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When, in the late 1800s, German psychiatrist Emil Kraepelin pointed to dementia praecox and manic depressive psychosis—now schizophrenia and bipolar disorder—as distinct diseases, he wasn’t blind to some overlap between the two. Psychosis marks both illnesses, of course. Their numbers are roughly the same, population-wise; they’re close in onset age. They strike both sexes similarly. Kraepelin, however, was looking at overall symptom patterns. And for a century or so, his distinctions have driven diagnosis, therapy and research.
But the wearying hunt for disease genes makes us use new eyes, says cognitive psychologist David Schretlen. Schretlen and others see singling out pure subgroups that overlap both illnesses as a way to tie symptoms to specific genes. As a bonus, he may find just how much biology schizophrenia and bipolar disorder (BD) share.
“Subtypes exist; we know that,” says Schretlen. In some schizophrenia patients, for example, delusions stand out; with others, it’s hallucinations. Or negative symptoms can rule—apathy, emotional blunting, not wanting to wash or talk. Likewise, he adds, some bipolar patients become psychotic in acute disease; others don’t. Mania can overshadow depression. Or not.
“Perhaps we need to set diagnostic labels aside,” he says, “and look at both illnesses instead in light of, say, the presence of psychosis or severity of mood symptoms or even the nature of cognitive losses.” Lately, Schretlen’s been focusing on that last one. His multi-center team has begun by getting a baseline of cognitive abilities in the two disorders. Recently, they gave 106 schizophrenia patients, 66 BD patients and 316 healthy adults unusually wide-ranging tests of cognition—attention, concentration, memory, reasoning, visual perception, processing speed and the like. This early work deserves kudos for overcoming the classic obstacle to studying cognition in these disorders, namely, the way that age, sex, IQ and other demographics skew results. “Bipolar patients, for example, are often so far above average intellectually that they can mask their cognitive problems.” So, he uses statistics to level the field.
Schretlen found that both illnesses harm patients’ thinking, though it’s more intensely so in schizophrenia. Surprisingly, the kinds of problems in schizophrenia and bipolar disorder almost parallel each other. “That,” says Schretlen, “suggests common biology.”
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