Obtaining high quality visual event-related potentials is crucial for the early detection of abnormal development within the central nervous system and is important for successful early intervention implementation in at-risk newborns. VEPs are objective, non-invasive, and sensitive to structural and functional brain damage. Recording during polysomnography-identified active sleep reduces variability allowing reliable VEPs to be obtained.
The day before the study, make sure that the baby's head is washed with neutral soap so that the hair is clean and dry. 30 minutes before beginning the study, allow the parent to begin feeding the newborn before burping the baby and wrapping the baby in a sheet so that the baby sleeps easily and spontaneously. Before handling the neonate, wash the hands carefully and put on a sanitary mask.
Gently wipe the scalp of the newborn with alcohol to remove any residual dirt and superficial grease before the neonate falls asleep and measure the distance between the nasion and inion and between both preauricular pits. Calculate 10%and 20%to ensure proper placement of the cranial electrodes according to the International 10-20 System of Electrode Placement and cover the newborn's entire head with a tubular elastic mesh for the correct attachment of the EEG and VEP electrodes, leaving the face fully free and exposed. Mark the location of the surface electrodes on the mesh and use a swab to perfectly separate the newborn's hair at the sites at which each electrode will be placed, then lightly rub the skin with abrasive gel for neurophysiological studies.
For electrode placement, place an elastic band sensor on the baby's chest to record the thoracic respiratory expansion and locate individual surface disc electrodes for the EEG through the mesh at leads F3, F4, C3, C4, O1, and O2.To record ocular movements, place one electrode through the mesh one centimeter above the external canthus of the left eye and place another electrode one centimeter below the external canthus of the right eye. Fix the electrodes to the skin with medical adhesive tape and attach the electrodes for surface electromyogram recording on both sides of the chin referenced against each other. Place the ground electrode on the right mastoid and set the one channel of the VEP equipment with the Oz versus Cz leads, then set the analysis time for the VEP registration at 600 milliseconds and wait to begin the VEP recording until the impedance values are below 5 kilo ohms.
With the newborn sleeping in a hospital crib, prolong the EEG recording for 60 to 90 minutes until active sleep is identified. Begin the EEG recording while carefully observing the characteristics of neonatal sleep to identify the active sleep stage during which the VEPs will be recorded, according to the criteria summarized in the table. When the neonate brings well-defined active sleep, allow one minute of EEG recording before positioning handheld goggles with an LED matrix two centimeters directly above each eye to apply monocular light stimulation.
Observe whether the infant's eyes are closed during the VEP registration in active sleep and begin presenting 20 to 40 luminous stimuli. During the recording, check the reproducibility of the recorded averages and identify the PII component as the maximal positive peak between 120 and 300 milliseconds preceded by a negative NII wave and followed by a maximal NIII negativity between 200 and 400 milliseconds. Stop the averaging if the newborn moves excessively, wakes up, or changes to another sleep stage distinct from active sleep.
Finish the registration after two averages with reproducible VEP are attained or when six averages occur without a recognizable VEP. At the end of the analysis, evaluate the reproducibility of the VEPs by similar appearance and measurements between the two averaged curves and use the device cursors to measure the absolute latencies of the NII, PII, and NIII waves. Calculate the inter-peak latencies in milliseconds, including the differences between the absolute PII-NII, NII-NIII, and PII-NIII latencies, then measure the peak-to-peak amplitudes in microvolts for the NII-PII and PII-NIII components and compare the latency in amplitude values obtained to the normal or expected values estimated for a population of healthy, similarly aged newborns.
In a healthy, full-term newborn, a clear positivity around 200 milliseconds compatible with the PII component can be observed. NII, which corresponds to a preceding negative small potential, is evident at about 130 milliseconds. The NIII component follows PII as a negativity of approximately 300 milliseconds.
Here three epochs of sleep EEGs with typical aspects of active sleep, quiet sleep, and trace alternant are shown. In a full-term newborn, typical VEP waveforms demonstrate a clear PII, an immature response observed in pre-term newborns that is normal at this age, and this tracing from a 36-week old preterm newborn with periventricular leukomalacia a non-replicable waveform with waves that do not exactly reproduce the shape of the previous average can be observed, making it impossible to reliably measure the true latency or amplitude of the components. Active sleep must be carefully observed when attempting this procedure.
VEP recording during a state change must be avoided and reproducibility must be achieved in at least two average waveforms. Using the appropriate sensory stimulation to obtain somatosensory and auditory middle-long latency evoked responses can provide a sensitive and non-invasive means for evaluating pathological conditions in other sensory pathways.
