Muscle atrophy or wasting can occur due to many different reasons and scenarios. In the context of peripheral nerve disorders it occurs, generally, due to denervation or disuse. Note that atrophy due to disuse or nerve compression is not time sensitive when it comes to reinnervation. Generally speaking, there are enough nerve fibers in continuity to preserve the ability of the muscle to be reinnervated. However, atrophy due to nerve transection is time sensitive because no nerve fibers are in continuity to preserve the muscle. This must be reinnervated before the muscle becomes unresponsive.
Atrophy in a Specific Injury Patterns:
• Complete brachial plexus injury at the proximal root level may cause wasting of muscles that derive their innervation directly from the roots (rhomboid and serratus anterior muscles) as well as all of the muscles to the upper extremity.
• Upper plexus injury will show wasting of the C5 and C6 innervated musculature, which includes supraspinatus, infraspinatus, deltoid, biceps, and brachialis muscles.
• Axillary nerve injury will have visible wasting only of the deltoid (you can’t see the teres minor atrophy, which is also innervated by the same nerve)
• A proximal ulnar nerve injury shows wasting of the flexor carpi ulnaris and a hollow at the ulnar or medial aspect of the forearm as well as more distal wasting of the hand intrinsic muscles.
• A distal ulnar nerve injury will only have the hand intrinsic muscle wasting. This can be seen, especially, at the first dorsal interosseous where there will often be a significant hollow or depression.
• A partial distal median nerve injury (such as that seen in severe carpal tunnel syndrome) may demonstrate only mild atrophy at the thenar musculature depending on both the extent of muscle loss as well as the degree of cross-innervation of the thenar musculature by the ulnar nerve.