Severe canavan disease is a rare inherited brain disorder that usually becomes clear in early infancy. Early signs of Severe canavan disease often include poor head control, low muscle tone, feeding trouble, and delays in development. As Severe canavan disease progresses, many children develop severe movement problems, seizures, vision problems, and difficulty swallowing. The condition is lifelong, and the most severe form can shorten life, though survival varies with complications and supportive care. Treatment does not cure Severe canavan disease, but care may include feeding support, seizure medicines, physical therapy, and help with breathing and comfort.
How many people live with Severe canavan disease?
Exact counts for severe Canavan disease are not reported separately, but Orphanet classifies Canavan disease overall as affecting fewer than 1 in 1,000,000 people, which suggests fewer than about 8,000 people worldwide (Orphanet, 2024) and fewer than about 340 people in the United States (Orphanet, 2024). Because the severe form that starts in infancy appears to make up most known cases and the milder form is much rarer, the number living with severe Canavan disease is likely close to those totals, but this is an estimate rather than a direct count.
Overview
Severe canavan diseaseSymptoms
Severe Canavan disease usually becomes noticeable in early infancy. Common features include poor head control, very low muscle tone, feeding problems, developmental delay, and an unusually large head. Many children later develop stiffness, seizures, and vision loss.
Outlook and prognosis
Severe Canavan disease usually causes profound developmental disability from infancy and often shortens life, with many children dying in childhood. Some live longer with skilled supportive care, but they need full-time help with feeding, movement, and breathing.
Causes and risk factors
Severe Canavan disease is caused by harmful changes in both copies of the ASPA gene. Risk is highest when both parents carry a changed gene; it can be inherited or happen as a new genetic change.
Genetic influences
Genetics are central to severe Canavan disease. It is caused by harmful changes in the ASPA gene, usually inherited from both parents, and genetic testing can confirm the diagnosis and help guide family planning.
Diagnosis
Severe Canavan disease is often suspected from early clinical features and delayed development. The diagnosis of Severe Canavan disease is confirmed with genetic tests, and brain imaging may show changes that support it.
Treatment and drugs
Severe Canavan disease is treated with supportive care to ease symptoms and protect comfort, including seizure medicines, feeding support, and help with breathing. Physical, occupational, and speech therapy may help with movement, stiffness, and daily care needs.
Symptoms
Severe Canavan disease often affects day-to-day basics like feeding, holding up the head, and getting comfortable. Early on, this might look like a baby who feels unusually floppy, has trouble with head control, or is not gaining motor skills as expected. Many of the early features of severe Canavan disease become clear in the first months of life, and more serious problems with movement, vision, or seizures may appear over time. This severe form usually causes major developmental and physical challenges, though the exact pattern can still vary somewhat.
Babies with severe Canavan disease often feel very floppy, especially in the neck and trunk. This can make head control, rolling, sitting, and comfortable positioning much harder.
Many living with severe Canavan disease do not develop motor skills at the usual pace. Skills like reaching, rolling, sitting, or speaking may be very limited or may not develop.
Sucking and swallowing can be hard, so feeds may take a long time or feel tiring. Some babies cough, choke, spit up often, or do not gain weight well.
The head may grow faster than expected during infancy. In severe Canavan disease, this can become one of the more noticeable physical signs.
How people usually first notice Severe canavan disease?
The first signs of severe Canavan disease are usually noticed in early infancy, often within the first few months of life, when a baby seems unusually floppy, has poor head control, or is not meeting expected early motor milestones such as lifting the head or rolling. Families and doctors may also notice feeding problems, irritability, an unusually large head size that becomes more apparent over time, and later a loss of skills rather than steady development. Because severe Canavan disease is a genetic condition present from birth, how severe Canavan disease is first noticed often comes through these early developmental concerns at routine checkups rather than symptoms appearing much later in childhood.
Types of Severe canavan disease
No distinct types of Severe canavan disease are widely recognized.
Can I slow down the onset of symptoms?
No. Right now, there is no good evidence that lifestyle changes or standard medical treatment can meaningfully delay the start of symptoms in severe Canavan disease, which is a serious inherited brain condition that usually begins in early infancy because the gene change is present from birth. Supportive care for feeding, breathing, movement, and seizures can help with comfort and day-to-day care, but it does not prevent the condition or stop symptoms from starting; research on newer treatments is ongoing, so while there may be more options in the future, the best step now is early specialist care and honest, steady support.
What genetics can tell you about symptoms
In severe Canavan disease, genetics can help explain why symptoms start very early and why they are so serious, because changes in the ASPA gene affect how much working enzyme the body can still make. Variants that leave little or no enzyme activity are usually linked with the classic severe form, with symptoms often beginning in infancy and including major delays in head control, movement, and development. Even so, genetics cannot predict every detail, and children with the same condition can still differ somewhat in which symptoms stand out and how quickly problems progress.
Causes and risk factors
For families, severe Canavan disease is caused by inheriting two nonworking copies of a gene called ASPA. This gene change leads to too little of an enzyme that helps keep brain white matter healthy. The risk of severe Canavan disease in future pregnancies is highest when both parents carry one changed ASPA gene, and family history can be an important clue. Risk is also higher in some groups, including people of Ashkenazi Jewish ancestry, though the condition can happen in any family. Diet, exercise, and everyday exposures are not known to cause severe Canavan disease, and genetic testing can sometimes clarify your personal risk.
What are the most common causes or risk factors?
Severe Canavan disease is almost always caused by inheriting two changed copies of the ASPA gene, one from each parent, so it is a genetic condition present from birth. The main risk factors are having a family history of Canavan disease or having parents who both carry one changed copy of this gene, even if they are healthy themselves; the chance is also higher in people of Ashkenazi Jewish background because carrier rates are higher in that group. There are no well-established non-genetic causes, and it is not thought to be caused by anything a parent did or did not do during pregnancy.
Environmental and biological risk factors
When people look into risk factors for severe Canavan disease, the answer is a little different from many other conditions. Awareness of both biological and environmental influences helps you feel prepared, but for severe Canavan disease, the established risks are mainly inherited biological ones present before or at conception. No outside exposure has been clearly proven to cause it.
Severe Canavan disease most often occurs when both biological parents carry the same condition-linked change and both pass it on in one pregnancy. If only one parent is a carrier, a child may carry the change but usually does not have the disease.
Having a child or close relative with severe Canavan disease can raise the chance that both biological parents are carriers. This is why family history often leads doctors to discuss carrier screening before or during pregnancy.
Carrier rates are higher in people of Ashkenazi Jewish ancestry than in many other groups. That means the chance of severe Canavan disease can be higher in some families from this background, although it can occur in any ethnic group.
When biological parents are related by blood, even if the connection seems distant, they are more likely to carry the same rare inherited change. This can increase the likelihood of severe Canavan disease in a pregnancy.
Genetic risk factors
When families ask about the genetic causes of severe Canavan disease, the answer usually centers on a gene called ASPA. Some risk factors are inherited through our genes. Severe Canavan disease usually happens only when a child inherits two nonworking copies of ASPA, one from each parent. This is why carrier status, family history, and ancestry can all affect risk.
ASPA gene: Severe Canavan disease is caused by harmful changes in the ASPA gene. When both copies are not working properly, the brain cannot process a natural substance as it should, which harms white matter.
Recessive inheritance: Severe Canavan disease follows a recessive inheritance pattern. This means a child is usually affected only after inheriting two altered ASPA copies, while someone with one altered copy is typically a healthy carrier.
Carrier parents: If both parents carry an ASPA change, each pregnancy has a 1 in 4 chance of severe Canavan disease. The same pregnancy also has a 1 in 2 chance of a child who is an unaffected carrier and a 1 in 4 chance of a child who inherited no altered copies.
Family history: A family history of Canavan disease can raise the chance that relatives also carry an ASPA change. This can matter even when no one else in the family has clear symptoms.
Ashkenazi ancestry: Certain ASPA changes are more common in people of Ashkenazi Jewish ancestry. Because of this, carrier rates are higher in this group than in the general population.
Related parents: When parents are biologically related, there is a higher chance they carry the same rare ASPA change from a shared family line. This can increase the chance of severe Canavan disease in their children.
Lifestyle risk factors
Severe Canavan disease is an inherited neurologic disorder, so lifestyle habits do not cause it. Still, how lifestyle affects Severe Canavan disease can be medically important because daily care routines may influence comfort, feeding safety, breathing, skin health, and the risk of complications. In practice, discussions of lifestyle risk factors for Severe Canavan disease focus on supportive habits and caregiving patterns that can worsen or ease day-to-day problems.
Unsafe feeding routines, rushed meals, or poor positioning can increase choking and aspiration risk in Severe Canavan disease. Careful pacing and posture during feeds may help reduce lung complications and improve nutrition.
Inadequate calorie, fluid, or nutrient intake can worsen poor growth, weakness, and overall frailty in Severe Canavan disease. Consistent nutrition planning may help support energy needs and reduce stress from feeding difficulties.
Lack of regular stretching, positioning changes, and guided movement can worsen stiffness, contractures, and discomfort in Severe Canavan disease. Gentle physical therapy routines may help preserve range of motion and improve daily comfort.
Skipping airway clearance routines or allowing prolonged shallow positioning can make mucus retention and chest infections more likely in Severe Canavan disease. Breathing support strategies and upright time may help protect lung function.
Risk prevention
For families affected by severe Canavan disease, true prevention of the condition itself is usually not possible once a child has inherited the changed gene from both parents. Prevention is mainly about lowering the risk of complications, protecting comfort, and supporting growth, breathing, feeding, and daily care as much as possible. Some prevention is universal, others are tailored to people with specific risks. If severe Canavan disease runs in a family, genetic counseling and carrier screening can also help clarify the chance of future pregnancies being affected.
When severe Canavan disease is known in a family, carrier testing before or during pregnancy can help parents understand reproductive risk. In some families, this is the main way to prevent another affected pregnancy rather than prevent the disease after birth.
Many living with severe Canavan disease have trouble sucking, swallowing, or gaining weight, which raises the risk of choking and poor growth. Early feeding evaluation, texture changes, and sometimes tube feeding can help reduce aspiration and support nutrition.
Weak airway protection and limited movement can make chest infections more likely in severe Canavan disease. Positioning, airway clearance, prompt treatment of colds, and watching for early symptoms of severe Canavan disease such as worsening feeding or breathing can help lower complications.
Some children with severe Canavan disease develop seizures that can interrupt sleep, feeding, and comfort. Regular neurology care and taking anti-seizure medicine as prescribed can reduce seizure-related emergencies.
Can you prevent Severe canavan disease?
Severe Canavan disease cannot be prevented once a baby has inherited the gene changes that cause it. Because it is a genetic condition passed down when both parents carry the changed gene, the main way to lower the chance before pregnancy is family-planning care such as blood or saliva testing to see whether both parents are carriers, along with genetic counseling, especially if there is a family history or Ashkenazi Jewish ancestry. These steps can help people understand their risk and options, but they cannot guarantee prevention.
Transmission
Severe canavan disease is not infectious, so it does not spread from one person to another through touch, coughing, kissing, sex, food, or blood. Instead, the genetic transmission of Severe canavan disease happens when a child inherits two nonworking copies of the ASPA gene, usually one from each biological parent.
Many living with Severe canavan disease were born to parents who are healthy carriers and may not know it. This pattern is called autosomal recessive inheritance, and for each pregnancy between two carriers, there is a 25% chance the child will have Severe canavan disease, a 50% chance the child will be a carrier, and a 25% chance the child will inherit neither altered copy.
When to test your genes
For severe Canavan disease, gene testing is most useful if a baby shows early signs such as poor head control, low muscle tone, or developmental delay, or if there is a family history. It is also important before or during pregnancy for parents at risk, especially those with known ASPA variants.
Diagnosis
Severe Canavan disease is usually recognized in infancy, when a baby is not meeting expected movement and development milestones and starts to have low muscle tone or feeding trouble. Over time, the head may grow larger than expected, and skills such as head control, rolling, or sitting may be very delayed or absent. Doctors usually combine the pattern of symptoms with brain imaging and lab testing, then confirm the diagnosis of Severe Canavan disease with genetic tests.
Doctors look closely at early signs such as poor head control, marked low muscle tone, feeding difficulties, and delayed development. In severe Canavan disease, these findings often appear within the first months of life and help raise concern for a serious brain disorder.
A detailed exam checks muscle tone, reflexes, vision, alertness, and how a baby moves and responds. This helps show how the nervous system is being affected and can guide the next tests.
MRI often shows a characteristic pattern of widespread changes in the brain's white matter. These imaging findings strongly support the genetic diagnosis of Severe Canavan disease and help rule out some other causes.
Doctors may measure a substance called N-acetylaspartic acid in the urine, which is often much higher than usual in Canavan disease. A high result supports the diagnosis, although it is usually confirmed with genetic testing.
How is Severe canavan disease diagnosed?
Doctors usually suspect severe Canavan disease in a baby who is not meeting early milestones and may have low muscle tone, feeding problems, or a head that is growing larger than expected. The diagnosis is usually based on the pattern of symptoms together with a brain scan, most often an MRI, and lab tests that look for very high levels of a certain chemical in the urine or blood. Genetic testing can then help confirm the diagnosis by checking for changes in the gene linked to Canavan disease.
Stages of Severe canavan disease
Severe canavan disease does not have defined progression stages. This is because it is usually understood by the pattern of symptoms over time, rather than by a standard stage system used across all patients. Doctors usually start with a conversation about the early symptoms of Severe canavan disease, such as poor head control, low muscle tone, feeding trouble, and delayed development, then add a neurologic exam and brain imaging. Genetic testing and other lab tests are often used to confirm the diagnosis and help monitor how the condition is changing.
Outlook and prognosis
The outlook for severe Canavan disease is poor, with symptoms usually beginning in early infancy and progressing rapidly over the first years of life. Affected children typically develop profound developmental impairment, severe hypotonia that can evolve into spasticity, feeding difficulties, seizures, and progressive loss of motor function. Life expectancy is markedly reduced, and many children with the severe form die in childhood, although survival into the teenage years or longer can occur with intensive supportive care. Mortality is often related to respiratory complications, infections, aspiration, or complications of severe neurologic disability rather than a sudden event from the disease itself. There is currently no curative treatment, so prognosis depends largely on symptom severity and the quality of nutritional, respiratory, and seizure management.
Long-term effects
For families, severe Canavan disease often affects nearly every part of daily life as a child grows, from movement and feeding to breathing and communication. Severe Canavan disease usually leads to major, lasting neurologic disability that begins in infancy and becomes more apparent over time. Long-term effects can involve the brain, muscles, swallowing, vision, and breathing, and these changes strongly shape life expectancy in severe Canavan disease.
Many children with severe Canavan disease have profound delays in learning motor, language, and social skills. Sitting, walking, and spoken communication often do not develop, and most remain fully dependent on others for daily care.
Low muscle tone early in life often shifts into stiffness, tight muscles, and trouble controlling movement. Over time, this can make positioning, comfort, and basic physical care more difficult.
Swallowing can become unsafe or inefficient, so feeding may take a long time and may not provide enough nutrition. Poor weight gain, reflux, and choking episodes can become ongoing health concerns.
Weak muscle control and swallowing problems can raise the risk of mucus buildup, aspiration, and repeated chest infections. Many living with severe Canavan disease develop chronic breathing difficulties, especially as neurologic impairment advances.
How is it to live with Severe canavan disease?
Living with severe Canavan disease usually means profound disability from early infancy, with major challenges in movement, head control, feeding, communication, and development, and many children also have seizures or trouble with muscle tone as the condition affects the brain’s white matter, the tissue that helps nerve signals travel smoothly. Daily life often centers on round-the-clock care, medical appointments, feeding support, physical comfort, and watching closely for breathing or swallowing problems, so families and other caregivers are deeply involved in nearly every part of the child’s routine. For the people around them, this can bring intense love and closeness, but also physical, emotional, and financial strain, which is why strong support from medical teams, therapists, and community resources can make a real difference.
Treatment and drugs
Severe Canavan disease does not yet have a cure, so treatment focuses on comfort, safety, and helping with everyday needs as the condition affects the brain and muscles over time. Many living with severe Canavan disease need a mix of care, such as medicines to control seizures, help with muscle stiffness, feeding support if swallowing is hard, and physical, breathing, and positioning therapy to reduce complications like chest infections and pressure sores. Treatment plans often combine several approaches, and support from family, home nurses, and specialists can make day-to-day care more manageable. Research is ongoing, including studies of gene-based treatments, but for now the main goal is to treat symptoms early and give the best possible quality of life.
Can you cure Severe canavan disease?
No, severe Canavan disease cannot be cured right now. Treatment focuses on easing symptoms and supporting daily needs, such as help with feeding, breathing, movement, seizures, and comfort. These treatments may improve quality of life, but they do not stop or reverse the disease.
Non-drug treatment
Day-to-day care for severe Canavan disease usually centers on comfort, feeding, movement, breathing, and communication rather than curing the condition. Supportive therapies can help many living with severe Canavan disease stay as comfortable and safe as possible while reducing complications. Because needs vary over time, care is often coordinated by a team that includes rehabilitation specialists, nutrition experts, breathing specialists, and palliative care professionals.
Gentle stretching, guided movement, and positioning can help reduce stiffness and lower the risk of contractures in severe Canavan disease. For many, this can mean easier daily care, safer transfers, and more comfort when lying down or sitting.
Occupational therapy focuses on posture, hand use, seating, and practical comfort in everyday life. It may also help families choose supportive equipment such as molded seats, braces, or standing devices.
Speech and feeding specialists can assess swallowing and help lower the risk of choking or food going into the lungs. This is often part of feeding support for severe Canavan disease, especially when sucking and swallowing become hard.
Dietitians help make sure calories, fluids, and nutrients match a child’s needs when feeding is slow or tiring. If weight gain is poor or aspiration risk is high, tube feeding may be discussed as a non-drug way to support growth and comfort.
Your genes may have an impact on how drugs work for you
In severe Canavan disease, there is no standard medicine that fixes the underlying condition, so treatment usually focuses on comfort and day-to-day problems such as seizures, muscle tightness, trouble sleeping, or pain. For many living with severe Canavan disease, how well these medicines work can vary from person to person, but this is usually shaped more by the child’s overall health, age, feeding needs, and how advanced the condition is than by known inherited differences that doctors use in routine care. In other words, genetic testing for medicine response is not a usual part of treatment here, and care teams more often adjust medicines slowly based on what they see in real life, such as whether seizures settle down or a medicine causes too much sleepiness.
Pharmacological treatments
For many living with severe Canavan disease, medicines are mainly used to ease day-to-day symptoms such as seizures, painful muscle tightness, reflux, and excess secretions. There is no approved drug that reverses the underlying brain disease, so treatment for severe Canavan disease is usually supportive and tailored to each child’s needs. Drug treatments are not about fixing everything at once, but about improving comfort, feeding, sleep, and safety as much as possible.
Anti-seizure drugs such as levetiracetam, valproic acid, clobazam, or phenobarbital may be used if severe Canavan disease causes seizures. The choice depends on seizure type, age, and side effects, and some children need more than one medicine.
Baclofen, diazepam, or tizanidine may help reduce severe stiffness and spasms, which can make dressing, positioning, and sleep harder. If swallowing is difficult or side effects are limiting, doctors may adjust the dose slowly or choose another option.
Some children with severe Canavan disease have twisting movements or abnormal postures, sometimes called dystonia, and medicines such as trihexyphenidyl or clonazepam may help. These drugs are used carefully because they can also cause sleepiness, dry mouth, or constipation.
Famotidine or proton pump inhibitors such as omeprazole are often used when reflux causes arching, vomiting, coughing, or discomfort during feeds. In everyday life, this can mean fewer painful feeding times and better sleep for both the child and caregivers.
Genetic influences
For families facing severe Canavan disease, genetics are central to why it happens. This condition is caused by changes in the ASPA gene, which means the body cannot make enough of an enzyme needed to handle a natural brain chemical called N-acetylaspartate. When that process fails, the chemical builds up and the brain’s white matter does not develop normally, which is why severe Canavan disease begins early and can cause major problems with movement, muscle tone, feeding, and development. A carrier means you hold the gene change but may not show symptoms. Severe Canavan disease is inherited in an autosomal recessive pattern, so a child develops it only when they inherit one changed copy of ASPA from each parent. If both parents are carriers, each pregnancy has a 1 in 4, or 25%, chance of severe Canavan disease, which is why genetic counseling and family testing can be helpful.
Is Severe canavan disease genetic?
Genetics are central to severe Canavan disease. It is caused by harmful changes in the ASPA gene, usually inherited from both parents, and genetic testing can confirm the diagnosis and help guide family planning.
Having a genetic risk is not necessarily a diagnosis
Genetic variants are a normal part of human biology and are widely found across the general population. Most of these variants do not directly affect health and are simply part of natural human diversity. For many genes, having a variant associated with a condition suggests a possible link rather than a certain diagnosis, especially when several genes and environmental factors are involved. In some situations, though, a single well-studied variant in a particular gene can be enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
When families look for how genetics affect treatment for severe Canavan disease, the first step is usually genetic testing, because the condition is caused by changes in the ASPA gene and the result helps confirm the diagnosis and guide care planning. Different gene changes may be linked with very low enzyme activity, which can help doctors discuss likely needs such as feeding support, seizure treatment, and physical therapy. Not every difference in response is genetic, but genetics can also affect whether someone may be eligible for research studies, including experimental gene-based treatments. For day-to-day treatment, medicines used for symptoms in severe Canavan disease are usually adjusted based on what helps most and what side effects appear, rather than on a standard drug-gene test. This means genetic information is important, but it works alongside careful follow-up and symptom-based care, not in place of it.
Interactions with other conditions
In daily life, severe Canavan disease often affects many body systems at once, so other health problems can have a bigger impact than they would otherwise. Children living with severe Canavan disease may be more likely to have seizures, trouble swallowing, reflux, and repeated chest infections, and each of these can make comfort, feeding, and breathing harder. Doctors call it a comorbidity when two conditions occur together. For example, if a child develops pneumonia or another respiratory illness, weakness, poor head control, and swallowing problems linked to severe Canavan disease can make recovery more difficult and may raise the risk of needing hospital care. Over time, tight muscles, limited movement, and poor nutrition can also contribute to bone weakness, constipation, or spine curvature, which can add to pain and everyday care needs. Interactions can look very different from person to person, so care is usually tailored with input from neurology, nutrition, lung, and rehabilitation teams.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
What other conditions can cause similar symptoms to Severe canavan disease?
Severe Canavan disease can look similar to other rare inherited brain disorders that affect a baby's development and muscle control, especially Krabbe disease, Tay-Sachs disease, and Alexander disease. Doctors may also consider cerebral palsy and some other rare inherited conditions that affect how the body makes or uses energy, because these can also cause low muscle tone, feeding trouble, seizures, and delayed milestones.
History
Families and doctors first began to recognize this condition when babies who had seemed well at birth started missing expected milestones in the first months of life. They might have trouble gaining head control, seem unusually floppy, or become less able to do things they had just started to learn. First described in the medical literature as a serious disorder affecting brain development, severe Canavan disease was gradually separated from other childhood conditions that could look similar at first.
As medical science evolved, doctors learned that severe Canavan disease affects the brain’s white matter, the part that helps nerve signals travel smoothly. Earlier on, diagnosis depended mainly on a child’s symptoms, the exam, and changes seen after death in brain tissue. Later, lab testing and brain scans made it much easier to recognize the pattern during life, which helped families get clearer answers and support sooner.
Researchers also noticed that severe Canavan disease appeared more often in some communities, which led to broader study and better awareness. Over time, scientists confirmed that the condition has a genetic cause, although the history of severe Canavan disease is really a story of many small advances rather than one single discovery. This progress helped explain why the early symptoms of severe Canavan disease can overlap with those of other neurologic disorders in babies.
Today, the condition is usually identified much earlier than it was in the past. That has not changed how serious severe Canavan disease can be, but it has improved diagnosis, family counseling, and access to supportive care. Knowing this history helps explain why older reports sometimes used different terms or grouped children together in ways that would be considered less accurate now.