Takeaway
An atypical phenotype of fatal familial insomnia (FFI) is likely caused by a different strain of prion from typical cases; the atypical FFI prion may also be present in people with sporadic familial insomnia (sFI).
Why this matters
FFI is a rare prion disease typically exhibiting severe neuronal loss in thalamic and olivary regions; however, atypical presentations of FFI occur marked by spongiform encephalopathy of the cortex.
Understanding the genetic and pathological differences between the two forms of FFI may lead to improvements in treatment and clinical outcomes.