What causes intellectual disability?

Intellectual Disability or mental retardation is an umbrella term which is seen in a wide variety of conditions. Strictly speaking the clinician should treat intellectual disability as clinical sign suggestive of an underlying pathology rather than as a disease in itself. Given below is a list of conditions that can lead to intellectual disability

Causes of intellectual disability

(from Bherman Et al (ed) Nelson's Textbook of Pediatrics)

Category	Type	Examples
Prenatal (causes before birth)	Chromosomal disorders	Downs syndrome*, Fragile X syndrome, Prader Wili syndrome, Klinefelters syndrome
	Single gene disorders	Inborn errors of metabolism, such asgalactosemia*, phenylketonuria*,mucopolysaccaridoses Hypothyroidism*, Tay- Sachs disease        Neuro-cutaneous syndromes such as tuberous     sclerosis, andneurofibromatosis Brain malformations such as geneticmicrocephaly, hydrocephalus and  myelo-meningocele* Other dysmorphic syndromes, such as Laurence  Moon Biedl syndrome
	Other conditions of genetic origin	Rubistein Tabi syndrome De Lange syndrome
	Adverse material / environmental influences	Deficiencies* , such as iodine deficiency and folic acid deficiency Severe malnutrition* in pregnancy Using substances * such as alcohol (maternal alcohol syndrome), nicotine, and cocaine during early pregnancy Exposure* to other harmful chemicals such as  pollutants, heavy metals,abortifacients, and harmful medications such as thalidomide,phenytoin and warfarin sodium in early pregnancy Maternal infections such as rubella*,syphillis*,    toxoplasmosis,cytomegalovirus and HIV       Others such as excessive exposure to radiation*, and Rh incompatibility*
Perinatal  (around the time of birth)	Third trimester ( late pregnancy)	Complications of pregnancy* Diseases* in mother such as heart and kidney  disease and diabetes Placental dysfunction
	Labour (during delivery)	Severe prematurity, very low birth weight, birth  asphyxia Difficult and/or complicated delivery*  Birth trauma*
	Neonatal (first four weeks of life)	Septicemia, severe jaundice*, hypoglycemia
Postnatal  (in infancy and childhood)		Brain infections such as tuberculosis, Japanese  encephalitis, and bacterial meningitis Head injury* Chronic lead exposure* Severe and prolonged malnutrition* Gross understimulation*