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Small Vessel Brain Disease is a non-specific finding on MRI that sometimes, but not always, results in clinically detectable symptoms. It is very common after 50, and is often accelerated in persons with diabetes, high blood pressure, high cholesterol, and in smokers, as well as in an assortment of rheumatologic conditions and the like.
It can lead to focal neurological signs (e.g. numbness. weakness, trouble swallowing, etc) but more often than not results in more diffuse and subtle changes in cognition (e.g. memory loss, poor concentration, impaired analytical skills, etc), sometimes progressing to dementia.
The prognosis depends on the cause or causes and how well one is able to reduce one's risk factors. As it is not directly treatable or curable, the prognosis can be highly variable and may be influenced by one's genetics, one's environment, and one's other illnesses. Strict control of blood pressure, cholesterol, sugar levels and stress levels can all be important parts of reducing the progression. Some patients take a baby aspirin every day to thin the blood and help improve circulation in the small blood vessels of the brain.
The exact cause is not known in most cases. Basically, it comes down to the health of the tiny blood vessels supplying oxygen and nutrients to the nerve cells that comprise the brain (and control our mental and bodily functioning). These vessels can get damaged or thready or become the victims of small clots from upstream. Any process that negatively impacts the health of these vessels and their ability to transport blood will be a factor in this disease process.
Just as some people with normal BP and normal cholesterol still get vascular disease and have heart attacks and strokes, so too can this process occur in those with no identifiable risk factors. Some people have undiagnosed vasculitic conditions, for example, that result in inflammation in the blood vessel walls, damaging them from "the inside" rather than damaging the lumen itself. A person might have a chemical sensitivity or allergy that is not severe enough to cause gross symptoms, but may be enough to trigger chemical damage to the blood vessels or to provoke antibodies against components of the blood vessels (i.e autoimmunity).
Ultimately, how our bodies respond to "insults" from outside, including how well they are able to fend off injury and repair damage (which is occurring continuously in one form or another from the moment we are born), and for how long, is a product of our genetic make-up (i.e. the blue-print we are handed down from our parents)