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MARCH | APRIL 2017

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88 m a s s a g e & b o d y w o r k m a r c h / a p r i l 2 0 1 7 innervation for a particular nerve is always smaller than a dermatome because the dermatome includes fiber innervation from other nerve roots as well. A good comparison is to look at the C8 dermatome (Image 5), which extends the length of the upper extremity, and compare that with the cutaneous innervation of the ulnar nerve. Cutaneous innervation of the ulnar nerve is only on the ulnar aspect of the hand, but does not include the medial aspect of the forearm or upper arm that is included in the entire C8 dermatome. There can be some variation in these dermatome locations, so maps may differ slightly. A single nerve root has fibers that blend into different peripheral motor nerves. The group of muscles supplied by fibers from a single nerve root is referred to as a myotome. If a nerve root is being compressed, weakness or atrophy in any of the muscles that have fibers supplied from that nerve root (the nerve root myotome) could exist. Recognizing dermatome or myotome symptom patterns is a key facet of locating a site of peripheral nerve injury. The second key term of nerve injury is peripheral neuropathy. Pathology farther along the length of the nerve is called a neuropathy. Neuropathy literally means "damage to or disease affecting the nerves." Peripheral indicates that the injury is in the peripheral nerves, distant from the nerve roots and spinal cord. Many nerve compression syndromes, such as thoracic outlet and carpal tunnel syndromes, are examples of peripheral neuropathies. Both motor and sensory symptoms can occur from peripheral neuropathy. In some cases, the symptom pattern may give an indication of the severity of the injury. For example, there is a greater percentage of sensory fibers in the distal median nerve compared to motor fibers. That is why people who develop carpal tunnel syndrome tend to develop sensory symptoms first. If motor symptoms (weakness with grip strength) are present, this indicates a greater degree of nerve injury because more fibers (including more motor fibers) are affected. Peripheral neuropathies also produce motor symptoms. If the motor nerve root fibers are being compressed, weakness or atrophy of the muscles being supplied by that nerve will result. The location of nerve entrapment or impairment has a direct bearing on which muscles will be affected. The more proximal a nerve compression pathology is, the greater the number of muscles that will be affected. For example, there are three muscles innervated by the same nerve indicated in Image 6. If compression is occurring at Site A, all three muscles could be affected with weakness or atrophy. If compression is occurring at Site B, weakness or atrophy would only be expected in muscles two and three. Knowledge of common nerve entrapment sites is therefore crucial to knowing what type of symptoms should be expected. If a nerve compression is more proximal, there is a greater likelihood that more motor and sensory regions would be impacted. The more distal the site of nerve compression is, the fewer motor or sensory fibers will be affected. Knowing when a nerve is purely motor or purely sensory is also helpful because the pathology may produce only motor or only sensory symptoms because the affected nerve is not a mixed nerve. BIOMECHANICS OF NERVE INJURY Nerve injury can occur from either compression or tension (pulling force on the nerve). Compression injuries are more common and make up the large majority of peripheral neuropathies that you hear about, such as carpal tunnel syndrome or thoracic outlet syndrome. Nerve injuries generally develop from compressive loads, such as a direct blow to the nerve or a chronic lower-level compression. Tension injuries on nerves don't receive as much attention as compression injuries, yet are increasingly recognized as a likely cause of many nerve-related symptoms. With excess tensile stress, the overall diameter of the nerve decreases, thereby compressing the fibers within the nerve; this condition is called adverse neural tension. Recognizing the biomechanical forces of compression or tension on nerves helps determine how they may be involved in various clinical presentations. C8 dermatome and cutaneous innervation of the hand. 5 Cutaneous innervation of ulnar nerve C8 dermatome

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