(CNN) -- The American Psychiatric Association is in the midst of redesigning a document often called the Bible of Psychiatry. It's known more officially as the Diagnostic and Statistical Manual of Mental Disorders, or DSM for short.

For practical purposes, including insurance reimbursement, the DSM determines what does and does not qualify as a psychiatric illness in the United States. Because of this, changes to the document can lead to profound effects on patients' lives. Changing criteria can dictate who and who cannot be considered to have a mental illness worthy of treatment... and insurance coverage.

Nowhere have proposed changes to the upcoming edition of the DSM generated more angst, or media coverage, than in the area of autistic disorders.

Anyone who has a family member with severe autism, or has known someone with the condition, might be surprised by this. Of all brain conditions for which we don't know the cause, none are more obvious than autism in terms of symptoms or impairment.

Autism strikes right where it hurts most, and that is the ability to understand the emotions and behaviors of other people. In addition to severe impairments in social understanding and behavior, people with autism also typically engage in pointless repetitive behaviors and have obsessive, narrow interests, often centering around technology.

Occasionally autistic people demonstrate skills, as seen in the movie "Rainman." One famous autistic patient can tell you the day of the week of any day in history. I had a patient years ago who had exactly memorized all bus routes schedules for the greater Los Angeles area using a method he inexplicably called the "liver system." He could - and did whenever prompted - recite exact times and stops over hundreds of routes for hours on end.

So the question remains: how could changes in diagnostic criteria change who does and doesn't have such an obvious disease?

To answer this question, let's do a thought experiment. Imagine you are at the doctor's office receiving news of your latest medical testing. Would you rather be told that you had a large breast lump that was benign or a very small one that was cancer? Or consider this: You go to the ER with chest pain. Would you be very comforted if the doctor told you that you'd only had a small heart attack?

What these examples make clear is that some medical conditions are best thought of as either being present or absent. Either you have a disease or you don't. Now consider this:

We all know that high blood pressure is a silent killer, and that many lives have been saved by the development of safe and effective treatments. Setting aside what the number exactly mean, most of us probably also know that the upper range of normal for blood pressure is 140/90. Suppose your doctor takes your blood pressure and it is 141/91. Knowing that this is high, she asks you to relax and then takes it again five minutes later. This time it is 139/89. Would any of us say that you had a disease based on the first reading, but fortunately were perfectly normal based on the second?

All psychiatric diseases are like blood pressure. They are on a continuum without gaps. If one administered questions to identify any condition and rate its severity in a large enough group of people, one would find someone occupying every value from zero to the maximum score.

The problem in psychiatry is that our entire diagnostic system is based on the idea that mental illnesses are like cancer and not like blood pressure. The DSM provides minimum criteria for each disorder, which means that no matter how close you are to having any given condition, if you fall below the line you technically don't have it.

I suspect you can see the problem immediately. If nature does not provide clear guidelines for where normal stops and mental illness begins, how does the line get drawn? The quick answer is that it gets drawn in much the same way the voter redistricting lines get drawn: based on some data, a lot of fighting, and finally some not entirely satisfactory compromise.

So back to autism. Like all other mental disorders, it runs along a spectrum from people who most of us would have called nerdy when I was a kid to people who spend their lives unable to speak, rocking back and forth for hours on end. We all agree that the silent, rocking folk are ill and need care. But where does extreme nerdiness and social awkwardness give way toAsperger's syndrome? When is someone autistic enough to deserve the label?

This is the rub. As with all spectrum conditions, there are far more people with mild autism than with its more severe forms. Again think of blood pressure. Lots of us run 150/92 - not many of us run at 200/110 (and those of us who do often don't live to tell about it).

What the new DSM proposes to do is make it harder to meet criteria for autism, so that probably the majority of people who fall on the mild end of what people often call simply "the spectrum" will now be declared non-autistic.

Proponents of this move argue that much of the autism epidemic that has been observed in the last several decades may result at least in part from the fact that less and less severely affected people are being diagnosed. Making the criteria stricter will reverse this trend. It will also reduce the risk of stigmatizing people who in former times might have been viewed as eccentric, but non-diseased.

Opponents of the changes argue that many young people have been helped by receiving the diagnosis and that this help will vanish if they are no longer considered to have a "real" and "billable" condition.

What is the answer to this dilemma? Like all real problems in the world it falls along a spectrum.

The opinions expressed in this post are solely those of Charles Raison.

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