Diagnostic overshadowing: definition and clinical significance
Diagnostic overshadowing in intellectual disability occurs when a person's established intellectual disability becomes an overly persuasive explanation for symptoms that may arise from a separate, treatable physical or mental health condition. A clinician may attribute persistent sadness to a person's disability, interpret acute pain as a behavioral problem, or explain a marked loss of functioning as an inevitable feature of lifelong cognitive limitations. In each case, the error is the premature attribution of a new clinical finding to an existing diagnosis before plausible alternative causes have been evaluated. The concept is especially important because intellectual disability and other health conditions can coexist, while differences in communication, symptom expression, and access to services complicate recognition. Dell'Armo and Tassé, 2024; NICE NG54.
The practical principle is straightforward: a diagnosis of intellectual disability explains a particular developmental profile; it does not independently establish the cause of a new symptom, change in behavior, or decline in skills. Proper clinical care therefore asks what has changed, when it changed, what the person is experiencing, and what evidence supports each possible explanation. This approach can identify treatable conditions while also protecting people from unsupported psychiatric labels, invasive testing without indication, and interventions chosen for convenience rather than clinical need. NICE NG11.
Intellectual disability and the boundaries of an existing diagnosis
Intellectual disability is characterized by significant limitations in intellectual functioning and adaptive behavior arising during the developmental period. Intellectual functioning encompasses reasoning, learning, and problem solving; adaptive behavior encompasses conceptual, social, and practical skills used in everyday settings. AAIDD specifies onset before age 22 in its current defining criteria. Its guidance also requires attention to assessment error, culture, linguistic diversity, individual strengths, and the influence of supports. These features establish the person's developmental context and help guide appropriate assessment; they cannot be used as a blanket explanation for later medical or psychological changes. AAIDD defining criteria; AAIDD clinical FAQs.
The distinction between baseline limitations and acquired change is foundational. A person may have longstanding difficulty organizing daily tasks yet develop new confusion during an infection. Another person may usually need help identifying emotions yet become markedly withdrawn after bereavement. A child whose communication has always been limited may suddenly resist meals because of oral pain. The developmental history supplies the baseline against which these changes become visible. An accurate assessment takes seriously both intellectual and adaptive function, including the level of communication support the person needs, without allowing either to replace a medical or psychiatric evaluation.
What diagnostic overshadowing is and what it is often confused with
The core mechanism is attribution bias: evidence of intellectual disability changes the interpretation of other clinical evidence. Closely related problems can produce the same outcome, including inadequate history taking, inaccessible communication, fragmented records, insufficient specialist training, limited appointment time, and diagnostic tools that have not been validated for the person's communication or cognitive profile. These are distinct mechanisms and may coexist. A missed diagnosis alone does not prove that a clinician exhibited a particular bias; establishing overshadowing as a causal explanation requires evidence that disability status distorted judgment. Dell'Armo and Tassé, 2024; NICE evidence review on identification and assessment.
Diagnostic overshadowing also differs from diagnostic masking. Masking refers to one condition or symptom pattern obscuring another, whether or not a clinician makes an attribution error. Communication limitations, overlapping behavioral manifestations, medication effects, and sensory or neurological conditions can make a disorder difficult to recognize even when a clinician is alert to the possibility. The distinction matters for prevention: correcting a judgment bias requires reconsideration of assumptions, whereas correcting limited access to information may require augmentative communication, additional informants, time for observation, and adapted measures.
An important further distinction is between a symptom and its cause. Aggression, sleep disruption, repetitive actions, self-injury, or social withdrawal are descriptions of what happens. They may reflect pain, distress, a psychiatric condition, environmental demands, communication barriers, medication effects, learned behavioral functions, or more than one contributor. They are not diagnoses by themselves. Functional behavioral assessment can clarify the conditions under which a behavior occurs, while clinical assessment determines whether illness or injury is present. Both are often necessary. NICE NG11.
What the scientific evidence actually shows
The 2024 systematic review: a real concern with heterogeneous findings
The strongest directly relevant synthesis is the systematic review by Kristin Dell'Armo and Marc J. Tassé, published in the American Journal on Intellectual and Developmental Disabilities in 2024. The authors reviewed 25 studies of diagnostic overshadowing of psychological disorders in people with intellectual disability, including peer-reviewed publications and dissertations. Around one third of the included studies found no overshadowing. The reviewers assessed the overall quality of evidence as low, identifying recurring problems such as small convenience samples, dated research, artificial vignette methods, and statistical weaknesses. Their finding warrants careful interpretation: diagnostic overshadowing is a plausible and documented pattern under some experimental and clinical conditions, but the literature does not justify treating it as an inevitable response by every clinician or estimating a universal real-world rate from the available studies. Dell'Armo and Tassé, 2024.
The populations and methods matter. When researchers ask clinicians to evaluate a brief simulated case, they can manipulate whether the same symptom information accompanies a label of intellectual disability. This experimental control helps isolate attribution bias. Yet a vignette omits important features of real practice: repeated visits, evolving examination results, collateral histories, longitudinal records, supported communication, and multidisciplinary review. Conversely, a retrospective chart study may reflect real care while offering less control over the reasons a diagnosis was or was not made. These differences limit direct comparisons among results. Research findings on psychiatric diagnostic judgments should also not be uncritically generalized to every physical illness or healthcare setting.
The 2025 psychologist experiment: a more precise account of possible bias
A later study by Dell'Armo and Tassé tested 318 licensed psychologists using simulated clinical material that either did or did not indicate intellectual disability. Participants reviewed schizophrenia-related information through a written or video vignette and gave diagnostic impressions. On rating scales, they judged schizophrenia slightly less likely when intellectual disability was indicated, with a small effect. Open-ended responses revealed a more nuanced pattern: 96% of respondents assigned to the intellectual-disability condition considered a mental health diagnosis, and the study did not find evidence of complete diagnostic overshadowing in those open responses. Video presentation did not materially change diagnostic impressions. These data support the possibility of a small shift in the perceived likelihood or specificity of a diagnosis, while showing substantial recognition of mental illness among participating psychologists. Dell'Armo and Tassé, 2025.
This study strengthened the field by using a larger professional sample and more than one response format. Its simulated cases still cannot establish the frequency of harmful diagnostic errors during ordinary patient care. It also tested impressions of a particular psychiatric presentation, so its results should not be converted into numerical claims about depression, epilepsy, gastrointestinal disease, or all clinical disciplines. The evidence supports better diagnostic reasoning and appropriately adapted assessment. It does not support blanket accusations about professional groups.
Earlier experimental research and its continuing relevance
Mason and Scior reported a British vignette study in 2004 involving psychiatrists and clinical psychologists. The cases described comparable presenting problems while varying information about intellectual functioning; among respondents, symptom recognition tended to be lower when intellectual disability was described. This study helped demonstrate how an intellectual-disability label could influence clinical judgments within an experimentally controlled setting. The response pattern and historical context limit inference about current real-world practice, but the design illustrates the underlying question that newer studies continue to investigate: does a developmental diagnosis alter recognition of a potentially independent health condition when the symptoms remain otherwise comparable? Mason and Scior, 2004.
The overall state of evidence is therefore differentiated. The concept and the need to assess co-occurring conditions are established in professional guidance. Controlled studies provide evidence of attribution bias in some circumstances, with mixed findings and limited external validity. Claims about exact prevalence, comparative performance across every specialty, or the effectiveness of a single anti-bias training package remain uncertain. Keeping these levels of certainty distinct strengthens clinical recommendations rather than weakening them. NICE NG54; Dell'Armo and Tassé, 2024.
How health conditions are missed: linked mechanisms
Anchoring on the established diagnosis
Diagnostic anchoring occurs when the first available explanation dominates subsequent reasoning. In intellectual-disability care, the developmental diagnosis is often prominently documented, while the person's usual presentation, recent health changes, and personal account are less accessible. New irritability may be described as a longstanding behavioral feature; difficulty concentrating may be attributed to cognitive limitation; reduced mobility may be treated as reduced motivation. The corrective step is to describe the current problem independently of the existing diagnosis, then compare it with the person's individual baseline and the timeline of change.
Communication mismatch and underreported symptoms
A person may communicate through speech, gestures, behavior, picture symbols, assistive technologies, or a combination of methods. Some can name a symptom precisely but need additional processing time. Others experience pain or emotional distress but communicate its presence indirectly. Standard interview questions may be too abstract, fast, or linguistically complex. Healthcare teams can create a misleading impression of absent symptoms simply by failing to offer an accessible response method. NICE recommends adapting communication and considering input from people who know the individual while including the patient directly in assessment and decisions. NICE NG54; ASHA intellectual disability practice portal.
A communication supporter can help explain familiar expressions, recognize changes, and facilitate augmentative and alternative communication. The supporter is an additional source of information, while the person's own communication, preferences, privacy, and consent remain central. Different reporters may legitimately notice different phenomena across settings. Disagreements between the person, family, and staff are clinically useful information to explore, rather than automatic grounds to dismiss any participant. NICE NG54.
Atypical or indirect clinical presentation
Mental health conditions may be expressed through changes in sleep, appetite, activity, social engagement, self-care, irritability, or previously mastered skills. Physical disorders can also cause those changes. The absence of a conventional verbal description of sadness, fear, pain, or hallucinations does not exclude clinically important distress. Equally, a behavioral change does not establish a psychiatric disorder. Clinical assessment tests different hypotheses through history, observation, examination, proportionate investigations, and appropriate specialist input. NICE identifies loss of skills, social withdrawal, irritability, avoidance, agitation, and reduced interest in usual activities as changes that can justify mental-health evaluation. NICE NG54.
Fragmented services and information loss
A person may receive care from primary care, psychiatry, neurology, dentistry, education, residential support, and several hospitals. Symptoms can be interpreted differently by each service, especially if no one shares a clear account of the previous baseline, recent medication changes, communication supports, or pending test results. Repeated referrals may increase the number of encounters without producing an integrated explanation. Clear responsibility for coordinating assessment, recording working hypotheses, communicating findings, and reviewing outcomes helps reduce that fragmentation. NICE recommends coordinated pathways and explicit roles across intellectual-disability and mental-health services. NICE NG54.
Expectations about behavior and diagnostic labels
Terms such as 'challenging behavior' describe the impact or context of an action, not its medical cause. Such language can encourage an overly narrow response when the team begins with behavior management instead of asking what the behavior communicates or what has changed physically and psychologically. The reverse error is also possible: attributing all distress to pain may delay recognition of depression, anxiety, psychosis, or trauma. A sound formulation considers the interaction among medical factors, emotional experience, learned contingencies, sensory needs, relationships, and environmental conditions. NICE NG11.
Psychiatric conditions most vulnerable to misinterpretation
Depression and loss of functioning
Depression may involve sadness, reduced enjoyment, disturbed sleep or appetite, fatigue, hopelessness, and changes in psychomotor activity. In a person with intellectual disability, one of the first observable signs may be reduced participation in formerly preferred activities, new dependence in daily routines, withdrawal, or irritability. The diagnosis depends on the duration, pattern, associated symptoms, degree of change, differential diagnosis, and clinically significant impact. Persistent developmental support needs are not equivalent to an acquired depressive episode. Assessment should incorporate the person's account in an accessible form and information from people familiar with their usual functioning. NICE includes adapted depression measures among options for symptom monitoring; these supplement a clinical evaluation rather than making the diagnosis automatically. NICE NG54.
Anxiety, threat perception, and avoidance
Anxiety can be missed when avoidance is interpreted simply as refusal to cooperate, or when agitation is described solely as a behavior to suppress. New reluctance to attend a setting may reflect anxiety, painful experiences, sensory overload, bullying, a change in routine, or a combination of these. The diagnostic task is to explore feared situations and physiological arousal in a developmentally and communicatively appropriate way, while also considering environmental and physical explanations. Patterns across settings and changes over time can help distinguish a persistent anxiety condition from a specific stressful situation.
Psychosis and misinterpretation of communication
Psychosis requires particular care because unusual statements, imaginative experiences, repetitive speech, communication differences, and limited abstract understanding can be mistaken for delusions or hallucinations. The opposite error occurs when genuinely new perceptual disturbances, disorganization, or loss of functioning are attributed to longstanding intellectual disability. Assessment should establish a developmental baseline, elicit the meaning of experiences through accessible communication, evaluate onset and context, consider medical or substance-related causes, and involve clinicians skilled in both intellectual disability and psychiatric diagnosis. The 2025 experimental study examined impressions of schizophrenia-related symptoms and found only a small shift in likelihood ratings, illustrating the importance of calibrated conclusions about bias. Dell'Armo and Tassé, 2025; NICE NG54.
Trauma and safeguarding concerns
Fearfulness, sleep changes, avoidance, agitation, or regression can follow adverse experiences, including abuse, coercion, bullying, and disrupted relationships. The ability to disclose what occurred may depend on opportunities for private, supported communication and a safe response from professionals. The assessment should consider the person's safety, potential exploitation, environmental circumstances, and access to appropriate safeguarding procedures. Behavioral observations alone cannot establish a trauma diagnosis or substantiate an allegation; skilled interviewing and appropriate safeguarding processes are needed. NICE directs clinicians to evaluate vulnerability to exploitation and to apply safeguarding protocols when indicated. NICE NG54.
Physical illness, pain, and neurological conditions
Diagnostic overshadowing is often discussed in relation to psychiatric symptoms, but an equally consequential clinical problem occurs when physical complaints are not investigated because disability or behavior is assumed to explain them. A new behavioral disturbance may accompany dental disease, constipation, infection, injury, impaired vision or hearing, adverse medication effects, or another medical condition. The particular medical explanation must be established through appropriate history, examination, and testing; neither intellectual disability nor the behavior itself supplies the diagnosis. NICE explicitly calls for review of physical health, pain, medication interactions and side effects when assessing challenging behavior. NICE NG11.
Pain can be particularly difficult to recognize when the person cannot describe location, intensity, or quality using standard language. Changes in movement, sleep, appetite, vocalization, touch tolerance, facial expression, or participation may prompt an assessment, but individual patterns and clinical examination remain essential. Clinicians should ask the person directly in an accessible way, seek familiar observers' descriptions of a baseline change, and avoid treating a numerical pain score as the only evidence. The evidence base for recognizing pain in people with different cognitive and communication profiles requires careful attention to the tools and settings used.
Epileptic seizures, adverse drug reactions, sleep disorders, sensory changes, and acute delirium may alter behavior or cognition and require distinct assessment pathways. A sudden change in alertness or consciousness, a new focal neurological deficit, prolonged or repeated seizures, serious injury, or signs of severe acute illness requires urgent medical evaluation. These findings should never be normalized as part of a pre-existing intellectual disability. The severity and urgency of the clinical presentation, rather than the diagnostic label, guide escalation.
Cognitive decline and dementia
Adults with intellectual disability may experience age-related illness and neurocognitive change, while their baseline cognitive performance varies widely. Standard dementia tools may have limited interpretability when applied without appropriate norms and knowledge of longstanding abilities. New loss of practical skills, additional prompting, or a change in behavior may prompt assessment, especially for adults with Down syndrome. NICE recommends asking about such changes at annual health checks and considering adapted measures of cognitive and adaptive function during a dementia assessment. A longitudinal individual baseline is therefore often more informative than a single raw cognitive score. NICE NG54.
Clinical assessment that reduces missed diagnoses
First establish an individualized baseline
An effective assessment begins with what the person was like before the current concern. Document usual communication, sleep, appetite, mobility, social engagement, self-care, work or school functioning, coping methods, preferred activities, and any longstanding behaviors. Clarify the time course: sudden, intermittent, progressive, or situational changes suggest different investigative priorities. Record what the person says or shows, who observed the change, and whether it occurs across environments. The aim is to identify new findings without assuming that every deviation is a symptom of a single disorder.
Then assess physical health, mental health, and context together
The clinician should take an accessible history and perform an examination suited to the presenting problem. Potential medical causes, pain, medication effects, sleep, sensory impairment, recent illness, psychiatric symptoms, environmental demands, bereavement, and trauma-related concerns should be considered according to the actual presentation. This is a hypothesis-driven differential diagnosis, not an indiscriminate package of laboratory tests or a requirement that every condition be excluded before mental-health care begins. NICE recommends assessing the person's developmental and medical history, current medication, family and social circumstances, and potential psychiatric disorders beyond the first reported symptoms. NICE NG54.
Adapt the interview without surrendering diagnostic rigor
Useful accommodations include plain language, one question at a time, processing time, visual or pictorial choices, a quiet room, familiar routines, and supported communication technologies where helpful. A clinician should ask how the person prefers to communicate, check comprehension, avoid suggestive questions, and offer opportunities to speak privately. Questions need to be fitted to the person's language and cognitive profile while retaining the specificity needed to identify symptoms. ASHA describes communication assessment and individualized supports as core components of intellectual-disability practice. ASHA practice portal.
A trusted family member or support worker may supply a detailed longitudinal history, especially when expressive communication is limited. Their interpretation should be treated as one perspective that can be checked against the person's own account, direct observation, medical records, and information from other settings. Respecting supported decision-making means enabling the patient to participate at the highest feasible level, including choices about who is present and what information is shared. Consent and confidentiality requirements remain applicable.
Use adapted instruments for the question they can answer
Screening instruments identify possible problems requiring further evaluation; symptom scales can monitor change; diagnostic interviews and clinical judgments integrate criteria, time course, impairment, exclusion of alternative explanations, and context. A positive screen is not a diagnosis, and a low score on an unvalidated instrument cannot safely rule out a disorder. NICE advises considering assessment measures developed or adapted for people with intellectual disability and interpreting ordinary instruments cautiously. For adults with milder intellectual disability, it identifies the Glasgow Depression Scale as an option for self-report; its carer supplement may assist across a wider range of support needs. Tools must be selected according to the intended construct and the population for which evidence is available. NICE NG54.
A measure of adaptive behavior helps characterize everyday support needs and baseline functioning. It does not establish the cause of a recent change in health or mood. Likewise, an IQ score neither diagnoses depression nor determines whether physical pain is present. Conflating intellectual assessment, psychiatric screening, and medical diagnosis creates measurement errors that can mimic or magnify diagnostic overshadowing. The assessment method should match the clinical question and the person's communication and cultural context. AAIDD clinical FAQs.
Develop a multidisciplinary formulation and plan for review
When symptoms are complex, a written formulation should summarize findings, the most plausible explanations, the evidence for and against them, immediate risks, missing information, and next steps. A plan may involve primary care, nursing, psychiatry, psychology, neurology, speech-language pathology, occupational therapy, dentistry, or other services, depending on the clinical need. Care teams should designate responsibility for follow-up and communicate results in accessible terms. If a treatment does not work or the person's condition changes, the initial explanation needs to be reopened. NICE calls for coordinated care, reassessment when new information emerges, and care plans that identify goals, interventions, outcomes, and responsibilities. NICE NG54.
Challenging behavior, functional assessment, and the medical differential
A functional behavior assessment examines antecedents, behavior, consequences, patterns across environments, and factors that may maintain an action. It can reveal communication functions, situational demands, sensory contingencies, or socially mediated patterns. It does not substitute for evaluation of pain, a psychiatric condition, or another health problem. A person can have a learned pattern of behavior and an acute illness simultaneously. Clinicians therefore need to integrate the functional account with medical and psychological assessment and revise it when evidence changes. NICE NG11.
When repetitive self-injury or severe aggression occurs, protecting immediate safety and assessing relevant medical and psychiatric contributors take priority. A careful risk assessment addresses the person's safety, possible harm to others, vulnerability to exploitation, and the least restrictive appropriate response. The onset and context of behavior are especially informative: new or sharply intensified behavior after a medication change, illness, social disruption, or possible injury warrants reconsideration of the causal explanation. Behavior-support plans should emphasize quality of life, communication, environmental modifications, and person-centered goals. NICE NG11.
Medication as a potential source of error
Psychotropic medication may be indicated for a diagnosed psychiatric disorder, and carefully monitored treatment can be clinically appropriate. Diagnostic problems arise when a sedating medication is used as a substitute for investigating distress, when a behavioral label becomes the sole rationale for ongoing prescribing, or when adverse effects are themselves interpreted as manifestations of disability. NICE recommends clear target symptoms, specialist oversight and monitoring, periodic review, and particular restrictions on antipsychotic prescribing for behavior that challenges. The treatment decision belongs to a qualified clinician working with the person and their chosen supporters; medication should not be started, stopped, or adjusted solely on the basis of an educational article. NICE NG11.
How families, supporters, and clinicians can respond to concerns
A useful concern statement describes observable change rather than assigning a diagnosis: 'For the last two weeks, she has stopped enjoying music, wakes repeatedly, and needs much more help dressing; this is different from her usual behavior.' Such information helps a clinician build a differential diagnosis. The person and their supporter can ask which physical and mental health explanations have been considered, what examination or investigation is appropriate, how current medicines might affect symptoms, and what findings would require reassessment. Requesting that the reasoning and follow-up plan be documented is an effective way to maintain continuity between appointments.
For a person who uses limited speech, sharing an accessible communication profile, preferred pain indicators, reliable ways to express yes and no, usual routines, and a chronological account of changes may be particularly valuable. Supporters should avoid assuming they know every experience the person has had. Offering direct, private communication when possible can reveal problems that are difficult to report in front of familiar people, including relationship conflict or abuse. Patients should receive explanations in formats they can understand and participate meaningfully in decisions about testing, treatment, and referrals. NICE NG54.
If symptoms persist despite reassurance, asking for a review or second opinion is appropriate. The request can focus on the incomplete explanation and changed baseline rather than presuming misconduct or bias. Severe deterioration, significant new confusion, serious self-injury, suspected stroke, signs of sepsis, or immediate threats to safety require emergency care based on local procedures. For less acute concerns, coordinated follow-up through primary care and relevant specialists can reduce repeated evaluations that never reach a clear plan.
Improving services and research: what would count as success?
Better clinical systems make the person's developmental history and current condition visible together. Practical improvements include accessible appointment materials, communication accommodations, sufficient assessment time, health checks that examine mental and physical health, shared baseline records, medication review, cross-service referral pathways, and specialist consultation when needed. NICE provides a guideline-based foundation for these measures. Their presence is a quality-of-care expectation; it should not be represented as proof that any specific intervention eliminates diagnostic overshadowing or reduces mortality unless that outcome has been directly studied. NICE NG54; NICE NG11.
Research needs to distinguish clinician attribution bias from broader pathways to missed diagnosis. Stronger studies could examine real patient encounters prospectively, include people with different levels of intellectual disability and communication support, measure independently verified diagnostic accuracy, and report follow-up outcomes rather than only stated intentions in vignettes. Trials of clinical decision supports, training, communication accommodations, and continuity systems should identify the specific outcome improved and the circumstances in which that improvement occurred. Participatory research involving people with intellectual disability and their supporters can improve the relevance of research questions and outcome measures. The systematic review's conclusion about low-quality, heterogeneous evidence makes these methodological priorities especially important. Dell'Armo and Tassé, 2024.
At the individual level, successful care means that new symptoms are evaluated according to evidence, the person can communicate and participate, clinically important conditions are recognized and treated when present, unnecessary treatment is avoided, and care plans are revisited when the course differs from expectations. At the service level, useful indicators include access to appropriate evaluation, delayed or revised diagnoses, medication safety, continuity of follow-up, patient-reported experience in accessible formats, and meaningful health or functioning outcomes. A system focused only on the number of additional diagnoses could create overdiagnosis; accuracy and patient benefit are the proper objectives.
Questions about diagnostic overshadowing
Does intellectual disability itself cause depression, psychosis, or physical pain?
Intellectual disability describes developmental limitations in intellectual and adaptive functioning. People with intellectual disability can also experience depression, psychosis, chronic pain, infections, neurological disorders, and other conditions. Each requires assessment on its own evidence. The presence of intellectual disability neither proves nor rules out an independent condition. AAIDD; NICE NG54.
Is a sudden behavioral change evidence of mental illness?
It is a reason to investigate. A change in behavior can accompany mental distress, pain, a medical problem, medication effects, social or environmental change, or several factors together. The assessment should use the person's baseline, the timeline of change, accessible communication, and appropriate clinical examination. Screening and functional assessment can assist but do not replace diagnosis. NICE NG11.
How common is diagnostic overshadowing?
There is no reliable single prevalence estimate that applies to all clinicians, settings, and health conditions. A 2024 systematic review found mixed evidence across 25 studies, including about one third that found no overshadowing, and rated the overall evidence as low quality. A 2025 experimental study found a small effect on schizophrenia likelihood ratings but substantial recognition of potential mental illness in open responses. The careful conclusion is that misattribution is a clinically relevant risk whose frequency depends on the question, population, and method used. Dell'Armo and Tassé, 2024; Dell'Armo and Tassé, 2025.
Can a caregiver diagnose a missed condition?
A caregiver may recognize a meaningful change and contribute critical contextual information, but clinical diagnosis requires appropriately qualified evaluation. The person's own testimony and preferences matter, and other sources should be combined rather than treated as substitutes for the person's voice. Documentation of changes, accessible communication, a review of alternative explanations, and a clear follow-up plan make clinical evaluation more informative. NICE NG54.
Do adapted screening tools solve the problem?
Adapted tools can improve the relevance of symptom measurement, but validity depends on the population, communication profile, construct, and setting. A screening result is evidence to interpret within a clinical assessment. It cannot by itself distinguish every mental health condition from physical illness, pain, environmental distress, or lifelong adaptive limitations. NICE NG54.
When is urgent evaluation warranted?
New severe confusion, difficulty breathing, a suspected stroke, a prolonged seizure, serious injury, major loss of consciousness, or immediate danger to self or others requires urgent medical or emergency assessment. Clinicians should assess the acute presentation according to its severity and should not explain away emergency signs because the person has an intellectual disability. Where there is immediate danger, use local emergency services.
Scientific conclusion
Diagnostic overshadowing identifies a specific failure of clinical inference: a known developmental diagnosis is allowed to explain symptoms that warrant independent investigation. The scientific response is to establish the person's baseline, recognize change, adapt communication, assess physical and mental health together, and reconsider conclusions when evidence is incomplete or treatment fails. Contemporary research documents possible attribution effects while also showing variability across methods and clinicians. Clinical practice can therefore address the risk precisely, by improving diagnostic reasoning, access to relevant information, and accountability for longitudinal care. Dell'Armo and Tassé, 2024; Dell'Armo and Tassé, 2025; NICE NG54.
Related Articles
Autism and Intellectual Disability in Adults: Differences and Support — Examines the distinct diagnostic profiles of autism and intellectual disability in adulthood, including the support implications of co-occurrence.
IQ: What Intelligence Quotient Measures and What It Means — Explains general intellectual testing and interpretation, a different question from recognizing co-occurring medical and psychological conditions.
Mental Health Problems in People With Intellectual Disability: Recognition and Care — Broad co-occurring mental health diagnostic and care pathway.
Depression in People With Intellectual Disability: Signs and Treatment — Recognition and treatment of depressive disorders in ID.
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