Causes of Disabilities
There are thousands known causes of disabilities, but in a great number of cases the exact cause of the disability is never known. Several factors may combine to create a disability.
A disability may be developmental or acquired and may arise from prenatal damage, perinatal factors, acquired neonatal factors and early childhood factors. These may include genetic factors, infections, traumatic or toxic exposure or nutritional factors which result in perinatal or postnatal damage.
Risk Factors for Developmental Delay
Developmental disabilities can occur singly or concurrently in one person. Developmental disabilities can involve a cognitive or sensory difficulty, social or communications/language-related problem, a motor impairment, adaptive delay or some combination of these. The Global Disease Control Priorities Project estimates that 10% to 20% of individuals worldwide have a developmental disability of some kind. In the U.S. alone, it is estimated that 9% of children younger than 36 months of age have a possible developmental problem, while 13.87% of children 3 to 17 years of age have a developmental disability.
Health care providers who see newcomer families have a pivotal role to play in identifying and initiating early treatment for developmental disabilities. Developmental disabilities may last a lifetime but early recognition of their existence, a timely diagnosis and an appropriate treatment plan can make a difference for the children and families involved. In many parts of the world, sub-optimal conditions and care during pregnancy and childbirth can have a range of impacts on developmental health.
The lack of maternal and child health care is one of the significant causes of disabilities in developing countries. Lack of health insurance and inadequate access to health care for newcomers in Canada could similarly adversely affect health outcomes. Prenatal risk factors include chronic maternal illness, certain maternal infections, toxin exposures and nutritional deficiencies.
Some infections that the mother suffers can damage the infant when the disease organisms cross the placental barrier. For example, maternal HIV/AIDS increases the risks for prematurity and being small for gestational age (SGA). Both effects are associated with increased risk of mortality and developmental delay. HIV enters the central nervous system days to weeks after primary exposure. The virus causes neuronal damage and cell death, leading to progressive encephalopathy with motor disabilities, as well as to microcephaly and brain atrophy, with cognitive and language delays.
CMV is a herpes virus spread by close interpersonal contact with saliva, blood, genital secretions, and urine or breast milk. Maternal transmission to the fetus from a new or reactivated infection can occur at any gestational age but is highest with a primary infection compared to a reactivated infection. Ten per cent of affected infants show signs of infection at birth, with a substantial risk of neurological squeal such as Sensori-neural Hearing Loss (SNHL), intellectual delay, microcephaly, seizure disorders and cerebral palsy. CMV is the leading nonhereditary cause of SNHL, which may be progressive, absent at birth, unilateral or bilateral.
Genital herpes is one other virus that affects the newborn baby. Infection of the fetus with this virus usually occurs late in pregnancy, probably during delivery and can result in severe neurological damage. When infection occurs several weeks prior to birth, a variety of congenital abnormalities can result.
Congenital toxoplasmosis occurs at a rate of 1.5 cases per 1000 live births and causes neurocognitive deficits such as intellectual disability, seizures and visual impairment caused by chorioretinitis.
One of the most serious viral diseases during the first three months of pregnancy is rubella, which may produce heart malformations and mental retardation. Maternal infection during pregnancy transmits the rubella virus to the fetus, causing deafness, congenital cataracts, microcephaly, seizures and intellectual disability.
It was estimated in 2008 that over 1.3 million cases of syphilis had occurred in pregnant women worldwide, with a large proportion being untreated or inadequately treated. Like all the preventable STIs, syphilis has been linked to preterm labour, low birth weight and death. Congenital syphilis can cause deafness, microcephaly, intellectual disability and visual impairment through interstitial keratitis.
Maternal smoking during pregnancy increases the risk of placenta Previa, placental abruption, and preterm labour. It also has adverse effects on fetal growth.
Excessive exposure to alcohol in the utero is the most common teratogenic cause of developmental disabilities, including microcephaly, cognitive disability, learning disabilities, Attention Deficits Hyperactivity Disorder (ADHD) and behavioral challenges. Fetal Alcohol Spectrum Disorder (FASD) occurs worldwide in approximately 1.9 per 1000 live births.11 Alcohol causes interruption of neuronal production and secondary destruction, with faulty migration of neurons causing microcephaly.
Maternal exposure to other toxins, including recreational drugs and certain medications can also cause developmental disabilities. These drugs can cross the placenta and adversely affect the developing child. For example, around the 1960s, thalidomide was introduced as a medication to control nausea in pregnant women. But unfortunately, it turned out to cause severe malformations in the legs and/or arms of the developing child.
Many other drugs are suspected of producing birth defects when taken during pregnancy. Substances that produce such effects are called teratogens. There is a long list of substances known or suspected to be teratogens, including legal drugs such as alcohol, nicotine and caffeine. Prescription drugs such as hormones, steroids, anticonvulsants as well as tranquilizers are also known to causes disabilities at prenatal stage. Environmental pollutants such as lead and mercury as well as Illegal drugs such as cocaine, heroin and marijuana are known causes of disabilities at this stage.
The amino acid phenylalanine is a neurotoxin to the developing fetal brain. Untreated PKU, both maternal and postnatal in the infant, causes intellectual disabilities.
Excessive exposure to radiations such as X-rays can affect the fetus.
Exposure to lead, mercury and chemical compounds such as polychlorinated biphenyls (PCB) and alcohol can be identified as a contributing cause of intellectual disability in 4% to 5% of cases. The dose and timing of exposures are variables in predicting neurotoxic outcomes.
Illnesses such as diabetes, hypertension, renal disease and autoimmune disorders are associated with complications to pregnancy that can adversely affect a fetus or newborn child. Maternal diabetes increases the risk of fetal anomalies, macrosomia (a birth weight >4000 grams), subsequent birth injury and hypoglycemia, all of which can negatively impact developmental outcomes in the infant. Hypertension, alone or combined with a renal or autoimmune disorder, can cause placental insufficiency and inadequate fetal growth.
Maternal malnutrition, before and during pregnancy, can have a negative impacts on infant birth weight and development. Maternal nutrition affects the developing child. Deficiencies in iron, vitamins and calorie intake can place the baby at risk. Severe maternal malnutrition may impair the child’s intellectual development in addition to having adverse effects on physical development. Folic acid deficiency is associated with neural tube defects. Severe deficiency is associated with intellectual disability, growth failure and cretinism. This risk factor for developmental disability may also be combined with increased exposure to prenatal toxins.
The WHO and the Society of Obstetricians and Gynaecologists of Canada define the perinatal period as commencing at 22 weeks gestation and ending 7 days after birth. The inadequate care in this critical period puts mothers and children at higher risk for several pregnancy-related complications, such as birth trauma, hypoxia and ischemia, hypoglycemia, hyperbilirubinemia and various serious infections.
Disabilities originating at perinatal stage are biomedical in nature. They may result from drugs taken during labour and delivery, prematurity, injury, oxygen deprivation, or infections acquired during the trip through the birth canal.
STIs: Congenital transmission of herpes viruses 1 and 2 is associated with a high risk of long-term neurological problems. Without treatment, 30% to 50% of infants born to mothers with untreated gonorrhea, and up to 30% with untreated chlamydia, will develop ophthalmic neonatorum, which can lead to blindness if not treated early.
Bacterial infections can be transmitted from mother to child trans-placentally, during pregnancy or during delivery, by passage through the birth canal. Congenital bacterial infections leading to neonatal sepsis and meningitis are an important cause of neonatal morbidity in developing countries.
Undiagnosed or untreated Rh Iso-immunization is associated with anemia and severe hyperbilirubinemia, and may result in seizures, deafness, cognitive delays and cerebral palsy in infants who survive.
Preterm birth (<37 weeks gestation) is a global problem. Risk factors for preterm delivery include: multi-fetal pregnancy, uterine abnormalities, placental bleeding, prenatal drug exposure, chronic maternal illness, hypertensive disorders, prolonged rupture of the membranes and bacterial vaginosis.
Lack of prenatal care, under immunization and inadequate treatment for maternal infections or other medical issues, including STIs, can all contribute to developmental disabilities in a preterm.
This condition is also known an anoxia. It may occur during a prolonged or difficult birth, and because the brain suffers damage very quickly without a fresh and adequate supply of oxygen, brain damage can result. The neurons of the central nervous system require oxygen, if they are deprived of it, some cells may die and this can cause physical and psychological defects. If too many neurons die, the infant may suffer serious brain damage. Anoxia in a newborn is more likely to damage cell of the brain stem than those of the cortex and to result in motor defects. In extreme cases, the child may experience paralysis of the legs and arms.
Postnatal causes occur after birth. The environment is a major factor in many of these situations. Congenital illnesses or infections such as meningitis, cerebral malaria and other childhood illnesses may cause mental retardation. Meningitis if not treated may cause brain damage. Other environmental factors include:
Preconceptional causes of developmental disability relate predominantly to genetic disorders or malformation syndromes. Genetic disorders are the most commonly identified causal factor for intellectual and other disabilities, and include single gene disorders, multifactorial and polygenic disorders, and chromosomal abnormalities. Genetic disorders associated with developmental delay include sickle cell disease. Sickle cell is a blood disorder caused by recessive genes. Children only have the disorder if they receive the gene from both parents. If the gene is paired with a normal one, the individual does not have the condition, but can pass it on to his or her descendants. These individuals are called carriers. Other examples of genetic causes of disabilities include:
The fragile X syndrome is an inherited disability caused by a mutation on the X chromosome and it is now recognized as the most commonly known inherited cause of mental retardation, affecting about 1 in 4,000 males and 1 in 8,000 females. A common associated condition is recurrent otitis media (middle ear infection) with resulting hearing and language problems.
Cognitive disabilities can also be severe. Many of these individuals are challenged by limited attention span, hyperactivity, stereotypic behaviours (such as hand flapping or hand biting), and an inability to relate to others in typical ways. It is believed that almost half of individuals with fragile X syndrome have coexisting autism.
Another example of a genetic cause for mental retardation due to a chromosomal abnormality is Down syndrome (a chromosomal disorder wherein the individual has too few or too many chromosomes). The nucleus of each human cell normally contains 23 pairs of chromosomes (a total of 46). In the most common type of Down syndrome, trisomy 21, the 21st set of chromosomes contains three chromosomes rather than the normal pair. Certain identifiable physical characteristics include an extra flap of skin over the innermost corner of the eye.
The degree of mental retardation varies, depending in part on how soon the disability is identified, the adequacy of the supporting medical care, and the timing of the early intervention. A great majority of people with Down syndrome have a high incidence of medical problems. For example, about half have congenital heart problems, and these individuals have a 15 to 20 times greater risk of developing leukaemia. Although people with Down syndrome have intellectual disabilities, they have fewer adaptive behaviour challenges than many of their peers with mental retardation.
Some genetic causes of disabilities are not so definite but rather result from interplay between genes and the environment. Phenylketonuria (PKU), also hereditary, occurs when a person is unable to metabolize phenylalanine, which builds up in the body to toxic levels that damage the brain. If untreated, PKU eventually causes mental retardation. Changes in diet (eliminating certain foods that contain this amino acid, such as milk) can control PKU and prevent mental retardation, though cognitive disabilities can be seen in both treated and untreated individuals with this condition
Overcoming challenges
Although some conditions or causes of disabilities cannot be prevented, at least at the present time, the impact of the condition can be reduced substantially by directly addressing the cause. For example, PKU is a genetic reason for mental retardation but that it takes factors in the individual’s environment for damage to be devastating. Even more cases are preventable.
Education and access are at the heart of many prevention measures. For example, education about the prevention of HIV/AIDS can be effective with all adolescents, including those with mental retardation. Public education programs can also help pregnant women understand the importance of staying healthy.
Other prevention strategies involve testing the expectant mother, analysing the risk factors of the family (genetic history of disabilities or various conditions), and taking action when necessary; screening infants; protecting children from disease through vaccinations; creating positive, nurturing, and rich home and school environments; and implementing safety measures. The incidence of disabilities, including mental retardation, has been greatly reduced by immunization against viruses such as rubella, meningitis, and measles.
People must not underestimate the importance of prenatal care. For example, FAS is 100 per cent preventable. Pregnant mothers who do not drink alcohol prevent this condition in their children. Staying healthy also means taking proper vitamins and eating a well-balanced diet. For example, folic acid reduces the incidence of neural tube defects. By eating citrus fruits and dark, leafy vegetables (or taking vitamin supplements), one receives the benefits of folic acid—a trace B vitamin that contributes to the prevention of conditions such as spina bifida and anencephaly.
Prevention of disabilities
| For Pregnant Women | For Children | For Society |
| Obtain early prenatal medical care. | Guarantee universal infant screening. | Eliminate the risks of child poverty. |
| Seek genetic counselling. | Ensure proper nutrition. | Make early intervention programs universally available. |
| Maintain good health. | Place household chemicals out of reach. | Provide parent education and support. |
| Avoid alcohol, drugs, and tobacco. | Use automobile seatbelts, safety seats, and cycle helmets. | Protect children from abuse and neglect. |
| Obtain good nutrition. | Provide immunizations. | Remove environmental toxins. |
| Prevent premature births. | Prevent or treat infections. | Provide family planning services. |
| Take precautions against injuries and accidents. | Have quick and easy access to health care. | Provide public education about prevention techniques. |
| Prevent or immediately treat infections. | Prevent lead poisoning. | Have universal access to health care. |
| Avoid sexually transmitted diseases. | Guarantee proper medical care for all children. | Vaccinate all children. |
| Provide early intervention programs. | ||
| Eliminate child abuse and neglect. |
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