Respiratory Physiology of Seizures

Infantile ictal apneas in a child with williams-beuren syndrome

Williams-Beuren syndrome is a genetic disorder rarely associated with seizures. The few described cases of Williams-Beuren syndrome and epilepsy have primarily involved infantile spasms and deletions extending beyond the common deletion region for this disorder. We present the case of a 5-week-old child with ictal apneas and typical Williams-Beuren syndrome deletion. Diagnosis was challenging, because the child had cardiac, respiratory, and gastrointestinal abnormalities typically associated with Williams-Beuren syndrome, which are also associated with cyanotic episodes. The results of interictal electroencephalography were normal, illustrating that prolonged electroencephalography is often essential in evaluation of suspected ictal apneas. Seizure freedom was achieved with carbamazepine. Sudden death is seen in Williams-Beuren syndrome, and this case raises the question whether some of these cases may be related to ictal apneas and could potentially be preventable with appropriate pharmaceutical intervention.

Differences in serotonin receptor expression in the brainstem may explain the differential ability of a serotonin agonist to block seizure-induced sudden death in DBA/2 vs. DBA/1 mice

DBA mice are models of sudden unexpected death in epilepsy (SUDEP) that exhibit audiogenic generalized convulsive seizures (GCS), ending in death due to respiratory arrest (RA). Serotonin (5-HT) normally enhances respiration in response to elevated CO(2) levels, which occur during GCS in patients. Fluoxetine, a selective serotonin reuptake inhibitor (SSRI), blocks GCS-induced SUDEP in both DBA/2 and DBA/1 mice. This study examined the effects of a 5-HT(2B/2C) agonist (m-chlorophenylpiperazine, mCPP) to test the generality of serotonergic effects on DBA mice. In DBA/2 mice mCPP pre-treatment [5 or 10 (but not 2) mg/kg, i.p.] significantly reduced RA incidence without blocking seizure susceptibility. However, in DBA/1 mice mCPP in doses up to 40mg/kg was ineffective in blocking seizure-induced RA, and 60mg/kg was toxic. The cause of this strain difference was perplexing. Previous studies showed that brainstem 5-HT receptor protein expression was abnormal in DBA/2 mice. Therefore, expression of 5-HT receptor proteins in the medial-caudal brainstem of DBA/1 mice was evaluated using Western blots. In DBA1/mice 5-HT(2C) and 5-HT(3B) receptor expression levels were significantly reduced, as seen previously in DBA/2 mice. However, 5-HT(2B) receptor expression was also reduced in DBA/1 mice, contrasting with the 5-HT(2B) receptor elevation seen in DBA/2 mice. This difference may explain the differential effects of the 5-HT(2B/2C) agonist in these SUDEP models. mCPP blocked RA in DBA/2 mice and concomitantly reduced tonic seizures, which also occurs. Fluoxetine is the only agent tested that blocks RA selectively in these SUDEP models, which may be clinically relevant.

The serotonin axis: Shared mechanisms in seizures, depression, and SUDEP

There is a growing appreciation that patients with seizures are also affected by a number of comorbid conditions, including an increase in prevalence of depression (Kanner, 2009), sleep apnea (Chihorek et al., 2007), and sudden death (Ryvlin et al., 2006; Tomson et al., 2008). The mechanisms responsible for these associations are unclear. Herein we discuss the possibility that underlying pathology in the serotonin (5-HT) system of patients with epilepsy lowers the threshold for seizures, while also increasing the risk of depression and sudden death. We propose that postictal dysfunction of 5-HT neurons causes depression of breathing and arousal in some epilepsy patients, and this can lead to sudden unexpected death in epilepsy (SUDEP). We further draw parallels between SUDEP and sudden infant death syndrome (SIDS), which may share pathophysiologic mechanisms, and which have both been linked to defects in the 5-HT system.

Sudden unexpected death in epilepsy

Epilepsy is one of the most common serious neurological conditions worldwide, and sudden unexpected death in epilepsy (SUDEP) is the main cause of epilepsy-related death. Although the exact pathophysiological causes of SUDEP are still unknown, potential risk factors include young age of epilepsy onset, duration of epilepsy, frequently recurrent seizures, winter temperatures and cardiac abnormalities such as arrhythmias during and between seizures. Unfortunately available pharmacological treatment for epilepsy remains limited in seizure control and consequently in SUDEP control, so alternative treatment should be considered in patients with refractory epilepsy. This article will revisit the possible related causes and preventative actions for SUDEP.