Developmental Delay
Nathan J. Blum
Child development is a dynamic interaction of biologic and environmental factors. Typically, over a period of many years, an infant who is initially entirely dependent on others to meet life-sustaining needs such as food and shelter develops into an independent adult capable of caring for others. Growth of a child’s skills occurs across multiple domains of development, including motor, language, cognitive, and socioemotional domains, and allows for slowly increasing independence. Within any single domain, the sequence of new skills acquired is fairly consistent, but there is significant individual variability in the rate of development within and across the different domains that complicates attempts to distinguish children with normal variations in development from children with developmental delays. However, early detection of developmental delays is essential to promote early intervention that can improve developmental outcomes and family adjustment. During well-child care, clinicians are the professionals most likely to interact with a child early in development and thus have a unique opportunity and responsibility to detect children with developmental delays. The approach to the evaluation of developmental delay is discussed further in Chapters 185 and 547.
DEFINITIONS AND EPIDEMIOLOGY
The sequences in which children gain new skills in motor, language, cognitive, and socioemotional development are precisely delineated, and for many skills, the age at which 50% of children will be able to accomplish a specific task is known. A child’s developmental age is defined as the age at which approximately 50% of children would demonstrate similar functioning. Since developmental age is determined on the basis of the child’s functioning, it could be higher or lower than the chronological age.
A global developmental delay occurs when a child’s developmental age lags behind chronological age across all the domains of development. The degree of delay is often quantified by calculating the developmental quotient (DQ = developmental age/chronological age × 100). When a child is developing at the expected rate, the DQ is 100. However, if a 2-year-old child is functioning at a 1-year-old developmental age, the child’s DQ is 50, indicating that the child is developing at half the expected rate. A DQ less than 75 to 80 represents a significant developmental delay. When skills in one domain lag behind (or are more advanced than) skills in other domains, a dissociation in the domains of development occurs, such as seen with language disorders, coordination disorders, and learning disabilities.
Intellectual disability (formerly mental retardation) is defined as a significant deficit in both cognitive skills and adaptive functioning that develops prior to 18 years of age.1 The deficit in cognitive skills is determined by the intelligence quotient (IQ) on a standardized IQ test that is more that 2 standard deviations below the mean. Since IQ tests are designed to have a mean score of 100 and a standard deviation of ±15 points, an IQ below 70 with a similar magnitude of deficits on tests of adaptive functioning is necessary to make a diagnosis of intellectual disability. Adaptive functioning refers to how independently and effectively an individual meets the demands of daily life compared to what is expected for his or her age. It is critical to recognize that factors other than cognitive skills effect adaptive functioning. For instance, an individual’s behavior, motivation, social skills, and education will effect adaptive functioning, as will the opportunities, demands on the individual, and supports available to assist the individual.
The importance of adaptive functioning is further highlighted by the epidemiology of intellectual disability. Approximately 1.5% of school-aged children in the United States are diagnosed with intellectual disability compared to approximately 1% of adults.2,3 Children may be diagnosed with intellectual disability because they struggle with the academic demands of school, and the diagnosis may help provide access to special education services. Once out of school, if the individual is employed, living independently, and socially successful, he or she may no longer demonstrate the deficits in adaptive functioning required for the diagnosis.3
Intellectual disability is often further sub-classified on the basis of IQ test score: mild intellectual disability (IQ: 50–55 to 70); moderate intellectual disability (IQ: 35–40 to 50–55); severe intellectual disability (IQ: 20–25 to 35–40); profound intellectual disability (IQ: below 20–25).4 At the boundaries, the IQ scores overlap to account for the fact that individuals with similar IQ scores can differ significantly in their functioning (eTable 91.1). 
Increased recognition that the level of adaptive functioning is significantly affected by environmental conditions and support available to assist individuals and by the stigma associated with the term mental retardation led to a change in the name to intellectual disability in 2007. Intellectual disability and mental retardation are currently both used to describe the same disability.
ETIOLOGY AND GENETICS
Across the spectrum of cognitive impairments, the influences of biologic and psychosocial factors is not uniform. The brain requires appropriate stimulation in order for development to proceed (see Chapter 81). Children living in poverty, who are more likely to experience poorer schooling, less responsive parenting, and other psychosocial stressors, are at increased risk for cognitive impairments, particularly milder range impairments.5 Genetic abnormalities are the most commonly identified biologic abnormality found to cause intellectual disability (see Chapter 185).
Prenatal or postnatal injury to the developing brain can also cause intellectual disability. Low birth weight (<1500 grams) is associated with a 3-fold increase in the risk of intellectual disability. In utero alcohol exposure is the most common teratogen exposure known to cause intellectual disability.6 Congenital infections, postnatal infections such as encephalitis or meningitis, and traumatic brain injury cause a small percentage of cases of intellectual disability.
CLINICAL FEATURES AND DIFFERENTIAL DIAGNOSIS
Children with developmental delays are seen by pediatricians for a variety of reasons. Most commonly, parents are concerned that the child is not meeting age-appropriate expectations for development. In other cases, parents may be concerned about behavioral difficulties, problems with peer interactions, or poor school performance. It is important for pediatricians to consider a developmental delay during all these clinical encounters.
Early identification of children with developmental delays is easier for more severe delays. Children with severe delays are likely to be detected in the first 6 months of life when they may be noticed to have difficulty tracking visual stimuli and responding consistently to auditory stimuli. They may have difficulty sucking and feeding, abnormalities in muscle tone, and/or a delay in smiling at caregivers, rolling over, or sitting.
Children with less severe delays may be brought to the pediatrician during the second and third years of life due to lack of attainment of language milestones. This creates a challenge for pediatricians in distinguishing children with global developmental delays who are at high risk for intellectual disability from the more prevalent language disorders (see Chapter 86). Finally, quite a few children with mild intellectual delays are not detected until they are having difficulty in school.
A delay in language development is often the main concern of parents of a child with both a global delay and an isolated language delay. Many children with mild delays may not be brought to the physician’s attention by parents. Developmental screening tests for early detection of delays are now a standard of care during pediatric health supervision visits (see Chapter 82 and Table 82-1).
When children are found to be developmentally delayed, the clinician should consider possible causes other than intellectual disability. The possibility of a language disorder has been discussed. It is particularly important for physicians to consider the possibility of a hearing or vision deficit because these deficits may interfere with development and require specific interventions. In addition, motor delays (eg, children with cerebral palsy) may interfere with performance on a developmental test and not allow a child to demonstrate their true cognitive potential. Autism spectrum disorders interfere with communication and social interaction in a manner that often makes accurately assessing cognitive ability particularly challenging (see Chapter 92).
In assessing the result of a developmental screening test, psychosocial, cultural, and behavioral factors also must be considered. Beliefs about when children should attain milestones such as toilet training, knowing colors, and putting on shoes can vary significantly among cultures and influence when children attain these milestones.7 Assessing a child raised in a bilingual environment in only the culture’s dominant language may unfairly limit performance. Finally, young children’s performance on a test may be affected by anxiety, fatigue, hunger, illness, or oppositional behavior.
Table 91-1. Risk Factors for Developmental Delay
