Erb’s palsy is a medical condition that affects movement in the arm, most frequently occurring as a birth injury. This condition results from damage to the bundle of nerves in the neck and shoulder known as the brachial plexus. Understanding the frequency and contributing circumstances provides context for its management and prevention. This article examines the incidence rates of Erb’s palsy and the primary factors involved in its development.
What is Erb’s Palsy?
Erb’s palsy is a brachial plexus injury that involves the upper nerve roots, typically the fifth and sixth cervical nerves (C5 and C6). These nerves form the upper trunk of the brachial plexus, which transmits movement and sensation signals from the spinal cord to the shoulder, arm, and hand. When these upper nerves are stretched or torn, communication to the associated muscles is disrupted, leading to weakness or paralysis.
The functional impact is most noticeable in the shoulder and upper arm, often resulting in a characteristic presentation. The arm may hang limply by the side, internally rotated, with the forearm pronated. This posture is sometimes referred to as the “waiter’s tip” position, and it reflects the loss of function in muscles like the deltoid and biceps. Though the hand and lower arm are generally spared in Erb’s palsy, the inability to lift the arm or bend the elbow significantly limits function.
Global and Regional Incidence Rates
Erb’s palsy is considered one of the more common birth injuries, with reported incidence rates varying across different studies and regions. Globally, the estimated prevalence of Erb’s palsy, also known as brachial plexus birth palsy, ranges from approximately 0.9 to 2.6 per 1,000 live births. This range suggests that thousands of newborns are affected by this condition each year.
The variation in reported statistics stems from several factors, including differences in how injuries are reported and the distinction between transient and permanent cases. Many mild injuries, which are often temporary, may not be consistently tracked across all healthcare systems, potentially skewing the lower end of the incidence estimates. Rates of permanent injury, where function does not return, are significantly lower, sometimes reported between 0.1 and 1.6 per 1,000 live births.
Primary Contributing Factors
The primary cause of Erb’s palsy in newborns is mechanical trauma during birth, resulting from excessive stretching of the neck and shoulder. This stretching occurs when the infant’s head is moved forcefully away from the shoulder, straining the brachial plexus nerves. The most significant circumstance leading to this trauma is a complication known as shoulder dystocia.
Shoulder dystocia occurs when the baby’s head delivers, but a shoulder becomes lodged behind the mother’s pubic bone, physically obstructing the rest of the delivery. Attempts to free the shoulder, such as lateral traction or pulling on the head and neck, can cause the damaging stretch injury.
Several maternal and fetal characteristics increase the risk of shoulder dystocia and Erb’s palsy. Fetal macrosomia (high infant birth weight) is a major contributor, as a larger baby is more likely to become impacted in the birth canal. Maternal diabetes, whether pre-existing or gestational, is often associated with the birth of a larger infant, further elevating the risk.
Other factors relate to the mechanics of labor. A prolonged second stage of labor increases the potential for complications. Instrumental deliveries, where forceps or a vacuum extractor are used, can also introduce forces that stretch the nerves if not applied correctly.
Injury Classification and Severity Levels
The long-term outlook depends heavily on the extent of the nerve damage. Injuries to the brachial plexus are generally classified into three main types based on the severity of the trauma to the nerve fibers. This classification helps medical professionals predict recovery and determine the appropriate treatment path.
Neurapraxia
This is the mildest form, involving a temporary block of nerve conduction without any anatomical tearing of the nerve structure. These stretch-related injuries are the most common and often allow for spontaneous and complete recovery, typically within the first few months after birth.
Axonotmesis
This is a moderate injury where the nerve fibers are damaged, but the protective outer sheath remains intact. Recovery is still possible as the inner nerve fibers can regrow, but this process takes much longer, occurring at a slow rate of about one millimeter per day.
Neurotmesis
This is the most severe category, representing a complete rupture or tear of the nerve, including both the axon and the surrounding connective tissue. This type of injury requires surgical intervention and has no chance of spontaneous recovery, often leading to a permanent loss of function if not promptly addressed. An even more severe form is avulsion, where the nerve root is completely torn away from the spinal cord.