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There are several types of classifications and assessments that may be useful when working with people with intellectual disabilities. These may address the following areas:

• the level of a person's ability
• a person's skills, functioning and community participation
• the cause of a person's intellectual disabilities
• epilepsy
• the range of mental disorders experienced by people with intellectual disabilities.

Mental disorders are commonly experienced by people with intellectual disabilities - the point prevalence has been measured as 40% (Cooper and Bailey, 2001). A person with intellectual disabilities is therefore considerably more likely to have an additional mental disorder than the average person from the general population. This is not surprising when one considers the likely causes of mental disorders. As well as having all the risk factors that are relevant for the whole population, people with intellectual disabilities may have extra risk factors. These can be considered using a biological-psychological-social-developmental framework (see Figure 1: Possible risk factors for mental disorders).

Assessment
The integration and interpretation of information collected during a thorough psychiatric assessment typically comprises three stages:

• measurement of psychopathology
• classification of psychopathology into diagnostic groups
• determining the likely aetiology.

Methods - the methods of assessment used with the general population are also relevant when working with people with intellectual disabilities, supplemented by some additional special considerations. This includes spending time with the person to hear their concerns and experiences and to conduct an examination; taking a collateral history from a close relative or other close person; reviewing previous medical case notes; and speaking to professionals from other disciplines, such as community nurses, psychologists and social workers, who are involved in supporting the person. The information gathered should cover the usual psychiatric headings of full history, mental state examination, physical examination and special investigations. The findings can then be integrated and interpreted into the three-stage framework to summarize the relevant psychopathology (positive and negative findings), classify and describe likely aetiology. It is important to conduct a thorough, detailed assessment so that the treatment/intervention/support plan can be devised in such a way as to optimize the chance of best possible outcome for each individual in his or her particular set of circumstances.

Special assessment considerations
Psychiatric assessment of a person with intellectual disabilities has much in common with assessments undertaken with people of average ability, but there are a number of additional special considerations (see Figure 2: Psychiatric assessment of a person with an intellectual disability).

Capacity to consent: a person's intellectual disabilities and/or additional mental disorder may influence his or her capacity to consent to an intervention. Assessment should therefore include:

• the person's understanding of a proposed intervention and what it involves
• the reason for the proposed intervention
• the potential benefits of the intervention
• the potential risks of the intervention
• the potential consequences of declining the intervention
• the range of alternative interventions, including their potential benefits and risks.

A person may have capacity to consent to or refuse some interventions (e.g. a blood test to screen for hypothyroidism) but not have capacity to consent to or refuse more complex interventions (e.g. cardiac surgery or electroconvulsive therapy). When a person does not have capacity to consent to a particular intervention, it is good practice to offer as much information and explanation as possible in as accessible a format as possible, to enable the person to partially consent/decline as far as they have capacity to do so. It is then important to include the person's next-of-kin, advocate, legal representative/guardian, professionals from other disciplines involved in the person's care and key support workers in decision-making. The exact process will depend on the area of jurisdiction; for example, in Scotland the legal framework provided by the Adults With Incapacity (Scotland) Act 2000 should be followed (see also Keywood and Flynn 2003: pp.59-62). In some circumstances, implementation of mental health legislation may be appropriate.

Communication: a number of factors can influence a person's ability to understand and to express opinions and information. The extent and type of a person's intellectual disabilities influences communication ability and needs, as can a range of additional factors, such as:

• neurological factors
• mental disorders
• drug side-effects
• life experiences that have shaped personality development (e.g. leading to social phobias or lack of confidence)
• social circumstances
• the setting in which the communication is taking place
• the communication style of the other person(s)

It is important to consider all these factors in order to enhance communication. (see Figure 3: Communicating with a person with an intellectual disability).
Clinicians should remain aware of possible communication limitations; even the most able person with intellectual disabilities may have difficulties remembering and communicating the temporal sequence of past events or symptoms. Acquiescence and suggestibility may also remain a limitation to the assessment of information. It should be remembered that a person with limited or no verbal communication skills may use other methods of communicating their feelings. This may include a change in their usual behaviour, or even aggression if this is the only or easiest way to communicate pain, distress or fear (see also Joyce 2003: pp.17-20).

Collateral history: taking a history from an informant is an integral part of psychiatric assessment. Sometimes several informants are necessary. The more severe the person's intellectual disabilities, the greater the reliance on carers for information; for people with profound intellectual disabilities and no verbal communication skills, most of the assessment information will be provided by carers.

Information from parents - parents are usually able to provide detailed information from the past. This helps to provide an understanding of how the person developed through their childhood, the adverse experiences they encountered, their achievements and relationships that shaped the person they have become as an adult. This information is also important to distinguish the usual behaviour and traits of the person from behaviours that are new (i.e. symptoms) and may be part of mental ill-health. Important background information may also include:

• past psychiatric or medical information
• past drug and other treatments, and their effectiveness
• allergies
• family history of psychiatric and medical problems
• details of the person's skills and abilities, against which current function may be compared (regression of skills is often a feature of mental ill-health, but its detection requires knowledge of previous level of skills).

Information from others - if the person no longer lives with their parents, the parents will not be able to provide detailed information on daily aspects of life. The level of detail and accuracy of information from support workers may depend on various factors, such as:

• how much individual time the support worker spends with the person and how well they know her/him
• how many other people the worker also supports
• how well information is shared between members of the support teams
• how well organized care is (e.g. 'key-worker' system, identified responsibilities)
• how well the employing organization supports and develops its support workers.

If a person's support package has only recently been established, the support workers may be able to provide limited information only. Understandably, support workers may volunteer information on needs that present the greatest challenge to care (e.g. aggression) and omit symptoms such as social withdrawal or loss of energy. It is important, therefore, that a thorough assessment is always conducted of all possible psychopathology, using a semi-structured approach, after the person and their support worker have given the history in their own words.

Where necessary, additional potential informants, such as employers, day centre officers or support workers from previous placements, should be approached, and limitations and gaps in the gathered information should be recognized.

Pathoplastic effect of developmental level on psychopathology: psychopathology is modified by developmental level. This pathoplastic effect is greater the more severe the person's intellectual disabilities. Understanding of intellectually complex concepts such as guilt requires a developmental age of about 7 years. An adult with profound intellectual disabilities will learn throughout her/his life, will develop skills, and will have adult biological drives and motivations and therefore is not the same as a 0-3-year-old (the equivalent mental age to profound intellectual disabilities), but she/he will not acquire the mental capacity to understand abstract concepts requiring a higher developmental age. Similarly, psychopathology such as hopelessness, worthlessness, suicidal ideation, delusional perception, and distorted body image do not present at more severe levels of intellectual disabilities. Additionally, some psychopathology (e.g. psychotic symptoms) requires good verbal communication skills to describe its presence, hence it may not be elicited in people with profound intellectual disabilities.

Some symptoms that occur during mental ill-health in adults with intellectual disabilities are rare in adults of average ability. Examples include loss of skills, impairment of communication, and onset of or exacerbation of pre-existing problem behaviours. Irritability is often the core mood symptom presenting in depressive episodes. Developmental level can also have an impact on the content of abnormal phenomena. For example, delusions may have a 'childlike' content, reflecting the intellectual capacity and life experiences of the person.

Suggestibility can influence the characteristics of psycho­pathology. Regarding misery, for example, the assessor may be able to cheer up the person to the point of smiling or laughing during the interview. Characteristically, the person is unable to sustain the cheerful mood state, and returns to misery when not being talked out of it. Delusions may not be 'unshakeable' - i.e. the assessor may be able to gain the person's acquiescence that they are not really true; however, the person returns to the delusional belief once not being actively persuaded otherwise.

Developmental history: assessment of development serves several functions.

• Measurement of developmental level enables appropriate interpretation of psychopathology (in the context of the pathoplastic effect of intellectual disabilities on psychopathology).
• Determining the usual pattern of behaviours and skills for the person is essential to distinguish symptoms of mental ill-health from long-standing traits (e.g. sleep disturbance may be a long-standing trait, or one of several symptoms of depressive episode, requiring treatment).
• Diagnosis of autistic spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD) requires knowledge of developmental history (in addition to present information) (see also Howlin 2003; pp. 24-8).
• A good understanding of development reduces the likelihood of 'diagnostic overshadowing' (the tendency to attribute all problems to the person's intellectual disabilities - e.g. failing to identify, and therefore treat, a person's superimposed depressive episode or anxiety disorder). This is particularly relevant for people who in addition to intellectual disabilities have ASD or ADHD; depression and anxiety are thought to occur commonly in people with ASD.
• For some individuals, developmental models may be relevant in understanding the aetiology of problem behaviours: e.g. head-banging is a developmental phase at about 18 months of age; oppositional behaviour is common at about 2 years. For an adult with a similar level of mental capacity, developmental models may help explain aetiology, with behaviour development arrested at certain stages such that it is a problem for the person.
• Plans of treatment/interventions/supports need to be devised in a way that is developmentally appropriate.
• Determining the underlying cause of a person's intellectual disabilities identifies genetic causes. An awareness of the associated health factors allows for early detection and/or prevention. For example, hypothyroidism is common in Down's syndrome, as are sensory impairments and cardiac anomalies, all of which can be screened for.

Behavioural phenotypes: many genetic causes of intellectual disabilities have specific behavioural phenotypes. This area is covered in O'Brien 2003: pp.33-38.

Epilepsy is common, occurring in about 25% of adults with intellectual disabilities (see also Bernal 2003: pp.69-73). Compared with epilepsy among the general population, people with intellectual disabilities often have mixed seizure types and seizure frequency that is difficult to control. This is of direct relevance to psychiatric assessment, and often requires differential diagnosis between seizures (particularly complex partial seizures), mental ill-health (such as panic attacks, problem behaviours and depressive episodes) and drug side-effects (e.g. side-effects of anti-epileptic drugs can mimic depressive episodes or problem behaviours). As mental disorders and epilepsy are both common in this population, they often co-exist. People who have epilepsy may also have a higher rate of mental disorders than those who do not. They may have psychological sequelae of the epilepsy itself, and a proportion additionally experience pseudo-seizures. Psychiatric assessment should include an assessment of epilepsy in order to address these issues, and therefore requires a knowledge of epilepsy, its associations and treatments, in addition to its interface with psychiatric disorders.



Assessment of epilepsy is also required to inform the development of treatment plans in view of the effects of psychotropic drugs on seizure threshold, and drug interactions between anti-epileptic drugs and other psychotropic drugs.

Physical health: for many reasons, physical disabilities and disorders are common in people with intellectual disabilities, who are also more likely to be prescribed medication. There may be psychiatric sequelae to some of these physical health needs, and side-effects of drugs and drug combinations. Assessment of physical health and drug histories therefore assumes a more important component of psychiatric assessment than it might for a person of average ability.

Classification
Classification of mental disorders is undertaken for a variety of reasons. In the mid-20th century it was recognized that there was considerable international variation in psychiatric practice, and the introduction of operationalized diagnostic criteria focused attention on improved, more standardized psychiatric assessments and diagnostic procedures. It could be contended that while such issues are no longer contemporary in general psychiatry, they remain relevant for psychiatric practice with people with intellectual disabilities. Classification can therefore improve clinical practice and so benefit individuals. Classification systems also provide a tool to use in the description of whole population needs and in service planning. Classification also has an essential role in research. Inclusion criteria must be clearly described in order to:

• allow comparison of findings between different studies
• replicate studies
• evaluate effectiveness of treatment in one group compared with another
• study the epidemiology of mental disorders in the popu­lation
• identify potentially modifiable vulnerability and protective factors for mental disorders.

All of the above requires the use of operationalized diagnostic criteria as described in a classificatory system.

The development of DC-LD
Classification of mental disorders among the general population typically draws on the diagnostic criteria within ICD-10 or DSM-IV. However, while these systems may be appropriate for people with mild learning [intellectual] disabilities, they lack validity and utility for people with more severe learning [intellectual] disabilities or with ASD or ADHD in addition to mild learning [intellectual] disabilities. DC-LD (Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities; Royal College of Psychiatrists, 2001) provides more appropriate operationalized diagnostic criteria within a classification system specifically designed for use with adults with learning [intellectual] disabilities, and is complementary to ICD-10 (see Figure 4: Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning [Intellectual] Disabilities [DC-LD]).

The development of DC-LD addressed the following issues:

• the pathoplastic effect of increasing severity of learning [intellectual] disabilities on psychopathology
• limitations in eliciting psychopathology from informant histories (and hence the increased likelihood of inaccuracy introduced by the extensive sub-classifications within ICD-10 main categories)
• inconsistencies within ICD-10 and DSM-IV regarding classification of problem behaviours, features that are recognized as part of a behavioural phenotype, and use or otherwise of the 'organic' categories
• the need to distinguish between features due to level of learning [intellectual] disabilities, cause of learning [intellectual] disabilities, developmental disorders, mental illness, personality disorders and problem behaviours.

DC-LD introduces a hierarchical approach to improve clinical practice and reduce the risks of diagnostic overshadowing. It provides further information on assessment and classification of mental disorders in adults with learning [intellectual] disabilities.

REFERENCES
Bernal J, Epilepsy in People with Learning Disabilities. Psychiatry 2003; 2:9; 69-73.

Cooper S-A, Bailey N M. Psychiatric disorders amongst adults with learning disabilities - prevalence and relationship to ability level. Ir J Psychol Med 2001; 18: 45-53.

Howlin P, Autistic Spectrum Disorders. Psychiatry 2003; 2:8: 24-28.

Keywood K, Flynn M, Healthcare Decision-making by Adults with Learning Disabilities: Some Levers to Changing Practice. Psychiatry 2003; 2:9: 59-62.

Joyce T, Functional Analysis and Challenging Behaviour. Psychiatry 2003; 2:8: 17-20.

O'Brien G, Behavioural Phenotypes in Adulthood. Psychiatry 2003; 2:8: 33-38.

FURTHER READING
Books Beyond Words series. London: Gaskell.
(A series of purpose-designed publications presenting pictorial information to facilitate work with people with intellectual disabilities. They cover a range of topics and are published by Gaskell.)

Royal College of Psychiatrists. DC-LD (Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities/Mental Retardation). London: Gaskell, 2001.
(A new classificatory system designed specifically for use with adults with intellectual disabilities; it is complementary to ICD-10. As well as presenting operationalized diagnostic criteria, the text discusses issues relevant to psychiatric assessment with this population.)

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