Booklets Examining People with Learning Disabilities, Section one:

Prevalence, Classification and Causes

In order for us to understand how the eye-care needs of people with learning disabilities differ from the rest of the population we need to understand what defines learning disability, its prevalence and its causes. This section aims to describe these, along with some history of how people with learning disability have been cared for through the ages, to help understand our own prejudices and misunderstandings, as well as those of the wider society.

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Historically, the classification of what constituted a learning disability varied widely and whether you were so labelled often depended on luck, the amount of support your family could provide and on the particular regime or individual assessing you. With the development of modern social care, a need for more accurate and consistent assessment of individuals was required. This led to the adoption in the UK and many other countries, of the idea of applying an IQ threshold below which an individual would be classified as learning disabled.

It was assumed that the distribution of intelligence in a population follows a normal Gaussian distribution, or ‘bell shaped’ curve. Most IQ tests set the mean at 100 and the level for classification as learning disabled 2 standard deviations below this mean, or below an IQ of 70. This has been adopted in the UK, so that someone with an IQ score of 69 or less would be considered as having a learning disability (if the cause of this impairment had been acquired under the age of 18). Further classification of the severity of the learning disability is made depending on IQ score. The International Community have put together a variety of scoring systems to classify disease and the Diagnostic and Statistical Manual Of Mental Disorders, 4th Edition (DSM-IV): and the International Classification of Mental and Behavioural Disorders, 10th Revision (ICD-10) have arrived at similar scores as seen below:

Level	ICD- 10	DSM-IV
Mild	50 - 69	50 - 55 to 70 -75
Moderate	35 - 49	35 - 40 to 50 - 55
Severe	20 - 34	20 -25 to 35 - 40
Profound	< 20	<20 - 25

Prevalence of learning disability in a population using this statistical model would be 2.23%, although actual data differs from this figure sometimes quite considerably. There are a variety of reasons for this such as how the tests were conducted, whether the tests were age appropriate or appropriate for the culture and language of those tested. Other reasons include prevalence variations reported by different governments and often their statistics depend largely on the accuracy of data collection in their country. In some cases political influence may also affect reported figures. A large scale literature review conducted by Kiely in 1987 showed that the prevalence rates for learning disability also varied due to age, sex and socioeconomic status. In most studies the prevalence is slightly higher in males and prevalence data sorted according to age tends to show an increase from birth to the mid teens as more cases are identified in the school age population and then diminishes with increasing age as people with milder disabilities are assimilated into society and no longer show up on census or survey data. This pattern is demonstrated in the Kavanaugh and Opit (1999) secondary analysis of the Surveys of Disability of Children and Adults, which are carried out by the Office of Population Censuses and Surveys in the United Kingdom. These surveys focused on reports of mental handicap as an underlying condition and resulted in estimates of 3.5 per 1,000 in the general population, with age-specific rates as follows:
 

Age	Rate per 1,000 Population
0 - 4 years	1.7
5 - 9 years	2.3
10 - 15 years	4.1
16 -19 years	6.3
20 -24 years	5.1
25 -34 years	5.3
35 - 44 years	3.9
45 - 54 years	2.5
55 - 65 years	2.1
65 - 74 years	2.7
75 years and older	2.3

The rate for people with severe and profound disability remains much more stable however at around 0.05% or 5 in 10000.

In order for clinicians and others to understand the expected level of functioning of individuals who have been categorised with various degrees of learning disability the World Health Organisation adopted the following table based on ‘developmental age norms’. This classification system has been criticised by some people working in the field of disability as being too vague due to variations in normal behaviour and also because it applies age inappropriate ‘labels’ to individuals; so care must be exercised when applying its information.

• Mild Mental age 8.5 to 11.0 years
• Moderate Mental age 6.0 to 8.5 years
• Severe Mental age 3.75 to 6.0 years
• Profound Mental age 0 to 3.75 years

Causes of Learning Disability

The term learning disability is the term we use to describe the outward manifestation of (or symptoms produced by) a variety of underlying neuro-developmental disorders. From the clinical point of view this term gives little information as to the cause of the neuro-developmental disorder and no information on its likely effects on other structures within the body. Often when dealing with a person with a learning disability the clinician will have no reason or diagnosis of the cause of the learning disability and indeed often this has never been diagnosed or even investigated fully. With increasing knowledge of the science behind brain function and increased availability of diagnostic tests and imaging then more accurate diagnoses are becoming more common.

Does this matter? Knowing the cause of the neuro-developmental disorder allows us to extrapolate the likely effect on vision or the visual system, for instance an injury caused to one hemisphere of the brain in early life from a brain tumour may lead us to suspect hemianopia, whereas a genetic cause such as Down’s syndrome will affect every cell in the body, affecting the development and ageing of ocular structures.

To list all the possible causes of learning disability is outside the scope of this article but outlined below are the major causes and some examples of each.

Examples of Neuro-developmental Disorders

Genetic

Chromosomal – Fragile X, Down‘s syndrome, Prader-Willi
Structural – Tuberous Sclerosis, Neurofibromatosis, Sturge-Weber Syndrome

Metabolic

Phenylketonuria, Metachromatic Leukodystrophy

Acquired

Infection – Rubella, Toxoplasmosis, Syphilis, Meningitis
Toxic Insult – Fetal Alcohol Syndrome, Maternal substance abuse, Lead Toxicity
Trauma – Structural Damage, Cerebral Palsy
Neoplasms

It should be noted that with the acquired disorders the age or developmental stage at which the insult occurs may dramatically affect the overall severity of the resulting disorder.

All of these conditions will have secondary complications due to the disorder. These complications fall within a few large categories. They include:

1. Impairment of cognitive processing (as in intellectual disability)
2. Impairment of sensation (as in blindness or deafness)
3. Psychiatric illness (as in schizophrenia or chronic depression)
4. Impairment of physical movement (as in spasticity, rigidity or other neuromotor dysfunctions)
5. Seizures
6. Abnormal impulse behaviour

Each of these complications may carry with it secondary health consequences; for example, intellectual disability and sensory impairment may increase chances of traumatic accidental injury; psychiatric illness may lead to poor hygiene and self-neglect; impairment of physical movement may lead to osteoporosis; seizures may lead to bone fracture or aspiration; and abnormal impulse behaviour may lead to self injury. It should also be remembered that these in turn may lead to visual problems, so the cause of someone’s cataract or corneal opacity may not primarily be related to their neuro-developmental disorder but to self injurious behaviour or neglect and poor hygiene.

When we examine patients with good communication skills where full and detailed histories are available to us many of the causes and complications of ocular and visual problems quickly become apparent. With people where the information provided, and their communication skills are more limited greater thought and consideration of different scenarios may be needed to reach the correct conclusion. Having an insight into the more commonly encountered conditions will help this process but in general people with learning disability are much more likely to have disorders of the eye and visual system. Some examples of commonly encountered problems in different conditions are outlined below:

Cerebral Palsy

• Refractive error 50-64%
• Amblyopia 15-32%
• Strabismus 50-69%
• Nystagmus 8.6-18%
• Visual field defect 1-11%
• Optic atrophy 4-10%
• Cortical visual impairment 3.1%

Down’s Syndrome

• Refractive error 30-42%
• Strabismus 27-36%
• Cataract 4-27%
• Keratoconus
• Reduced visual acuity
• Impaired accommodation
• Blepharitis

Conditions affecting normal development of the neural system and brain before birth may also affect the development of the eye and visual pathway. In these individuals developmental disorders such as those listed below may be observed.

• Anophthalmos (no eye) or microphthalmos
• Coloboma of iris, retina, optic nerve
• Corneal sclerosis and anterior chamber defects
• Aniridia and associated congenital glaucoma
• Cataract (and secondary aphakia or lenticular remnants following needling procedures as infants)
• Dislocation of lens
• Retinopathy of prematurity and dragged macula
• Optic nerve hyploplasia
• Optic atrophy
• Strabismus
• Nystagmus

This list is not exhaustive but logical extrapolation and understanding of normal embryology and visual development will help to explain some of the conditions encountered when examining these patients and subsequently enable the practitioner to explain possible causes for acquired behaviours and discuss modifications to environments and care processes, which may help the individual.

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