FormalPara Key Summary Points

Why carry out this study?

 Breast surgery, especially radical mastectomy, is often accompanied by moderate to severe acute pain.

 Serratus anterior plane block (SAPB) can better target the nerve network innervating the breast.

 This paper investigates the clinical application of SAPB in breast surgery.

What was learned from the study?

 SAPB has significant benefits in different breast surgery and facilitates rapid recovery of patients. The clinical application of SAPB in different breast surgery will be more extensive.

Introduction

Breast surgery, especially radical mastectomy, is often accompanied by moderate to severe acute pain. Acute pain is closely related to the occurrence and duration of postoperative chronic pain, which significantly reduces postoperative quality of life [1]. There is an urgent need to alleviate postoperative pain and achieve rapid recovery. Serratus anterior plane block (SAPB) provides regional analgesia on the anterolateral chest wall by blocking T2–9 intercostal nerves passing through the intercostal and serratus anterior muscles as well as the long thoracic and dorsal thoracic nerves [2]. It is mainly used in breast surgery, rib fracture treatment, and thoracotomy [3,4,5]. SAPB can target the neural network innervating the chest wall and breast well [6], and there have been some studies on the use of SAPB in breast surgery in recent years. In this review, we begin with the technical characteristics of SAPB, then discuss the perioperative background of different breast surgeries, and comprehensively review the impact of SAPB on acute and chronic pain, speed of recovery, and likelihood of tumor recurrence after breast surgery. The direction of future SAPB development is then discussed, followed by prospects of SAPB for broader application in breast surgery.

This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.

Methods

A systematic search of the PubMed databases was performed using a combination of keywords and MeSH terms, including “serratus anterior block,” “serratus anterior plane block,” “SAP block,” “breast surgery,” “breast cancer surgery,” “mastectomy,” “pectoral nerve block,” AND “regional block.” Furthermore, we manually searched for the references in the relevant literature. All studies from 2013 to 2022 were considered. Inclusion criteria comprised randomized controlled trials, prospective, retrospective cohort studies, and case series.

Discussion

Serratus Anterior Plane Block (SAPB)

The serratus anterior is a large and flat muscle close to the lateral wall of the thoracic cage; it originates from the outside of the eighth to ninth ribs and ends on the medial surface and lower corner of the scapula. The serratus anterior plane is a fascial space, and SAPB is a fascial plane block technique proposed by Blanco et al. [2] in 2013, in which local anesthetic is injected on the midaxillary line between the latissimus dorsi and the serratus anterior or on the deep surface of the serratus anterior under ultrasound guidance. This technique blocks the lateral cutaneous branch of the intercostal nerves (T2–9) passing through the serratus anterior muscle and infiltrates the thoracic dorsal and long thoracic nerves, providing good analgesia in the anterior and lateral walls of the chest.

Different Puncture Approaches

SAPB can be categorized according to the injection level and puncture approach as superficial serratus anterior block, deep serratus anterior block, or modified serratus anterior block. Superficial serratus anterior block refers to injection of local anesthetic between the latissimus dorsi and serratus anterior muscles at the level of the fifth rib on the midaxillary line (Fig. 1), which blocks the lateral cutaneous branch of the intercostal nerve and the long thoracic nerve. Deep serratus anterior block refers to injection of local anesthetic between the serratus anterior muscle and the external intercostal muscle at the level of the fifth rib on the midaxillary line (Fig. 2), where the lateral cutaneous branch of the intercostal nerve passes through the external intercostal muscle. Modified serratus anterior block involves injection of local anesthetic between the latissimus dorsi and serratus anterior at the level of the sixth rib on the posterior axillary line.

Fig. 1
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Ultrasound imaging of the superficial serratus anterior plane block

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Fig. 2
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Ultrasound imaging of the deep serratus anterior plane block

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Blanco et al. compared superficial and deep layers of serratus anterior block and reported that the former appeared to be more effective in terms of anesthesia diffusion, sensory distribution, and duration of action [2]. Biswas et al. performed dye injections at different layers of the serratus anterior muscle in fresh cadavers and found that at the same injection dose, superficial and deep serratus anterior injections had the same diffusion area but that the former could prevent excessive advancement of the needle tip into the pleura, with greater safety [7]. Overall, deep SAPB can block more branches and collaterals before the intercostal nerves pass through the superficial layer of the serratus anterior muscle, which is more conducive to analgesia, though the optimal level for serratus anterior block remains a matter of debate.

Factors Influencing the Effect of SAPB

SAPB is a fascial plane block. The local anesthetic reaches the target in a volume-dependent manner. The range of SAPB is mainly affected by the volume of local anesthetic. The onset time is usually within 30 min. Kunigo et al. studied the relationship between the volume of local anesthetic and SAPB range. Compared with 20 mL, 40 mL of 0.375% ropivacaine achieved a wider block range and additionally blocked the T1 and T8 intercostal nerves, but the effective duration of postoperative analgesia was similar. Considering the innervation range of the mammary gland (T2–T6) and the small injection volume required to avoid local anesthetic poisoning, an injection volume of 20 mL is sufficient to meet the needs of analgesia in breast surgery [8]. In general, the concentration of local anesthetic is an important factor affecting both the efficacy and duration of analgesia. Huang et al. performed a randomized controlled trial to compare different concentrations of ropivacaine on SAPB for postoperative analgesia in patients undergoing breast surgery. The results showed that the duration of sensory block increased with concentration. The duration of 0.375%, 0.5%, and 0.75% ropivacaine was 7.8 ± 2.3 h, 11.0 ± 3.3 h, and 13.9 ± 4 h; the areas under the curve (AUCs) of the numerical rating scale (NRS) pain intensity scores on movement were 71.8 ± 12.3, 53.0 ± 15.9, and 48.2 ± 13.3, respectively. Although the analgesic effect of 0.5% and 0.75% ropivacaine was better than that of 0.375% ropivacaine, there was no significant difference in analgesic effect between 0.5% and 0.75%. Therefore, SAPB with 0.5% ropivacaine is an appropriate concentration for postoperative analgesia in breast surgery [9].

Further research is required to balance the onset and duration of blockade with the risk of systemic toxicity of narcotic drugs and determine the optimal dosing regimen, i.e., the volume and concentration of local anesthetic, to guide clinicians in designing the best analgesic plan for patients [10].

Continuous SAPB

Continuous SAPB can provide a good analgesic effect in the anterior and lateral chest, and its use in thoracoscopy [3], rib fracture treatment [4], and breast cancer surgery [5] has been reported in the literature. Both the superficial and depth of the serratus anterior fascia can be catheterized, with a puncture position most commonly located at the fifth rib on the midaxillary line. The position of the catheter is easier to fix in deep serratus anterior block, and firm catheter fixation is key to the success of continuous peripheral nerve block [11].

Methods of Anesthesia for Breast Surgery

Breast Innervation

Adult female breasts are located in front of the second to sixth ribs; most often the location is in front of the pectoralis major muscle, with a small proportion located on the superficial surface of the serratus anterior and rectus abdominis sheath. The skin of the breast is innervated by the second to sixth intercostal nerves and the supraclavicular nerve (Fig. 3) [10]. The supraclavicular nerve innervates the skin of the upper part of the mammary gland, the anterior cutaneous branch of the intercostal nerve innervates the inner skin of the mammary gland [12], the lateral cortex of the intercostal nerve innervates the outer skin of the mammary gland [13], and the axillary region and the inner side of the upper arm are innervated by the intercostobrachial nerve (formed by the confluence of the lateral branch of the second intercostal nerve and the lateral branch of the third intercostal nerve) [14]. The intercostobrachial nerve exhibits variable height in extrathoracic anatomy, can accept the confluence of other intercostal nerves (T1, T4), and can form various confluences with branches of the brachial plexus (forearm medial cutaneous nerve, posterior forearm nerve). The nipple–areola complex is mainly innervated by the fourth intercostal nerve [15], though the exact innervation of the nipple–areola complex remains controversial because of numerous anatomical difficulties and anatomical variations. Most of the skin of the breast is innervated by the intercostal nerves. The thoracic wall muscles are innervated by branches of the brachial plexus nerves; among them, the lateral pectoral nerve innervates the upper part of the pectoralis major, whereas the medial pectoralis nerve innervates the lower part of the pectoralis major and the pectoralis minor.

Fig. 3
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(Reproduced with permission from Dr B. Versyck)

Innervation of the anterolateral chest wall and axillary region. The lateral, intermediate, and medial supraclavicular nerves (purple) are branches of the superficial cervical plexus that supplies the skin of the upper part of the mammary gland. The medial brachial cutaneous nerve (blue) and lateral and pectoral nerves (black) are branches of the brachial plexus. The intercostobrachial nerve (blue) is a branch of the T2 intercostal nerve that supplies the skin of the axilla together with the medial brachial cutaneous nerve. The lateral cutaneous branches (green) and anterior cutaneous branches (red) of the T2–T6 intercostal nerves are otherwise responsible for most of the cutaneous innervation

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Nerve Block Requirements for Different Types of Breast Surgery

Understanding the anatomy and extent of resection for each type of breast surgery is important for planning anesthesia and perioperative pain relief. Surgery involving the breast varies greatly in terms of the extent of tissue resection, anatomical changes, and the innervating nerves involved. Hence, each type of breast surgery needs to be discussed separately. The nerve blocks required for various breast surgeries are summarized in Table 1.

Table 1 Nerve blocks required for various breast surgeries
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Benign lumpectomy: In this procedure, a breast lump and some surrounding breast tissue are simply removed, and to block the nerves in the surgical area, the lateral cutaneous branch or anterior branch of the intercostal nerve is selectively blocked according to whether the lump is lateral or medial to the nipple [16].

Simple mastectomy: The scope of this procedure extends up to below the clavicular plane, down to above the rectus abdominis muscle, medial to the parasternal side, and lateral to the anterior border of the latissimus dorsi muscle. The breast tissue is completely removed from the fascia of the pectoralis major, and a drainage tube is placed. The surgical scope is extensive; the pectoralis major fascia remains intact, and the supraclavicular nerve and lateral and anterior cutaneous branches of the intercostal nerves need to be blocked.

Modified radical mastectomy: The entire breast, pectoralis minor muscle, and axillary lymph nodes are removed, involving a large area. The axillary area is jointly innervated by the intercostal brachial nerve and the long thoracic nerve. The medial thoracic, lateral thoracic, and supraclavicular nerves also need to be blocked, as do the lateral and anterior cutaneous branches of the intercostal nerves [17].

Breast reconstruction: Breast reconstruction mainly includes prosthesis placement and autologous tissue reconstruction. Prosthesis placement involves placing a prosthesis between the pectoralis major and minor muscles. The operation requires blunt separation of the pectoralis major and pectoralis minor, stretching of the serratus anterior, and injury to or rupture of the medial pectoralis nerve, lateral pectoralis nerve, and lateral cutaneous branch of the intercostal nerves [18]. These nerves need to be blocked simultaneously. Lower abdominal flaps and latissimus dorsi myocutaneous flaps are most commonly used for autologous tissue reconstruction. When latissimus dorsi flaps are used, the thoracic and dorsal nerves should be blocked. Modified serratus anterior block is particularly suitable in such cases.

Breast augmentation and breast reduction surgery: Pain due to submuscular breast augmentation mainly originates from stretching of and damage to the pectoral muscle, and mainly involves the medial and lateral thoracic nerves [19]. Many surgeons prescribe muscle relaxants to relieve pain.

Methods of Anesthesia and Analgesia for Breast Surgery

Comprehensive factors, such as the type of surgery, anatomical condition, postoperative analgesia, and individual patient factors, need to be considered to select the appropriate block method, and a large number of clinical trials are needed for verification. Thoracic epidural block can be employed as an independent anesthesia method for breast surgery, with an accurate analgesic effect. However, the difficulty of puncture and surgery risk are high because of the influence of coagulation function. Additionally, there is the potential for complications such as epidural hematoma, spinal cord injury, total spinal anesthesia, and hemodynamic instability, among others, which are difficult to manage. Therefore, this method is rarely used.

General anesthesia is the most common anesthesia method for breast surgery. Opioids are an essential part of general anesthesia and intravenous analgesia. Nevertheless, opioid use leads to many side effects, such as delayed awakening, hyperalgesia, nausea, vomiting, itching, and respiratory depression, which affect the quality of life during recovery.

Regional block techniques are often used in combination with general anesthesia in breast surgery. With the popularization of ultrasound, the scope of clinical application of regional block has been greatly increased. Under ultrasound, nerves, blood vessels, muscles, and fascia can be clearly identified, and the travel path of the puncture needle and the diffusion of local anesthetic solution can be observed in real time. The probability of the puncture needle injuring blood vessels and surrounding tissues by mistake is very small. It is safer to conduct puncture, effectively control complications, and ensure the accuracy and reliability of the blocking effect. As a result of the complex distribution of nerves in the mammary gland and surrounding areas, there are a variety of nerve block techniques for analgesia. Regional block techniques, such as intercostal nerve block, erector spinae plane block, paravertebral block, and thoracic nerve block, have been advocated for the management of postoperative pain in patients with breast cancer [20, 21]. The pectoral nerve block I (PECS I) block involves the medial and lateral thoracic nerves, and the pectoral nerve block II (PECS II) block involves the lateral cutaneous branch of the intercostal nerve, the intercostal brachial nerve, and the long thoracic nerve. The block covers the axilla and breast region, but the block is close to the surgical area, which may damage the surgical tissue in the axilla [22].

SAPB is a widely used regional block technique in breast surgery. The serratus anterior plane shows obvious muscle layers under ultrasound, and the position of needle insertion is superficial and away from the pleura and blood vessels. Compared with other regional block techniques, SAPB is less difficult, the potential complications are less serious, and the single-point block range is sufficiently wide to provide regional analgesia for breast surgery.

Application of SAPB in Breast Surgery

In mastectomy, the injection site of the superficial serratus anterior block is close to the surgical area, which may cause muscle edema or hematoma, affecting the operation; furthermore, the local anesthetic injected may be flushed away and suctioned, impacting the effect of the block. In contrast, deep serratus anterior block is not affected by surgery. Edwards et al. reported that deep serratus anterior block improved postoperative analgesia to a greater extent and reduced opioid consumption than superficial serratus anterior block in mastectomy patients [23]. Additionally, Piracha et al. suggested the use of deep serratus anterior block in breast surgery to avoid long thoracic nerve block that may occur in superficial block, and to avoid the blockage of nerve response leading to unclear nerve identification and impairment of scapular function [24]. Modified serratus anterior block can effectively block the thoracodorsal nerve, which is especially suitable for patients undergoing breast reconstruction with a latissimus dorsi myocutaneous flap [25].

Xiao et al. applied SAPB before breast cancer surgery and for 3 days thereafter. The authors reported that preoperative continuous serratus anterior block can reduce postoperative pain and adverse reactions in these patients, reduce anxiety, and improve postoperative outcomes. Regarding the quality of early recovery, analgesia and rehabilitation effects are better than postoperative intervention [5]. Regardless, there are some problems when continuous SAPB is utilized for modified radical mastectomy. First, continuous SAPB may increase the risk of catheter-related infections. Second, continuous SAPB may delay the patient’s early postoperative activity, and moving the affected upper limb early may shift the position of the indwelling catheter; the position of the catheter is easier to fix in deep serratus anterior block.

Deep SAPB with an indwelling catheter may be suitable for use in modified radical mastectomy for breast cancer. SAPB alone can provide regional analgesia for nearly all anterolateral breast procedures. If the surgery involves the medial area of the breast, an anterior cutaneous branch of the intercostal nerve can be added to increase medial analgesia during breast surgery [26]. If the surgery involves the pectoral muscles, the combination of a type I thoracic nerve block and SAPB can provide good regional analgesia in the surgical area.

Efficacy of SAPB in Breast Cancer Surgery

Modified radical mastectomy is the main treatment for breast cancer, which involves the breast and axillary region, has a wide range and large trauma, and is accompanied by moderate to severe postoperative pain. The problems of postoperative pain and rapid recovery of patients with breast cancer need to be solved urgently. At present, there have been some reports on the use of SAPB for postoperative analgesia and rapid recovery of breast surgery, which are summarized in Table 2.

Table 2 Overview of clinical application of serratus anterior plane block in breast surgery
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Effect of SAPB on Acute Pain After Breast Cancer Surgery

Patients with breast cancer experience moderate to severe pain. A series of prospective clinical studies have shown that preoperative SAPB can reduce postoperative pain scores after breast cancer surgery, the probability of rescue analgesia, and the need for perioperative opioids, suggesting that SAPB can provide good postoperative analgesia in patients with breast cancer [27,28,29,30]. A systematic review of regional block techniques for postoperative analgesia in breast tumors showed that paravertebral block and SAPB had a high probability of reducing pain at 24 h after major oncologic breast [31]. Gupta et al. reported that although patients in the classic thoracic paravertebral block and serratus anterior block groups had similar intraoperative fentanyl requirements and similar postoperative visual analog scale (VAS) scores and both blocks exerted good analgesic effects, the duration of muscle blockade and analgesia was shorter in the serratus anterior block group (245.6 ± 58 min vs. 346 ± 57 min). Considering the difficulty and related complications of thoracic paravertebral block, SAPB can be used as an alternative for postoperative analgesia in breast cancer surgery [32].

Application of SAPB in PMPS

Some studies have shown that approximately 25–60% of mastectomy patients may experience progression to postmastectomy pain syndrome (PMPS) [33]. The current main treatment methods for PMPS include drug therapy, physical therapy, and minimally invasive therapy [34], with drug therapy as the classic method. When pharmacological treatment is ineffective, a regional block technique is recommended as an alternative; among these techniques, serratus anterior block better targets the neural network that innervates the chest wall and breast [6] and is considered beneficial.

Prevention of Pain by SAPB Before Surgery

In the basic concept of pain physiology, a block of nociceptive nerve fibers before a stimulus will reduce the central sensitization and wind-up of pain, and reduce perceived pain for a long period [35]. Preoperative SAPB conforms to this rationale. Fuzier et al. suggested that SAPB before breast surgery appeared to be a protective factor for the risk of pain at 3 months [36]. In a prospective controlled study of SAPB, Qian et al. found that the incidence of chronic pain syndrome 3 months after the operation was 46/89 (51.7%) in the control group and 22/99 (25.6%) in the SAPB group, suggesting that preoperative SAPB reduced the incidence of postoperative chronic pain syndrome in patients with breast cancer [1]. Most of the analgesic benefit of the preoperative block was short-lasting and modest, and it may be that an even longer block period utilizing catheter techniques and repeated local analgesia dosing for several days is necessary to yield maximal benefit [37].

SAPB After Occurrence of Postoperative Chronic Pain Syndrome

Takimoto et al. applied the serratus anterior block technique in a patient with PMPS with a pain score of 10. Sustained pain reduction occurred, and the patient did not require any interventional management for pain for 11 months after the last injection [38]. Silva et al. [39] conducted a retrospective study. On the basis of analysis of pain data, 74.1% of the patients reported at least a 30% decrease in pain intensity after 24 h, 44% a 30% decrease in pain intensity after 1 week, and 71.4% a decrease in pain intensity of at least 50% after 1 month. It is worth noting that SAPB has not been effective in all studies [40]. Studies have shown that patients with implant-filled breast reconstruction who describe pain as tightness have good results with SAPB; in contrast, patients with iatrogenic scar tissue have more fascia surrounding the scar tissue, which is thicker and less flexible and hinders full diffusion of the anesthetic and thus renders SAPB less effective [38, 41].

The hypothesis that SAPB can alleviate PMPS is a research area worthy of further exploration. Overall, there is a lack of consensus regarding the measurement of PMPS; there have been few studies on SAPB for PMPS, and most to date have been case reports. More such trials are needed to evaluate the efficacy of SAPB for PMPS. Future research should focus on determining which types of PMPS are most suitable for SAPB intervention and which level of SAPB is most suitable to guide doctors in making the best choice for patients.

Acceleration of Rapid Patient Recovery

SAPB reduces perioperative opioid use and the incidence of nausea and vomiting to some extent and improves patient comfort [29]. SAPB can also provide a good postoperative analgesic effect, reduce the use of opioids, the acute inflammatory response, and the release of serum inflammatory factors, and accelerate the time to first flatus and ambulation. Yao et al. conducted a prospective study to observe the effect of preoperative SAPB on the quality of recovery after breast cancer surgery. The results showed significantly higher 40-item recovery quality scores in the SAPB group than in the control group at 24 h after surgery, suggesting that SAPB improved the quality of recovery after breast cancer surgery [42]. Hards et al. also reported that SAPB could control nausea and vomiting after mastectomy well and was helpful for reducing the time to ambulation, with only 64% of patients in the control group able to ambulate on the first day after surgery [28].

The use of SAPB for adequate postoperative analgesia and minimal opioid use can further help to control postoperative nausea and vomiting, promote early recovery of gastrointestinal function, and facilitate early activity and rapid postoperative recovery [43].

Effect of Regional Blockade on Cancer Recurrence and Patient Prognosis

In terms of cancer recurrence, regional blockade may offer some benefits: (1) reduction of the stress response may reduce associated immunosuppression [44], (2) pain reduction may reduce opioid use and thus the possible adverse effects of opioids on cancer recurrence [45], and (3) amide local anesthetics may have direct antitumor effects [46, 47].

Most studies in which the authors are evaluating the effect of regional blockade on recurrence and prognosis in patients with breast cancer have been retrospective, with uncertainty regarding the direction of the association and conflicting results [48,49,50]. First, there are many interfering factors. Complications, nosocomial infection, tumor biology, immunity, and timely tumor treatment (radiotherapy, chemotherapy) after surgery all have an impact on recurrence and prognosis [51]. Second, regarding the prognosis of patients with tumor recurrence, the follow-up period required is long, and it can be difficult to track details over long periods. Another limitation in the study of breast cancer recurrence and prognosis is that the recurrence rate is not high, and small sample sizes will increase the difficulty of determining a statistically significant difference.

Overall, proving the efficacy of a single intervention (regional block) in a complex multifactorial tumor setting is very challenging. A well-designed, standardized, prospective randomized controlled trial with a sufficient sample size is required to determine whether SAPB may have clinical benefits in terms of tumor recurrence and patient prognosis.

Conclusion

SAPB is a simple, safe and effective emerging regional block technique. Compared with other regional block techniques, it can target the nerve network innervating the chest wall and breast well and provide good analgesia in the anterolateral chest wall. It is expected to be used as an independent regional anesthesia in minimally invasive breast surgery. The application of SAPB in breast surgery can reduce the use of opioids in the perioperative period, relieve acute pain after surgery, reduce the incidence of postoperative PMPS to a certain extent, and is conducive to rapid patient recovery. Nevertheless, the impact on tumor recurrence and patient prognosis remains controversial, and more trials are needed to demonstrate the effect of SAPB on the prognosis of recurrent tumors. With the promotion of day surgery and fast track surgery, the clinical application of SAPB will be more extensive.