In the diagnosis and monitoring of multiple sclerosis, magnetic resonance imaging plays a crucial role. Images of the brain show inflammatory lesions at different locations in MS patients, primarily in the so-called white matter of the cerebrum (often arranged around the ventricular system of the cerebrospinal fluid = periventricular lesions).In my consultation hours, I usually also show the patients their MRI images, so they get an impression of their inflammatory lesion load. Some patients are then quite surprised about the “many white spots”. Often, the question is asked “what exactly is affected and inflamed” and “which functions of the brain are affected by the inflammatory foci.” This is a comprehensible question, but practically not so easy to answer.
Function of Different Brain Areas
There are areas in the brain whose function is fairly well defined. Modern medicine has gained knowledge about the function, among other things, from examinations of skull-brain trauma patients, where certain areas were destroyed by the injury. This enabled important insights into the function of different brain areas to be gained. For instance, we know that in humans, language is located in the left temporal lobe. The occipital lobe’s cerebral cortex is responsible for seeing. And the memory function is largely controlled by the so-called hippocampus in the basal temporal lobe.
However, one must understand that in practice the individual assignment of functions to certain brain regions can vary greatly. This is related to the plasticity of the brain. Impressive examples of this plasticity are examples from pediatric epilepsy surgery, where parts of the brain are removed to treat severe seizure disorders. Their function is then taken over by the remaining brain areas, and the children can develop almost normally.
Inflammatory Lesions in the Brain
Because of these plastic capabilities, it is therefore more sensible to view the human brain not as a combination of modules with different tasks, but rather as a complex network that can constantly rewire itself to meet its requirements. This works particularly well in childhood. However, plasticity, albeit much less pronounced, persists into old age. This is also a reason why rehabilitation can be very successful in later life, e.g. after a stroke.
Due to the complex network, a single MS lesion in the brain rarely leads to a defined loss of function. This usually happens when a descending tract system is directly affected, or the lesion is located in the optic nerve. In such a case, the inflammation then leads to noticeable failures, which the person affected then experiences as a “clinical MS relapse”.
In most cases, an inflammatory lesion remains clinically silent (ratio clinical relapse : inflammatory lesion in MRI approx. 1 : 10), but contributes to the overall damage of the entire network. In the initial phase of the disease, this network damage can be compensated to a certain degree. But it is quite plausible that the clinical equivalent of network damage is the fatigue reported by many patients, i.e. the abnormal tiredness. More brain performance is required to perform the same tasks as a non-affected person (despite the impairment of the brain network). Starting from this understanding, it is less the single lesion that leads to a dysfunction, but rather the sum of network damage that occurs over the course of the disease.
Inflammatory Lesions in the Spinal Cord
The situation with inflammatory lesions in the spinal cord is different. In the spinal cord, many tract systems, which are important for the motor and sensory function of the limbs, run side by side in a confined space. The spinal cord is, so to speak, the main supply line that connects the brain with the rest of the body. If an inflammation occurs here, it usually has immediate effects on the affected person: They experience disturbances such as abnormal sensations, sensory disturbances, coordination problems or paralysis in the area of the limbs below the inflammatory lesion. Bladder and rectal function, as well as sexual function, can also be affected. Consequently, this means that spinal cord lesions, unlike lesions in the brain, can usually be clearly assigned to a specific failure pattern.
This difference between lesions in the brain and the spinal cord is also the reason (I have pointed this out several times) why we often only use the brain for MRI routine control, as here “silent” lesions, which cannot be assigned to any specific failure symptomatology, occur much more frequently.
