Progressive multifocal leukoencephalopathy
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Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease which results from the reactivation of John Cunningham virus (JC virus) infecting oligodendrocytes in patients with compromised immune systems. It is considered the most common clinical manifestation of JC virus infection in the brain 7, and is seen in three clinical contexts:
PML due to immunocompromised state
PML-s-IRIS: simultaneous development of Immune Reconstitution Inflammatory Syndrome (IRIS) and PML due to immune reconstitution
PML-d-IRIS: immune reconstitution worsens pre-existing PML
Progressive multifocal leukoencephalopathy is strongly associated with immunosuppressed states, and primary PML developing in an immunocompetent patient is very rare.
Classically, PML occurred in patients with AIDS, typically developing in patients with CD4 counts of 50-100 cells/μL, and is found in approximately 5% of autopsies of patients who died from AIDS 2.
However, the incidence of PML in the non-HIV setting is thought to be increasing 4,17:
post-transplant: bone marrow or solid organ transplants
solid organ malignancies
isolated CD4 lymphocytopaenia
immunosuppressive monoclonal antibody therapy, such as natalizumab (an IgG monoclonal antibody used in the treatment of relapsing-remitting multiple sclerosis 5), efalizumab, and rituximab 20
In addition to being seen in immunocompromised patients, it is also seen in patients whose immune function is recovering, and PML is one of the classic conditions encountered in immune reconstitution inflammatory syndrome (IRIS) 16.
Patients with progressive multifocal leukoencephalopathy present with various neurological symptoms. It typically spares the optic nerve and the spinal cord. The most frequently encountered symptoms include:
altered mental status
limb and gait ataxia
visual symptoms (diplopia and hemianopia)
seizure (as PML can also involve the grey matter in later stages)
The final diagnosis is established with brain biopsy (specificity: 100%, sensitivity: 65-95%).
PML lesions are secondary to progressive demyelination due to John Cunningham (JC) virus (usually reactivation) which infects oligodendrocytes. In PML seen in HIV/AIDS, reactivation is typically seen when CD4 cell count drops below 100/μL 17.
Lesions tend to have a confluent, bilateral but asymmetrical supratentorial white matter and thalamic involvement 7. However, basal ganglia, brainstem and cerebellum also can be involved. Subcortical frontal and parieto-occipital regions are common locations. However, isolated posterior fossa involvement has also been reported 15. While the condition invariably involves white matter, subcortical U-fibers and in late stages of disease grey matter involvement is also seen 6.
Histology reveals demyelinating plaques involving the white matter and subcortical U-fibers. Other findings include infected oligodendrocytes with enlarged amphophilic nuclei located at the periphery of the lesions, macrophages containing phagocytosed cellular debris and myelin, and reactive gliosis with enlarged astrocytes 13.
Asymmetric focal zones of low attenuation involving the periventricular and subcortical white matter. This is in contradistinction to more symmetrical hypoattenuation seen in HIV encephalopathy.
Typically seen as multifocal, asymmetric periventricular and subcortical involvement. There is little or no mass effect or enhancement 10 and the subcortical U-fibers are commonly involved with a predilection for the parieto-occipital regions 17. In fact, even though subcortical U-fiber involvement is an important feature of lesions of multiple sclerosis, these are relatively uncommon and if new lesions have a subcortical U-fiber distribution, this favors PML 21. Typically, the lesion presents a sharply demarcated peripheral border along the subcortical U fibers and a hazy and ill-defined inner border.22 The corpus callosum may be involved 17.
T1: involved regions are usually hypointense
T1 C+ (Gd)
typically there is no enhancement 10
enhancement can be seen in PML-IRIS, AIDS with ART, and in patients on natalizumab 16,17
when enhancement is present, it may be associated with improved survival 3,17
DWI/ADC: peripheral patchy diffusion restriction particularly at the leading edge 14
MR spectroscopy: according to one study, spectra of PML lesions were characterized by significantly reduced NAA, lactate presence, and by significantly increased choline and lipids compared with control group values 8
elevated at the leading edge of lesions
most commonly seen in progressive non-IRIS PML 18
Treatment and prognosis
Prognosis is generally poor with an inexorable neurological decline leading to coma and death occurring in the majority of patients with PML 9. If untreated, PML is usually fatal within one year, often within 2 to 6 months 17. Treatment with antiretroviral therapy (ART) may prolong survival, although in some instances this leads to PML-IRIS. High-dose glucocorticoid therapy is recommended for patients with PML-IRIS. Some reports also state some benefit with cytarabine or mirtazapine, the latter especially in natalizumab-associated PML.
new demyelinating lesions in patients with multiple sclerosis
early in the disease MS and PML may appear similar 21
periventricular location or well-defined borders favor new MS lesions 21
HIV/AIDS encephalopathy: often diffuse white matter disease, with atrophy, symmetric, spares the subcortical U-fibers
differing clinical history (e.g. hypertension)
can involve both grey and white matter
differing clinical history (e.g. recent infection/vaccination)
can involve both grey and white matter
lesions usually enhance
cerebral toxoplasmosis: usually enhance
immunodeficiency-associated CNS lymphoma: usually enhance
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