Epilepsy Fellow Washington University in St. Louis St. Louis, Missouri
Rationale:
To report a rare finding of EEG voltage attenuation due to calvarial thickening in osteopetrosis.
Studies on the effect of physiologic variations in skull thickness on EEG amplitude have shown variable results. However, literature on pathological skull thickness is limited with only one prior report of osteopetrosis causing low voltage.
Methods:
Forty-nine-year-old woman with history of osteopetrosis and epilepsy underwent an EEG during an admission for evaluation of seizures in settings of recurrent osteomyelitis. During fully awake state, EEG background showed all voltages of 12-14 microvolts. Head CT showed severe calvarial thickening and bilateral extradural hyperdensities consistent with extramedullary hematopoietic tissue.
Results:
Literature is limited on low voltage EEG in fully awake adults. Studies have shown an association with Huntington’s chorea, Creutzfeldt-Jakob disease and hypothyroidism. Heritable low voltage EEG trait has been localized to the distal part of chromosome 20q; which in turn has been shown to have an association with alcohol use disorder and anxiety disorder. There have been varying results of studies assessing effect of skull thickness on EEG voltage with the most recent study showing no association between physiologic variations in skull thickness within an individual to account for variation in amplitudes between anatomical regions. Our report shows that significant pathological increase in skull thickness can affect the EEG amplitude. Alternatively, the presence of bilateral extramedullary hematopoietic tissue in this condition may be adding another layer, which is contributing to the diffuse attenuation.
We hypothesize that EEG amplitude depends not only on the strength of the cortical current, but also on the conductive properties of the volume between the cortex and scalp surface. A greater electrical resistance due to the thickness of the layers likely results in reduced EEG amplitudes.
Conclusions:
With this rare case of EEG amplitude attenuation due to pathological increase in skull thickness in osteopetrosis, we hypothesize that the degree of skull thickness may determine EEG amplitude.
