Hyperventilation revisited: Physiological effects and efficacy on focal seizure activation in the era of video-EEG monitoring
Guaranha MSB, Garzon E, Buchpiguel CA, Tazima S, Yacubian EMT and Sakamoto AC;
Commented by , 18 Feb 2005
Background
Hyperventilation (HV) is widely used as a diagnostic procedure to activate epileptiform EEG discharges, especially in patients with generalized epilepsies (ref. 1; ref. 2). Its potential value as a seizure activator during video-EEG monitoring has been little studied.
Aim
To assess the effectiveness of HV as a seizure-provoking procedure during video-EEG monitoring in epilepsy surgery candidates.
Methods
Retrospective analysis of all patients with refractory partial epilepsy who underwent video-EEG monitoring (average duration, 3 days) over a 13-year period. Excluded: patients with frequent seizures, mental retardation, age < 10 years.
AEDs were gradual discontinued. HV (> 20 breaths/min for 5 min) was repeated every 3 hours (except between midnight and 6:00 AM.) and until seizures were recorded.
Seizure activation was defined as a clinical seizure associated with ictal EEG changes occurring during HV or within 5 min of HV completion.
Results
Of 97 eligible patients, 24 showed seizure activation during HV, mostly in the first 4 min. Both spontaneous and activated seizures occurred in 21 patients and, with two possible exceptions, did not differ in their clinical, EEG or SPECT characteristics.
Activation occurred in 30% of 63 patients with temporal lobe epilepsy and 5% of those with frontal lobe epilepsy. No other predictors of susceptibility to activation were identified.
Professor Perucca's comments
The ability of HV to activate seizures in focal epilepsies is probably underestimated (ref. 3; ref. 4).
This study, the largest conducted in the presurgical setting, suggests that HV is safe and effective in activating seizures. The mechanims of activation are not well understood, but may include reflexes triggered by mechanical stimulation of the olfactory epitelium (ref. 5) and/or systemic metabolic changes (ref. 6; ref. 7)
Given the cost of presurgical video-EEG monitoring, any procedure facilitating the occurrence of seizures is welcome. The authors suggest that repeated HV tests should be routine during presurgical video-EEG monitoring, particularly in patients with temporal lobe epilepsy who represent a large fraction of epilepsy surgery candidates.
These findings are a welcome reminder that advances in science and technology may not reduce the value of simple and time-honored procedures. This study has important practical implications, but interpretation should take into account some methodological shortcomings:
- The design was retrospective, which could involve selection and assessment bias
- It is unclear how exclusion of some patients (particularly those with a history of "frequent" spontaneous seizures, not otherwise defined) impacted on the results
- The relation between susceptibility to activation and location of the epileptogenic zone was not well characterized, due to small size of subgroups
- Only 8 patients with activated seizures had SPECT data, prompting caution in interpreting the claim that SPECT findings are unaffected by HV
Most important of all, the study did not address the most important question, i.e. whether repeated HV tests shorten the duration of video-EEG monitoring, or affect any clinically relevant diagnostic or outcome measure.
This issue is most important, because to my knowledge repeated HV tests are not universally included during presurgical video-EEG monitoring: some centres do them routinely, others do not them at all.
</>Ideally, the clinical usefulness of HV during video-EEG monitoring should be addressed in a well designed prospective randomized controlled study: this can be done easily, and it would impact greatly on the optimization of presurgical investigations. Is any of our readers willing to undertake this challenge?
References
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2. Dalby MA. Epilepsy and three per second spike and wave rhythms: A clinical electroencephalographic and prognostic analysis of 346 patients. Acta Neurol Scand 1969; suppl 40; 3+
3. Schuler P, Claus D, Stefan H. Hyperventilation and transcranial magnetic stimulation: two methods of activation of epileptiform EEG activity in comparison. J Clin Neurophysiol 1993;10:111-5
4. Miley CE, Forster FM. Activation of partial complex seizures by hyperventilation. Arch Neurol 1977; 34 (6); 371-3
5. Servit Z, Kristof M, Strejckova A. Activating effect of nasal and oral hyperventilation on epileptic electroencephalographic phenomena: Reflex mechanisms of nasal origin. Epilepsia 1981; 22 (3); 321-329
6. Sherwin I. Alterations in the non-specific cortical afference during hyperventilation. Electroencephalogr Clin Neurophysiol 1967; 23 (6); 532-8
7. Van der Worp HB, Kraaier V, Wieneke Gh et al. Quantitative EEG during progressive hypocarbia and hypoxia: Hyperventilation induced EEG changes reconsidered. Electroencephalogr Clin Neurophysiol 1991; 79 (5); 335-41