Neurodevelopmental disorder

Intellectual disability

Intellectual disability, also disorder of intellectual development,^[13] and known as learning difficulty (in the United Kingdom and different from learning disability),^[14] formerly mental retardation, is a neurodevelopmental disorder characterized by significant impairment in intellectual and adaptive functioning.^[15][16][17] The level of impairment may be mild, moderate, severe or profound.^[3][13] Causes may be either developmental or genetic. Children with intellectual disability typically have an intelligence quotient (IQ) below 70 and deficits in at least two adaptive behaviors that affect everyday living.

Communication disorders

A communication disorder is any disorder that affects an individual's ability to comprehend, detect, or apply language and speech to engage in dialogue effectively with others.^[18] This also encompasses deficiencies in verbal and non-verbal communication styles.^[19] The delays and disorders can range from simple sound substitution to the inability to understand or use one's native language.^[20]

Autism spectrum disorder

Autism, also known as autism spectrum disorder (ASD), is a condition characterized by impairment in social communication and interaction,^[21][22] as well as a need or strong preference for predictability and routine, sensory processing differences, focused interests, or repetitive behaviors. Features of autism are present from early childhood and the condition typically persists throughout life.^[23][24] Autism is classified as a neurodevelopmental disorder,^[25] and a diagnosis requires professional assessment that these characteristics cause significant challenges in daily life beyond what is expected given a person's age and social environment.^{[26][27][28][21]} Because autism is a spectrum disorder, presentations vary and support needs range from minimal assistance to full-time, 24-hour care.^{[27][21][29][30]}

Attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by executive dysfunction occasioning symptoms of inattention, hyperactivity, impulsivity and emotional dysregulation that are excessive and pervasive, impairing in multiple contexts, and developmentally-inappropriate.^[31][32][33]

ADHD symptoms arise from executive dysfunction,^[42] and emotional dysregulation is often considered a core symptom.^[46] Difficulties in self-regulation such as time management, inhibition and sustained attention may cause poor professional performance, relationship difficulties and numerous health risks,^[47][48] collectively predisposing to a diminished quality of life^[49] and a direct average reduction in life expectancy of 13 years.^[50][51] ADHD is associated with other neurodevelopmental and mental disorders as well as non-psychiatric disorders, which can cause additional impairment.^[33]

Motor disorders

Motor disorders are neurodevelopmental disorders that cause abnormal and involuntary movements. They can result from damage to the motor system. They were redefined in DSM-5.^[52] Motor disorders include developmental coordination disorder, stereotypic movement disorder, and tic disorders including Tourette's.^[52] Motor disorders are distinct from movement disorders.

Specific learning disorders

Deficits in any area of information processing can manifest in a variety of specific learning disabilities (SLD). It is possible for an individual to have more than one of these difficulties. This is referred to as comorbidity or co-occurrence of learning disabilities.^[53]

Social deprivation

Deprivation from social and emotional care causes severe delays in brain and cognitive development.^[64] Studies with children growing up in Romanian orphanages during Nicolae Ceauşescu's regime reveal profound effects of social deprivation and language deprivation on the developing brain. These effects are time-dependent. The longer children stayed in negligent institutional care, the greater the consequences. By contrast, adoption at an early age mitigated some of the effects of earlier institutionalization.^[65]

Genetic disorders

A prominent example of a genetically determined neurodevelopmental disorder is trisomy 21, also known as Down syndrome. This disorder usually results from an extra chromosome 21,^[66] although in uncommon instances it is related to other chromosomal abnormalities such as translocation of the genetic material. It is characterized by short stature, epicanthal (eyelid) folds, abnormal fingerprints and palm prints, heart defects, poor muscle tone (delay of neurological development), and intellectual disabilities (delay of intellectual development).^[67]

Less commonly known genetically determined neurodevelopmental disorders include Fragile X syndrome. Fragile X syndrome was first described in 1943 by Martin and Bell, studying persons with family history of sex-linked "mental defects".^[68] Rett syndrome, another X-linked disorder, produces severe functional limitations.^[69] Williams syndrome is caused by small deletions of genetic material from chromosome 7.^[70] The most common recurrent copy number variation disorder is DiGeorge syndrome (22q11.2 deletion syndrome), followed by Prader-Willi syndrome and Angelman syndrome.^[71]

Immune dysfunction

Immune reactions during pregnancy, both maternal and of the developing child, may produce neurodevelopmental disorders. One typical immune reaction in infants and children is PANDAS,^[72] or Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infection.^[73] Another disorder is Sydenham's chorea, which results in more abnormal movements of the body and fewer psychological sequellae. Both are immune reactions against brain tissue that follow infection by Streptococcus bacteria. Susceptibility to these immune diseases may be genetically determined,^[74] so sometimes several family members may have one or both of them following an epidemic of Strep infection.^{[citation needed]}

Infectious diseases

Systemic infections can result in neurodevelopmental consequences, when they occur in infancy and childhood, but would not be called a primary neurodevelopmental disorder. For example HIV^[75] Infections of the head and brain, like brain abscesses, meningitis or encephalitis have a high risk of causing neurodevelopmental problems and eventually a disorder. For example, measles can progress to subacute sclerosing panencephalitis.

A number of infectious diseases can be transmitted congenitally (either before or at birth), and can cause serious neurodevelopmental problems, as for example the viruses HSV, CMV, rubella (congenital rubella syndrome), Zika virus, or bacteria like Treponema pallidum in congenital syphilis, which may progress to neurosyphilis if it remains untreated. Protozoa like Plasmodium^[75] or Toxoplasma which can cause congenital toxoplasmosis with multiple cysts in the brain and other organs, leading to a variety of neurological deficits.

Metabolic disorders

Metabolic disorders in either the mother or the child can cause neurodevelopmental disorders. Two examples are diabetes mellitus (a multifactorial disorder) and phenylketonuria (an inborn error of metabolism). Many such inherited diseases may directly affect the child's metabolism and neural development^[76] but less commonly they can indirectly affect the child during gestation. (See also teratology).

In a child, type 1 diabetes can produce neurodevelopmental damage by the effects of excess or insufficient glucose. The problems continue and may worsen throughout childhood if the diabetes is not well controlled.^[77] Type 2 diabetes may be preceded in its onset by impaired cognitive functioning.^[78]

A non-diabetic fetus can also be subjected to glucose effects if its mother has undetected gestational diabetes. Maternal diabetes causes excessive birth size, making it harder for the infant to pass through the birth canal without injury or it can directly produce early neurodevelopmental deficits. Usually the neurodevelopmental symptoms will decrease in later childhood.^[79]

Phenylketonuria, also known as PKU, can induce neurodevelopmental problems and children with PKU require a strict diet to prevent intellectual disability and other disorders. In the maternal form of PKU, excessive maternal phenylalanine can be absorbed by the fetus even if the fetus has not inherited the disease. This can produce intellectual disability and other disorders.^[80][81]

Nutrition

Nutrition disorders and nutritional deficits may cause neurodevelopmental disorders, such as spina bifida, and the rarely occurring anencephaly, both of which are neural tube defects with malformation and dysfunction of the nervous system and its supporting structures, leading to serious physical disability and emotional sequelae. The most common nutritional cause of neural tube defects is folic acid deficiency in the mother, a B vitamin usually found in fruits, vegetables, whole grains, and milk products.^[82][83] (Neural tube defects are also caused by medications and other environmental causes, many of which interfere with folate metabolism, thus they are considered to have multifactorial causes.)^[84] Another deficiency, iodine deficiency, produces a spectrum of neurodevelopmental disorders ranging from mild emotional disturbance to severe intellectual disability. (see also congenital iodine deficiency syndrome).^[85]

Toxicants

Different toxicants can adversely affect fetal development. Alcohol consumption for example can cause fetal alcohol syndrome associated with developmental delay.^[86]

Physical trauma

Brain trauma in the developing human is a common cause of neurodevelopmental syndromes. It may be subdivided into two major categories, congenital injury (including injury resulting from otherwise uncomplicated premature birth)^[87] and injury occurring in infancy or childhood. Common causes of congenital injury are asphyxia (obstruction of the trachea), hypoxia (lack of oxygen to the brain), and the mechanical trauma of the birth process itself.^[88]

Placenta

A growing number of studies are linking the placenta to fetal brain development.^[89]