Embryological basis of variation:
The positional variations in the scalene muscles and BP may be explained by their embryological development. The scalene muscles arise from the inferior embryonic segment of the cervical myotome, and the common scalene muscle is formed in the eighth week of embryonic development. The whole process is influenced by the surrounding neurovascular structures.[3] During the 4th week of embryogenesis, the ventral rami of C4-C8 and T1, which eventually develop into the BP, grow from the neural tube and extend toward their corresponding sclerotomes and myotomes. The developing BP later penetrates the common scalene muscle along with the SA, dividing it into the ASM and the MSM. This results in considerable variability in the relationship between the components of the BP and the ASM, including the situation mentioned in this case.[4]
Clinical Significance:
The possibility of BP variation is very high, and the variation comes in many forms. Therefore, clinicians are required to have a solid knowledge base of anatomy to make accurate diagnoses and to perform precise surgeries.
Thoracic outlet syndrome (TOS), which is caused by compression of the BP and subclavian vessels as they exit the thoracic chest, often presents with neurological and vascular symptoms. TOS is now classified into three categories, including neurogenic thoracic outlet syndrome (NTOS), arterial thoracic outlet syndrome (ATOS) and venous thoracic outlet syndrome (VTOS), among which NTOS is the most common type, accounting for 94–95% of all TOS cases. Diverse variations in the pathway between the BP and ASM, under which circumstances the BP is easier to compress, are considered to be an important pathogenesis for NTOS.[3, 4] Many case studies have found that C5 and C6 can penetrate through the ASM together[5], and Anjali Aggarwal presented several cases in which the BP traveled lateral to the axillary artery[6]. Niki T. Patel collected 78 cadavers and discovered nine different patterns of BP variation.[7] However, there are very few cases presenting variations in which C5 travels anterior to the ASM. In some case series studies, the researchers found a small proportion of subjects with a variation in which C5 passes anterior to the ASM[8], and some of the researchers attached ultrasonic photographs of the variation, but their paper still lacks a detailed description of the variation and photographs of the actual cadaver. It is worth noting that Gagandeep Kaur Aheer et al. found a case in which both sides of C5 passed anterior to the ASMs[9], but a case in which the left C5 passed anterior to the left ASM and the right C5 passed through the right ASM is new. The case presented herein reveals a new potential anatomical cause of NTOS. Furthermore, the author presenting the case referenced previous studies and reminded clinicians that it is important to note the bilateral differences when diagnosing or treating patients with TOS.
A BP block for anesthesia is essential for shoulder surgery and upper limb surgery. The anatomical characteristics of the BP branches are the basis of a BP block. A BP block can be performed in the interscalene pathway, supraclavicular pathway, or axillary area pathway. For patients with variations in the pathway between the BP and the ASM, a BP block between the interscalene pathway may lead to an inadequate BP block during shoulder surgeries. Therefore, it is important to confirm whether the patient has any regional anatomy variations by performing an ultrasound prior to BP block to reduce the possibility of complications. Furthermore, an interscalene BP block also has postoperative analgesic benefits for patients who suffer from tremendous pain after undergoing shoulder or upper limb surgery. Thus, it is possible for patients with BP variations to have poor analgesic effects due to an interscalene BP block.[10]
BP injuries are a series of severe, life-changing illnesses that usually occur after high-intensity injuries in working-age men. Most surgeons choose to routinely explore the supraclavicular BP and its root in patients with upper trunk or upper limb injuries to ensure that an accurate and conclusive diagnosis is determined, so as to identify available donor nerve roots for nerve grafting, estimate the time of recovery, construct appropriate plans for distal reconstruction, and determine the potential need for neurolysis. Having a solid knowledge base of the probable variations of the BP helps surgeons to explore the BP more thoroughly, thereby avoiding missing lesions or missing potential donor nerve roots.[11] Additionally, it is worth noting that BP injuries can occur in newborns during labor, which is specifically called a ‘brachial plexus birth injury (BPBI)’. Erb’s palsy (upper BP injury involving C5–C6 ± C7) is the most common type of BPBI, and alternative treatments include conservative interventions (gentle passive movement training and sensory stimulations), electrostimulation of the lesion, and surgeries including nerve transfer and nerve grafting. Thus, understanding potential BP variations is crucial for BPBI management because the therapeutic site is determined by the patient’s specific anatomical structure.[12]
Limitations and Prospects
This case report not only presents a new precaution for clinical practice but also reflects educational significance, which reminds clinical medical students to note the possible differences between actual human bodies and anatomy descriptions in textbooks. Considering that photographs of the actual cadaver are provided, the accuracy of the descriptive case report can be guaranteed. However, since the report includes only one middle-aged female cadaver from Zhejiang Province of China, further studies with larger and more variable samples of cadavers from different regions and countries are needed. Additionally, the lack of a detailed medical history of the body donor limits our ability to explore other possible clinical manifestations related to BP variation. In addition, the molecular mechanisms of different BP variations that occur during embryological development, as well as the possibility of variations in other segments of the BP, need to be explored in more studies.