Erb's palsy (Erb-Duchenne palsy) is caused by a sudden tension on the roots of C5 and C6 following a sudden and marked increase in the angle between the shoulder and the neck. This results in a lower motor neuron injury affecting certain muscles in the upper extremity.
Clinical features of the "flail arm":
(1) The arm hangs limply at the side, straight down (loss of shoulder extension and elbow flexion).
(2) The arm is internally rotated so that the palm faces posterior (loss of external shoulder rotation and elbow supination).
(3) The fingers are partially flexed (impaired extension of wrist and fingers).
(4) The small muscles of the hand are preserved.
Preceding events may include:
(1) shoulder dystocia during delivery
(2) motorcycle accident
(3) blunt trauma during contact sports
(4) other trauma to the shoulder
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