Rasmussen's encephalitis (RE) is an exceedingly rare progressive neurologic disorder, first described in 1958 by Rasmussen et al. [1]. Clinically, it presents with intractable, pharmacoresistent focal seizures, hemiparesis, and cognitive decline. Studies from Germany have estimated its incidence to be 2.4 cases/10 million individuals under 18 years of age [2]. In one series of 115 surgically treated epilepsy patients, RE represented 21 cases in a mostly young pediatric cohort (18.3 %) [3]. RE is a pediatric disorder with a mean age of presentation between 6 and 8 years; however, adult-onset cases have also been reported [[4], [5], [6], [7], [8]]. The etiology of RE is still unknown, with genetic factors, viral infections, and autoimmunity all possibly playing a role in its pathogenesis [4,[9], [10], [11], [12], [13], [14], [15]].

The histologic findings of RE are well described and are staged according to the study by Pardo et al. [16] In short, Pardo stage (PS) 1 is defined by discrete foci of inflammatory cells without significant evidence of cortical or neuronal injury, while PS 2 exhibits increased inflammatory activity and evidence of neuronal injury. Extensive neuronal damage defines PS 3, while PS 4 is characterized by complete cortical destruction with vacuolation and scant residual inflammatory infiltrate.

Some patients with RE can have a second disease process that can also independently explain the seizure clinical picture. This phenomenon is termed “dual” or “double” pathology and has been well described in the literature. Reported second pathologies include ganglioglioma, cavernous hemangioma and FCD [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]] Although reported, the coexistence of RE and FCD has not been previously studied in a large cohort, mainly due to the rarity of RE. This study examines how often these two pathologies may be seen together. We provide a rationale for the observed co-occurrence of RE and FCD in a number of cases, while acknowledging technical challenges in histologic assessment of either diagnosis. As entirely submitting the brain resection specimens from RE patients is not always performed, representative sampling leaves room for undocumented findings - both RE and FCD are not grossly apparent. Interobserver variability in the diagnosis and typing of FCD is also an important caveat (see below). Nonetheless, both RE and FCD are causes of pharmacoresistent epilepsy and their surgical treatment prompts histologic investigation of their co-occurrence.