2 shows a patient before and after bunny line treatment. If LLSAN has a large role in the formation of bunny lines, and the patient is a candidate for LLSAN treatment (as discussed below), that area can be treated concomitantly.