Effects of Prolonged Seizures on Basal Forebrain Cholinergic Neurons: Evidence and Potential Clinical Relevance

Nicotinic acetylcholine receptors and epilepsy

Despite recent advances in understanding the causes of epilepsy, especially the genetic, comprehending the biological mechanisms that lead to the epileptic phenotype remains difficult. A paradigmatic case is constituted by the epilepsies caused by altered neuronal nicotinic acetylcholine receptors (nAChRs), which exert complex physiological functions in mature as well as developing brain. The ascending cholinergic projections exert potent control of forebrain excitability, and wide evidence implicates nAChR dysregulation as both cause and effect of epileptiform activity. First, tonic-clonic seizures are triggered by administration of high doses of nicotinic agonists, whereas non-convulsive doses have kindling effects. Second, sleep-related epilepsy can be caused by mutations on genes encoding nAChR subunits widely expressed in the forebrain (CHRNA4, CHRNB2, CHRNA2). Third, in animal models of acquired epilepsy, complex time-dependent alterations in cholinergic innervation are observed following repeated seizures. Heteromeric nAChRs are central players in epileptogenesis. Evidence is wide for autosomal dominant sleep-related hypermotor epilepsy (ADSHE). Studies of ADSHE-linked nAChR subunits in expression systems suggest that the epileptogenic process is promoted by overactive receptors. Investigation in animal models of ADSHE indicates that expression of mutant nAChRs can lead to lifelong hyperexcitability by altering i) the function of GABAergic populations in the mature neocortex and thalamus, ii) synaptic architecture during synaptogenesis. Understanding the balance of the epileptogenic effects in adult and developing networks is essential to plan rational therapy at different ages. Combining this knowledge with a deeper understanding of the functional and pharmacological properties of individual mutations will advance precision and personalized medicine in nAChR-dependent epilepsy.

Evidence supporting deep brain stimulation of the medial septum in the treatment of temporal lobe epilepsy

Electrical brain stimulation has become an essential treatment option for more than one third of epilepsy patients who are resistant to pharmacological therapy and are not candidates for surgical resection. However, currently approved stimulation paradigms achieve only moderate success, on average providing approximately 75% reduction in seizure frequency and extended periods of seizure freedom in nearly 20% of patients. Outcomes from electrical stimulation may be improved through the identification of novel anatomical targets, particularly those with significant anatomical and functional connectivity to the epileptogenic zone. Multiple studies have investigated the medial septal nucleus (i.e., medial septum) as such a target for the treatment of mesial temporal lobe epilepsy. The medial septum is a small midline nucleus that provides a critical functional role in modulating the hippocampal theta rhythm, a 4-7-Hz electrophysiological oscillation mechanistically associated with memory and higher order cognition in both rodents and humans. Elevated theta oscillations are thought to represent a seizure-resistant network activity state, suggesting that electrical neuromodulation of the medial septum and restoration of theta-rhythmic physiology may not only reduce seizure frequency, but also restore cognitive comorbidities associated with mesial temporal lobe epilepsy. Here, we review the anatomical and physiological function of the septohippocampal network, evidence for seizure-resistant effects of the theta rhythm, and the results of stimulation experiments across both rodent and human studies, to argue that deep brain stimulation of the medial septum holds potential to provide an effective neuromodulation treatment for mesial temporal lobe epilepsy. We conclude by discussing the considerations necessary for further evaluating this treatment paradigm with a clinical trial.

Status epilepticus without impairment of consciousness: Long-term outcomes according to duration

OBJECTIVE

The point after which non-convulsive status epilepticus (NCSE) can cause permanent damage remains to be elucidated. The aim of this study was to analyze the association between time to resolution and long-term outcomes in NCSE.

METHODS

We performed a retrospective study of all patients with focal NCSE without consciousness impairment at two tertiary care hospitals in Spain. All the data were registered prospectively and the study period was December 2014-May 2018. We collected information on demographics, SE etiology, time to administration of different lines of treatment, time to NCSE resolution, and outcomes at discharge, 1 year, and 4 years. Clinical outcome was prospectively categorized as good (return to baseline function) or poor (new disability and death).

RESULTS

Seventy-four patients with a mean (±SD) age of 63.4 ± 17.5 years and a mean follow-up time of 2.4 ± 2.2 years were studied. A poor outcome at discharge was associated with a potentially fatal etiology (p < 0.001), EMSE score (Epidemiology-based Mortality Score in Status Epilepticus) (p = 0.012), lateral periodic discharges on EEG (p = 0.034), and occurrence of major complications during hospitalization (p = 0.007). An SE duration of >100 h was clearly associated with a worse outcome (p < 0.001). In the multiple regression analysis, the only independent predictors of a poor outcome at discharge were an SE duration of >+100 hours (p = 0.001), a potentially fatal etiology (p = 0.001), and complications during hospitalization (p = 0.010). An SE duration of >100 hours retained its value as the optimal cutoff point for predicting poor outcomes at both 1 year (p = 0.037) and 4 years (p = 0.05). Other predictors of poor long-term outcomes were a potentially fatal etiology (p < 0.001) and EMSE score (p = 0.034) at 1 year, and progressive symptomatic etiology at 4 years (p = 0.025).

SIGNIFICANCE

In patients with focal NCSE without consciousness impairment, a potentially fatal etiology and an SE duration of >100 h were associated with poor short-term and long-term outcomes.