Evaluation of the distortion of EEG signals caused by a hole in the skull mimicking the fontanel in the skull of human neonates

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Abstract

Objective

Interpretation of Electroencephalography (EEG) signals from newborns is in some cases difficult because the fontanels and open sutures produce inhomogeneity in skull conductivity. We experimentally determined how EEG is influenced by a hole mimicking the anterior fontanel since distortion of EEG signals is important in neurological examinations during the perinatal period.

Methods

Experiments were carried out on 10 anesthetized farm swine. The fontanel was mimicked by a hole (12×12 mm) in the skull. The hole was filled with 3 types of medium differing in conductivity (air, 0 S/m; sucrose–agar, 0.017 S/m; saline–agar, 1.28 S/m). Three positions of the snout were stimulated with a concentric bipolar electrode to activate cortical areas near the middle, the edge, and the outside of the hole. The somatic-evoked potential (SEP) was recorded by a 4×4 electrode array with a 4 mm grid spacing. It was placed on the 4 quadrants of a 28×28 mm measurement area on a saline-soaked filter paper over the skull, which served as artificial scalp.

Results

The SEP over the hole was clearly stronger when the hole was filled with sucrose– or saline–agar as compared to air, although paradoxically the leakage current was stronger for the sucrose– than saline–agar. The current leaking from the hole was strongly related to position of the active tissue. It was nearly negligible for sources 6–10 mm away from the border of the hole. The distortion was different for 3 components of the SEP elicited by each stimulus, probably indicating effects of source distance relative to the hole.

Conclusions

EEG is strongly distorted by the presence of a hole/fontanel with the distortion specifically dependent on both conductivity of the hole and source location.

Significance

The distortion of the EEG is in contrast to the lack of distortion of magnetoencephalography (MEG) signals shown by previous studies. In studying brain development with EEG, the infant's head and sources should be modeled accurately in order to relate the signals to the underlying activity. MEG may be particularly advantageous over EEG for studying brain functions in infants since it is relatively insensitive to skull defects.

Introduction

Brain functions of human infants, including pre-term and term babies, have been traditionally evaluated with EEG as part of a neurological examination. The evaluation is useful in infants with risk factors such as complications during labor and delivery (Sunshine, 1997). EEG is also useful for staging the development of the nervous system, since differences in the functional organization of certain brain regions are already present in newborns (Eiselt et al., 2001), and for differential diagnosis of seizures from non-seizures in the paroxysmal motor behavior (Tharp, 1997, De Weerd, 1995). However, neonatal EEG monitoring is still in need of development, particularly in relating EEG signals to the underlying brain functions.

Interpretation of the underlying physiology and pathophysiology is complicated by the presence of the fontanels and sutures in the skull. The anterior and posterior fontanels are present at the bregma and the lambda. The skull is not present within the fontanels. The brain below the fontanels is protected by the dura, which is thicker within the opening. The fontanels and sutures are squeezed together during delivery, but become larger during the first months and then eventually close. The anterior fontanel may be large enough to admit an adult's thumb. The posterior fontanel is closed within the first 2 month. The anterior fontanel stays open as long as 16 months or, in abnormal cases, even for several years. The sutures can be quite wide near the fontanels. The mean width of the coronal and lambdoidal sutures at their midpositions is 3–4 mm for infants between 0 and 60 days after birth (Eramie and Ringertz, 1976). The sutures may stay unfused for several years (Hansman, 1966). EEG signals may be profoundly affected by the fontanels and sutures, which are effectively skull defects, since they represent areas of high conductivity relative to the skull. Volume currents might be affected by these openings in the skull. Thus, it is necessary to evaluate the effect of these skull openings on EEG in order to accurately infer physiological functions in human infants.

This report describes distortions of the somatosensory-evoked potentials (SEPs) by a hole in the skull of neonatal pigs. Although the EEG signal distortion by skull defects can be studied using mathematical models of the head as discussed later, we chose to study the problem in an experimental preparation since such study is rare and empirical assessment always provides a solid foundation for future modeling studies. Our results may be useful for interpreting distortions of EEG signals in humans arising from skull defects like the fontanels and sutures in human infants, since the head of the piglet is large and the scalp and skull are comparable to those of human infants in thickness. The neonatal farm swine of 1–3 weeks of age has a skull thickness of 2–3 mm near the vertex, similar to the thickness in human newborns (Hansman, 1966). Unlike human neonates, the piglets do not have a fontanel and the sutures are nearly closed at birth. A square hole was, therefore, created in the skull to mimic a fontanel. The SEP distortion was evaluated as a function of distance of active tissue from the hole and for 3 different conductivities of the hole.

Section snippets

Preparation

The protocol used in this study was approved by the Animal Use Committees of the Albuquerque Veterans Affairs Medical Center and the University of New Mexico School of Medicine. Animals were treated according to the National Institute of Health ‘Guide for the Care and Use of Laboratory Animals, Revised 1996’.

Experiments were carried out on 10 farm swine (Sus scrofa). The weight ranged between 8 and 16 kg and the age between 4 and 6 weeks. Pigs were chosen because of their large gyrencephalic

Experiment 1

The circle in the inset of Fig. 1 shows the estimated source location in the rostrum region (shaded area delineated by the circular coronal sulcus) of the SI cortex activated by the snout stimulation. This area was within the square 12×12 mm hole created in the skull. The SEP measurements were carried out within the dashed square area. The SEP over the hole was larger in the sucrose1–agar condition compared to the air1 condition in which the hole was filled with a non-conducting medium (namely

Discussion

The above results clearly demonstrate that a hole in the skull of the neonatal piglet may produce a strong distortion of EEG signals. The leakage of current through the hole and the surrounding skull was clearly affected by the conductivity of the hole and distance of the active tissue producing the SEP. The distortion was different for the different components of the SEP.

In the case of a hole filled with air, the volume current from an active tissue below the hole cannot flow through the air

Acknowledgements

This work was supported by the German Academic Exchange Service (DAAD), the European Community (IST 1999-10378), and by the NIH Grant (R01-NS30968). Special thanks go to Yoshio Okada for the great opportunity to perform this scientific work at his laboratory.

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