Abstracts

Rationale: Among malformations of cortical development (MCD), polymicrogyria is estimated to be one of the most common brain malformations, accounting for approximately 16% of MCDs. However, the prevalence and incidence of polymicrogyria (PMG) in population-based cohorts are insufficiently studied. PMG is a highly epileptogenic malformation and can be caused by genetic and environmental factors.

Methods: We performed a systematic search of MRI scans at two pediatric neuroradiology department databases in Stockholm using the keyword polymicrogyria. The study population included all children living in the Stockholm region born January 2004 to October 2021 with PMG. Information on the number of children living in the region during 2004-2021 was collected from publicly available records from Statistics Sweden. The number of births for each year during the study period was collected from the Medical Birth Register. All MRI scans were re-evaluated, and malformations were classified by the same pediatric neuroradiologist. The prevalence and yearly incidence were estimated. Clinical data on PMG etiology and epilepsy were collected from medical records. The study was approved by the Regional Ethics Committee Stockholm.

Results: A total of 109 individuals with PMG were included in the study, whereof 67 (61%) were males. The overall PMG prevalence in Stockholm was 2.3 per 10,000 children. The overall estimated yearly incidence of PMG between 2004 and 2020 was 1.85 per 10000 person-years. The most common PMG distribution was in the frontal lobe (67%), followed by the parietal lobe (39%). PMG in the perisylvian region was observed in 54 individuals (50%). Genetic testing was performed in 89 patients revealing pathogenic or likely pathogenic variants in 37%. Additionally, 13% had genetic findings with unclear clinical significance. Five patients had a confirmed congenital infection, and in two individuals the PMG cause was assumed to be vascular. Epilepsy was diagnosed in 54%. Seizure onset during the first year of life was observed in 44%. The median age at first seizure was 19 months. We could not observe any difference in age of seizure onset in unilateral, bilateral, or perisylvian PMG. The most common seizure type was focal seizures with impaired awareness, followed by epileptic spasms and focal to bilateral tonic-clonic seizures. Epilepsy in relation to lobar distribution will be compared. Thirty-three out of 59 patients with epilepsy (56%) were treated with more than two antiseizure medications, indicating that therapy-resistant epilepsy is common in PMG patients.

Conclusions: This is, to our knowledge, the first population-based study on PMG prevalence and incidence. Confirmed genetic etiology was present in one-third of children with PMG. Epilepsy occurred in 54% and the majority had pharmacoresistant epilepsy. Our findings increase knowledge of PMG and will help in counseling patients and their families.

Funding: Swedish Research Council, Stockholm County Council, Karolinska Institutet; Frimurarna Barnahuset Foundation; Linnea and Josef Foundation, Margarethahemmet Foundation; Sällskapet Barnavård