What is demyelination?
Most neurons (nerve cells) are covered by a protective insulating layer called myelin. Similar to the insulating wrapping around electrical wires, myelin sheaths allow electrical impulses to transmit quickly and efficiently along the neurons. Demyelination occurs when the myelin sheaths around the neurons damage and breakdown. This slows or stops the transmission of electrical impulses along neurons, resulting in neurological symptoms manifested in demyelinating diseases. Common symptoms of demyelinating diseases include altered mental status, muscle weakness, spasms, sensory changes, vision loss, problems walking (ataxia) and bowel or bladder control.
Although the precise cause of demyelination is unknown, the genesis is thought to be destructive autoimmunity in which the immune T- and B-cells inappropriately target the myelinated neurons, damaging not only the myelin sheaths but also the surrounding glial cells and the neurons themselves. For this reason, most demyelinating diseases are considered autoimmune-related disorders, such as multiple sclerosis (MS), Devic’s disease (aka neuromuscular optica), optic neuritis and transverse myelitis. On the other hand, demyelinating diseases like Guillain-Barre syndrome and acute disseminated encephalomyelitis (ADEM) are thought to be caused by preceding viral infections that trigger the immune system via molecular mimicry. The antigens of the pathogens mimic the antigens of the body’s cells, thus confusing the immune system and causing it to attack itself even when the pathogens have long been eliminated from the body. Since demyelination is primarily caused by the inappropriate activation of immune responses, the mainstay treatment of demyelination is immunosuppression. The most common medication to suppress the immune system is corticosteroid, like methylprednisolone. Another medication reported to be effective in treating demyelination is intravenous immunoglobulin (IVIG), particularly in ADEM and MS in children.
Does demyelination cause brain cancer in the first place?
To date, only a few case studies have reported on the relation between demyelination and brain cancer. However, none of them suggest or conclude that demyelination causes brain cancers. A study published in 2016 BMC Neurology reported a 57-year-old woman who presented with a six week history of progressive fatigue, confusion, headache and visual problem (right inferior homonymous quadrantanopia). She was diagnosed with acute multifocal demyelination with brain biopsy. Despite a good initial response to corticosteroid and rituximab, the patient’s condition deteriorated rapidly. A repeat brain biopsy done six months later showed that she actually had diffuse large B-cell lymphoma, a rare type of brain cancer known as PCNSL.
This study did not suggest that her brain cancer was caused by demyelination, but drew a conclusion that rare brain tumours like the primary central nervous system lymphoma (PCNSL) can be preceded by sentinel lesions indistinguishable from the demyelination of MS or ADEM. Furthermore, the mistreatment of steroids before biopsy can obscure a diagnosis of PCNSL. This case highlights that not all histologic and radiologic evidence of demyelination correspond to a primary demyelinating disease. Certain brain tumours can simulate demyelinating diseases, confusing the clinician judgements and diagnosis.
On the flipped side, demyelinating diseases can also simulate brain tumours. A study published in the 2006 Indian Journal of Medical Science reported on the histological assessment of seven cases where demyelinating diseases presented as brain tumours were indistinguishable clinically and radiologically. Even in brain biopsy, they mimic the cells of brain cancer (astrocytic neoplasm), reminding clinicians to have a high degree of suspicion when diagnosing these diseases. Another study in the American Journal of Surgical Pathology reported on 17 patients who presented with symptoms mimicking brain and spinal cord tumours were eventually diagnosed as demyelinating diseases. Despite CT scans and MRI results consistent with brain tumours, the study emphasizes on the need to perform special stains on biopsied samples in order to demonstrate the evidence of demyelination.
In conclusion, there is no evidence or study that has shown that demyelination causes brain cancer. However, it seems that the diagnosis of demyelinating disease and brain cancer may be difficult to differentiate and clinicians are reminded to stay vigilant when handling such cases.