The motor impairment in cerebral palsy comes from the motor area of the brain’s outer layer (called the cerebral cortex), the part of the brain that directs muscle movement and affects the control of posture. It is not caused by problems in the muscles – these are effects.
Causes before birth
Current research suggests that for most children with cerebral palsy, the difficulty arose before birth and they were born with it, although it may not be detected until months or years later.
In some cases, the cerebral motor cortex has not developed normally during foetal growth (cerebral dysgenesis). Incorrect cell development early in pregnancy, or any interruption of the normal process of brain growth during foetal development, can cause brain malformations that interfere with the transmission of brain signals. This might be because of mutations in genes that control brain development, or some other reason. The foetal brain is particularly vulnerable during the first 20 weeks of development.
Periventricular leukomalacia (PVL) is a type of damage to the white matter of the brain. The white matter is responsible for transmitting signals inside the brain and to the rest of the body. PVL damage looks like tiny holes in the white matter of an infant’s brain. These gaps in brain tissue interfere with the normal transmission of signals. There are a number of events that can cause PVL, including maternal or foetal infection. There is a period of selective vulnerability in the developing foetal brain, between 26 and 34 weeks of gestation, in which periventricular white matter is particularly sensitive to insults and injury.
Intracranial hemorrhage describes bleeding inside the brain caused by blocked or broken blood vessels. A common cause of this kind of damage is foetal stroke. Some babies suffer a stroke while still in the womb because of blood clots in the placenta that block blood flow. Other types of foetal stroke are caused by malformed or weak blood vessels in the brain or by blood-clotting abnormalities. Maternal high blood pressure (hypertension) is a common medical disorder during pregnancy that has been known to cause foetal stroke. Maternal infection, especially pelvic inflammatory disease, has also been shown to increase the risk of foetal stroke.
Lesions and other alterations to the brain do not always show up on scans. Scans that are done early in infancy, often show more than those done later in childhood.
Although cerebral palsy is essentially a motor disorder, other alterations to normal development can occur in many cases. They can affect mental capacity, the senses and parts of the body. For example, many young children with cerebral palsy have missing enamel (enamel hypoplasia) on their primary teeth, resulting from disturbed development of enamel and dentin.
Causes at the time of birth
The second most common cause is thought to be birth complications, including asphyxia. However, it is worth noting that many babies who experience asphyxia during birth grow up without having cerebral palsy or any other neurological disorder.
Asphyxia, a lack of oxygen in the brain caused by an interruption in breathing or poor oxygen supply, happens in labour and delivery. But even though a newborn’s blood is equipped to compensate for short-term low levels of oxygen, if the supply of oxygen is cut off or reduced for lengthy periods, an infant can develop a type of brain damage called hypoxic-ischaemic encephalopathy (HIE), which destroys tissue in the cerebral motor cortex and other areas of the brain. This kind of damage can also be caused by severe maternal low blood pressure, rupture of the uterus, detachment of the placenta, or problems involving the umbilical cord. Again, often (though not always), other organs of the body are also affected.
It is also possible for traumatic damage to the brain to occur at birth.
Causes after birth
Thirdly, a small number of children have acquired cerebral palsy, which means the disorder begins after birth. It is often possible to pinpoint a specific reason for it, such as brain damage / head injury in the first few months or years of life, or direct brain infections such as bacterial meningitis and viral encephalitis, which trigger the abnormal release of brain chemicals. It is only if the infection or injury occurs in the very early years that neurological damage leads to cerebral palsy; and not every infection or injury does lead to permanent damage.
Rhesus incompatibility, also known as Rhesus Disease or Haemolytic Disease of the Newborn, can cause damage in the first 24 hours after birth. It occurs when a mother’s blood group is rhesus negative and the baby’s is rhesus positive; and in addition something in the mother’s history, such as during a previous pregnancy, has caused her to produce antibodies of a rhesus-D type, which have crossed the placenta to the baby. They have then caused haemolysis (breaking open) of the baby’s red blood cells. The results for the baby can range from mild jaundice and anaemia to a fatal reaction. It can sometimes also cause problems before that, for the foetus.
In jaundice there is a build-up of yellowish bilirubin in the blood, too much for the baby’s liver to deal with. Many babies have jaundice, but in some cases, when the bilirubin accumulates in regions of the brain (a phenomenon known as kernicterus) it causes permanent damage, to which a newborm baby is particularly susceptible. It leads to deafness as well as cerebral palsy.
Stroke can cause cerebral palsy after birth as well as before it. (Perinatal stroke is defined as between 28 weeks of gestation and 7 days after birth.)
The causal network and causal pathways
This description of the causes of cerebral palsy might make it seem as if it would be easy to work out what had happened to an individual child, and to develop preventive strategies. However, what happens in practice is that causal factors are interdependent, making things more complicated. The most obvious example of this is pre-existing damage to the foetus going on to cause premature birth or a difficult delivery in which more complications may arise. These causal pathways are a part of the study of the epidemiology (influential factors) of cerebral palsy. One causal factor occurring on its own may not result in cerebral palsy for the child, but it may do so if it interacts with another. The manner in which one causal factor can influence another, in order to make cerebral palsy more likely, is known as synergism. Those seeking to prevent or reduce the severity of cerebral palsy in that early period of life, take the approach of trying to reduce or treat synergistic risk factors.
Risk factors
What will increase a baby’s risk of being born with cerebral palsy? Monitoring thousands of expectant mothers, following them through childbirth, and monitoring their children’s early neurological development, has helped to establish these risk factors. If a mother or her baby has any of these risk factors, it does not mean that cerebral palsy is inevitable, but it does increase the chance for the kinds of brain damage that cause it.
Low birthweight and premature birth. The risk of cerebral palsy is higher among babies who weigh less than 5 ½ pounds at birth or are born less than 37 weeks into pregnancy. Many babies born extremely early, for example at 24 weeks, can survive with intensive care, but with medical problems that put them at risk for cerebral palsy. Many term or near-term babies also develop cerebral palsy because of other factors.
Multiple births. Twins, triplets, and other multiple births are at increased risk.
Infections during pregnancy. Infectious diseases caused by viruses, such as toxoplasmosis, rubella (German measles), cytomegalovirus (CMV), and herpes, can infect the womb and placenta. Researchers currently think that maternal infection leads to elevated levels of immune system cells called cytokines that circulate in the brain and blood of the foetus. Cytokines respond to infection by triggering inflammation. Inflammation may then go on to cause central nervous system damage in an unborn baby. Maternal fever during pregnancy or delivery can also set off this kind of inflammatory response.
Exposure to toxic substances. Mothers who have been exposed to toxic substances during pregnancy, such as methyl mercury, are at a heightened risk of having a baby with cerebral palsy.
Mothers with thyroid abnormalities, learning disability, or seizures. Mothers with any of these conditions are slightly more likely to have a child with cerebral palsy. Antiepileptic medication can also produce “Foetal Anticonvulsant Syndrome” in children.
There are some medical conditions during labour and delivery, and immediately after delivery that act as warning signs for an increased risk of cerebral palsy. A close eye needs to be kept on those who face a higher risk. Warning signs include:
Breech presentation. Babies with cerebral palsy are more likely to be in a breech position (feet first) instead of head first at the beginning of labour.
Complicated labour and delivery. A baby who has vascular or respiratory problems during labour and delivery.
Small for gestational age. Babies born smaller than normal for their gestational age are at risk for cerebral palsy because of factors that kept them from growing naturally in the womb.
Low Apgar score. The Apgar score is a numbered rating that reflects a newborn's condition. It measures a baby's heart rate, breathing, muscle tone, reflexes, and skin colour during the first minutes after birth. Clinicians assign points; the higher the score, the more normal a baby's condition. A low score at 10-20 minutes after delivery is considered to be an important sign of potential problems such as cerebral palsy.
Seizures. An infant who has seizures faces a higher risk of being diagnosed later in childhood with cerebral palsy.
Risk factors for the level of severity of cerebral palsy are the size, location and timing of the insult / injury and the nature of the brain’s response to it (cortical re-organisation).
