Erb’s Palsy (Erb-Duchenne paralysis) is weakness or paralysis in the arm caused by an injury to the brachial plexus. This network of nerves near the neck sends signals to the arm, hand, and shoulder. The condition most commonly occurs during a difficult childbirth when the infant’s neck is stretched, causing trauma to the upper nerve group within the plexus. The physical presentation is distinct, reflecting the specific muscle groups that have lost their nerve supply.
The Classic Physical Appearance
The most recognized sign of Erb’s Palsy is a specific, resting posture of the affected limb, often referred to as the “waiter’s tip” position. This name comes from the way the arm hangs by the side, resembling a waiter holding out their hand to receive a tip. The shoulder is typically adducted (held close to the body) and rotated internally.
The elbow joint is usually extended, or straightened, rather than held in a natural, slightly flexed position. The forearm is also pronated, meaning the palm faces backward or downward. This combination of shoulder adduction, internal rotation, and forearm pronation creates the characteristic limp and unnatural appearance.
The affected arm may appear flaccid, lacking the spontaneous movement seen in the unaffected limb. In newborns, the Moro reflex (a startle response) will be absent on the injured side. The hand and fingers, which are controlled by nerves lower in the plexus, generally retain their ability to grasp.
Functional Deficits Causing the Posture
The unique physical appearance results directly from damage to the upper nerve roots of the brachial plexus, specifically the fifth and sixth cervical nerves (C5 and C6). These nerve roots supply muscles responsible for fundamental shoulder and elbow movements. The resulting weakness or paralysis in these groups leaves stronger, opposing muscles unopposed, pulling the limb into the “waiter’s tip” posture.
Damage to the nerves supplying the deltoid muscle causes an inability to lift the arm away from the body (shoulder abduction). The supraspinatus and infraspinatus muscles, necessary for shoulder external rotation, are weakened, leading to constant internal rotation. The biceps and brachialis muscles, which bend the elbow, lose function due to injury of the musculocutaneous nerve.
This loss of elbow flexor strength leaves the arm extended, maintained by unopposed extensor muscles. The lack of biceps function also prevents the forearm from turning the palm upward, resulting in the pronated position. This entire presentation reflects the specific motor functions lost due to the C5 and C6 nerve root injury.
Variations in Presentation and Associated Signs
The presentation of Erb’s Palsy varies widely depending on the severity and extent of the nerve injury. Mild cases may involve only slight shoulder weakness or limited range of motion, rather than a fully flaccid arm. More severe cases can involve complete paralysis of the entire arm, indicating injury that extends beyond C5 and C6.
Over time, secondary visual signs develop due to the lack of muscle activity. Muscle atrophy (wasting and shrinking of affected muscles like the deltoid and biceps) can become noticeable. This atrophy contributes to a visible difference in size and appearance between the affected and unaffected limbs.
The injury can also lead to sensory deficits, such as numbness or decreased sensation along the outer part of the upper arm and forearm. These deficits correspond to the skin areas supplied by the damaged nerves. In cases where the injury is significant and occurs early in life, impaired bone growth can lead to a noticeable limb length discrepancy.