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. On the one hand, direct injury to the nerves above the collarbone, such as in automobile accidents or fractures, seems to intuitively explain why there is pain. On the other hand, because it is more difficult to understand why light movements with the arms extended (such as when using a keyboard) would cause pain, many physicians discount these activities as causal. My initial experience with this disorder also caused me a great deal of puzzlement until I began helping a surgeon during his thoracic-outlet dissections. It then became apparent that, in the vast majority of cases, the offending agents that kept these large nerve trunks irritated were coming from inside these nerves and that the surrounding tissues were only secondary, not primary, to the creation and worsening of symptoms. These agents produced scar like adhesions on the surface of the nerve covering, which (when examined under the microscope and with specific immune stainings) revealed the presence of pain-causing nerves in these same fibrotic tissues. Once these tissues were removed, the abnormalities in the nerve dramatically decreased. Barring frank trauma, the primary cause for symptoms and dysfunction is, thus, a localized irritation of the brachial plexus