Un liquor particolarmente informativo

A 3-year-old child was admitted to our Emergency Department with neurological multifocal symptoms. Magnetic resonance imaging showed pulvinar and lenticular nucleus involvement. The parents reported an infection from Varicella Zooster Virus (VZV) a few days earlier. After taking lumbar puncture, the patient was hospitalized with a preliminary diagnosis of viral encephalitis. Cerebrospinal fluid (CSF) examination was negative for albumin quotient, quantitative IgG synthesis, isoelectric focusing and Polimerase Chain Reaction (PCR) for Herpes viruses. High CSF intrathecal total IgM fraction (48% r.v. ≤0) was detected as well with highly positive IgM antibody indices specific for VZV. CSF free light chain (FLC) kappa and lambda indices were highly positive: respectively 34.12 (r.v. <6.39) and 17.78 (r.v. <5.51). The diagnosis of VZV encephalitis, was possible only after specific CSF serology and CSF FLC measurement. IgG isoelectric focusing is considered the best screening test to detect immune activation in CNS, however, in this patient we observed mainly an IgM response. Our case indicates that CSF FLC determination is perhaps a better test to screen for an immune activation of the central nervous system, as it becomes positive also in case of IgM intrathecal synthesis.