Issue link: https://www.massageandbodyworkdigital.com/i/918051
A B M P m e m b e r s e a r n F R E E C E a t w w w. a b m p . c o m / c e b y r e a d i n g M a s s a g e & B o d y w o r k m a g a z i n e 89 This brings up an interesting anatomical factor regarding neural anatomy. There are actually very small nerve fibers that innervate the outer sheath of any nerve. Essentially, this is sensory nerve supply to the nerve itself. The small fibers that innervate the outer sheath of the nerve are called the nervi nervorum. The nervi nervorum fibers are sensitive to both compression and stretching of the nerve. One of the common complaints from those experiencing DSN pathology is pain when performing overhead activities with the arm. Because of shoulder mechanics and the pathway of this nerve, it is likely that the DSN nerve is getting stretched during these activities. In addition, if there is even a slight degree of compression on the nerve, stretching or pulling the nerve fibers taut could aggravate the compressive force. When you do activities that either stretch or compress the nerve, you are reinforcing those signals of irritation to the central nervous system. The more those signals get reinforced, the less it takes to set them off, which can lead to perpetual and chronic pain and irritation of the DSN and the muscles it innervates. One anatomical study found a communicating branch between the DSN and the long thoracic nerve, which innervates the serratus anterior muscle and can also be implicated in scapular winging when it is compressed. 2 This connecting branch between the two adjacent nerves will increase the likelihood that nerve compression or 2 tension sensations would be felt in areas innervated by either of those two nerves. Compression of Small Nerve Branches The second possible cause of DSN pathology is compression of any of the small branches of the nerve by taut bands of muscle in the upper thoracic region. Myofascial trigger points are frequently associated with these taut bands and muscle tightness that constricts the small branches of the nerve, that may then cause mid-thoracic and medial scapular border pain (along the path of the DSN). The symptoms of nerve compression, whether from taut bands of muscle in the mid-thoracic region or by the middle scalene muscle, could be identical and difficult to distinguish. For that reason, any comprehensive treatment aimed at addressing potential nerve compression in this area should thoroughly address the cervical, shoulder, and upper-back regions. Dysfunctional Feedback Loop The third major contributing factor to DSN pathology is a dysfunctional feedback loop that happens with muscular weakness. Compression of the nerve interrupts motor signals and causes weakness and atrophy in the levator scapulae and rhomboid muscles. As noted earlier, the most prominent clinical indicator of this motor impairment is winging of the scapula. When the scapula pulls away from the thoracic rib cage in the scapular winging dysfunction, the medial border lifts up in a lateral direction away from the spine during certain movements. As the bone moves out in this position, it pulls on the skin and superficial cutaneous nerves of the upper-back region. These superficial nerves innervating the skin can produce mid- thoracic back pain simply because they are being tugged on. In this case, upper-back pain can be caused both by compression of the DSN as well as tension or tugging on the superficial cutaneous nerves of the upper back. The tugging or tensile forces applied to these superficial cutaneous nerves can also be aggravated in certain postural distortions. Individuals with upper thoracic kyphosis or forward-head posture and forward-rounded shoulders already have increased tensile loads across the upper back's superficial tissues. All these factors can combine to produce pain and irritation in the upper-back region. Most likely, any one of these factors alone may not produce a significant problem. It is often the compounding of these factors together that may create the pain and impairment. Other Causes of DSN Neuropathy While not common, there are some other potential causes of DSN injury that can lead to muscle weakness or pain. Myofascial trigger points are frequently described as occurring in the upper-thoracic region, and trigger point injection and dry needling have both been used as treatment approaches to address this pain. Damage of the nerve has been reported from trigger point injections, dry needling, and nerve block injections in the scalene region. 3 The pathway of the DSN is very close to that of the long thoracic nerve. There is a fair amount of literature describing pathological factors that lead to long thoracic nerve pathology. One of the common factors is heavy straps worn across the top of the shoulders, such as that from bras, backpacks, handbags, or heavy equipment bags. If your client is reporting any type of heavy strap or weight worn across the shoulder, that is a likely factor in The DSN curves around the posterior scalene muscle before descending to the back. Image is from 3D4Medical's Complete Anatomy application. Posterior scalene DSN