Diagnosis of Exclusion
There exist many conditions, mine among them, for which science presently lacks an understanding of aetiology. At some level this is true for many diseases. Doctors can identify cancerous cells as the cause of a patient's symptoms but usually lack a complete understanding of how cancer is caused. Other diseases, like strep throat, on the other hand, are extremely well understood. We are aware of the infectious agent, we understand its method of transmission, and we have reliable tests to determine its presence in a patient. Conveniently, we also have proven treatments. But here, I address specifically those diseases about which we have only the vaguest understanding of the underlying mechanism.
Consider again strep throat. In a small fraction of patients, untreated or under-treated cases can lead to rheumatic fever. The same underlying disease can present differently in different patients. As concerns many autoimmune diseases, however, due to the lack of an understanding of aetiology, the unity of many conditions would be impossible to grasp in the context of differing presentations.
In my case, I have experienced an autoimmune disease of the brain, specifically one responsive to steroids. My diagnosis is based not on any blood test or biopsy, but instead on a combination of strong suspicion, responsiveness to treatment and exclusion of other diseases like lyme that could cause similar symptoms. The symptoms are verifiable but the aetiology is not understood.
The responsiveness of a disease to immune suppression and steroids does strongly indicate that its cause is an inflammatory process. But 'Steroid-Responsive Encephalopathy associated with Autoimmune Thyroiditis' is not the only known autoimmune brain disease. Multiple sclerosis is believed to have an autoimmune aetiology and in the short run attacks can respond to treatment with corticosteroids. Similarly, many other diseases exist, such as neurosarcoidosis, which are both steroid-responsive, of autoimmune aetiology and effect the brain. Another is vasculitis of the central nervous system.
These diseases each have a different clinical presentation and a different set of correlations with antibodies, but the diagnosis of one and not the other is not made with the same specificity as a diagnosis of cancer or bacterial infection. Here, diagnoses are made by excluding diseases that are dectable with more reliable diagnostic processes, and then by classifying according to the clinical presentation. Multiple sclerosis generally shows evidence of demyelination on imaging. Primary angiitis of the central nervous system has a certain characteristic presentation. A neurologist might say, “you don't have vasculitis of the central nervous sytem; if you did, we wouldn't be having this conversation.” You probably could not imagine a doctor similarly excluding a diagnosis of HIV on the basis that the severity of a patient's symptoms didn't fit a particular profile. Here, however, that sort of diagnostic criteria is not possible.
Given the absence of a more robust system for grouping and treating patients across this spectrum of disorders, this is most likely the best that can be done. Fortunately, the treatment profiles for most of these conditions generally follow the same pattern of corticosteroids alongside immunosuppression. Still there is something immensely unsatisfying about not having any tangible understanding (if a microbe or antibody can be considered tangible) of what precisely is causing my malady. More disturbing is that in the absence of a critical mass of patients, it is hard to imagine a realization of the funding and controlled studies that would be required to procure such information, even if it became technically feasible.
