Moderate and severe learning disabilities are defined as conditions in which intellectual capacity is compromised, with the likelihood that there will be no catch-up development.Profound learning disability is defined. according to the ICD 10 guidelines, as having IQ < 20, severe learning difficulties as having IQ 20 to 36,.moderate as havinglQ 35 to 49, and mild as having IQ 50 to 69. The prevalence of individuals with an IQ < 50 is about IS0 per 100000 of the population, and for those with a milder learningdisability, 1225 per 100 000 of the population.In the 1')30s, a distinction was made between 'pathological', severc learning disability, and 'familial' (usually mild) learning disability. Severely affected individuals were found to have reduced fecundity and life-expectancy, and usually had demonstrable cerebral abnormalities. In keeping with the fact that about one-third have a syndrome caused by a genetic defect, their unaffected first-degree relatives tend to have normal intelligence. The first-degree relatives of individuals with mild learning disabilities have a mean 1Q below the mean for the rest of the population, suggesting a multifactorial aetiofogy I.'.Idiopathic learning disability has been shown to have average wcighted correlations of 0.86 for monoqgotic twins and 0.61 for dizygotic twins and average coqklations between first-degree rela!ivcs of 0.4, suggesting an overall heritability of SO%?.Currently only SO% o f severe, and 20% of mild learning disability have identifiable causes, and the risk of recurrence for undiagnosed cases varies widely. The risk o f recurrence for idiopathic mild learning disabilities has been estimated at 5 to 35% -higher with a male index case -if therc is no other family history of the condition, and 37 t o 39% risk if there is an affected parent or older sibling".'. The risk.. quoted for recurrencc of severe learning disability are 3 to 10% for a $bling of an isolated case, again higher if the index case is male. l'his rises to 1 in 4 if two siblings were affected previously. These recurrence risk.. are based upon the 'pooling' of a great number of different genetic conditions which result in learning disability through different genetic mechanisms, making a true estimate of risk in any one family for an apparently isolated caw without diagnostic clues.*US usage 'mental retardation'. NH. A specific learning disorder (US, learning disorder) is used when a person has an IQ above 70 but has difficulty in performing academic taks.
Genetic causes of learning disabilitiesThere are many different causes of learning disabilities.
SING1.E-GENE DISORDERSThere are many well-recognised mcndelian conditions which are associated with learningdisorders. These are characterised by autosomal recessive, autosomal dominant, or Xlinked patterns of inheritance.Autosomal \recessive disorders are manifested in the. homoqgote, b h o has inherited .the gene from clinically unaffqcted carrier parents (heterozygotes), and the recurrence'risk for full siblings of an affected child is 1 in 4.
