Activation of seizures by HV was first reported in 1924, even before the discovery of the EEG. This technique became widely used in the diagnosis of absence seizures. HV responses can vary widely depending on age of patient and the amount of individual effort put forth. Common HV
From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ
responses in adults include no EEG change or mild slowing temporally, frontally, or diffusely. In younger individuals, in contrast, responses can be particularly dramatic, with extremely high voltage synchronous delta waves in bursts or runs. This can be further exaggerated if it has been many hours since the person has eaten a meal (relative hypoglycemia) (Fig. 1). Unless there are definitive spikes embedded within the synchronous delta activity or the record fails to return to baseline within 1 to 2 min of verified completion of over-breathing, the response should be interpreted as negative, because there is large variation in the normal response. Consistent focal features, whether epileptiform or not, are interpreted as abnormal.
With HV, there is a rise in PaO2 and a drop in PaCO2. To compensate for the resultant hypocapnia, the cerebral blood vessels constrict. The mechanism of the EEG response to HV is not yet understood, but several theories are reviewed by Takahashi. These include inadequate compensatory vasoconstriction, cerebral hypoperfusion as a result of vasoconstriction, increased neuronal excitability from respiratory alkalosis, synchronous activity of the thalamocortical projections that are enhanced by hypocapnia, and decreased activity of the mesencephalic reticular formation. The more dramatic changes noted in children suggest that immature autoregulation may explain the HV response, or that HV slowing is independent of cerebral blood flow. Whatever the true mechanism, HV is an accepted, standard technique for activation of the EEG.
When there are no epileptiform discharges uncovered during a routine EEG, the HV procedure is explained to the patient. Those with severe cardiac or pulmonary disease, uncontrolled hypertension, or a recent vascular event, such as myocardial infarction, stroke, or transient ischemic attack, should not be exposed to this procedure, because hypocapnia and alkalosis may cause vasospasm or decrease cerebral perfusion. To begin the procedure, the technologist instructs the patient to breathe deeply and rapidly for 3 min. Patients are usually lying flat during a routine EEG, although it has been noted that the effects are enhanced by an upright posture, possibly because of the relative cerebral hypoperfusion. Patients should be told that they might experience symptoms of lightheadedness and tingling, particularly around the mouth and fingertips, although they can be reassured that this is reversible. As mentioned, the time since the last meal should be documented, because low glucose may enhance the response. The EEG is usually recorded on a bipolar montage with the patient's eyes closed, and should be run on the same montage, at least 1 min before starting the hyperventilating and continued for up to 3 min afterward, documenting the effort that the patient puts forth.
Syncope may lead to prolonged post-HV slowing, and patients with the rare vascular disease known as Moyamoya can have a delayed "re-buildup" referring to high-voltage diffuse slowing, which can occur even after HV is completed. Longer duration of HV (up to 6 min)
was shown by Adams and Luders to have a higher yield than 6 h of continuous EEG monitoring, justifying continuation of this technique when no epileptiform discharges are noted at 3 min, particularly in patients with absence seizures. Occasionally, patients become mildly confused and have difficulty discontinuing the procedure and may require gentle reminders to stop. If this is ineffective, rebreathing CO2 with a paper bag or oxygen mask placed over the mouth can be useful. For the pediatric population, it is helpful to have children blow at a pinwheel, although for the very youngest, HV may be captured during the all too common periods of sobbing in the EEG laboratory. HV should be discontinued if repetitive discharges or seizures are elicited.
HV can enhance subtle but significant abnormalities, rendering insignificant ones unchanged or diminished. Additionally, it can be used as an induction technique for nonepileptic seizures. Previous studies have shown that, in patients with partial seizures and a normal baseline EEG, focal interictal discharges were elicited in 6 to 9% of patients using HV, whereas, in children with absence epilepsy, generalized spike and wave activity appeared in 80% of cases. A recent study by Holmes et al. however, suggested that even in known epilepsy patients, seizures were elicited by HV in less than 0.5%.
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