Despite its rarity, this is an interesting topic to cover for a couple of reasons. First, in one of my health system administrative roles, I’ve noticed a large uptick in evaluation for this entity. And, then, second, it was picked up by the NEJM Emergency Medicine Journal Watch. So, paradoxically, the purpose here is both to heighten awareness of this disease process while at the same time throwing cold water on it.
The hullabaloo is all about autoimmune encephalitis – and the subset involved in this study concerns the typical onset after herpes simplex encephalitis. A non-trivial number of patients recover from an initial HSV encephalitis only to have recurrence of seizures and/or cognitive decline. Some of these cases can be attributed to reactivation of latent virus, while the others were of uncertain etiology. It now seems clear there is a significant autoimmune link, with increasing reports of antibodies against synaptic receptors detected in CSF.
In part of this very small, prospective study out of Spain, they identified 51 index cases of HSV encephalitis. None of these patients had neuronal antibiodies present at the index presentation. However, within one year follow-up, a quarter developed new onset encephalitis with neuronal antibodies detectable in the CSF, primarily NMDA-receptor type. The typical presenting features of these patients differed between young children and adults, but most notably included seizures, behavior changes, and psychosis.
Recognition of this disorder is important because patients can have profound improvement with treatment and immunosuppression. That said, this is still an uncommon complication of a fairly rare disease – it took these authors 3 years to prospectively enroll their 51 patients, and even their retrospective case review across 6 years only managed to pick up 48 patients. These antibody responses are extremely rare, and recency bias should not dramatically change your practice. However, in the context of new-onset psychosis, an LP may be reasonable – and, if the remainder of the clinical evaluation supports it, antibody testing could dramatically alter treatment in a small cohort of patients.
“Frequency, symptoms, risk factors, and outcomes of autoimmune encephalitis after herpes simplex encephalitis: a prospective observational study and retrospective analysis”
https://www.thelancet.com/journals/laneur/article/PIIS1474-4422(18)30244-8/fulltext