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Infectious Disease and Microbiology – Progressive Multifocal Leukoencephalopathy
Progressive multifocal leukoencephalopathy (PML) is a rare, subacute demyelinating disease of the brain white matter caused by primary infection or reactivation of JC virus (JCV). First identified in 1958, PML primarily affects patients with AIDS or other forms of impaired cell-mediated immunity. The disease is characterized by rapidly progressive focal neurologic deficits and is often fatal. PML is considered an AIDS-defining illness.
The incidence of PML increased significantly during the HIV/AIDS epidemic but declined after the widespread introduction of highly active antiretroviral therapy (HAART). Before the HIV era, reported deaths due to PML were approximately 1.5 per 10 million persons in 1974, increasing to 6.1 per 10 million by 1987. Among untreated HIV-positive individuals, 1–4% may develop PML. In 1992, approximately 2% of HIV-related deaths were attributed to PML. A Danish study demonstrated declining incidence rates following the introduction of HAART: 3.3 cases per 1000 patient-years during the pre-HAART era (1995–1996), 1.8 cases per 1000 patient-years during early HAART (1997–1999), and 1.3 cases per 1000 patient-years during late HAART (2000–2006).
JC virus infection is extremely common in the general population. Seroprevalence among adults in the United States and Europe ranges from 60–80%. Primary infection usually occurs during childhood between 10 and 14 years of age and is generally asymptomatic. Although most individuals remain asymptomatic carriers, active viral replication is more commonly observed in immunocompromised patients, particularly those infected with HIV. Asymptomatic urinary shedding of JCV and BK virus has been documented in immunosuppressed individuals, pregnant women, and the elderly.
The most important risk factor for PML is advanced HIV infection, especially with CD4 counts below 100/mm³. More than 60% of PML cases occur in this setting. Other risk factors include congenital or acquired immunodeficiency, organ transplantation, hematologic malignancies, chronic corticosteroid therapy, and immunosuppressive treatment for autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, and sarcoidosis. Rare cases have occurred without identifiable immunodeficiency. Cases have also been reported in patients with multiple sclerosis or Crohn’s disease receiving natalizumab therapy, with risk increasing with prolonged treatment duration.
The exact mode of transmission and incubation period of JCV are not fully understood. Primary infection has been documented in renal transplant recipients receiving organs from seropositive donors. In PML, the virus infects oligodendrocytes within the central nervous system, leading to widespread demyelination. Neurologic deterioration progresses rapidly over weeks and typically occurs without signs of increased intracranial pressure. In some HIV-positive patients, initiation of HAART may paradoxically trigger inflammatory PML as part of immune reconstitution inflammatory syndrome (IRIS).
JC virus is a DNA virus belonging to the family Papovaviridae and genus Polyomavirus. Although BK virus is another known human polyomavirus, it is not associated with PML.
Patients with PML typically present without constitutional symptoms such as fever. Common manifestations at presentation include hemiparesis, visual field deficits such as homonymous hemianopia, cognitive impairment, aphasia, ataxia, cranial nerve deficits, sensory abnormalities, seizures, confusion, personality changes, and dementia. The spinal cord is usually spared. As the disease progresses, patients may develop profound neurologic deficits including cortical blindness, quadriparesis, severe dementia, coma, and marked motor weakness. Clinical manifestations are often more severe than expected from imaging or pathological findings. The presentation is similar in both HIV-positive and HIV-negative individuals.
The diagnostic gold standard is demonstration of JC virus antigen or genomic DNA within brain tissue combined with characteristic pathological findings. Detection methods include immunocytochemistry, in situ hybridization, and polymerase chain reaction (PCR). Because brain biopsy carries substantial risk, PCR detection of JCV DNA in cerebrospinal fluid is the preferred diagnostic test. In patients not receiving HAART, PCR sensitivity ranges from 72–92% and specificity from 92–100%. Sensitivity decreases after initiation of HAART because viral replication declines. Cerebrospinal fluid findings are nonspecific and may include pleocytosis, elevated IgG, and oligoclonal bands. Serologic testing is generally not useful. Viral culture is impractical because JCV grows very slowly and susceptible cell lines are difficult to obtain.
Imaging studies play a central role in diagnosis. CT scans typically demonstrate hypodense, non-enhancing subcortical white matter lesions without edema or mass effect, often involving the periventricular regions, centrum semiovale, parieto-occipital areas, and cerebellum. MRI is more sensitive and usually reveals multiple asymmetric subcortical white matter lesions with high T2 signal intensity. Inflammatory PML is characterized by contrast-enhancing lesions. Electroencephalography may show focal or diffuse slowing, and abnormalities may occasionally precede CT changes.
Pathologically, PML results from direct JCV infection and destruction of oligodendrocytes, leading to multifocal demyelination throughout the central nervous system with minimal inflammatory response. Reactive gliosis, bizarre astrocytes with lobulated nuclei, and lipid-laden macrophages are commonly observed. The cerebral hemispheres, cerebellum, and brainstem may all be affected, while spinal cord involvement is uncommon. Electron microscopy reveals polyomavirus inclusions within enlarged oligodendrocyte nuclei.
The differential diagnosis includes HIV encephalopathy, opportunistic CNS infections such as cytomegalovirus, neurosyphilis, cryptococcosis, tuberculous meningitis, and toxoplasmosis, as well as primary CNS lymphoma and Kaposi’s sarcoma. Other considerations include acute multiple sclerosis, acute hemorrhagic leukoencephalitis, herpes simplex encephalitis, multifocal varicella-zoster leukoencephalitis, postinfectious encephalomyelitis, and cyclosporine-induced neurotoxicity.
There is currently no specific antiviral therapy for PML. In HIV-positive patients, the primary treatment strategy is aggressive antiretroviral therapy to restore immune function. Patients with dementia may require supervised medication administration to ensure adherence. In individuals with PML related to immunosuppressive therapy, reduction or discontinuation of immunosuppressive agents is recommended whenever possible. Intravenous or intrathecal cytarabine may provide some benefit in patients with hematologic malignancies, although evidence is limited. Cidofovir has not demonstrated clinical benefit. Corticosteroids may be useful in inflammatory PML associated with immune reconstitution. In cases associated with natalizumab therapy, plasma exchange combined with discontinuation of natalizumab has been attempted.
Patients require frequent neurologic and clinical follow-up to monitor for disease progression or recurrence. HIV-positive patients also require ongoing monitoring of HIV disease activity and immune status.
The prognosis of PML remains poor. Death typically occurs within approximately six months of diagnosis. However, some HIV-positive patients may experience spontaneous fluctuations or prolonged survival over several years, particularly if immune function improves substantially with aggressive antiretroviral therapy.
Progressive multifocal leukoencephalopathy (PML) is a rare, subacute demyelinating disease of the brain white matter caused by primary infection or reactivation of JC virus (JCV). First identified in 1958, PML primarily affects patients with AIDS or other forms of impaired cell-mediated immunity. The disease is characterized by rapidly progressive focal neurologic deficits and is often fatal. PML is considered an AIDS-defining illness.
The incidence of PML increased significantly during the HIV/AIDS epidemic but declined after the widespread introduction of highly active antiretroviral therapy (HAART). Before the HIV era, reported deaths due to PML were approximately 1.5 per 10 million persons in 1974, increasing to 6.1 per 10 million by 1987. Among untreated HIV-positive individuals, 1–4% may develop PML. In 1992, approximately 2% of HIV-related deaths were attributed to PML. A Danish study demonstrated declining incidence rates following the introduction of HAART: 3.3 cases per 1000 patient-years during the pre-HAART era (1995–1996), 1.8 cases per 1000 patient-years during early HAART (1997–1999), and 1.3 cases per 1000 patient-years during late HAART (2000–2006).
JC virus infection is extremely common in the general population. Seroprevalence among adults in the United States and Europe ranges from 60–80%. Primary infection usually occurs during childhood between 10 and 14 years of age and is generally asymptomatic. Although most individuals remain asymptomatic carriers, active viral replication is more commonly observed in immunocompromised patients, particularly those infected with HIV. Asymptomatic urinary shedding of JCV and BK virus has been documented in immunosuppressed individuals, pregnant women, and the elderly.
The most important risk factor for PML is advanced HIV infection, especially with CD4 counts below 100/mm³. More than 60% of PML cases occur in this setting. Other risk factors include congenital or acquired immunodeficiency, organ transplantation, hematologic malignancies, chronic corticosteroid therapy, and immunosuppressive treatment for autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, and sarcoidosis. Rare cases have occurred without identifiable immunodeficiency. Cases have also been reported in patients with multiple sclerosis or Crohn’s disease receiving natalizumab therapy, with risk increasing with prolonged treatment duration.
The exact mode of transmission and incubation period of JCV are not fully understood. Primary infection has been documented in renal transplant recipients receiving organs from seropositive donors. In PML, the virus infects oligodendrocytes within the central nervous system, leading to widespread demyelination. Neurologic deterioration progresses rapidly over weeks and typically occurs without signs of increased intracranial pressure. In some HIV-positive patients, initiation of HAART may paradoxically trigger inflammatory PML as part of immune reconstitution inflammatory syndrome (IRIS).
JC virus is a DNA virus belonging to the family Papovaviridae and genus Polyomavirus. Although BK virus is another known human polyomavirus, it is not associated with PML.
Patients with PML typically present without constitutional symptoms such as fever. Common manifestations at presentation include hemiparesis, visual field deficits such as homonymous hemianopia, cognitive impairment, aphasia, ataxia, cranial nerve deficits, sensory abnormalities, seizures, confusion, personality changes, and dementia. The spinal cord is usually spared. As the disease progresses, patients may develop profound neurologic deficits including cortical blindness, quadriparesis, severe dementia, coma, and marked motor weakness. Clinical manifestations are often more severe than expected from imaging or pathological findings. The presentation is similar in both HIV-positive and HIV-negative individuals.
The diagnostic gold standard is demonstration of JC virus antigen or genomic DNA within brain tissue combined with characteristic pathological findings. Detection methods include immunocytochemistry, in situ hybridization, and polymerase chain reaction (PCR). Because brain biopsy carries substantial risk, PCR detection of JCV DNA in cerebrospinal fluid is the preferred diagnostic test. In patients not receiving HAART, PCR sensitivity ranges from 72–92% and specificity from 92–100%. Sensitivity decreases after initiation of HAART because viral replication declines. Cerebrospinal fluid findings are nonspecific and may include pleocytosis, elevated IgG, and oligoclonal bands. Serologic testing is generally not useful. Viral culture is impractical because JCV grows very slowly and susceptible cell lines are difficult to obtain.
Imaging studies play a central role in diagnosis. CT scans typically demonstrate hypodense, non-enhancing subcortical white matter lesions without edema or mass effect, often involving the periventricular regions, centrum semiovale, parieto-occipital areas, and cerebellum. MRI is more sensitive and usually reveals multiple asymmetric subcortical white matter lesions with high T2 signal intensity. Inflammatory PML is characterized by contrast-enhancing lesions. Electroencephalography may show focal or diffuse slowing, and abnormalities may occasionally precede CT changes.
Pathologically, PML results from direct JCV infection and destruction of oligodendrocytes, leading to multifocal demyelination throughout the central nervous system with minimal inflammatory response. Reactive gliosis, bizarre astrocytes with lobulated nuclei, and lipid-laden macrophages are commonly observed. The cerebral hemispheres, cerebellum, and brainstem may all be affected, while spinal cord involvement is uncommon. Electron microscopy reveals polyomavirus inclusions within enlarged oligodendrocyte nuclei.
The differential diagnosis includes HIV encephalopathy, opportunistic CNS infections such as cytomegalovirus, neurosyphilis, cryptococcosis, tuberculous meningitis, and toxoplasmosis, as well as primary CNS lymphoma and Kaposi’s sarcoma. Other considerations include acute multiple sclerosis, acute hemorrhagic leukoencephalitis, herpes simplex encephalitis, multifocal varicella-zoster leukoencephalitis, postinfectious encephalomyelitis, and cyclosporine-induced neurotoxicity.
There is currently no specific antiviral therapy for PML. In HIV-positive patients, the primary treatment strategy is aggressive antiretroviral therapy to restore immune function. Patients with dementia may require supervised medication administration to ensure adherence. In individuals with PML related to immunosuppressive therapy, reduction or discontinuation of immunosuppressive agents is recommended whenever possible. Intravenous or intrathecal cytarabine may provide some benefit in patients with hematologic malignancies, although evidence is limited. Cidofovir has not demonstrated clinical benefit. Corticosteroids may be useful in inflammatory PML associated with immune reconstitution. In cases associated with natalizumab therapy, plasma exchange combined with discontinuation of natalizumab has been attempted.
Patients require frequent neurologic and clinical follow-up to monitor for disease progression or recurrence. HIV-positive patients also require ongoing monitoring of HIV disease activity and immune status.
The prognosis of PML remains poor. Death typically occurs within approximately six months of diagnosis. However, some HIV-positive patients may experience spontaneous fluctuations or prolonged survival over several years, particularly if immune function improves substantially with aggressive antiretroviral therapy.
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