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Brachial plexus injuries can happen to anyone at any age due to some physical trauma or accident. In infants, this type of injury is caused by difficulties or complications during the process of childbirth. The brachial plexus is the bundle of nerves that controls muscle movement and sensation in the shoulders, arms, wrists, and hands. When damaged, the affected areas may lose sensation and even movement. 
Most infants do not end up with a permanent disability; they typically heal within a few months and have no lasting symptoms. Severe injury can happen in rare cases and may need to be corrected with surgery. As birthing procedures have improved over the last few decades, brachial plexus injuries have decreased in newborns, but they can still be caused by a number of factors.
Brachial Plexus Damage
The nerves of the brachial plexus are attached at one end to the spinal cord and at the other end to smaller nerves in the arms. They run through the neck, so if the neck gets pulled too much by stretching the head and shoulder in opposite directions, these nerves can be injured.
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Damage may range from mild stretching to small and large tears and, in the worst cases, complete separation of the nerves from the spinal cord.
Each of the five nerves of the brachial plexus control different parts of the arm, so the signs of injury will correlate with the particular nerves that have been damaged. The extent of injury dictates how severe the symptoms will be, which include numbness, weakness, loss of sensation, and paralysis. For infants that have experienced a brachial plexus injury, most will heal naturally with time, others will need some physical therapy, and a few will need surgery to repair more significant dysfunction.
There are several risk factors for brachial plexus injuries in infants. One of these is a large size at birth, which can lead to a difficult delivery if the neck gets stretched too far during the birth process.
When the infant is too large to fit easily through the birth canal, this stretching can happen to varying degrees. The medical term for a large fetus is large for gestational age and is considered to be any baby with a birth weight greater than the 90th percentile.  The average weight for a baby at birth is seven pounds.
A woman’s doctor should be estimating the birth weight as the pregnancy proceeds. It is not an exact science, but various types of measurements can lead to a relatively accurate estimation of how big the infant will be at birth. This helps a doctor decide if a woman will need to have a cesarean section to avoid complications such as an injury to the brachial plexus.
Difficult or Obstructed Labor
Another risk factor for brachial plexus injury is any difficult or prolonged labor. Any time labor becomes a challenge, for whatever reason, and goes on for much longer than normal, there is a risk that a birth injury like brachial plexus damage will occur. This may be from an obstructed birth or simply from mistakes made by the delivery team, including excessive pulling or misuse of instruments.
Normal delivery is when the head comes out first, but sometimes the shoulder gets stuck after the head emerges. This is called shoulder dystocia, and it is one of the most common causes of brachial plexus injury.
The damage occurs as the baby is being pulled out; the pressure can stretch the brachial plexus nerve bundle to the point of injury. Up to 15 percent of infants born with this complication will end up with injuries to the brachial plexus.
A breech birth is not an ideal way for a baby to be born. A baby should come out head first, but when the buttocks emerge first from the birth canal, it’s called a breech birth, and there is potential for complications. As the infant is proceeding out from the birth canal in this direction, the arms are pulled above the head. 
This makes it more difficult to get the entire body out, and the pressure applied can cause stretching of the brachial plexus nerves. As with a large fetus, a doctor should be able to recognize the risk of breech birth and determine if a cesarean section (c-section) delivery would be a safer option for the mother and baby.
Mishandling During Birth
Doctors and other caregivers are better trained in delivering babies than ever before, but they can still make mistakes. Those mistakes can lead to birth injuries, including brachial plexus damage. Whether the birth is normal or difficult, the person delivering the baby may pull too hard on an arm or shoulder and stretch out the baby’s neck. It’s also possible to use instruments, such as forceps or vacuum, inappropriately, and cause damage to the nerves.
Other Brachial Plexus Injuries
Birth-related brachial plexus injuries are the most common cause of associated palsies, but they can also be the result of trauma and injury in adults and children. A fall, an automobile collision, a bullet wound, and even playing contact sports can all lead to the kind of stretching or other trauma that damages the brachial plexus.
For most infants with this type of injury, regardless of the cause, the outlook is good. They will heal and completely recover with time. In some cases, however, the damage is too great to heal naturally, and surgery may be needed. Surgery cannot always correct all of the dysfunction, however, and in rare cases, a child may live with the signs of brachial plexus injury for the rest of their lives.
- Elizabeth G. Baxley|Robert W. Gobbo. (2004, April 1). Shoulder Dystocia. AAFP American Academy of Family Physicians.
Retrieved from: https://www.aafp.org/afp/2004/0401/p1707.html
- Large for Gestational Age (LGA). (n.d.). Children's Hospital of Philadelphia.
Retrieved from: https://www.chop.edu/conditions-diseases/large-gestational-age-lga
- Brachial Plexus Injury in Newborns: MedlinePlus Medical Encyclopedia. (n.d.). MedlinePlus - Health Information from the National Library of Medicine.
Retrieved from: https://medlineplus.gov/ency/article/001395.htm