No, it cannot be treated. As for fatal, it depends on the cause, the extent and if more is forming.
White matter disease can form from headaches to high blood pressure to dementia to MS- so it would really be the underlying disease that would cause the death. Since there are so many causes from minor to major, it is not possible to really comment on it as you really need a diagnosis from a doctor.
So, what you would want to do is try to determine what is causing the white matter changes and treat what you can to try to at least slow the process, though it won't reverse what's there already.
Examples:
If you have hypertension, you'll want to work with your doctor on getting on the right blood pressure medication to lower the blood pressure. Or if your blood pressure is borderline high, you may work on getting into a regular exercise regime and take dietary measures to see if that gets it to optimum range first before trying medication.
If you have a lipid panel that has high flags on it, you want to get your HDL, LDL and triglycerides into optimum range for your age category, if possible to try to prevent the plaque formation that can cause microvascular ischemic changes in your brain (the lack of oxygen caused by tiny blood vessels being blocked off by the plaque).
If you suffer migraine headaches, if you can find a daily preventative medication regime with your neurologist that limits how many migrianes you get, that would be excellent!