Progressive multifocal leukoencephalopathy as the presenting feature in a patient with occult low-count monoclonal B-cell lymphocytosis.

Introduction: Low Count Monoclonal B-Cell Lymphocytosis (LC-MBL) is a relatively poorly understood entity which has been suggested to be very common in asymptomatic adults and possibly related to infectious complications despite not progressing to CLL.

Methods: We describe the first case of Progressive Multifocal Leukoencephalopathy (PML) presenting in a 72-year-old man with LC-MBL but no other immunocompromising conditions.

Results: A diagnosis of PML was confirmed with classic MRI findings in association with a high CSF John Cunningham polyomavirus (JCV) viral load (4.09' 10⁵ copies/mL). An extensive search for underlying immunocompromising conditions only demonstrated LC-MBL representing approximately 4% of total leukocytes (0.2' 10⁹/L).

Discussion: This is the first report of PML in association with LC-MBL. Careful review of peripheral blood flow cytometry results is necessary to identify this disorder. Further study of the epidemiology and infectious complications of LC-MBL are warranted.