The Transdiagnostic Dimensional Approach: Another Way of Understanding Mental Illness

You are probably familiar with the names of the various mental illnesses, such as major depressive disorder and schizophrenia. These names reflect what is called the “categorical” approach to mental illness, which is the dominant approach used in practice.

However, emerging research is finding that there is a new approach to understanding mental illness that could be more accurate — the transdiagnostic dimensional approach — which thinks about mental illness as an extreme and stressful version of common experiences, rather than as categories of distinct experiences.

So, what is the difference between these two approaches and their applications?

The Categorical Approach to Mental Illness

The categorical approach is essentially how we approach physical illness. Each possible diagnosis represents a category of illness, and clinicians try to figure out which one the symptoms belong to. When making diagnoses, mental health professionals ask about the person’s experience to answer questions like, “Does this person have this mental illness or not?” or “Which mental illness does this person have?”

This approach is concrete and intuitive: One can either have a broken bone or not, and a person either has depression or does not. Also, just like someone could have a broken bone or arthritis, a person can have depression or schizophrenia. Mental health professionals go through a list of symptoms and algorithms to make these categorical diagnoses.

The categorical approach has been helpful in many ways. For example, it helps mental health professionals communicate. When we hear a given diagnosis, we can make some guesses as to what difficulties the person is having. Additionally, diagnoses are often made to help the individual with mental illness in planning for treatment, looking for resources and finding support groups. It also gives some explanation for the person’s behaviors and challenges, which can be helpful for family members.

The Problems with the Categorical Approach

Researchers have noticed problems with this approach and its assumptions. First, in the categorical approach, someone with a mental illness is assumed to be “different” from someone who does not have a mental illness, such as in brain mechanism. Yet the boundary of “have vs. does not have” for mental illness is unclear.

Imagine two people who have the same difficulties because of low mood, but one had low mood for 14 days and the other for 13 days. This one-day difference could lead to different diagnoses or potentially no diagnosis for the second person. However, this difference probably does not indicate a different underlying brain mechanism of mood.

Second, you may know someone with multiple diagnoses. For example, people who have anxiety disorder have a high chance of also having depression. This means that different categorical mental illnesses are not completely distinct as assumed, but rather share experiences and/or mechanisms underlying multiple illnesses. This shows that the categorical approach might not capture the true nature of mental illness.

The Transdiagnostic Dimensional Approach

Growing evidence suggests that a “transdiagnostic dimensional” approach reflects mental illness more accurately. Many researchers and organizations, such as the National Institute of Mental Health, are moving toward this approach in mental health research.

Let’s start with the “dimensional” part. Think of sadness, an emotion often associated with depression. The categorical approach would ask you to determine whether you are feeling sad or not at any given time. However, it is most likely easier (and more accurate) to instead describe the dimension of your sadness, such as “a little” or “very,” which can then be captured on a scale of, say, 0 to 10. This is the dimensional approach, and like temperature and blood pressure, everybody lies somewhere on the dimensions.

Viewing human experiences in this way, people with mental illness would rate their experiences at the extreme end of the scale (9 or 10). This also means that while someone might not meet the threshold for a diagnosis of depression (like 6 or 7), they might be experiencing difficulties because of similar mechanisms as someone with the diagnosis. This idea of dimension could be applied to the existing categories, but research seems to indicate that such dimensions are “transdiagnostic” — in other words, they are not bound by the categorical groups and can occur across disorders.

One example of a transdiagnostic dimension is negative affect, a broader dimension than sadness that also includes anxiety and general distress. Negative affect seems to be experienced by most individuals with categorical mental illnesses, such as depression, anxiety and more. If you are high on the negative affect dimension, then you have a higher chance of having any or multiple mental illnesses that share this dimension. This kind of transdiagnostic dimension could explain the high probability of having multiple diagnoses. This also means that treating negative affect could treat many categorical mental illnesses.

Research is now trying to understand these transdiagnostic dimensions to better prevent and treat mental illness. Because all of us, including people without mental illness, are somewhere on each dimension at any given time, we can also integrate research on basic human functioning, such as cognition and personality, which were traditionally distinct research fields. This makes the mental illness research team much bigger than ever before.

What This Means for You

In practice, the categorical approach will continue to play an important role in the diagnosis of mental illness for the foreseeable future. So how does the transdiagnostic dimensional approach apply to you and your loved ones who have mental illness?

Whether we have a mental illness or not, we all lie somewhere on the same dimensions and can imagine similar difficulties. The transdiagnostic dimensional approach helps validate each person’s place on the spectrum, regardless of whether you “technically” meet the threshold for a particular diagnosis.

Additionally, if a mental health professional thinks you may have multiple diagnoses, or cannot give a clear diagnosis, please do not think that you have too many problems or that the difficulties you are facing are not real. These challenges are more likely a reflection of the current diagnostic system, which fails to capture every way mental illness can be experienced.

The transdiagnostic dimensional approach shows us that we still have much to learn about mental illness. By thinking about mental illness in this new way, we can better appreciate a person’s experiences and the effect of their symptoms on their lives. Ideally, this perspective will one day help guide our decisions around diagnosis and treatment.

The best minds in research and many organizations (including NAMI) are working diligently to improve our understanding of mental illness so that we can better help you and your loved ones. Stay tuned!

Takakuni Suzuki, Ph.D., is a NAMI Unger Research Fellow postdoc at the University of Michigan Department of Psychiatry. His research interests are in understanding mental illnesses in a broad transdiagnostic dimensional model by leveraging advanced statistical techniques and multiple measurement methods (e.g., questionnaires and electroencephalograms).

Note: This piece was originally published in the Fall 2020 issue of the Advocate.