Progressive Multifocal Leukoencephalopathy

Progressive multifocal leukoencephalopathy is a rare, severe demyelinating disease of the central nervous system caused by reactivation of JC virus. JC virus is a polyomavirus to which the large majority of the adult population has been asymptomatically exposed, and it establishes lifelong latency.

Disease occurs almost exclusively in the setting of significant impairment of cell mediated immunity:

• advanced HIV infection
• haematological malignancy
• certain immunomodulatory and biologic therapies, most classically natalizumab used in multiple sclerosis

The last is a recognised complication of growing clinical importance. Risk stratification and monitoring in patients receiving these therapies is a central practical concern.

Pathophysiology

Reactivation of latent JC virus, under conditions of significantly impaired cell mediated immune surveillance, leads to lytic infection of oligodendrocytes, the myelin producing cells of the central nervous system. This produces progressive, often multifocal areas of demyelination.

There is no surrounding inflammatory response of the kind typically seen in other central nervous system infections. This reflects the underlying profound impairment of the cellular immune response that permits this unusual pattern of injury.

Presentation is typically subacute, developing over days to weeks. The clinical features reflect the location of the demyelinating lesions.

Common features:

• progressive weakness or sensory disturbance
• visual field deficit or cortical blindness
• cognitive decline and behavioural change
• less commonly, ataxia or aphasia

The overall clinical picture is frequently multifocal and progressive rather than static.

Seizures can occur, particularly where lesions involve the cortex or subcortical white matter close to the cortical surface.

There is no fever or other systemic feature of infection, and the clinical course is generally painless and non inflammatory. This is characteristic and can lead to initial consideration of an alternative, non infective diagnosis.

A specific and important scenario is new or worsening neurological symptoms in a patient on natalizumab or another biologic therapy with recognised progressive multifocal leukoencephalopathy risk. Any new neurological symptom must be taken seriously and prompt urgent investigation. It must not be attributed by default to the underlying condition being treated.

This applies particularly to multiple sclerosis, where the clinical distinction from a disease relapse can be genuinely difficult.

Magnetic resonance imaging is the key investigation. It shows characteristic asymmetric, non enhancing (or only minimally enhancing) white matter lesions on T2 and fluid attenuated inversion recovery sequences, typically without significant mass effect. Lesions often involve subcortical U fibres and cross the corpus callosum.

This pattern helps distinguish the condition from other demyelinating and inflammatory processes. Imaging alone is not definitively diagnostic and requires correlation with cerebrospinal fluid findings.

Cerebrospinal fluid polymerase chain reaction testing for JC virus deoxyribonucleic acid is the key confirmatory test. A positive result in the appropriate clinical and radiological context is considered diagnostic. A negative result, particularly with a low viral burden, does not entirely exclude the diagnosis. Repeat testing or, in cases of persistent diagnostic uncertainty, brain biopsy may be required.

Assessment of the underlying immunosuppressive state is an essential component of both diagnosis and ongoing risk assessment. It includes HIV testing where not already known.

For patients on natalizumab specifically, test anti JC virus antibody status. A positive antibody status indicates substantially elevated risk, particularly at high titre and in combination with prolonged treatment duration and prior immunosuppressant use.

There is no specific antiviral treatment with proven efficacy against JC virus. Management centres entirely on reversal or reduction of the underlying immunosuppression. This permits immune reconstitution and, ideally, clearance or containment of the virus by a restored cellular immune response. It is the only intervention consistently associated with improved outcome.

In HIV associated disease, immediate initiation or optimisation of antiretroviral therapy is the essential management step. The aim is rapid and effective viral suppression and CD4 count recovery.

Immune reconstitution inflammatory syndrome is a risk as immune function recovers. It can paradoxically cause clinical and radiological worsening as an inflammatory response develops against the previously immunologically silent viral infection. It sometimes requires corticosteroid treatment.

In natalizumab associated disease, the drug must be stopped immediately on diagnosis or strong clinical suspicion. Plasma exchange or immunoadsorption is used in some protocols to accelerate clearance of the drug from the circulation and hasten immune reconstitution.

This itself carries a significant risk of triggering immune reconstitution inflammatory syndrome. It is managed with corticosteroids (typically high dose methylprednisolone) where it develops. Very close monitoring is required during this recovery phase, given the potential for significant clinical deterioration during the inflammatory reconstitution process. Paradoxically, this represents evidence of an immune response against the virus.

For progressive multifocal leukoencephalopathy occurring in the context of other immunosuppressive medications or underlying haematological malignancy, the primary strategy is reduction or discontinuation of the causative or contributing immunosuppressive therapy, where clinically feasible. Individualise according to the specific clinical context, balanced against the risks of withdrawing treatment for the underlying disease.

Supportive care is provided alongside efforts at immune reconstitution. It includes physiotherapy, occupational therapy and management of specific neurological deficits (seizure control where relevant, swallowing assessment where bulbar function is affected).

Overall prognosis remains guarded, particularly in HIV associated disease diagnosed late or with a low CD4 count. Outcomes have improved substantially with earlier recognition and prompt initiation of effective antiretroviral therapy compared with the pre antiretroviral therapy era.

Risk stratification and monitoring protocols for patients on natalizumab and other high risk biologic therapies are an important preventive and early detection strategy. They include regular clinical review, periodic magnetic resonance imaging surveillance (with the specific interval determined by individual risk factors including JC virus antibody status and treatment duration), and antibody titre monitoring. The aim is to identify progressive multifocal leukoencephalopathy at the earliest possible, most treatable stage.