Preventable and unpreventable causes of childhood-onset epilepsy plus mental retardation.

Objective: The objective of this study was to determine the causes of childhood epilepsy associated with mental retardation and determine whether these causes are preventable.

Methods: We selected all patients from the Nova Scotia population-based childhood epilepsy cohort (n = 692) who had mental retardation and had epilepsy onset between 1977 and 1985. Causes and family history were determined by chart review and caregiver interview after 18.8 (SD: +/-7) years of follow-up.

Results: Overall, 147 patients had mental retardation and epilepsy (21% of all childhood epilepsy). Standard psychological testing was available for 57%; 38.5% were too impaired for testing, which left 4% with the degree of mental retardation assessed clinically. Severe/profound mental retardation predominated (mild: 24%; moderate: 23%, severe/profound: 53%). Fifty-nine percent had additional severe neurologic deficits, most often associated with severe mental retardation. Epilepsy syndromes were symptomatic generalized (n = 73), partial (n = 58), and other (n = 16). Most had a brain imaging study: 91% had a computed tomography scan, and 12% had an MRI scan. Sixty-three percent had a defined cause; 37% had an unknown cause. A defined cause was more likely in those with severe mental retardation (60 of 78 vs 31 of 65). Identified causes were prenatal or genetic (65%), perinatal (8%), or complications of prematurity (13%). Only 11 (7%) had an acquired cause that was potentially preventable. Many (36%) had a first- or second-degree relative with epilepsy, more often in those without a clear cause (54% vs 30%) and without additional neurologic disability (57% vs 26%).

Conclusions: Approximately 20% of children with epilepsy have mental retardation. The cause is prenatal or genetic in nearly two thirds, and only 7% have an acquired, preventable cause. Important genetic influences may be present, especially in the absence of a defined cause.