Co-Occurring Diagnoses

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Co-Occurring Diagnoses

Mental Illness and Developmental Disabilities

Authors Note: This post includes references to self-harm and abuse. 

For most mental health professionals, the term “dual diagnosis” is used to describe people who have a mental illness and a substance use disorder. However, another seldom-recognized population is also dually diagnosed. These individuals have co-occurring intellectual and developmental disabilities and mental illnesses. Surprisingly, few professionals are trained in this specialty or know how frequently the two conditions coexist.

If mental illnesses are so common in people with intellectual and developmental disabilities, why do few professionals know about this form of dual diagnosis? 

Unfortunately, standard clinical graduate programs and medical schools do not include dual diagnosis in their training curriculum. When a clinician sees such a case in practice, there is often a tendency to recognize only the developmental delay and attribute any other behaviors to that condition. This phenomenon, known as overshadowing, blinds the clinician to the possibility that a mental illness could be the cause of the person’s behaviors.

Barriers to Receiving a Mental Illness Diagnosis 

While the examples listed below are true, the names have been changed to protect the patrons’ privacy. 

1. An inability to accurately communicate one’s thoughts and feelings. 

For those who do not use words to communicate, this limitation makes it difficult or impossible for them to describe their own experiences accurately to their clinician. They may have no words to explain their feelings and what they perceive, yet words are the essential tool of a social worker or other mental health clinician trying to understand what is happening in a patient’s mind. 

I met Kate three years ago. Kate has cerebral palsy and uses a letter board to communicate. The letter board is a piece of paper with the alphabet stuck inside the glossy cover of a three-ring binder. When she told me she wanted to be an author, I agreed to help by typing the words as she spelled them letter by letter. The text below is shared with permission from the original author. 

Having CP is not only hard on me, but it is also hard on my family and friends. I have days where I ask God why he made me like this. God says he made plans to prosper, not to harm you; plans to give you hope and purpose. So why do I have to hurt not only physically but also emotionally? I get mad at this pain and tell it to go away. I know I should be more grateful for what I have, but sometimes I cannot see the good things. My body is not the only thing I have a hard time with; I also have depression. This is the pain I mean when I say emotional pain.

Having CP makes me feel trapped, and then I get thoughts of self-harm because I do not want to be trapped all my life. Sometimes I do think about ending my life, but, at the same time, I know I cannot do that because there is another life.

2. A lack of social exposure and education in mental illness.

This makes it difficult to discern if what they’re experiencing is normal. Especially in the cases in which the person has a limited support system or no support system, the person has a narrower range of everyday experiences against which he can compare his own. It may be difficult to discern that what they are hearing or seeing is outside normal experience bounds.

When I first met Ashley, she was staying in a rundown hotel with only enough money for one more night. Ashley has cerebral palsy as well but can communicate verbally. She shared with me that she thought she was pregnant. When I took her to the doctor’s office for a test, the results were negative. After running her records, the doctor saw that Ashley frequently took pregnancy tests. The doctor engaged in dialogue with Ashley for a while. Slowly, Ashley started opening up about her home life; the doctor and I immediately identified physical, verbal, and emotional abuse in her story, but she shared the accounts as if her father’s actions were perfectly normal. 

3. A lack of sameness across all disabilities.

Every person with an intellectual and developmental disability is unique! People with disabilities have good and bad days; they get mad and sad, just like everyone else. These highs and lows–often referred to as an increase or decrease in behavior–can look different for each person, even if they both have the same diagnosis. These baseline anomalies are routinely mistaken for learned behaviors that are part of the developmental delays and not assessed as possible symptoms of an emerging mental illness.

Aiden is a friend and fellow committee member who is also on the spectrum. He is well-spoken and dresses professionally; he is married and has a house and a job. It’s not immediately noticeable that Aiden has autism, resulting in people making assumptions about him, such as using his disability to excuse his behavior. On one occasion, Aiden had a piece of candy in his hand. While we were talking, a young child came over and took the candy out of his hand. Aiden immediately started yelling and rocking back and forth, repeating that the candy was his. 

Without specific training in dual diagnosis, clinicians are unaware of the possibility of co-occurring conditions, fail to ask the appropriate clinical questions, and create circumstances in which these individuals are either untreated, undertreated, or, more alarmingly, treated with ineffective or inappropriate methods. Mental illness, trauma, and environmental factors should all be considered before a person’s behaviors are dismissed.


This post is written in memory of Kiernan and Justice, who left this world far too soon. 



Co-Occurring Mental Illness and Developmental Disabilities


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