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Intellectual Disability

Causes, symptoms and prevention methods of learning disability

Intellectual disability, also formerly known as mental handicap, mental retardation or learning disability, refers to a significant impairment of cognitive and adaptive functions, with age of onset before 22 years . Patients have different forms of intellectual disability, which are permanent disabilities and thus have no cure. Before the age of 22, patients will have obvious deficiencies in adaptive behavior, and their personal learning and social adaptability will also be limited. However, in terms other than intelligence, such as emotional or physical abilities, most intellectually disabled people are the same as ordinary people. 

Causes of Intellectual Disability 

Congenital factors
  • Chromosomal or genetic abnormalities
  • Metabolic system abnormalities
  • Consanguineous marriage or advanced maternal age 

Acquired factors

  • Severe bacterial infection, wrong medication, falls, malnutrition, alcoholism, smoking, drug abuse,during pregnancy (gestational factors). 
  • Preterm birth, dystocia, infant hypoxia, bacterial infection or small for gestational age can lead to poor brain development or damage in children (perinatal factors) 
  • Brain damage in infants and young children caused by bacterial infections, malnutrition or accidents 
  • Environmental toxins such as the heavy metals lead, mercury and cadmium 

Types of Intellectual Disability, the I.Q Test 

One of the tests used to evaluate intellectual disability is the Intelligence Quotient Test or more popularly known as an IQ Test. According to the  International Classification of Diseases, 10th Edition (ICD-10), intellectual disability based on IQ Test result can be divided into the following four levels:


IQ level is about 50 to 55 to approximately 70 (accounting for 85% of the total) 

Approximate synonyms: Intellectual disability, mild; mild intellectual disability; mild mental retardation 

  • Often presents as speech–language delay 
  • Often not diagnosed until later age 
  • Can develop social and communication skills 



IQ level is about 35 to 40 to 50-55 (accounting for 10% of the total) 

Approximate synonyms: Intellectual disability, moderate; moderate intellectual disability; moderate mental retardation

  • Poor social awareness 
  • Can profit from training in self-help 
  • Can talk or learn to communicate 



IQ level is about 20 to 25 to 35-40 (accounting for 3.5% of the total) 

Approximate synonyms: Intellectual disability, severe; severe intellectual disability; severe mental retardation 

  • Able to learn some self-help skills 
  • Has limited speech skills 
  • Can say a few words 



IQ level is below 20 -25 (about 1.5% of the total) 

Approximate synonyms: Intellectual disability, profound; profound intellectual disability; profound mental retardation 

  • May need nursing care due to limited self-care skills 
  • Extreme cognitive limitation 
  • Often sensory and/or physical impairments 


Diagnostic methods for Intellectual Disability 

The Maternal and Child Health Center under the Hong Kong Department of Health has a comprehensive children's physical and mental observation program to provide medical diagnosis, assessment and referral services to all children in Hong Kong. The purpose is to identify children with special educational needs as early as possible. Diagnostic methods may include: 

  • Various intelligence assessment scales

Preliminarily judge the patient's intelligence level by asking patients or accompanying family members relevant questions and cooperating with the patient to complete the scale's parameters for listening, speaking, reading, writing and other contents. 

  • Neurological examination

Comprehensive assessment of the patient’s sensory, motor and reflex skills also involves observing the patient’s gait and eye movements. 

Use of imaging techniques to determine organic brain conditions or malformations, its severity, and possible cerebrovascular disease 

  • Thyroid function test

Congenital hypothyroidism (abnormally low levels of functional thyroid hormones at birth) can cause intellectual disability, so blood levels of thyroid-stimulating hormone (TSH), tri-iodothyronine (T3) and thyroxine (T4) are measured and appropriate treatment initiated. 

  • DNA Testing

Some cases of unexplained learning Disability may be genetically related. Doctors need to understand the patient's medical history in detail, including family history, and request for genetic testing to identify which specific genetic disease the patient may have. If necessary, parents of intellectually disabled children may also need to undergo genetic testing. 

  • Lead and heavy metals screening

Excessive lead, mercury and cadmium build up in the body can damage the neurons and developing brain causing cognitive dysfunction and impaired patient behavior and adaptation. 


Treatment methods for the intellectually disabled 

There is no specific pharmacologic treatment for intellectual disabilty but drugs like the ones below can be used to relieve some of the symptoms and rehabilitation treatment can help integrate the patient into society: 

Medical treatment
  • Anti-psychotic drugs (e.g. risperidone, olanzapine, aripiprazole mostly prescribed to manage aggression and self-injury)
  • Other psychotropic drugs such as psychostimulants (e.g. Ritalin); anti-depressants and anxiolytics (e.g. diazepam, lorazepam), anti-convulsants/mood stabilisers (e.g. carbamazepine, valproic acid) 
  • N-methyl-D-aspartate receptor antagonist. Prevent nerve cell damage by preventing excessive increases in glutamate concentrations, especially in cases of chronic lead (Pb) exposure 


Cognitive rehabilitation and skills training 

  • Through cognitive and rehabilitation training, we help early-stage intellectually disabled patients maintain their memory and higher-order functions, and upgrade their skills, thereby helping the intellectually disabled achieve self-care, learn livelihood skills, and reduce family and social dependence. 


Minimising the Risk of Acquiring Intellectual Disability 

  • Avoid drinking and smoking during pregnancy 
  • Avoid consanguineous marriage or advanced maternal age 
  • Prevention of bacterial infections, malnutrition or drug abuse during pregnancy 
  • Prevent neurological damage in infants and young children caused by bacterial infections, malnutrition or accidents 
  • Regular prenatal check-ups and child development check-ups 


OT&P Advice 

Intellectual Disability is a challenging condition that entails multi-faceted approach to management to best prepare the patients for self-care and independence. Although patients may have limitations in their personal learning and social adaptability, they are usually no different from ordinary people in aspects other than intelligence, such as emotional or physical abilities. They need our understanding and assistance to better integrate into society, discover their strengths, and find jobs that suit them. If you find that your child or relative or friend may have intellectual disability, please feel free to inquire with our doctors. 

Please note that all medical articles featured on our website have been reviewed by qualified healthcare doctors. The articles are for general information only and are not medical opinions nor should the contents be used to replace the need for a personal consultation with a qualified medical professional on the reader's medical condition.

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