Slow wave production in the EEG, with reference to hyperpnoea, carbon dioxide and autonomic balance

Normal variants and artifacts: Importance in EEG interpretation

Overinterpretation of EEG is an important contributor to the misdiagnosis of epilepsy. For the EEG to have a high diagnostic value and high specificity, it is critical to recognize waveforms that can be mistaken for abnormal patterns. This article describes artifacts, normal rhythms, and normal patterns that are prone to being misinterpreted as abnormal. Artifacts are potentials generated outside the brain. They are divided into physiologic and extraphysiologic. Physiologic artifacts arise from the body and include EMG, eyes, various movements, EKG, pulse, and sweat. Some physiologic artifacts can be useful for interpretation such as EMG and eye movements. Extraphysiologic artifacts arise from outside the body, and in turn can be divided into the environments (electrodes, equipment, and cellphones) and devices within the body (pacemakers and neurostimulators). Normal rhythms can be divided into awake patterns (alpha rhythm and its variants, mu rhythm, lambda waves, posterior slow waves of youth, HV-induced slowing, photic driving, and photomyogenic response) and sleep patterns (POSTS, vertex waves, spindles, K complexes, sleep-related hypersynchrony, and frontal arousal rhythm). Breach can affect both awake and sleep rhythms. Normal variants or variants of uncertain clinical significance include variants that may have been considered abnormal in the early days of EEG but are now considered normal. These include wicket spikes and wicket rhythms (the most common normal pattern overread as epileptiform), small sharp spikes (aka benign epileptiform transients of sleep), rhythmic midtemporal theta of drowsiness (aka psychomotor variant), Cigánek rhythm (aka midline theta), 6 Hz phantom spike-wave, 14 and 6 Hz positive spikes, subclinical rhythmic epileptiform discharges of adults (SREDA), slow-fused transients, occipital spikes of blindness, and temporal slowing of the elderly. Correctly identifying artifacts and normal patterns can help avoid overinterpretation and misdiagnosis. This is an educational review paper addressing a learning objective of the International League Against Epilepsy (ILAE) curriculum.

Hyperventilation revisited: physiological effects and efficacy on focal seizure activation in the era of video-EEG monitoring

PURPOSE

Hyperventilation is an activation method that provokes physiological slowing of brain rhythms, interictal discharges, and seizures, especially in generalized idiopathic epilepsies. In this study we assessed its effectiveness in inducing focal seizures during video-EEG monitoring.

METHODS

We analyzed the effects of hyperventilation (HV) during video-EEG monitoring (video-EEG) of patients with medically intractable focal epilepsies. We excluded children younger than 10 years, mentally retarded patients, and individuals with frequent seizures.

RESULTS

We analyzed 97 patients; 24 had positive seizure activation (PSA), and 73 had negative seizure activation (NSA). No differences were found between groups regarding sex, age, age at epilepsy onset, duration of epilepsy, frequency of seizures, and etiology. Temporal lobe epilepsies were significantly more activated than frontal lobe epilepsies. Spontaneous and activated seizures did not differ in terms of their clinical characteristics, and the activation did not affect the performance of ictal single-photon emission computed tomography (SPECT).

CONCLUSIONS

HV is a safe and effective method of seizure activation during monitoring. It does not modify any of the characteristics of the seizures and allows the obtaining of valuable ictal SPECTs. This observation is clinically relevant and suggests the effectiveness and the potential of HV in shortening the presurgical evaluation, especially of temporal lobe epilepsy patients, consequently reducing its costs and increasing the number of candidates for epilepsy surgery.

Correlation between cerebral perfusion and hyperventilation enhanced focal spiking activity

Single photon emission computed tomography (SPECT) has frequently been used to investigate cerebral brain perfusion (CBP) occurring ictally and inter-ictally in epileptic patients. Several studies have addressed the multimodal analysis of the modifications occurring in cerebral areas involved in seizure activity, by correlating SPECT with electroencephalografic (EEG) recordings during ictal and inter-ictal epileptiform lateralized discharges (IELDs). Although these studies have yielded interesting results, variations in regional CBP (rCBP) observed during ictal events are difficult to interpret since the areas of altered rCBP might reflect not only events restricted to the epileptogenic focus, but also large fluctuations determined by seizure spread. Inter-ictal rCBP correlates with the area generating the local EEG epileptogenic activity in a limited percentage of studies. Hyperventilation (HPV) represents a well established EEG activation procedure aimed at enhancing epileptiform discharges. Since HPV-enhanced IELDs may help analyze the CBP pathophysiology in inter-ictal epilepsy, in the present study we investigate this specific aspect co-registering EEG with SPECT in subjects affected by partial epilepsy responding to HPV with IELD enhancement without seizure precipitation. This study suggests a correlation between localized increase in rCBP and HPV-induced IELDs and provides a tool to discuss uncommon aspects of the physiology of rCBP during the inter-ictal state in the epileptogenic areas.

Hyperventilation in normal subjects. A clinical, gas-analytic and EMG study

Clinical (paresthesiae and Trousseau's sign), EMG, blood biochemical (lactic acid and total Ca2+) and gas-analytic (pH, PCO2, PO2, HCO3) changes were studied during and after 5 min voluntary hyperventilation (HV) in 15 normal subjects. Paresthesiae and spontaneous motor activity were common manifestations (87% and 67% respectively) in our test. They were significantly related only to changes in pH, PCO2 and PO2, with paresthesiae arising earlier and at milder gas-analytic changes than motor activity. Paresthesiae and motor activity induced by HV cannot be used as reliable indicators of pathology. Their presence simply reflects the degree of the blood gas-analytic changes (pH, PCO2 and PO2) which therefore should be carefully monitored before drawing any conclusion from an HV test.

Effect of low concentration stable xenon on the EEG power spectrum

The effect on the central nervous system of inhaled stable xenon, at concentrations of 25%, 30% and 35%, was assessed by evaluating changes in power spectra computed on the electroencephalogram. Ten normal adult subjects were studied in a protocol designed as a repeated measures experiment. Synchronous changes in the EEG power spectra were observed with stable xenon inhalation. These changes were equivalent for symmetrical electrode pairs, but the time history of the changes differed depending on the cortical region being measure. This suggests regional effects of stable xenon inhalation on the mechanisms producing the EEG.

The standardization of hyperventilation on EEG recording in childhood. I. The quantity of hyperventilation activation

In thirty-seven children free of neurological symptoms, we attempted the standardization of hyperventilation on EEG. We also attempted to determine the quantity of hyperventilation activation necessary to produce equivalent degrees of EEG slowing at different ages. The respiratory rate (RR), total expiratory volume/min (VE), O2 consumption volume/min (VO2), expiratory CO2 volume/min (VCO2), tcpO2 and tcpCO2 were monitored before, during and after hyperventilation. The optimal conditions for adequate activation were found to be: a respiratory rate of 30/min, a 3-fold elevation of VE and a duration of 4 minutes. With this activation, the degree of EEG slowing was found to be nearly inversely proportional to the age (in the age range of 6 years to 17 years old). Therefore, this activation may be adequate and useful for evaluating the EEG development in childhood. As to the relationship between the appearance of EEG slowing and changes in respiratory factors, the pCO2 decrease and the cerebral blood flow decrease, which may be evoked by the pCO2 decrease, are the most fundamental factors that produce EEG slowing during hyperventilation. The difference in the response on hyperventilation between children and adults may be due to age-related CNS sensitivity to CO2 and/or cerebral vascular CO2 responsiveness.

EEG and end-tidal carbon dioxide concentration in the hyperventilation syndrome

The hyperventilation syndrome (HVS) is a functional disorder with repeated involuntary hyperventilation attacks together with symptoms of respiratory alkalosis. We have studied the EEG and end-tibial pCO2 in the resting state and during hyperventilation activation in 12 HVS patients in order to find out whether there is a greater susceptibility to cerebral vasoconstriction in HVS patients than in controls, as indicated by slowing of the EEG. A surprisingly high proportion (58%) of abnormal resting EEGs was found in HVS patients, although the patients were neurologically normal. More theta and beta background activity was usually revealed in a quantitative computer analysis, especially frontally. Although the hyperventilation activation caused the same degree of hypocapnia in HVS patients and in controls, peripheral symptoms like tingling and numbness of fingers, as well as carpopedal spasms, occurred much more often in HVS patients. However, the EEG changes due to hyperventilation were similar in both HVS patients and normal controls, and it thus seems that the reason for cerebral symptoms in HVS patients is not a greater susceptibility to cerebral vasoconstriction.

Birth asphyxia and delta response to over-breathing in non-epileptic children

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