The patient has injured the upper arm, usually by sleeping with the arm over the back of a chair. The patient now presents, usually the next day, holding the affected hand and wrist with the good hand and reports decreased or absent sensation on the radial and dorsal side of the hand and wrist and the inability to extend the wrist (wrist drop), thumb, and finger joints. With the hand supinated (palm up) and the extensors aided by gravity, hand function may appear normal, but when the hand is pronated (palm down), the wrist and hand will drop (Fig. 126.1). Symptoms can also begin several days after the initial insult, leading to a delayed presentation.