Motor Impairment Associated with Neurological Injury in Premature Infants
Neurologically based motor impairment in preterm infants
By Sheena L. Carter, Ph.D.
Children born prematurely are more likely than full-term children to develop motor problems during infancy. About 10% of the smallest (<1000 grams) preterm infants will develop cerebral palsy. Factors necessary for a diagnosis of cerebral palsy, and the developmental course of neurologically based motor problems in premature children are delineated in this article. Because having any neurological injury increases the likelihood that other areas of the brain have also been affected, children who have early motor impairments that resolve may also have continuing difficulties related to weak motor abilities relative to peers, learning disabilities, and sensory issues.
Impacts of Prematurity on Motor Development
Following discharge from the Neonatal Intensive Care Unit (NICU), children born very prematurely can be expected to differ from healthy full-term babies in many aspects of their development. Most notably, the development of most preterm babies follows more closely expectations for age "corrected" for prematurity than for actual age. Even when compared with other children of their corrected age, however, most children born more than a few weeks early will have at least some additional developmental differences associated with prematurity during infancy. While developmental differences for most children born prematurely are benign and will eventually resolve, some of these differences can be associated with long-term problems. In some cases, long-term difficulties can be minimized by helping families and professionals to better understand the underlying conditions which influence the child's development.
Motor development is one area of development frequently affected by prematurity. Children born prematurely are more likely than full-term children to develop motor problems during infancy. The primary reasons for this include the increased risk of injury to the fragile preterm infant's motor system, vulnerability to early illness, and the necessary medical intervention required during a time full-term infants are highly protected in the womb. This article specifically addresses the issue of neurologically based motor impairment in preterm infants.
Neurological Basis of Preterm Motor Problems
When children are born very prematurely, the developing brain can be injured easily. Injury to the brain around the time of delivery can result from a number of different causes, most having to do with disruption of blood and/or oxygen delivery to areas of the brain. Blood flow is difficult for the premature infant to regulate and can be especially problematic under stressful conditions (such as the reduced oxygen or excessive carbon dioxide likely to result from poor lung function). The cranium of the preterm infant is soft enough that external pressure can easily change its shape, and increased pressure within the cranium can interfere with blood flow. Severe bleeding from fragile blood vessels, can also result in decreased blood flow to otherwise uninjured areas.
In the prematurely born infant, the brain tissue most susceptible to injury includes the fragile area surrounding the ventricles. The motor fibers through which the brain communicates with the legs travel through this vulnerable region of the brain and, therefore, movement difficulty affecting the legs is a frequent consequence of brain injury in the preterm infant.. The larger the area of injury, the greater the likelihood of impaired muscle control. Motor fibers serving other muscle groups, as well as other neurological functions, can also be affected with more extensive injury.
Damage to various areas of the brain which contribute to movement difficulties
can sometimes be identified using imaging techniques such as MRI or cranial
ultrasound. Two of the more common forms of brain injury which can
sometimes be identified neonatally are periventricular leukomalacia (PVL)
and bleeding into the ventricles (often called "intraventricular
hemorrhage" or IVH). PVL is a specific condition which refers
to cell death of the white matter behind and to the side of the
lateral ventricles due generally to a combination of decreased blood flow to the brain and reduced oxygen in the blood. PVL is strongly associated with impaired movement, especially of the legs. It is frequently seen with severe bleeding into the ventricles (high grade intraventricular hemorrhage or "IVH"). Severe bleeding into the ventricles (with or without leukomalacia) is associated with increased risk of later developmental problems, including impairment of movement. The medical diagnosis given when significant movement problems do not resolve as the child develops is "cerebral palsy" (CP). The presence of PVL and/or severe IVH increase an infant's probability of eventually being diagnosed with cerebral palsy.
PVL and IVH are not the only identifiable conditions which can result in cerebral palsy, and it is not uncommon for children to develop motor problems with no apparent injury (using currently available imaging techniques). Further, the resulting motor impairment cannot be accurately predicted from either diagnosis. That is, even with clear evidence of PVL, cerebral palsy is not diagnosed unless and until a child develops difficulty with movement that does not appear likely to resolve. As the extent of the impairment cannot be predicted with accuracy from the severity of the injury, it is possible for a child with severe PVL to have less ultimate impairment in movement than a child with lesser apparent injury.
Impairments Related to Early Neurological Injury
Many children with neurologically based motor impairment have only movement problems without impairment to other functions controlled by the brain. Having obvious neurological injury, however, does increase the likelihood that other areas of the brain have also been injured as well. Auditory processing problems, cortical visual impairment, and mental retardation are examples of other neurologically based problems that may occur. There are also less severe symptoms which are frequently associated with early brain injury in general. Hypersensitivity to stimuli (increased sensitivity to touch, sound, rapidly changing visual input, etc.), attention and/or activity problems, difficulty regulating arousal (including sleep problems), seizures, speech problems, and various learning difficulties are among some of the most common symptoms reported for children with early brain injury. These non-motor symptoms associated with brain injury can emerge in children with normal or near-normal motor development, but are more common in prematurely born children who also have motor problems.
Diagnosis of Cerebral Palsy
Cerebral Palsy (CP) simply means that there has been some injury to the brain during development which has resulted in difficulty transmitting the necessary impulses from the brain to the muscles for coordinated movement.
Many children born prematurely will develop some movement difficulties related to early neurological injury. These impairments emerge slowly over time and are typically not evident during the newborn period. Most mild motor abnormalities noticeable during the first few months of life will improve and may completely resolve with time. When motor impairment persists, a diagnosis of cerebral palsy may be considered. About 10% of children born at birth weights of less than 1000 grams will eventually receive a diagnosis of cerebral palsy (McCarton, et. al, 1996; O'Shea, et. al., 1998), a permanent condition. Diagnosing cerebral palsy in children born prematurely is often a difficult process which requires observing the child's development over time.
For a diagnosis of CP, the following are necessary:
1) Movement of muscles has to be adversely affected.
As voluntary control of the muscles develops with age, it takes time to determine whether and to what extent movement of the muscles has been affected by brain damage (which may or may not have been detected neonatally). Even with significant PVL or other injury known to be related to cerebral palsy, cerebral palsy will not be diagnosed unless and until movement is affected.
2) The motor impairment has to be due to a neurological injury.
That is, the source of the problem is at the level of brain functioning, rather than being due to a problem with another part of the motor system such as the spinal cord or muscle. Neurological injury may not be apparent using imaging techniques, however, and may be assumed from associated signs and symptoms.
3) The injury or lesion must be static (not getting worse, but no longer resolving).
A diagnosis of cerebral palsy is not given if the source of the problem is a progressive or deteriorating neurological condition. Although relatively rare, it is important to rule out such conditions as treatment options may be very different. Because of the amazing ability of the very young brain to recover from injury, it is also important to wait until the healing process is complete before diagnosing a permanent condition.
The level of impairment may increase with development for children with cerebral palsy. That is, as a child develops, the impaired use of a muscle or muscle group can become more apparent as he or she is unable to keep up with age expectations. As the demands increase, the impairment becomes more obvious. This is the imapirment which is becoming worse with time; the injury to the brain remains the same.
4) The injury has to occur while the motor system is still developing (usually before, during, or right after birth).
For premature infants, the injuries resulting in CP are thought to most often happen during the perinatal period (right around the time of birth). For some children, however, the injury may have happened prenatally; for others, injury may have happened during the neonatal period. It can be very difficult to determine exactly when the injury occurred.
5) The impairment in movement does not resolve with time.
Many children with motor problems during infancy do "grow out of" their symptoms, even after the first year of life. This is not considered to have been cerebral palsy. (There are a variety of other motor problems associated with prematurity.) For this reason, medical professionals tend to be very cautious about diagnosing mild to moderate cerebral palsy in prematurely born children during infancy.
Types of Cerebral Palsy
Cerebral Palsy comes in a variety of different forms and with a continuum of severity. CP can be so mild that it is only noticeable when the individual is stressed or involved in certain activities. It can be so severe as to limit most voluntary movement. It can take several years for the full impact of a child's cerebral palsy to become apparent. However, children do not switch from one form of cerebral palsy to another, nor from one impairment level to another, after the condition is fully expressed. For example, a child well beyond infancy with mild cerebral palsy effecting the legs only is not at risk to develop a different or more severe form of cerebral palsy later in development.
Most former preemies with CP will have spastic diplegia. Spastic diplegia involves an involuntary increase of muscle tone (tightness or stiffness) that primarily affects the limbs, with legs and feet being much more affected than the arms and hands. Children with spastic diplegia from injury before or around the time of birth almost always have some difficulty with their hands, though the legs will be much more obviously affected.
Other subtypes of spastic cerebral palsy besides spastic diplegia include spastic hemiplegia and spastic quadriplegia. The prefixes "hemi-" and "quadri-," like "di-" refer to the location of the impairment. Children with hemiplegia have movement of the limbs on one side of the body, but not the other. The full extent of the motor disability impairment is not realized until fairly late with spastic hemiplegia, often as late as three years of age. Epilepsy is relatively more common in children with hemiplegia (Russman, 1992), but these children are also very likely to function well in general. Quadriplegia interferes with the impairment of all four limbs, typically with the head and trunk involved as well. Quadriplegia most often occurs in children who have had severe deprivation of oxygen to the brain because of reduced oxygen in the blood and decreased blood flow to the brain (Eicher & Batshaw, 1993). Quadriplegia tends to be identified earlier than other forms of cerebral palsy and can sometimes be diagnosed in the first six months of life.
There are other forms of cerebral palsy which also originate in the perinatal period and involve abnormal movement patterns and postures caused by poor regulation of muscle tone and coordination. (For example, athetoid cerebral palsy is characterized by writhing movements.) These are not typically associated with prematurity. Children born prematurely may, of course, develop problems unrelated to their prematurity, but having one type of cerebral palsy does not mean that a child is likely to develop an unrelated type.
Development and Recovery
The diagnosis of CP can be very complex. One reason for the difficulty is the simultaneous processes of development and healing (what would be called "recovery of function" in an older patient who had previously acquired skills). An infant leaving neonatal intensive care may have strong indications of neurologically based muscle tone abnormality, yet the brain's remarkable ability to adapt during the recovery process is most apparent during the early months of infancy. At the same time, however, the normal developmental process continues - proceeding from early infancy where motor control is minimal to the sophisticated coordination of muscle groups necessary for complex movement.
Normal newborns have little voluntary muscle control. New skills unfold in a predictable manner as the baby grows and develops. Just as it takes time for these abilities to develop, it takes time for it to become apparent which of them may have been affected by early injury. (Remember, the early injury may still be resolving or healing.)
Preemies often have very low muscle tone neonatally (e.g., they are "floppy") and the abnormally increased muscle tone (the "tightness" associated with spastic CP) is often not apparent until after the first few months at home. Tone may appear normal during this period of transition. The initial low tone often resolves with recovery; but, as a child with CP starts to need voluntary movement, it becomes apparent that there is going to be difficulty with some movements. As more muscles are called into voluntary action, further difficulties with movement may appear.
During the first year it can be very difficult to determine at what point the infant has maximized whatever recovery is going to be made, and at what point the emerging muscle tone problems are indications of long-term impairment. The timing can be very different from one child to another. For this reason, it is not unusual for a child with mild CP not to be diagnosed until after the second year. Since CP is a permanent condition, doctors like to wait until they are very sure that what they are seeing is not going to go away before they give a diagnosis. Usually only the more severe cases of CP can be diagnosed within the first several months corrected age, while milder cases tend not to be diagnosed until it becomes apparent that the muscle tone or control problems are not going to resolve with development.
When there is clear motor impairment from a neurological cause that DOES go away with time, this is not considered CP; however, abnormal tone or abnormal reflexes that eventually become normal are often associated with later learning difficulties (Nelson, 1982). The most likely reason for this association is that the early motor signs are indications that there has been early neurological damage to part of the brain that is involved in movement, and where there is such damage there is a greater likelihood of additional damage that may go undetected in the early years.
Because of the emergence of impairment with development and the resolution of early problems during the recovery period, developmental evaluations in the first few months following hospital discharge can be very misleading in terms of long-term implications for infants born prematurely. Infants who have been particularly ill may still be recovering physically and even early symptoms of neurological damage may still be present that will eventually go away during the healing process. Most prematurely born infants tend to look good for their corrected ages if they are healthy, and this also can be misleading as it is just too early to see any indication of the most common problems associated with prematurity.
By one year corrected age, most prematurely born infants have become medically stable and physically healthy. Some long-term problems will have become noticeable at this point. For example, if a child's motor system is so impaired that the legs cannot be coordinated for walking, this will be clear by the time the child should be putting one foot in front of the other. Still, it is too early to see the impact of damage to areas of the brain needed for higher reasoning skills and even too early to tell if language will develop normally.
As children grow, new problems may appear that were not apparent earlier and children with cerebral palsy may sometimes look "worse" as they grow, particularly if they are not walking during the preschool years. This does not mean that the cerebral palsy is worsening or that their neurological condition is becoming more severe. The impairment becomes stable sometime during the first few years and children can continue to develop skills needed to improve their level of disability and lessen the impact of any handicap it may cause.
For children who "outgrow" their early motor impairments, or for those who are never formally diagnosed because their condition is so mild as to escape being "labeled" as cerebral palsy, parents should be aware of the potential for continuing difficulties related to weak motor skills relative to age mates, learning disabilities, and sensory issues. Many former premature infants continue to have these relatively mild developmental difficulties into school-age and beyond. Less research attention has been focused on borderline or undiagnosed developmental difficulties, but there is some evidence that family stress is greatest when children continue to have problems that are considered "mild" or "low normal" (Cronin, et. al, 1995.) One explanation for the increased stress these families experience is that there are likely to be less supportive services available for children who have very mild difficulties than for those with clearly significant impairment.
Making Developmental Predictions
Children who are born prematurely have an increased risk of early brain injury that is often associated with developmental difficulties. Motor symptoms are usually the earliest manifestation of early neurological injury. (Although, it is important to note that motor symptoms in preemies are not always neurologically caused.) These symptoms may resolve as the child recovers or they may persist and eventually warrant a diagnosis of cerebral palsy. While early identification is essential to prompt early intervention, it can be very difficult to establish clear diagnoses of neurodevelopmental problems in developing infants. Parents and professionals who work with children at high risk for delays need to be aware of the developmental nature of emerging difficulties in order to more adequately support each child's development and nurture optimal potential for success.
Most importantly, it is necessary to understand the limitations of trying to use information available during infancy and early childhood to predict later development. The most sophisticated imaging technology cannot predict later motor functioning for an individual infant with 100% accuracy (except in extreme cases) and the specific impairments related to early brain injury will vary greatly for individual children with similar injury. It is, therefore, important to maintain a focus on the current impact of neurological injury (e.g., "How is this affecting my child's functioning today?) rather than focusing on trying to determine whether or not particular symptoms will or will not go away with time. In this way, it is possible to address relevant needs as they arise by working to enhance the child's strengths and to minimize the impact of any impairments as they emerge.