Objective:

To highlight a series of atypical Powassan encephalitis cases presenting as rhombencephalitis occurring within close chronologic proximity to each other.

Background:

Powassan is a tick-borne flavivirus with infections ranging from asymptomatic to severe neurologic disease characterized by encephalitis or meningitis. The typical imaging pattern seen in patients with Powassan encephalitis includes cerebellar lesions, however there have been few reports of rhombencephalitis in the past. The reported incidence of Powassan infections has increased over the past two decades, although it remains relatively rare with 49 cases (45 neuroinvasive) reported to the CDC in 2023. 

Design/Methods:

We describe three cases of rhombencephalitis presenting within nine days of each other in November to December 2023 at a tertiary care center in Maine. 

Results:

All three patients presented with an infectious prodrome followed by neurologic worsening. MRI findings similarly demonstrated inflammatory changes in the brainstem and cerebellum, which were consistent with rhombencephalitis. Although confirmatory CSF testing for Powassan virus was positive in two patients and negative in one patient, Powassan infection was highly suspected in the negative patient given the similar presentation and known tick bite. They received empirical treatment with 5 days of IVIG. Their length of hospital stay ranged from 43-89 days with the number of ICU days ranging from 7-56 and ventilatory support days ranging from 6-18. Two patients needed CSF diversion during this hospitalization and two needed gastrostomy tube placement. MRS on discharge ranged from 4-5. Overall outcome of these patients had varying degrees of residual neurologic deficits. 

Conclusions:

There should be a higher suspicion of neuroinvasive Powassan disease in patients presenting with rhombencephalitis and who are in or with recent travel to endemic areas with deer ticks.