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Editor,—I read with interest the excellent review on posterior leukoencephalopathy syndrome published in January.1 The author, however, has omitted an important differential diagnosis in his article—namely, progressive multifocal leukoencephalopathy (PML). This can mimic the appearances of posterior leukoencephalopathy on both computed tomography and magnetic resonance imaging (MRI) scans of the brain and needs to be high on the differential diagnosis especially in patients with AIDS.2PML was first described in 19583 and is characterised by widespread demyelination in the cerebral hemispheres. Today PML is seen most frequently in patients with AIDS but can also occur in patients with chronic neoplasia and immunosuppressed states. Intellectual changes, hemiparesis, visual field defects, ataxia, aphasia, and dementia are clinical features. Seizures are rare. The cerebrospinal fluid is usually normal. On computed tomography there may be low attenuation areas in the posterior fossae but MRI (T2 weighted) shows characteristic increased signal in the posterior fossae. No enhancement is seen after intravenous contrast medium administration. The condition is associated with JC virus and has a very poor prognosis.
The author responds:
I am grateful to Dr Banerjee for his interest in my article. I agree with his comment that PML should be included in the differential diagnosis of posterior leukoencephalopathy. Even in patients with PML computed tomography shows hypodense non-enhancing white matter lesions without associated oedema or mass effect. MRI is more sensitive than computed tomography and reveals similar hyperintense signals in the cerebral white matter on T2 weighted spin-echo images. The white matter lesions of PML also have a predilection for occipital and parietal lobes. Occasionally, white matter lesions may symmetrically involve bilateral occipital lobes and may clinically present with cortical blindness; in such patients the clinical and imaging picture is similar to that of posterior leukoencephalopathy.1-1 Moreover, both posterior leukoencephalopathy and PML can occur in HIV infected patients and in patients with various lymphoproliferative and myeloproliferative disorders. Certainly in HIV infected patient PML should always be considered as a diagnostic possibility.
As name implies, PML is a progressive disorder, the presenting symptoms include altered mental status, speech and visual disturbances, gait difficulty, hemiparesis, and limb incoordination. The clinical condition deteriorates progressively and the patient dies within six months.1-2 In patients with posterior leukoencephalopathy the symptoms develop rapidly, and after treatment the clinical features and imaging abnormalities resolve completely. The characteristic clinical manifestations of posterior leukoencephalopathy include seizures, headache, vomiting, confusional state, visual abnormalities, and infrequently focal motor and sensory neurological deficits. Dr Banerjee has rightly commented that the seizures are infrequent in patients with PML, while in patients with posterior leukoencephalopathy seizures (especially occipital lobe seizures) are dominant and a universal manifestation. Patients with posterior leukoencephalopathy usually have a predisposing cause, the most common being hypertensive encephalopathy, toxaemia of pregnancy, renal diseases, and treatment with cytotoxic and immunosuppressive drugs.
In my opinion PML in patients with an acquired immunodeficiency state can reliably be differentiated from posterior leukoencephalopathy on clinical grounds even if bilateral symmetrical demyelinating white matter abnormalities of parieto-occipital regions are present on neuroimaging.