Proximal "High" Radial Nerve Damage Caused by Humeral Fracture Due to the fact that our patient is the latest edition to a herd of seven horses that were recently relocated places the foal at a high risk for injuries associated with acclimation to the group's social order. After the arrival to the new location the adult horses engaged in aggressive activities to re-establish social order. After the social hierarchy was re-established the adult horses in the herd may have still felt threatened by the new addition that was becoming more independent and venturing further and further from the brood mare. Perhaps an adult horse deliberately kicked this foal to intimidate it and reinforce the social hierarchy. Another possibility would be that the foal injured itself through a traumatic incident in the pasture such as a fall. Since four weeks passed from the trailer ride to the onset of clinical signs we feel that it is unlikely that a traumatic incident in transit could be responsible for our patient's clinical presentation. Lets focus on the theory that another horse in the herd kicked our patient. The onset of clinical signs likely occurred immediately after this traumatic incident due to the severe nature of the injury. We are speculating that an adult horse used a hind leg to deliver a traumatic blow to the foal's right shoulder/elbow region. Please reference hypothesis three for the theory of solely soft tissue damage to this region. In our current hypothesis we are speculating that the blow was traumatic enough to not only elicit soft tissue damage but also to cause a complete displaced midshaft humeral fracture. A severe blow may have caused the distal portion of the humeral fracture to shift medially causing severe soft tissue damage and radial nerve avulsion. Recall that the radial nerve is present on the medial side of the proximal and midshaft humerus. A complete midshaft humeral fracture that shifted medially could have lacerated soft tissue as well as the radial nerve. Since two weeks have passed and the foal is continuing to shows signs of a radial nerve deficit we feel that complete avulsion of the radial nerve may have occurred rather than just soft tissue swelling. This speculation is based on the thought that if there was no bone fracture present and only soft tissue swelling a two week period may allow enough time for soft tissue swelling to reduce adequately and promote radial nerve function. Since the shoulder and elbow are heavily muscled it can be extremely hard to detect instability and crepitation when humeral fractures are present. Radiographs are required rule out the presence of humeral fractures if they are suspect. Horses with humeral fractures typically present with dropped elbows due to either complete separation of the proximal and distal humerus or concurrent radial nerve damage. Horses with "high" radial nerve deficits are not able to bear weight on the affected limb and also are not able to extend the joints of the lower leg. Since this horse is able to bear weight on the affected limb we feel that a "low" radial nerve deficit could be an alternate hypothesis. With a "low" radial nerve deficit the horse would still be able to extend the elbow but would not be able to extend the carpus or the digits. Since the radial nerve migrates to the lateral side of the leg just below the humerus a "low" radial nerve deficit could easily occur through a traumatic blow to that region. However for the "high" radial nerve damage hypothesis we are speculating that a combination of a humeral fracture with soft tissue swelling/inflammation and radial nerve avulsion led to the loss of function of the foal's right forelimb. The initial fracture of the humerus and soft tissue trauma would have caused hemorrhage and leakage of cellular components into the interstitium of affected muscle groups and overlying soft tissue structures. Cellular components released such as histamine and prostaglandins would have initiated a local vasodilation. Neutrophils using their chemotactic properties would migrate to the affected area and degranulate. Their degranulation would begin to break down the damaged cellular components and initiate more swelling and vasodilation. Macrophages chemotactic for neutrophil degranulation would also begin to migrate to the area and phagocytize necrotic tissue and exhausted neutrophils. Unfortunately this inflammatory reaction would not only phagocytize necrotic debris but it would also damage local intact tissue. The shearing of the radial nerve by the humeral fracture may have resulted in complete avulsion of a portion of the radial nerve. The inflammatory process described above would also phagocytize damaged and intact portions of the radial nerve. Without an intact radial nerve in the humeral region innervation to any of the muscles or sensory structures supported by the radial nerve distal to this site would no longer function. We would definitely expect fibrin deposition to have all ready occurred around the fracture and some degree of fibrous connective tissue may have also been deposited. If the blood supply is still intact around the displaced humeral fracture we may see mild hypertrophy just beginning to occur; however, since only two weeks have probably passed since the injury it is highly unlikely that any hypertrophy will be present. The damage to the radial nerve is proximal to its innervation of the triceps brachii muscle. This muscle serves to extend the elbow and flex the shoulder. The triceps can no longer support the limb and therefore the elbow appears dropped. The triceps appears atrophied because there is a lack of radial nerve innerveration. Muscle groups without innervation will atrophy unless innervation is re-established. The inability to extend the elbow without the use of the triceps brachii, leads to the inability to support weight on the affected limb. As the horse bears weight on the limb, the elbow will collapse the limb by flexing because the triceps cannot apply the forces necessary to keep it extended. The radial nerve also innervates the extensor carpi radialis, common digital extensor, and the lateral digital extensors. The extensor carpi radialis serves to extend the carpus, while the common digital extensor and the lateral digital extensor serve to extend the carpus and the digit. Due to the loss of innervation of these muscles, the foal looses the ability to extend the distal limb resulting in knuckling over of the carpus and digit.