Diagnosing which disorder is causing a neurological syndrome can be difficult because there are hundreds, if not thousands, of syndromes that that may be caused by hundreds, if not thousands, of neurological disorders, depending on how narrowly or broadly they are defined. To further complicate things, many neurological syndromes may be caused by multiple disorders, and many neurological disorders may cause multiple syndromes.
The first step in the diagnosis of neurological syndromes is to locate the lesion (the area of dysfunction) based on the symptoms and signs. This starts by deciding if the problem is in the nervous system, versus another system of the body, and then determining if the lesion is in the central nervous system, the peripheral nervous system, or both. Determining the likely lesion location first is important because different areas of the nervous system tend to be affected by different types of pathology. Unfortunately, there are some neurological syndromes where the location of dysfunction is unclear; for these syndromes pattern recognition is often necessary.
The second step in neurological diagnosis is to further shorten the list of likely types of pathology by factoring in the syndrome time course. The syndrome time course, or tempo, is the temporal pattern of symptoms and signs at onset and during the subsequent course of the syndrome. Particularly important is the speed of onset or progression of the syndrome. Different types of pathology tend to cause different syndrome time courses. Unfortunately, time course terminology is not used consistently. These terms are often used for the speed of syndrome onset: sudden or acute is often used for seconds to hours; subacute is often used for days to weeks; and gradual or chronic is often used for weeks to years. Other terms are often used for the subsequent time course of a syndrome: continuous or episodic, fluctuating or static, and resolving or progressive.
The third step in neurological diagnosis is to consider risk factors that might suggest certain specific disorders. Patient risk factors for neurological disorders may include age, sex, and many possible pieces of information found in the past medical history, social history, family history, medications, or allergies. By first narrowing down the list of likely pathologies to one or a few based on the lesion location and syndrome time course, one or more risk factors may strongly suggest one or a few specific neurological disorders as the most likely cause of the syndrome.
These steps create a list of likely disorders, ranked by probability, called the differential diagnosis, which will determine the strategy for tests and treatments, if any are needed. This approach is the same as for syndromes of other body systems, but the localization step tends to be more involved because of the size and complexity of the nervous system. Sometimes, skipping steps or performing them in a different order works fine, but this can increase the risk of misdiagnosis, particularly skipping the localization step or starting with risk factors.
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Introduction to Neurology 