Where does anxiety end and depression begin?

In 2017, the World Health Organization estimated that 4.4% of the global population — more than 300 million people — suffer from depression, and that 3.6% of the global population — more than 250 million people — suffer from an anxiety disorder. But one fact complicates this picture: Many of the people who suffer from depression symptoms also suffer from anxiety symptoms, and vice versa. This can make it difficult to reliably and consistently diagnose each disorder.

The current mainstream approach to diagnosing mental health problems is to use a checklist system, such as the most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). A clinician works through a symptom list, comparing it against a person’s reported experiences. A person can be diagnosed with major depressive disorder if they persistently experience symptoms such as depressed mood and feelings of worthlessness. For a diagnosis of generalized anxiety disorder, people typically experience persistent worry and irritability. But when looking through all the criteria for each disorder, the commonalities are obvious. For example, both depression and anxiety can hinder life activities, prevent sleep, and drain energy.

The main difference between the two disorders is the primary state of a person’s emotions: Extreme sadness qualifies as depression, while extreme fear or worry qualifies as anxiety. But it’s easy to see the link between those two worlds. When people feel sad about their life and past experiences, they can also become anxious about where their life is headed. And when they feel anxious about the future, they can also become sad about how their life is currently playing out.

Laura Hack, a research fellow in psychiatry at Stanford University, describes some of the visible clues that help to distinguish between depression and anxiety. People with depression typically speak slowly and express tearfulness, while people with anxiety speak quickly and show signs of nervous stress such as sweating and shaking. But Hack explains that in a practical setting, she “wouldn’t consider these ‘telltale’ signs of either condition,” because depression frequently comes with irritability, and anxiety frequently comes with crying and fatigue. Similarly, people with either disorder tend to withdraw from daily activities and interests, although their reasons are often different: Depression destroys feelings of motivation and enjoyment, while anxiety scares people into avoiding opportunities.

Because of the considerable overlap in symptoms, and because patients can express the same symptoms for different reasons, it’s important for clinicians to look at the whole story when making decisions about diagnosis. “DSM diagnoses of depressive or anxious disorders (or both) must be made by considering the entire clinical picture, not any one particular symptom or sign,” Hack says.

It seems likely that the majority of people who are diagnosed with either anxiety or depression also suffer from symptoms of the other disorder. As Roiser puts it: “Comorbidity is the norm.”

Can someone be anxious and depressed?

It’s very common to experience both clinical anxiety and clinical depression, an effect that scientists refer to as comorbidity. However, it’s not clear exactly what percentage of people suffer from both sets of symptoms. Jonathan Roiser, a professor of neuroscience and mental health at University College London, says, “Estimates of comorbidity of depression and anxiety vary quite a bit depending on the population tested and the definitions used.”

One of the estimates Roiser finds most convincing comes from a study in 2007, which followed a group of approximately 1,000 people born in New Zealand between 1972 and 1973 until the age of 32. Of the people who had been diagnosed with generalized anxiety disorder up to that point in 2007, 72% had also experienced major depressive disorder. Of the people who had been diagnosed with major depressive disorder, 48% had also experienced generalized anxiety disorder. It seems likely that the majority of people who are diagnosed with either anxiety or depression also suffer from symptoms of the other disorder. As Roiser puts it: “Comorbidity is the norm.”

The key to understanding the relationship between anxiety and depression is to look at how they manifest themselves in the brain. “There is a lot of overlap between the brain networks identified in patients with anxiety and depression,” Roiser says, but this overlap isn’t particularly surprising when considering that patients suffer from similar symptoms.

Researchers in another 2007 study attempted to pick apart depression and anxiety by scanning the brains of both groups of patients, while separately studying those with and without comorbid symptoms. Compared to healthy people, all of the patient groups had smaller brain volumes in the anterior cingulate cortex, an area of the brain involved in processing emotional and motivational information. But when looking at patients without comorbid symptoms, they also found some differences between depression and anxiety. They explained that the similarities in brain activity reflected the shared symptoms between each disorder, while the differences provided some good reasons to think of depression and anxiety as independent disorders with unique symptoms.

The genetic overlap between depression and anxiety highlights a potentially deeper connection. According to some twin studies, which compare identical twins and non-identical twins across different environments, generalized anxiety disorder and major depression share common genetic roots. Hack explains that those shared roots are likely responsible for the abnormalities researchers find in the “structure and function of neural circuits that contribute to both disorders.”

Despite the complications in diagnosing depression versus anxiety, they don’t compromise treatment. That’s because, as both Roiser and Hack point out, a psychiatrist’s first-line medical response to each disorder is typically identical: drugs that boost the availability of a neurotransmitter known as serotonin in the brain. Cognitive behavioral therapy is also a common option for both disorders, so prescribing the “wrong” treatment isn’t a huge issue.

That’s not to say that treatment is foolproof. Drugs can have side effects that make some symptoms worse rather than better, and there are people who don’t respond well to any of the commonly available treatments: a phenomenon known as treatment-resistant anxiety or depression, which can lead to chronic and even life-threatening consequences.

What if the labels go away?

A diagnosis of “depression” or “anxiety” relies on basic reports of symptoms. In the future, these labels could be replaced with more nuanced categories of mental health disorders based on biological data from genetics and neuroscience.

Hack is currently developing stronger disease categories that are informed by neuroscience. She refers to these categories as biotypes, and they don’t necessarily agree with the traditional categories of depression versus anxiety. She’s beginning to apply these biotypes in testing new treatments for the most severe forms of anxiety and depression — the forms that are resistant to normal drugs. By bringing together data from genetics, neuroscience, and behavioral science when diagnosing people, Hack believes she will be “better able to guide individuals with mood and anxiety disorders to tailored treatments.”

Regardless of how the details ultimately pan out, it’s clear that there’s plenty of room for reforming how clinicians approach the puzzle of depression and anxiety. Researchers are now developing the tools to disentangle one disorder from the other, and they’re likely to revolutionize the whole field of mental health in the process.