The effect of carbamazepine and ethosuximide on hyperoxic seizures

New insights into the mechanisms and prevention of central nervous system oxygen toxicity: A prospective review

Hyperbaric oxygen therapy (HBOT) elevates the pressure of life-sustaining oxygen (pO2), thereby saving lives. However, HBOT can also cause toxic effects like lung and retinal damage (peripheral oxygen toxicity) and violent myoclonic seizures (central nervous system (CNS) toxicity). The mechanisms behind these effects are not fully understood, hindering the development of effective therapies and preventive strategies. Herein, we critically reviewed the literature to understand CNS oxygen toxicity associated with HBOT to elucidate their mechanism, treatment, and prevention. We provide evidence that (1) increased pO2 increases reactive oxygen species (ROS) concentration in tissues, which irreversibly alters cell receptors, causing peripheral oxygen toxicity and contributing to CNS oxygen toxicity. Furthermore, (2) increased ROS concentration in the brain lowers the activity of glutamic decarboxylase (GD), which lowers concentrations of inhibitory neurotransmitter γ-aminobutyric acid (GABA), thereby contributing to the onset of HBOT-derived seizures. We provide long-overlooked evidence that (3) elevated ambient pressure directly inhibits GABAA, glycine and other receptors, leading to the rapid onset of seizures. Additionally, (4) acidosis facilitates the onset of seizures by an unknown mechanism. Only a combination of these mechanisms explains most phenomena seen in peripheral and CNS oxygen toxicity. Based on these proposed intertwined mechanisms, we suggest administering antioxidants (lowering ROS concentrations), pyridoxine (restoring GD activity), low doses of sedatives/anesthetics (reversing inhibitory effects of pressure on GABAA and glycine receptors), and treatment of acidemia before routine HBOT to prevent peripheral and CNS oxygen toxicity. Theoretically, similar preventive strategies can be applied before deep-sea diving to prevent life-threatening convulsions.

Exogenous ketone ester delays CNS oxygen toxicity without impairing cognitive and motor performance in male Sprague-Dawley rats

Hyperbaric oxygen (HBO2) is breathing >1 atmosphere absolute (ATA; 101.3 kPa) O2 and is used in HBO2 therapy and undersea medicine. What limits the use of HBO2 is the risk of developing central nervous system (CNS) oxygen toxicity (CNS-OT). A promising therapy for delaying CNS-OT is ketone metabolic therapy either through diet or exogenous ketone ester (KE) supplement. Previous studies indicate that KE induces ketosis and delays the onset of CNS-OT; however, the effects of exogeneous KE on cognition and performance are understudied. Accordingly, we tested the hypothesis that oral gavage with 7.5 g/kg induces ketosis and increases the latency time to seizure (LSz) without impairing cognition and performance. A single oral dose of 7.5 g/kg KE increases systemic β-hydroxybutyrate (BHB) levels within 0.5 h and remains elevated for 4 h. Male rats were separated into three groups: control (no gavage), water-gavage, or KE-gavage, and were subjected to behavioral testing while breathing 1 ATA (101.3 kPa) of air. Testing included the following: DigiGait (DG), light/dark (LD), open field (OF), and novel object recognition (NOR). There were no adverse effects of KE on gait or motor performance (DG), cognition (NOR), and anxiety (LD, OF). In fact, KE had an anxiolytic effect (OF, LD). The LSz during exposure to 5 ATA (506.6 kPa) O2 (≤90 min) increased 307% in KE-treated rats compared with control rats. In addition, KE prevented seizures in some animals. We conclude that 7.5 g/kg is an optimal dose of KE in the male Sprague-Dawley rat model of CNS-OT.

Increased Antiseizure Effectiveness with Tiagabine Combined with Sodium Channel Antagonists in Mice Exposed to Hyperbaric Oxygen

Hyperbaric oxygen (HBO₂) is acutely toxic to the central nervous system, culminating in EEG spikes and tonic-clonic convulsions. GABA enhancers and sodium channel antagonists improve seizure latencies in HBO₂ when administered individually, while combining antiepileptic drugs from different functional classes can provide greater seizure latency. We examined the combined effectiveness of GABA enhancers (tiagabine and gabapentin) with sodium channel antagonists (carbamazepine and lamotrigine) in delaying HBO₂-induced seizures. A series of experiments in C57BL/6 mice exposed to 100% oxygen at 5 atmospheres absolute (ATA) were performed. We predicted equally effective doses from individual drug-dose response curves, and the combinations of tiagabine + carbamazepine or lamotrigine were tested to determine the maximally effective combined doses to be used in subsequent experiments designed to identify the type of pharmacodynamic interaction for three fixed-ratio combinations (1:3, 1:1, and 3:1) using isobolographic analysis. For both combinations, the maximally effective combined doses increased seizure latency over controls > 5-fold and were determined to interact synergistically for fixed ratios 1:1 and 3:1, additive for 1:3. These results led us to explore whether the benefits of these drug combinations could be extended to the lungs, since a centrally mediated mechanism is believed to mediate hyperoxic-induced cardiogenic lung injury. Indeed, both combinations attenuated bronchoalveolar lavage protein content by ~ 50%. Combining tiagabine with carbamazepine or lamotrigine not only affords greater antiseizure protection in HBO₂ but also allows for lower doses to be used, minimizing side effects, and attenuating acute lung injury.

Central Nervous System Oxygen Toxicity and Hyperbaric Oxygen Seizures

INTRODUCTION

The use of hyperbaric oxygen (O2) as a therapeutic agent carries with it the risk of central nervous system (CNS) O2 toxicity.

METHODS

To further the understanding of this risk and the nature of its molecular mechanism, a review was conducted on the literature from various fields.

RESULTS

Numerous physiological changes are produced by increased partial pressures of oxygen (Po2), which may ultimately result in CNS O2 toxicity. The human body has several equilibrated safeguards that minimize effects of reactive species on neural networks, believed to play a primary role in CNS O2 toxicity. Increased partial pressure of oxygen (Po2) appears to saturate protective enzymes and unfavorably shift protective reactions in the direction of neural network overstimulation. Certain regions of the CNS appear more susceptible than others to these effects. Failure to decrease the elevated Po2 can result in a tonic-clonic seizure and death. Randomized, controlled studies in human populations would require a multicenter trial over a long period of time with numerous endpoints used to identify O2 toxicity.

CONCLUSIONS

The mounting scientific evidence and apparent increase in the number of hyperbaric O2 treatments demonstrate a need for further study in the near future.

Epilepsy and Recreational Scuba Diving: An Absolute Contraindication or Can There Be Exceptions? A Call for Discussion

Recreational scuba diving is a popular sport, and people with epilepsy often ask physicians whether they may engage in diving. Scuba diving is not, however, without risk for anyone; apart from the risk of drowning, the main physiological problems, caused by exposure to gases at depth, are decompression illness, oxygen toxicity, and nitrogen narcosis. In the United Kingdom, the Sport Diving Medical Committee advises that, to dive, someone with epilepsy must be seizure free and off medication for at least 5 years. The reasons for this are largely theoretical. We review the available evidence in the medical literature and diving websites. The risk of seizures recurring decreases with increasing time in remission, but the risk is never completely abolished. We suggest that people with epilepsy who wish to engage in diving, and the physicians who certify fitness to dive, should be provided with all the available evidence. Those who have been entirely seizure-free on stable antiepileptic drug therapy for at least 4 years, who are not taking sedative antiepileptic drugs and who are able to understand the risks, should then be able to consider diving to shallow depths, provided both they and their diving buddy have fully understood the risks.

Hyperbaric oxygen therapy in orthopedic conditions: an evaluation of safety

BACKGROUND

Because of the documented cellular and biochemical benefits of hyperbaric oxygen (HBO), HBO therapy is applied now with increasing frequency to various orthopedic conditions. Many traumatologists and orthopedic surgeons might refer their patients for adjuvant HBO therapy. However, the potential risks and risk-benefit ratio have often been underemphasized in therapeutic trials.

METHODS

From October 2002 to September 2004, 240 patients with a total of 4,638 treatments received HBO therapy at the hyperbaric medicine center of our institution on an identical treatment protocol. HBO therapy patient treatment logs were reviewed to analyze the incidence of complications during HBO treatment.

RESULTS

The overall incidence of complications was 1.83%. Over 94% of treatment complications were mild to moderate and designated as minor complications; fewer than 6% were severe or life threatening and designated as major complications. The incidence of major complications (central nervous system [CNS] oxygen toxicity in this series) was 0.109%. There was no mortality. Two patients with unusual presentation of CNS oxygen toxicity were observed during the study period.

CONCLUSIONS

HBO therapy in orthopedic conditions is considered as a safe treatment because of a very low complication rate; however, analysis of patients with CNS oxygen toxicity revealed its unpredictability and inevitability. Although it is common sense that patients who develop a seizure in the hospital need help from the medical staff, it cannot be done in a monoplace hyperbaric chamber because of pressure unequalization. Therefore, a multiplace chamber equipped with an antechamber for medical contingency is possibly the better facility in consideration of safety.

Attenuation of Cerebral Oxygen Toxicity by Sound Conditioning

HYPOTHESIS

Sound conditioning might reduce cerebral oxygen toxicity.

BACKGROUND

Cerebral oxygen toxicity is related to high levels of reactive oxygen species. Noise-induced hearing loss has been shown to result from ischemia-reperfusion, in which reactive oxygen species play a major role. Repeated exposure to loud noise at levels below that which produces permanent threshold shift prevented noise-induced hearing loss and was associated with significant elevation of the antioxidant enzymes measured in the inner ear. We tested the hypothesis that sound conditioning might reduce cerebral oxygen toxicity.

METHODS

Forty-five guinea pigs were prepared for electroencephalography and auditory brainstem recording. The auditory brainstem recording detection threshold was determined to confirm baseline normal hearing. The animals were divided into three equal groups and subjected to the following procedures: Group 1, electroencephalography electrode implantation and auditory brainstem recording only; Group 2, exposure to oxygen at 608 kPa (the latency to the first electrical discharge in the electroencephalogram preceding the appearance of seizures was measured); and Group 3, sound conditioning followed by oxygen exposure. The animals were killed, and the brains were excised and homogenized. Brain levels of superoxide dismutase, catalase, glutathione peroxidase, glutathione transferase, glutathione reductase, glucose-6-phosphate dehydrogenase, and thiobarbituric acid reactive substances were compared among the groups.

RESULTS

Latency to the first electrical discharge was compared between Groups 2 and 3, and was found to be significantly longer in Group 3 (27.9 +/- 11 versus 20.4 +/- 7.6 min, p < 0.03). No significant changes were found in brain levels of superoxide dismutase, catalase, glutathione peroxidase, glutathione transferase, glutathione reductase, glucose-6-phosphate dehydrogenase, or thiobarbituric acid reactive substances.

CONCLUSION

Our data show that sound conditioning prolongs the latency to oxygen-induced convulsions. This effect was not accompanied by significant changes in whole-brain antioxidant enzyme activity or the magnitude of lipid peroxidation.

The effect of flunarizine on central nervous system oxygen toxicity in rats

The toxicity of hyperbaric oxygen in the central nervous system is expressed by generalized tonic-clonic seizures. In the search for drugs effective against these seizures, we tested flunarizine, a calcium antagonist known to have antiepileptic properties and only minimal cardiovascular side effects. 49 rats with chronic cortical electrodes were injected i.p. with six different doses of flunarizine (10-300 mg/kg) or vehicle, before exposure to 0.5 MPa oxygen. Two doses of flunarizine and vehicle were given to rats exposed to oxygen with 5% CO2 at an absolute pressure of 0.5 MPa. EEG and spectral analysis of background EEG activity were monitored. The duration of the latent period before the appearance of electrical discharges in the EEG was used as an index of oxygen toxicity. There was no statistical difference between the duration of the latent periods for the seven groups treated by flunarizine or by vehicle on exposure to 0.5 MPa pure oxygen (P = 0.9 in ANOVA), but on exposure to oxygen with CO2, there was significant prolongation of the latent periods in comparison with vehicle (P < 0.001). Our results suggest that on exposure to hyperbaric oxygen, the antiepileptic effect of flunarizine might be masked, probably by its cerebral antivasoconstrictive effect.