One of the hallmarks of multiple sclerosis is supratentorial hyperintensity signals, which can be a sign of demyelination. The supratentorial area is located just above the tent-like covering over the top of the cerebellum called the tentorium cerebelli. In other words, it is the area of the brain above the posterior fossa, which contains the cerebellum. The signals are often round in shape and are found around the ventricles as in the brain scan pictured below. The ventricles are the white structures shaped like an H in the middle of the brain.
Hyperintensity signals, however, are not unique to brain. They show up in the spinal cord and many conditions in addition to MS. For instance, more recently they have been associated Alzheimer’s and Parkinson’s disease. Also, hyperintensity signals don’t always indicate demyelination. They can be found in tissues other than myelin.
Hyperintensity signals show up as increased brightness (white) on MRI using different scanning techniques. The nature of brain scans causes tissues with more water to give off brighter signals that appear whiter on the scans. More water can be caused by many factors for different reasons.
In this regard, myelin is made up mostly of fats with some proteins mixed in to give it strength like steel bars in concrete. Fats repel water, which is why soap floats and water runs off a ducks back. When the fat in myelin breaks down, as in demyelination, it gets leaky. The inferior tissue that replaces it takes in more water than myelin. Thus the area of demyelination shows up brighter on MRI scans using particular scanning techniques, special mathematical formulas and physics. Furthermore, using different scanning techniques, radiologists can even determine if the MS lesion of demyelination is new or old based on the amount of water. Older lesions tend to dry up and get less bright. So called enhancing techniques help the radiologist make the determination.
In addition to myelin, bright spots on brain scans can also indicate small vessel ischemic diseases and white matter diseases of the brain. The brain is mostly made of white matter, about sixty percent. The rest is gray matter. Most of the white matter is made of myelin. The rest of it is glial cells which are support cells made from structural fats, as opposed to circulating liquid types of fats like cholesterol.
The fat in myelin serves as an insulator for the conduction of nerve signals, which speeds them up. Breaks occur at certain intervals along the myelin called nodes. The fat in between the nodes allows the signal to span greater distances jumping from node to node to increase the speed of conduction. Myelinated nerve cells are very fast. The time between stimulation in the form of touch on the great toe and realization in the brain is near instantaneous.
In brief, both small vessel ischemic diseases and white matter diseases involve the small arterial blood vessels of the brain, which are the points of weakest blood supply in the brain. Much of the white matter of the brain gets its blood supply from the small blood vessels of the brain. What’s more, myelin is particularly sensitive to decreases in blood flow and oxygen levels. That’s why small vessel ischemic disease and white matter diseases of the brain are seen more frequently in people with diabetes, high blood pressure, degenerated blood vessels and heart disease. I suspect it can also be caused by low blood pressure or anything for that matter that decreases blood flow to the brain.
As mentioned above aside from demyelination, hyperintensity signals show up for other reasons as well, not just demyelination. In some cases the hyperintensity signals are obvious and the diagnosis all but certain, which is demyelination. In other cases the signals aren’t so clear and it takes an astute radiologist to make the determination taking into consideration the patient’s history, signs and symptoms.
In addition to being small, discrete and round in shape, in contrast to small vessel ischemic disease or white matter diseases of the brain, in MS the hyperintensity signals tend to be located around and follow the veins of the brain. This causes them to stick out perpendicular to the ventricles in the shape of fingers. They were named this after the scientist who first noted them and called them Dawson’s fingers. The classic shapes of Dawson’s fingers that differentiate MS are seen in the image on the right.
In any case, hyperintensity signals can indicate problems. In some cases they indicate arterial problems and ischmic injury due to poor blood flow and oxygen delivery to the cells. In other cases the are caused by increased spaces in the brain such as in a shrinking degenerating brain. One expert, Dr. Franz Schelling has suggested that in the case of MS, they may be due to venous backjets or back pressure against the brain. As scanning techniques continue to improve, especially with the addition of upright MRI, we will continue to learn more about the cause of hyperintensity signals.