Preterm EEG: A Multimodal Neurophysiological Protocol

Electroencephalography or EEG is a technique that measures electrical activity in the brain. Neonatal EEG is always measured non-invasively by using individual electrodes attached to the scalp or by using an EEG cap. EEG is nowadays widely used in neonatal units for assessment and monitoring of brain function in preterm and term babies.

It is most commonly used in the diagnosis of epileptic seizures and in the assessment of brain maturity and recovery from hypoxic ischemic events. Over the past few years, practices of EEG recording and the understanding of preterm EEG signals have dramatically improved. The aim of this presentation is to introduce the theory and practice of advanced EEG recording techniques that are available for neonatal units.

Bedside experience in neonatal units around the world has shown that use of EEG may significantly improve the neurological care of sick, preterm, and full term babies. At the end of the 1990s, a research network based in Helsinki started a full revision of neonatal EEG concepts and practice. As a result of this work, several new ideas were introduced to combine all the three domains of EEG.Together.

These give an EEG technique that is capable of a thorough assessment of brain functioning. The frequency domain was expanded to a full band, EEG, and the spatial domain was increased with the introduction of neonatal multi-channel EEG caps. In addition, a novel system domain was added by introducing preterm sensory testing during EEG recording.

This recording setting is compatible with a novel analysis framework that connects preterm EEG activity to the development of brain connections. The EEG records the brain's electric activity with electrodes attached to the scalp. An EEG signal is generated mainly by activity in the cortical parametal neurons.

They react to incoming nerve signals and generate fluctuations in electric fields that can be seen in the EEG after amplification. The number and location of electrodes may vary from a few to over a hundred as needed. Clinical neonatal studies are historically performed with four to 10 electrodes.

Locations are always defined according to international nomenclature that specifies the scalp areas based on a combination of letters and numbers. In neonates, polygraph channels are typically added to record respiration, eye movements, chin muscle tone, as well as cardiac activity. EEG signal is conventionally strongly filtered to exclude the slowest and fastest activities.

While this is successful in excluding many technical artifacts and extra cerebral bio signals, it can also result in severe distortion of brain signals. A comparison of the same preterm EEG signals with and without filtering shows that filters may severely distort the appearance of the preterm EEG. The term full band, EEG, or F-B-E-E-G refers to a technique that is capable of recording all frequencies without distortion.

This is important for detecting the slowest events such as the so-called spontaneous activity, transient or SATs in the preterm EEG. The multimodal EEG study includes simultaneous testing of sensory system. This may be very useful in clinical assessment because brain lesions of preterm babies are often limited to subcortical structures and may hence only affect sensory pathways.

The dense array EEG is a recording system where the number of electrodes is much higher than the conventional two to 10 channel. Setting application of 20 to 60 electrodes is doable even in the intensive care unit. By using EEG caps designed for neonates, each of the over a hundred electrodes will bring significant new information from the neonatal brain.

Cortical reactions to sensory stimuli in the preterm brain are very different from those seen at any later ages. For instance, in preemies, the amplitude of a soma to sensory response to tactile stimulus may be hundreds of times larger and the duration many times longer than a response seen in older children. Preterm sensory responses are therefore visible at single response level, making them readily measurable during an EEG recording.

Since a preterm cortex easily, it may need several seconds to recover between responses, whereas the brain of older children can respond more than 10 times per second. All these differences are due to the fundamentally unique neural network mechanisms that underlie ongoing activity and sensory responses in the preterm brain, understanding preterm responses makes sensory testing clinically useful During a preterm EEG recording, the SOMA to sensory evoked response or SE arm can be triggered by applying a sensory stimulus to practically any part of the body. The stimulus can be tactile, electric, or even proprioceptive, such as that evoked by wrist or ankle flexion.

A convenient way to deliver SER stimulus during EEG recording is by touching the palm or foot, so a touch or proprioceptive stimulus generates a signal that travels along the peripheral nerve to the spinal cord, jumps onto a second nerve ascending to the thalami on the opposite contralateral side of the body. Finally, the signal travels along the thalamocortical nerve to the brain cortex where it elicits an electrical field that is recorded by the EEG over central parietal areas. The visual evokes response or VE arm can be easily triggered with a flashing light that stimulates the retina signals from the retina will travel along the optic nerves into thalami on both sides.

The signal then jumps further to thal cortical nerves and will be transmitted to the visual cortex. The resulting electric fields are recorded by TAL EEG electrodes during the time period from early preterm to term age. Major structural developments take place and are reflected in the appearance of EEG signals.

The cortical surface of an extremely small preterm baby is almost flat and has a thick underlying subplate layer. The first thalamocortical and cortico cortical connections grow into a subplate and begin to invade The cortical plate. During subsequent Weeks, the cortex will assume a more rated surface and the subplate becomes thinner.

The lamo, cortical and cortico cortical connections are now approaching their cortical targets. Near Term age, a baby's cortex is heavily gyrated and the subplate zone has practically disappeared. The lamo, cortical, and cortico cortical connections have reached their final target layers, and they are ready to start processing sensory data from both the outside world and from within The body.

The stage of development In the cortico thalamic connections define how the preterm cortex and subplate will react to sensory stimuli. This animation shows a comparison of the scalp recorded response and its corresponding depth profile within the cortex. In the youngest preemies, a thalamic impulse mainly results in a slow and large response in the subplate layer, which is seen as a high amplitude slow response in the surface electrodes, some weeks later, the same thalamic signal would trigger a somewhat smaller subplate response.

In addition, there is an increasingly clear rapid component from the deep cortical layers, which is also seen in the scalp electrodes near term age. The subplate responses are no longer visible. The main sensory reaction comes from the final thalamocortical destination.

A layer of four neurons resulting in a response that already comes close to what is seen in the adult cortex. Recording the preterm EEG is challenging both practically and technically. EEG recordings need to comply with various nursing procedures.

Hence, the EEG recording may need to be discontinued or halted at any given moment. In addition, the intensive care unit is a challenging environment for the sensitive EEG devices. Babies are always physically connected to devices, wires, and lines that may cause significant electrical interference to the EEG recordings.

The first thing to do at bedside is to minimize potential sources of technical artifacts. One should move loose vital monitor wires and IV lines away from baby's head, EEG wires and the amplifier. Also, make sure that baby's head is not touching any wet textiles, which may readily transmit electrical interference from the cot to the baby.

Before placing the EEG cap, it is preferable to remove interfering wax and oil from the baby's scalp by swiping it with a cloth wetted with diluted alcohol or baby shampoo. EEG cap placement is easier with two persons, one, taking care of the cap and the other holding the head and possible intubation or nasal CPAP tubes. If the patient has A-C-P-A-P, it needs to be removed for the 10 to 20 seconds that it takes to place the EEG cap.

The CPAP straps can be wrapped over the EEG cap if needed. Before fastening the chin strap. Make sure that you have positioned The cap symmetrically over the head.

Electrode gel is applied through the electrode holes to establish electric contacts to the scalp. A bendit tip may be useful for geling contacts that are difficult to access directly. It is important to have enough gel to fill the whole gel cup if too much gel is injected.

However, it'll leak out and may form bridges with neighboring electrodes or even wet the pillow causing artifacts. A more solid gel formula may be practical. In this regard, the skin surface may often need additional mechanical preparation after gel application, this step removes outer epithelial layers to decrease the amount of electrical and movement artifacts.

This may be done by a gentle mechanical rubbing or by using the so-called Sure prep method. The quality of skin contact can be assessed in real time from the impedance values given by the EEG software. For practical reasons, it may often be easiest to complete the gel application and skin preparation for a whole hemisphere at a time when the impedance levels are acceptable.

As shown by the green dots in this EEG software, it is time to switch to the EEG recording and observe the EEG signal quality. The last step is to apply polygraph channels. Muscle tone is usually measured with two EMG electrodes attached under the chin.

The heart is recorded with ECG electrodes attached over the chest Or on the shoulders. Eye movements may be recorded Either with frontal EEG or with separate eye electrodes or EOGs attached near the lateral corners of the eyes. Respiration can be monitored with a stretch sensitive Belt wrapped around the trunk.

This needs to be Tight enough to follow respiratory movements and yet loose enough to allow unrestricted respiration in babies with spontaneous breathing. Special care is needed not to restrain breathing movements. It is also possible to add a movement sensitive piazzo sensor to any body part that might show suspicious movements.

The last step is to position an EEG synchronized video camera to see the whole baby well in the screen. Once the baby is fully set up, the EEG recording can start during the recording. It is important to keep an eye on the EEG signal quality and make corrections if any of the electrodes give poor signals.

Annotation should be written to file about any events that may be considered clinically Important. Recording length is Adjusted individually so that EEG will include all the vigilant states awake, active sleep, and quiet sleep. This means typically a recording time of 40 to 90 minutes.

It is common to all testing of preterm sensory reactivity that a sensory response in preterm cortex declines rapidly if repeated to frequently. Likewise, an ongoing EEG activity near sensory cortex may also block or damp sensory responses. Hence, sensory stimuli shown in this video should be given at moments when the EEG has been relatively silent for at least a few seconds.

Therefore, quiet sleep is an ideal period for testing visual responses. Visual evoked responses can be studied by giving single flashes. Since the eyes are very sensitive, the flash can be given even from a distance through the transparent incubator wall, and even while the eyes are shut.

Somato, sensory evoked responses can be studied by giving a tactile stimulus to the palm or sole of the baby. Alternatively, one may simply flex the wrist or ankle to evoke proprioceptive stimulus. It is helpful to use a device that generates a stimulus locked mark or a trigger into the EEG trace.

Otherwise, it is necessary to add manual annotations depicting the timing of stimuli. Auditory reactivity is easy to test with almost any sound. A nice cortical reaction can be generated, for instance, by hand clapping.

The traditional horn may also be useful, but the sound is often so loud that it may both activate the auditory system and lead to an arousal reaction. At the end of EEG recording session, the cap and other sensors are removed and a baby's Scalp is cleaned. The EEG cap and all the other sensors are mechanically cleaned and sterilized according to hospital instructions.

A thorough Review of the EEG is done offline in a workstation where EEG findings can be processed and described in more detail. A good understanding of the relationships between developing brain function and structure makes preterm EEG study a unique experience giving the possibility of assessing brain maturation at a time when baby cannot yet communicate with the outside world. Consequently, better brain care is likely to be of benefit to the preterm baby for its entire lifetime.