27/01/2022
Doctors generally view diagnoses as discrete, well-defined entities. You either have malaria, or you do not. You are pregnant, or you are not. There is no in-between. Settling upon one also rules out the others.
For example, if a CT on a patient with a splitting headache shows a brain tumour, that’s the diagnosis. The doctors don’t go hunting for other causes such as migraine or haemorrhage. While it’s possible for a patient to have two different causes of the same symptoms, in practice this scenario is so rare that it can safely be ignored. One diagnosis is more than enough.
When psychiatry entered the medical fold, it was assumed that the same model would apply. When a patient is diagnosed with say, depression or post-traumatic stress disorder, then that condition would explain their symptoms and guide their treatment. The patient would not have both depression and PTSD, or so the thinking went. Yet statistical analysis of mental health databases shows something entirely different, a finding that brings uncomfortable questions to the heart of psychiatry. Not only do most patients have more than one diagnosis, but each diagnosis makes every other diagnosis more likely. This means that a diagnosis of depression increases your chance of also having PTSD, schizophrenia, anorexia, bipolar … all of them. This is most certainly not how diagnosis is supposed to work.
If the same pattern held for medical illnesses, then diagnosing arthritis would mean you were more likely to have tuberculosis or diabetes. Having gallstones would increase the chances of haemophilia or herpes. In fact, having any illness at all would render you more likely to have any of the others!
This is precisely the opposite of how medical illnesses behave. When the CAT scan shows that the patient has a brain tumour, the diagnoses of migraine or cluster headaches are certainly not made more likely. They are ruled out. Yes, the patient may have many of the same symptoms, but that doesn’t matter, the diagnosis is brain tumour, end of story. Since the scenario is so preposterous, psychiatrists have begun to wonder what all this overlap means for the nature of mental illness.
The major difference between medical and psychiatric diagnosis is the handbook of mental illnesses, the Diagnostic and Statistical Manual, or DSM. Whereas a medical illness like pneumonia might be diagnosed by an x-ray and blood test, depression is defined by having a certain number of symptoms from a list in the DSM. These include a loss of appetite, insomnia, difficulty concentrating, and of course, depressed mood. You can immediately notice that these items on the checklist are broader and less well-defined than seeing a tumour on an x-ray, or a bug under a microscope. DSM criteria also have great potential for overlap. Take insomnia, for example, which could be caused by anxiety, depression, mania, psychosis, or something else entirely.
The only time when having one medical diagnosis makes having another more likely is when there is a clear causal link. For example, a brain tumour may cause a brain haemorrhage. The patient with both is not exceptionally unlucky; receiving two very unlikely diseases at once. He got unlucky with one, and it caused the other.
On the medical wards, it is common to see patients with a whole host of diagnoses. A typical medical admission may have diabetes, ischaemic heart disease, chronic obstructive airways disease, pneumonia and heart failure. Here the diagnoses are all interlinked, which is why they appear together so frequently. Diabetes leads to ischaemic heart disease, which leads to heart failure. Chronic obstructive airways disease leads to pneumonia, which in turns worsens the heart failure and the diabetes. At the beginning of it all may be smoking. The difference with psychiatry is that each medical illness is a clear category. Diabetes is elevated blood sugar; pneumonia is infected lungs. While they may be related in cause, they do not overlap in category.
The confusion in psychiatric diagnoses comes from overlap in both cause and category. Diabetes can lead to pneumonia, PTSD can lead to depression – so far so good. But both PTSD and depression can have the same symptoms of social withdrawal, negative thoughts, ruminating about the past, loss of appetite and energy, and many more. Deciding which symptoms are part of which diagnosis is not easy, and to make matters worse, there is no test to decide the issue. A physician can run a blood test or pull up an x-ray and actually see diabetes or pneumonia. Psychiatrists cannot ‘see’ depression. Even with the most advanced brain imaging, which is only used in research, PTSD and depression appear very much the same.
Different illnesses sharing the same symptoms partly explains why having one diagnosis increases the chance of having another. Another reason is that there are very few, if any, symptoms that occur in only one mental illness. Most crop up everywhere.
The cardinal symptom of schizophrenia, for example, is having auditory hallucinations. But these are also found in depression and mania. They may even appear in someone not mentally ill at all, but suffering from too much stress and not enough sleep. Since no one symptom can define schizophrenia, psychiatrists rely on others such as delusions or paranoia, but these too are found elsewhere. And that’s for schizophrenia, which is probably the most striking and clear-cut psychiatric diagnosis of them all, what Thomas Szasz called, “the sacred symbol of psychiatry”.
How to distinguish between depression, dysthymic disorder, and adjustment disorder? You can perhaps surmise that psychiatric diagnosis is very messy and complex, and wonder how psychiatrists ever decide anything at all.
Yet in practice, psychiatrists diagnose illnesses quickly and have little trouble drawing the line between well and unwell. A psychiatrist missing a major mental illness is very rare, at least as rare as a physician missing a major medical illness. While a patient with a clear brain tumour may not notice or show any symptoms for months, depression may be entirely obvious immediately.
Similarly, if a man suddenly tears off his clothes at work and declares himself to be the Witch King of Centrelink, there may be disagreement over whether the diagnosis is schizophrenia, bipolar disorder, or something else, but there is zero doubt that a diagnosis is warranted.
We can summarise these findings as follows:
Psychiatric diagnosis is easy in general, hard in specifics.
Distinguishing mental illnesses from normal is easy, hard from each other.
In other words, mental illnesses are real, but fuzzy. Psychiatrists have long understood this paradox, but until recently there has been little hope of a solution. Now they are turning to the burgeoning field of neuroscience to place psychiatry on a more secure footing. Studying genetics, brain imaging and neural pathways, researchers are slowly illuminating the physical basis of mental illnesses. Particularly striking is the finding that similar genes or brain changes are involved in several different conditions. For example, schizophrenia and autism share similar genetic and structural changes, which is exactly what their overlapping symptoms would predict.
What does this tell us about what mental illnesses really are?
One theory is that mental illnesses are best understood not as discrete entities but as a variety of overlapping dimensions that emerge from abnormalities in the brain. The nature and severity of these brain changes, and how they combine together, may ultimately explain how illnesses shade into one another. So instead of puzzling over whether a patient claiming to be Witch King of Centrelink has schizophrenia or bipolar disorder, a psychiatrist may diagnose his condition in terms of disturbance in mood, connection between thoughts, and reality perception.
Ideally each of these symptom types will relate to a particular neural circuit and an avenue for treatment. For example, the patient may be treated with lithium to stabilise his mood, a benzodiazepine to quell his anxiety, and olanzapine to streamline his disconnected thinking. As our imaging and understanding of the brain improves, there will be a tighter connection between the symptoms observed, the labels applied, and the treatments delivered. Just as occurs right now in physical medicine.
The reason why this approach is not already mainstream is that neuroscience lags far behind our knowledge of other organs, largely because the brain is vastly more complex. Here lies an irony, since we are fully aware of many of the brain’s operations – such as the choice to reach out for a glass – but how that choice comes about, and what permits us to experience both the choosing and the reaching, is a total mystery. Meanwhile we are unaware of processes of regulating blood pressure and digestion, yet researchers have detailed precisely how they happen. Brain science remains in its infancy, and stuck there with it is psychiatry.
Physical medicine was only recently in the same position. For centuries, doctors thought that malaria was from something in the air (hence the name, literally “bad air”) but only recently identified its precise relationship to the mosquito and the parasite it carries. Now doctors know the lifecycle of falciparum malaria, its effect on various organs, the best drugs to treat it, and the means to prevent it. Before then were treatments, such as quinine, or ‘Jesuit’s Bark’, that were discovered by chance and worked by processes unknown. The brain is so complex that when it comes to its afflictions, we remain at the stage of “bad air” and Jesuit’s Bark.
Yet there is reason for hope. Little more than a century ago, there was very little understanding of mental illness and almost no treatment. Since then, a variety of effective treatments have been discovered, and improving neuroscience is helping us understand why they work.
As we unravel the brain’s mysterious processes in ever more detail, better treatments will be discovered and not just by the fickle whim of chance. With genetics and imaging, researchers are beginning to design treatments tailored to specific brain circuits. This process goes both ways, as investigating the structures behind successful treatments can illuminate why they work, which then paves the way for even better treatments.
Psychiatry is still on the trail of its falciparum malaria, but the path is not as dark as it once was.
Neurocentrix is a private Melbourne mental health research centre established by directors Professor David Barton and Dr Peter Farnbach. Both directors have extensive experience conducting mental health research and neuroscience clinical trials.
