The double doughnut sign is a radiological sign described in MRI of viral encephalitis, typically due to dengue encephalitis.
The double doughnut sign refers to a pattern of signal changes in the bilateral thalami that have a doughnut-like appearance 1-4. This sign may be appreciated on either T2/FLAIR, diffusion-weighted imaging (DWI) or susceptibility weighted imaging (SWI), with bilateral round thalamic hyperintensities, each with a low signal core, that give the appearance of two (i.e. double) doughnuts 1-4. This appearance is thought to be due to bilateral thalamic hemorrhages occurring within surrounding edema and diffusion restriction 1-4.
This sign has been typically described, albeit rarely, in dengue encephalitis 1-4. However, there are case reports of its description in other flavivirus encephalitides (e.g. Japanese encephalitis) 1-4.
Differential diagnosis
trilaminar sign: three-layered appearance that may be seen in acute necrotizing encephalopathy (of childhood) (ANEC)