Progressive multifocal leucoencephalopathy (PML) has been described as a complication in various conditions which result in impaired cellular immunity—these include lymphoproliferative disorders and chronic granulomatous disorders such as sarcoidosis. Iatrogenic immunosuppression in post-transplant patients and patients undergoing cancer chemotherapy, as well as those with autoimmune disorders, is also a risk factor. The occasional case has been described in pregnant women which some may regard as being an immunosuppresssed state.1 A review in 1984 of 230 patients found that 69% were due to lymphoproliferative or myeloproliferative disorders, 7% granulomatous disorders, 6% postrenal transplant, and 4% occurred in patients with the acquired immunodeficiency syndrome (AIDS). About 6% of all patients with PML had no identifiable evidence of immunosuppression. These were all anecdotal reports from the early 1970s.2

Since the onset of the AIDS epidemic in 1981, the incidence of PML has increased significantly and now human immunodeficiency virus (HIV) associated cases account for up to 85% of all cases of PML. Before the introduction of highly active antiretroviral therapy (HAART), the estimated incidence in patients infected with HIV was 4%.

HAART is the term used for a combination of three or four anti-HIV drugs from the following classes—nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transciptase inhibitors, and protease inhibitors. The widespread availability and uptake, at least in the developed world, of HAART has had a dramatic impact on morbidity and mortality of patients infected with HIV. This is, in part, due to reduction in incidence of opportunistic infections. In the period 1995 to 1997, one United States study documented a fall in the combined incidence of Pneumocystis carinii pneumonia, Mycobacterium avium complex, and cytomegalovirus infection from 21.9 per 100 person-years to 3.7 per 100 person-years.3 In one London hospital, the number of cases of toxoxplasmosis fell from 19 cases in …