bubble_chart Overview

Mental retardation (MR) is a group of disorders that occur during the developmental period, characterized by significantly lower general intellectual functioning than the average level of peers, accompanied by deficits in adaptive behavior. An intelligence quotient (IQ) below the population mean by 2.0 standard deviations (the population IQ mean is set at 100, with one standard deviation equivalent to 15 IQ points) generally indicates significantly below-average intelligence, typically an IQ of 70 (or 75) or lower. Adaptive behavior includes both personal life skills and the ability to fulfill social responsibilities. The developmental period usually refers to individuals under 18 years of age. Mental retardation has various names. In psychiatry, it is referred to as "mental developmental delay," "mental developmental deficiency," or "mental deficiency." In education and psychology, it is called "intellectual disability" or "intellectual deficiency." In pediatrics, terms such as "mental retardation," "intellectual delay," or "intellectual developmental disorder" are used. Special education schools may refer to it as "intellectual disability" or "mental handicap."

bubble_chart Epidemiology

The prevalence of MR varies depending on the definition, diagnostic criteria, sampling methods, and psychological assessment methods used in different studies. According to reports from the American Association on Mental Deficiency (AAMD) and the WHO, the prevalence of MR in children is 1–2%. A national collaborative group conducted a survey of 85,170 children across administrative regions in North, Northeast, Northwest, East, Southwest, and Central China starting on May 1, 1988. The results showed that the overall prevalence of MR among children aged 0–14 nationwide was 1.20%. The urban prevalence was 0.70%, while rural prevalence was 1.41%. The prevalence among boys was 1.24%, compared to 1.16% among girls. For children under 3 years old, the prevalence was 0.76%; for those aged 3–7, it was 1.10%; for ages 7–11, it was 1.44%; and for ages 11–14, it was 1.50%, indicating an increasing trend with age. The prevalence also differed across socioeconomic and cultural conditions: urban areas had a rate of 0.78%, rural areas 2.41%, mountainous regions 3.84%, and ethnic minority groups 3.60%.

bubble_chart Etiology

The causes of disease include biomedical factors and psychosocial-cultural factors. The former refers to various adverse factors the brain encounters during its development (prenatal and perinatal periods), which can prevent the brain from reaching its expected developmental level and ultimately affect intelligence. The latter refers to factors such as cultural deprivation, improper upbringing, and sensory deprivation, which can lead to insufficient or inappropriate postnatal information input, thereby impacting intellectual development. Based on the WHO 1985 classification and the survey results of the national collaborative group, the causes of disease are divided into the following types.

1. Infections and intoxications: 12.3%. Infections refer to brain infections occurring before or after birth, such as rubella, cytomegalovirus, toxoplasmosis, herpes simplex virus, and other viral infections. Intoxications include hyperbilirubinemia, toxemia, lead poisoning, alcohol poisoning, and long-term excessive use of drugs such as phenytoin or phenobarbital.

2. Mechanical brain injury and hypoxia: 19.6%. Physical or mechanical factors before, during, or after childbirth can cause brain injuries, such as birth trauma or traumatic brain injury. Ischemia and hypoxia during the perinatal or postnatal period can also damage brain tissue, such as severe maternal blood loss, anemia, heart failure, lung diseases, neonatal asphyxia, intracranial hemorrhage, drowning, anesthesia accidents, or brain hypoxia following prolonged epileptic seizures.

3. Metabolic, nutritional, and endocrine disorders: 5.8%. Disorders in the metabolism of substances such as amino acids, carbohydrates, fats, mucopolysaccharides, and purines can affect the development and function of nerve cells, such as phenylketonuria or galactosemia. Prenatal or postnatal nutritional deficiencies, especially in proteins, iron, and other essential substances, can reduce the number or function of fetal or infant brain cells. Endocrine disorders, such as hypothyroidism, can also affect intellectual development.

4. Gross brain diseases: 0.7%. These include tumors, degenerative diseases of unknown origin, neurocutaneous syndromes, and cerebrovascular diseases.

5. Congenital brain malformations or hereditary syndromes: 9.5%. Congenital malformations include hydrocephalus, hydranencephaly, microcephaly, neural tube defects, and brain malformations. Hereditary syndromes include conditions like adrenoleukodystrophy.

6. Chromosomal aberrations: 5.1%. These involve changes in the number or structure of autosomes or sex chromosomes, such as Down syndrome, trisomy 18, C-group trisomy, cri-du-chat syndrome, fragile X syndrome, Klinefelter syndrome, and Turner syndrome.

7. Other perinatal factors: 11.8%. These include premature birth, low birth weight, intrauterine growth retardation, maternal nutritional diseases, and pregnancy-induced hypertension.

8. Associated with mental disorders: Such as infantile autism or childhood schizophrenia.

9. Psychosocial factors: 8.2%. These children show no organic brain lesions but are primarily affected by adverse environmental factors such as neuropsychological damage and sensory deprivation, including severe lack of early appropriate stimulation and education.

10. Special sensory defects: 5.1%. These include deafness, mutism, blindness, and other special sensory impairments.

11. Unknown causes: 21.9%. After thorough examination, no identifiable cause is found, hence classified as unknown.

bubble_chart Clinical Manifestations

Generally, MR is classified into grade I, grade II, grade III, and extreme grade III based on IQ and adaptive behavior deficits.

1. Grade I MR Also known as "feeble-mindedness" in psychiatry. IQ ranges from 50 to 70, with grade I adaptive behavior deficits. Early development is slightly delayed compared to normal children, and they are less lively, showing little interest in their surroundings. They may either follow rules rigidly or behave roughly. Language development is slightly delayed, with limited abstract vocabulary. Analytical skills are poor, and understanding of issues is superficial. Academic performance is worse than that of average children—they can memorize texts but struggle to apply them correctly and have difficulty solving arithmetic word problems. With special education, they can acquire practical skills, basic reading, and the ability to recognize signs. As adults, they can perform general household chores and simple, concrete tasks. They lack initiative, are highly dependent, struggle to adapt to external changes, and are easily influenced or controlled by others. They can adapt to society under guidance.

2. Grade II MR Also known as "moron." IQ ranges from 35 to 49, with grade II adaptive behavior deficits. Overall development is slower than in normal children. Language function is underdeveloped, with unclear articulation and limited vocabulary. They can only engage in simple, concrete thinking and struggle with abstract concepts. Their ability to discern their surroundings is poor, recognizing only superficial or fragmented aspects of things. They make no progress in reading or arithmetic. With long-term education and training, they can learn simple interpersonal skills, basic hygiene habits, safety habits, and simple manual skills.

3. Grade III MR Also known as "imbecile." IQ ranges from 20 to 34, with grade III adaptive behavior deficits. Early development is delayed in all aspects. Speech is unclear, with minimal verbal expression and poor self-expression. Abstract concepts are lacking, and comprehension is very low. Emotions are immature, and movements are extremely clumsy. They have some defensive abilities, such as avoiding obvious dangers. Through systematic habit training, they can develop simple living and hygiene habits but require care from others. As adults, they can perform fixed and very simple manual labor under supervision.

4. Extreme Grade III MR Also known as "idiot." IQ is below 20, with extreme adaptive behavior deficits. They cannot comprehend their surroundings. Lacking language function, they may at most utter words like "dad" or "mom" but cannot truly recognize their parents, often making unconscious cries. They lack self-preservation instincts and cannot avoid obvious dangers. Emotional responses are primitive. Sensory and perceptual abilities are significantly diminished. Motor functions are severely impaired, with poor coordination or lifelong inability to walk. Multiple disabilities and recurrent epilepsy are common. They cannot manage personal care, and most die early in life. Survivors may respond to training in basic hand and foot movements.

bubble_chart Diagnosis

First, the presence of MR should be determined based on IQ, adaptive behavior, and age of onset, followed by identifying the underlying causes of MR.

During the diagnosis process, a detailed history of the child's growth and development should be collected, and a comprehensive physical and neuropsychiatric examination should be conducted. The developmental indicators of children at different ages and stages should be compared with those of normal peers to assess their intellectual level and adaptive abilities, leading to a clinical judgment. Additionally, by employing appropriate intelligence testing methods, a diagnosis can be made, and the severity of MR can be determined.

1. History Collection

(1) Family history: Determine whether the parents are consanguineous and whether there is a family history of blindness, mutism, epilepsy, cerebral paralysis, congenital malformations, MR, or mental illness.

(2) Maternal pregnancy history: Inquire about viral infections, late abortion, bleeding, or injuries during early pregnancy, exposure to chemical drugs, toxins, or radiation, and whether the mother had hypothyroidism, diabetes, severe malnutrition, multiple pregnancies, polyhydramnios, placental insufficiency, or maternal-fetal blood group incompatibility.

(3) Birth history: Determine whether the child was born prematurely or post-term, whether the delivery method was abnormal, whether the birth weight was low, and whether there was asphyxia, birth trauma, intracranial hemorrhage, grade III jaundice, or congenital malformations after birth.

(4) Growth and development history: Include neuropsychiatric milestones such as the timing of gross motor skills (e.g., lifting the head, sitting, walking), fine motor skills (e.g., picking up small toys or daily items), language development (e.g., calling parents, understanding speech), and other intellectual behaviors (e.g., feeding, dressing, controlling bowel and bladder).

(5) Past and current medical history: Check for a history of head trauma, bleeding, central nervous system infections, severe systemic infections, or seizures.

2. Physical Examination

3. Developmental Examination

4. Neuropsychiatric Examination

5. Laboratory Tests Laboratory tests include blood, urine, cerebrospinal fluid analysis, skull X-rays and CT scans, cerebral angiography, electroencephalography, evoked potentials, hearing tests, chromosome analysis, pituitary, thyroid, gonadal, and adrenal function tests, as well as tests for viruses (e.g., cytomegalovirus, rubella virus), protozoa (e.g., toxoplasma), and antibodies. Relevant tests should be selected based on diagnostic needs.

6. Intelligence Testing and Behavioral Assessment Grade I MR is often assessed using intelligence tests, while grade III or higher MR may be difficult to evaluate with intelligence tests alone and requires behavioral rating scales. However, rating scales are less reliable than intelligence tests for distinguishing grade I MR. Therefore, both methods should be used together, and the results must be analyzed comprehensively.

Intelligence Testing Methods

1. Screening Methods Standard intelligence tests often require extended time (sometimes 1–2 hours or more), making them impractical for general pediatricians or child health screenings. Simplified screening methods are thus employed, with content typically derived from various classic intelligence tests. These screenings take less time and can preliminarily identify suspected cases. Screening results should only serve as a basis for determining whether further testing is needed and cannot be used for definitive diagnosis. Currently, the following screening methods are commonly used in China.

(1) Denver Developmental Screening Test (DDST): Suitable for newborns to 6-year-olds, this method is simple, time-efficient, and requires minimal tools, with good reliability and validity. It has been widely adopted worldwide. China began using this method in the early 1980s. Cities like Shanghai and Beijing adapted DDST to China's social, economic, linguistic, cultural, educational, and geographical characteristics and developed a standardized version called the DDST-R (Revised Developmental Screening Scale for Children).

(2) Draw-a-Person Test: Scores are assigned based on the drawn human figure to assess intellectual development level, suitable for intelligence screening in children aged 5 to 12. Younger children tend to have higher scores while older children show a tendency for lower scores. This test exhibits significant correlation with IQ scores obtained from other intelligence scale tests.

2. Diagnostic Methods

(1) Wechsler Intelligence Scale for Children-Revised (WISC-CR): Applicable to children aged 6 to 16.

(2) China-Wechsler Young Children Scale of Intelligence (CWYCSI): Applicable to children aged 4 to 6.5.

(3) Gessell Developmental Schedules (Gessell ScaleR): Applicable to children aged 0 to 3.

Adaptive Behavior Assessment Methods

1. The Infant-Junior High School Student Social Life Ability Scale is applicable to children aged 6 months to 13-15 years. This scale is an indispensable tool for diagnosing MR and grading.

2. Neonatal Behavioral Neurological Assessment (NBNA): The national association group has determined the normal range of NBNA for newborns nationwide through research, and its clinical application is gradually being implemented.

bubble_chart Treatment Measures

1. Disease Cause Treatment For those whose disease cause has been identified, such as chronic diseases, poisoning, long-term malnutrition, hearing and vision impairments, efforts should be made to eliminate the disease cause as much as possible to partially or fully restore their intelligence. For children with endocrine and metabolic abnormalities like hypothyroidism or phenylketonuria, early diagnosis is crucial, followed by early intervention with thyroid hormone replacement or special dietary therapeutics for phenylketonuria to improve their intellectual levels. For MR caused by socio-psychological-cultural factors, changing environmental conditions, allowing them to live in a friendly and harmonious family, and strengthening education can lead to significant intellectual progress.

2. Training and Rehabilitation By combining medical, social, educational, and vocational training measures, patients should be trained according to their age and the severity of MR to help them achieve the highest possible intellectual level.

It is advisable to have the child receive sustained comprehensive education and training in organized institutions (such as nurseries and kindergartens) as early as possible. It is more beneficial to care for the child alongside normal children rather than separately. Special classes or special schools should be established to improve physical health, train daily living skills and speech functions, and provide basic cultural learning. Work therapy stations can be set up to train adolescent patients in productive skills while ensuring employment opportunities. Additionally, specialized hospitals or sanatoriums (villages) can be established to accommodate MR patients of grade II or above.

bubble_chart Prevention

1. Primary Prevention includes the following: ① Health education and nutritional guidance; ② Prenatal and perinatal care (high-risk pregnancy management, neonatal intensive care, discouraging alcohol and smoking during pregnancy, avoiding or discontinuing medications that adversely affect fetal development); ③ Immunization against pestilence diseases (viruses, bacteria, protozoa); ④ Genetic and metabolic screening and counseling (avoiding consanguineous marriages, identifying carriers); ⑤ Environmental protection (preventing physical and chemical pollution, poisoning, and noise damage); ⑥ Reducing head trauma and accidents, proper treatment of brain diseases, and controlling epileptic seizures; ⑦ Strengthening preschool education and early training; ⑧ Prohibiting neglect and abuse of children. The purpose of these measures is to prevent the occurrence of MR.

2. Secondary Prevention measures include: ① Follow-up of high-risk newborns, early detection and treatment of diseases, with special attention to the positive effects of early nutrition (protein and trace elements such as iron and zinc) and appropriate environmental stimulation on intellectual development; ② Regular health check-ups for preschool children (physical, nutritional, mental and psychological development, vision and hearing); ③ Screening for neonatal metabolic disorders (e.g., hypothyroidism, phenylketonuria); ④ Prenatal diagnosis and amniocentesis (for chromosomal disorders, neural tube defects, metabolic diseases). Secondary prevention focuses on early diagnosis and specialized intervention.

3. Tertiary Prevention requires comprehensive collaboration among society, schools, and families. Early detection of MR, early intervention and stimulation; providing effective assistance to families, maintaining family structure integrity, and improving the functioning of children with MR.

The fundamental approach to prevention lies in continuously deepening research into the etiology of MR disease. Only by targeting the disease causes can prevention become more effective.