Progressive multifocal leukoencephalopathy is a rare but serious brain infection that affects people whose immune systems are severely weakened. Understanding its causes, symptoms, and risk factors can help those at risk recognize warning signs early and seek appropriate medical care.
What Is Progressive Multifocal Leukoencephalopathy?
Progressive multifocal leukoencephalopathy, commonly known as PML, is a rare brain infection that attacks the protective covering of nerve cells. This covering, called myelin, is an oily substance that insulates neurons in the white matter of the brain and spinal cord, much like the plastic coating around an electrical wire protects the metal inside. When myelin breaks down, the nerve cells cannot transmit signals properly, leading to serious neurological problems.[1]
The name itself tells us a lot about the disease. “Progressive” means the condition gets worse over time. “Multifocal” indicates that it typically affects multiple parts of the brain rather than just one area. “Leukoencephalopathy” refers to disease of the white matter in the brain. These three characteristics together describe a condition that spreads through different brain regions and worsens without intervention.[2]
PML causes significant brain damage that can lead to a range of neurological problems including difficulty thinking, vision loss, weakness, and changes in personality. Because it is progressive, symptoms typically start subtly but become more severe over weeks to months. The condition can be fatal, and even with treatment, many people are left with lasting disabilities.[1]
Epidemiology
Progressive multifocal leukoencephalopathy is considered a rare disease. However, its occurrence has changed dramatically over the decades due to shifts in the population at risk. Before the HIV/AIDS epidemic in the 1980s, PML was extremely uncommon and mainly seen in people with blood cancers like leukemia or lymphoma, and in those who had received organ transplants.[3]
The AIDS pandemic brought PML into much sharper focus. Studies estimate that before effective antiretroviral therapy became available, as many as five percent of people living with HIV eventually developed PML. For these patients, PML was considered an AIDS-defining illness, meaning its presence indicated that HIV infection had progressed to AIDS.[1]
The introduction of antiretroviral therapy dramatically changed the landscape. This treatment effectively restores immune system function in people with HIV, and as a result, as many as half of all people with HIV-related PML can now survive. While HIV remains the most common underlying condition for PML worldwide, accounting for approximately 85 percent of cases, the outlook has improved significantly with modern treatment.[10]
More recently, the increased use of certain immunosuppressive and immunomodulatory medications has led to a new population of people at risk for PML. These are patients being treated for autoimmune diseases like multiple sclerosis, rheumatoid arthritis, and lupus, as well as those receiving treatment to prevent organ transplant rejection. As these powerful medications have become more widely prescribed, the proportion of PML cases occurring in people without HIV has gradually increased.[7]
Causes
Progressive multifocal leukoencephalopathy is caused by a common virus called the JC virus, named after John Cunningham, the first person from whose tissue the virus was successfully identified. Scientists also refer to it as human polyomavirus 2 or polyomavirus JC. This virus is remarkably widespread in the general population.[2]
Studies show that between 39 and 90 percent of adults have been infected with the JC virus at some point in their lives. This means most people carry the virus without ever knowing it. The virus typically infects people during childhood, possibly through contaminated food or water, or through contact with urine. Once a person is infected, the virus establishes what doctors call a latent infection, meaning it remains in the body without causing symptoms or problems.[3]
In healthy people with normal immune systems, the JC virus causes no symptoms whatsoever. The immune system keeps the virus under control, preventing it from becoming active. The virus typically lives quietly in the kidneys, bone marrow, or possibly the brain, where it remains dormant for years or even a lifetime without causing any harm.[2]
PML only develops when something goes seriously wrong with the immune system. When immune defenses become severely weakened, the JC virus can reactivate and undergo genetic changes that allow it to attack the brain. The virus specifically targets and destroys oligodendrocytes, which are the cells responsible for producing myelin. As these cells die, the myelin coating on nerve cells breaks down, disrupting communication between different parts of the brain and causing the neurological symptoms characteristic of PML.[1]
Experts are still working to fully understand why some people with weakened immune systems develop PML while others do not, even when they carry the JC virus. It appears that the virus may need to be present in specific locations in the body, undergo particular genetic mutations, and find an opportunity when immune surveillance is at its lowest. Some research suggests that HIV itself may have effects on brain tissue or the JC virus that make PML more likely to occur in people with AIDS compared to other immunosuppressed conditions.[3]
Risk Factors
The single most important risk factor for developing PML is having a severely compromised immune system. Without adequate immune function, the body cannot keep the JC virus in check, and the virus can reactivate and attack the brain. Several different conditions and situations can weaken the immune system enough to put someone at risk.[2]
People living with HIV and AIDS represent the largest group at risk for PML. Before antiretroviral therapy became available, HIV was by far the leading cause of PML, and it remains the most common underlying condition today. Most people with HIV who develop PML have very low CD4 cell counts, typically below 200 cells per microliter, indicating severely weakened immunity. However, there have been rare reports of PML occurring in HIV patients with higher CD4 counts.[10]
Cancer patients are another high-risk group, particularly those with blood cancers and cancers of the lymphatic system. People with leukemia, Hodgkin’s disease, or lymphoma have immune systems that are compromised both by their disease and by the chemotherapy used to treat it. These patients were among the first identified cases of PML before the AIDS epidemic.[1]
People who have received organ transplants must take immunosuppressive medications for the rest of their lives to prevent their bodies from rejecting the transplanted organ. These medications intentionally weaken the immune system, and while this is necessary for the transplant to succeed, it also creates a window of vulnerability for the JC virus to reactivate.[2]
Individuals with autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus may be at risk, especially if they are treated with certain medications. Some of the most effective treatments for these conditions work by suppressing or modifying the immune system. In particular, a medication called natalizumab, used to treat multiple sclerosis, has been linked to an increased risk of PML. Other medications associated with PML risk include rituximab, efalizumab, and various other immunosuppressants and biological therapies.[3]
Patients taking certain medications for inflammatory conditions like psoriasis and Crohn’s disease may also face increased risk. The risk appears to increase with the duration of treatment and when multiple immunosuppressive therapies are used together. For medications like natalizumab, doctors monitor patients closely and may test for JC virus antibodies before starting treatment to help assess individual risk.[4]
Symptoms
The symptoms of PML vary considerably from person to person depending on which parts of the brain are affected by the infection and how extensive the damage is. For most people, symptoms begin gradually and subtly, then worsen progressively over the course of several weeks to months. This progression is one of the hallmarks of the disease.[1]
One of the earliest and most common symptoms is clumsiness or a lack of coordination. People may find themselves dropping things more often, bumping into objects, or having difficulty with tasks that require fine motor control. This happens because PML affects the parts of the brain that coordinate movement. Along with clumsiness, progressive weakness often develops, which may affect one side of the body more than the other or may be limited to a single limb.[2]
Difficulty with speaking and thinking are also prominent early symptoms. People may struggle to find the right words, speak more slowly than usual, or have trouble expressing their thoughts clearly. Thinking may become slower and less sharp, and people may have difficulty concentrating or solving problems. Family members might notice these changes before the affected person does.[1]
As the disease progresses, more severe symptoms typically emerge. Vision problems can develop, including partial blindness affecting half of the visual field, or double vision. Some people develop complete blindness if the virus damages the parts of the brain responsible for processing visual information. Speech difficulties may worsen to the point where a person loses the ability to speak altogether.[2]
Many people with PML experience personality changes or behavioral problems. They may become more withdrawn, show less interest in activities they once enjoyed, or display emotions that seem inappropriate for the situation. In some cases, people develop dementia, with significant memory loss and confusion. As the disease advances, many people become unable to care for themselves and may eventually become bedbound.[5]
While less common, some people experience headaches or seizures, particularly those with end-stage HIV infection. The range and severity of symptoms reflect the fact that PML can affect multiple areas of the brain simultaneously, causing damage in different locations. This multifocal nature of the disease is what makes its presentation so variable from one person to another.[5]
Without treatment, the progression of symptoms leads to life-threatening disability. Death commonly occurs within one to nine months after symptoms begin, though some people survive longer. The outlook depends heavily on what caused the immune system weakness in the first place and whether that underlying condition can be effectively treated.[5]
Prevention
Because the JC virus is so common and most people are infected during childhood, preventing initial infection with the virus is not a realistic strategy. There is no vaccine available for the JC virus, and the virus typically causes no problems in healthy individuals, so routine prevention measures for the general population are not necessary.[2]
For people at risk of PML due to weakened immune systems, prevention focuses on careful monitoring and risk assessment. Before starting certain immunosuppressive medications, particularly natalizumab for multiple sclerosis, doctors typically test patients for JC virus antibodies. This blood test can detect whether someone has been exposed to the virus and can help estimate the risk of developing PML. Patients who test negative have a lower risk, while those with positive tests, especially with high antibody levels, may face a higher risk.[15]
Even if initial testing is negative, doctors typically repeat the blood test every six months during treatment, because people can become infected with the JC virus at any time. For medications with the highest PML risk, healthcare providers may recommend limiting the duration of treatment or avoiding combinations of multiple immunosuppressive drugs, which can compound the risk.[15]
For people living with HIV, maintaining strong immune function through consistent use of antiretroviral therapy is the most effective prevention strategy. Keeping the viral load low and CD4 cell counts high helps keep the immune system strong enough to control the JC virus. Regular medical follow-up and adherence to prescribed medications are crucial.[1]
People at risk should be educated about the symptoms of PML so they can report any new neurological problems to their healthcare providers immediately. Early detection, while it does not prevent PML, can lead to earlier intervention and potentially better outcomes. Patients on high-risk medications should have regular neurological assessments and brain imaging as part of their monitoring plan.[2]
For patients already taking immunosuppressive medications who develop concerning symptoms, stopping the offending medication may be necessary. This decision must be carefully balanced against the risks of the underlying disease worsening, and should always be made in close consultation with healthcare providers. In some cases, a procedure called plasma exchange can be used to rapidly remove certain medications from the bloodstream, particularly natalizumab.[5]
Pathophysiology
The pathophysiology of PML involves a complex series of events that begin with JC virus reactivation and end with widespread destruction of the brain’s white matter. Understanding these changes helps explain why PML causes such devastating neurological damage and why treatment is so challenging.[7]
Under normal circumstances, the JC virus remains dormant in various body tissues, including the kidneys and possibly the bone marrow. The virus can be found in the urine of about 30 percent of healthy individuals, where it is shed intermittently without causing any symptoms. The immune system, particularly the cellular immune response involving T cells, keeps the virus under tight control and prevents it from becoming active.[10]
When the immune system becomes severely weakened, this control breaks down. The virus can then undergo genetic mutations and recombination events that allow it to acquire new properties. These changes enable the virus to cross the blood-brain barrier, enter the central nervous system, and specifically target oligodendrocytes—the cells responsible for producing and maintaining myelin in the brain.[4]
Once inside the brain, the JC virus infects oligodendrocytes and begins to replicate. As the virus multiplies, it destroys these critical cells from the inside out. When oligodendrocytes die, they can no longer produce or maintain the myelin sheaths that insulate nerve fibers. Without this protective coating, nerve cells cannot efficiently transmit electrical signals, leading to disrupted communication between different parts of the brain.[1]
The destruction of myelin, called demyelination, occurs in multiple locations throughout the white matter of the brain—hence the term “multifocal.” These areas of damage appear as lesions on brain imaging studies. The lesions can occur in virtually any part of the white matter, but they most commonly affect regions involved in motor control, vision, cognition, and speech. The specific location of the lesions determines which symptoms a person experiences.[2]
In addition to oligodendrocytes, the JC virus can also infect astrocytes, another type of brain cell that provides support and nutrition to neurons. Infection of astrocytes can lead to further brain damage and may contribute to the progressive nature of the disease. As more and more areas of white matter are destroyed, neurological function continues to decline.[7]
One particularly complex situation can occur in people with HIV who begin antiretroviral therapy. As their immune systems start to recover, they may develop what’s called immune reconstitution inflammatory syndrome, or IRIS. In this scenario, the recovering immune system recognizes the JC virus in the brain and mounts an inflammatory response against it. While inflammation is normally helpful for fighting infections, in the confined space of the brain, it can actually cause additional damage. These cases, called PML-IRIS, can occur anywhere from one week to 26 months after starting treatment.[4]
The challenge in treating PML lies in the fact that there are currently no antiviral medications that can effectively eliminate the JC virus from the brain without causing unacceptable toxicity. The primary treatment strategy therefore focuses on restoring immune function so the body’s own defenses can control the virus. However, this restoration must happen quickly enough to prevent fatal damage, while also managing the risk of inflammatory complications. This delicate balance makes PML a particularly difficult condition to treat.[1]
