Efficacy of Local Anesthesia in the Face and Scalp

A Prospective Trial

Tyler Safran, MD; Dino Zammit, MD; Jonathan Kanevsky, MD, FRCSC; Manish Khanna, MD, FRCPC

Disclosures

Plast Reconstr Surg Glob Open. 2019;7(5):e2243 

In This Article

Discussion

This prospective trial looking at the speed of onset of local anesthesia was successful in demonstrating a significant difference in time of onset between upper and lower regions of the face, when controlling for concentration and type of anesthesia. The upper face took significantly longer time (153.54 seconds), than both lower face (69.37 seconds) and ear (60.2 seconds), although showing no differences in amount injected nor type and size of lesions. In addition, the 12 patients who were excluded for having required additional anesthesia were all located in the upper face group. This would have further emphasized the longer time to full anesthetic effect. In demonstrating that more time is needed for adequate anesthesia in certain areas of the face and scalp, surgeons can better plan any local procedure for patients. This is the first study of its kind, comparing the speed of onset of local anesthesia in this anatomical region. Currently, surgeons will resort to overinjecting areas that are consistently difficult at becoming anesthetized. This has been hypothesized to possibly distort the anatomical plane and risk the surgeon losing their planned margins.[7,8]

Using the time to full anesthesia effect for each injection for all 102 patients, the authors were able to generate a heat map showing the distribution of timing for local anesthesia (Figure 1). As mentioned previously, the forehead and upper scalp areas took significantly longer for full anesthesia effect, whereas regions adjacent to these areas took a moderate amount of time for full anesthesia effect relative to the rest of the face. The rest of the face took significantly shorter to full anesthesia effect compared with the longer times of the forehead and upper scalp.

Figure 1.

Heat map of anesthetic timing (average seconds required for adequate anesthesia).

Local anesthesia is the mainstay of minor procedures and is consistently used as an adjunct in multimodal analgesia.[1] Understanding the pharmacology and mechanism of action of these medications can significantly help the surgeon to tailor their administration, understand how to improve the medication response, and help improve the patient's experience.

As an overview, local anesthetic agents reversibly disrupt the sodium ion channels in the neuron cell membranes, which inhibit the function of sodium channels.[6] This inhibition decreases both the rate of repolarization and depolarization of the nociceptive (pain) receptors.[6] The local anesthetic diffuses into cells via a gradient explained by the difference in pH. To diffuse into the neuronal cells, the chemical anesthetic needs to be in the proximity.[6] In the present study, the only factor that differed between the patients was anatomical location. With the advent of new imaging technology and increased access to cadavers, our ability to accurately map out and understand facial anatomy has increased dramatically.[5] Based on facial anatomy, the authors of the present study postulate 3 anatomical differences between the facial regions that can help explain the difference in time to adequate anesthesia.

First, the anesthetic agent works by diffusion into the receptors in an inverted relationship with the radius of the cross-section of the nerve. The smallest and fastest receptor to be anesthetized is the C fibers, which transmit pain.[6] The goal of local anesthesia administration is to inhibit these fibers; however, it must be explained to the patient that pressure sensation is not always affected. In vivo, different tissue layers diffuse the agent longer than others. If a major nerve crosses deep to the injection, it may take a longer amount of time to diffuse in proximity to the nerve for depolarization. This is one of the major factors that the authors have postulated in explaining the findings of this prospective study. The more layers impeding its diffusion the longer its onset of action. What differs in the upper region of the face is the trajectory the nerves travel. The forehead and scalp are innervated by V1 (supratrochlear and supraorbital), V2 (auriculotemporal), and the greater occipital nerve (C1 and C2).[4,9–15] As an example, the supratrochlear nerve emerges at the frontal notch, approximately 1 cm from midline, and supplies the conjunctiva and the skin of the upper eyelid, then curves up on the forehead, beneath the corrugator supercilii and frontalis muscles.[11] At this point, the supratrochlear nerve then ascends beneath the corrugator and the frontal belly of occipitofrontalis before dividing into branches that pierce these muscles to supply the skin of the lower forehead near the midline.[4,5] The supraorbital nerve pierces through mainly the supraorbital notch, in 90% of cases, and supplies palpebral filaments to the upper eyelid and conjunctiva.[4,12,16] It ascends on the forehead with the supraorbital artery and divides into medial (superficial) and lateral (deep) branches that supply the skin of the scalp as far back as the lambdoid suture.[4] These branches are at first deep to the frontal belly of occipitofrontalis. The medial branch perforates the muscle to reach the skin, whereas the lateral branch travels deep to the frontalis, without reliable branching patterns.[8] Both these major branches to the skin in the upper forehead run deep to muscle, and in some cases just along the border of the periosteum. The local anesthetic, injected into the dermal/subcutaneous plane, needs to diffuse downwards through fascia and muscle to arrive at the main cutaneous nerve. In addition, if injected in the sub/supraperiosteal plane, the local anesthesia can be dispersed and lost through plane hydrodissection, missing the nerves completely, given that they are all found above the galeae plane. In contrast, the infraorbital nerve, for midface sensation, exists from the infraorbital foramen, which lies between levator labii superioris and levator anguli oris.[4] Here, it becomes very superficial and branches into palpebral branches, nasal branches, and superior labial branches.[4] These branches are both superficial and very small in caliber compared with their counterparts from the forehead. Both these factors can lead to differences in time to anesthesia, given the superficial nature of the main branches and small caliber of sensory nerves, which helps improve diffusion.

Second, vascularity is a major factor in determining the effectiveness and longevity of local anesthesia. When local anesthesia is absorbed into the blood stream, it is ushered away from its primary site of action and unable to exert its effect on the target nerves. To counteract this effect, many have used epinephrine in their solution to cause vasoconstriction and thus an increase in the duration of onset. As Taylor et al. have demonstrated in their study of angiosomes, there is an abundance of vascular channels and vasculature in the scalp compared with other area of the head and neck.[17] The authors of the present study have hypothesized that the high density of vessels can actually lead to the increase in absorption and thus lower the effective concentration of anesthetic that can be left to diffuse into the nerves. The authors of this study used epinephrine in entire local anesthesia, instead of plain lidocaine, to better control for this increased vascularity and achieve hemostasis during the procedure.

Finally, pain receptors (C fibers) are joined by a multitude of other receptors that transmit information, such as vibration, pressure, and temperature. These receptors are arranged within the dermis of the skin and transmit their respective information to the main nerve in their proximity. These mechanoreceptors, however, have been showed to have different dispersion across the body. Although pain receptor distribution itself has not been studied, there have been many articles examining 2-point discrimination differences and other sensation differences in the body and face. The forehead, when compared against the facial locations, has been shown to be the least sensitive while using a pressure-specified sensory device.[18] In addition, when testing 2-point discrimination, the forehead compared with the rest of the face had the highest static, moving, and vibratory 2-point discrimination. This difference has been explained by relative differences in mechanoreceptor distribution. Many studies have compared the lips and nose to be the most sensitive areas of the body, comparable to the tips of the fingers.[19] The authors hypothesize that this difference in the concentration of mechanoreceptors correlates with a decrease in pain receptor endings, and thus less receptors to be acted on by the local anesthetic.

In all, these factors play important roles for any surgeon. Operative flow and patient preparation can be significantly impacted by organizing patients based on their anesthetic needs and potential for further interventions to ensure adequate anesthesia. By simply being conscious of certain anatomical variations as seen in this article, the surgeon can be more aware of which patients may require additional time before surgery. In addition, looking at these results, patients with lesions in the upper face, mainly the scalp and upper forehead, may simply benefit from a regional block in addition to the regular infiltrative anesthesia.

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