Cognitive outcomes following vagus nerve stimulation, responsive neurostimulation and deep brain stimulation for epilepsy: A systematic review

Long-term effects of vagus nerve stimulation therapy on cognitive functioning in patients with drug-resistant epilepsy

BACKGROUND

Despite the increasing use of vagus nerve stimulation (VNS) for drug-resistant epilepsy, its impact on cognitive functioning remains insufficiently investigated.

OBJECTIVE

We aimed to comprehensively assess changes in cognition after long-term VNS therapy in a prospective sample of adults with epilepsy.

METHODS

Between December 2019 and March 2023, patients scheduled for VNS implantation were invited for neuropsychological assessment, including tests of executive functions, working and short-term memory (recall of a verbal logical story and the Rey-Osterrieth complex figure (ROCF)), and social cognition. Participants were re-evaluated after a year of VNS therapy and the pre- and postoperative scores were compared by means of the Student's t or Wilcoxon's signed rank tests for paired samples. Patients available only after a longer follow-up (more than 24 months) were also re-examined and included in a secondary analysis.

RESULTS

The study included 28 PWE (16, 57.1% female, average age 33.7 ± 10.0 years). Twenty-two PWE followed-up at 14.5 ± 4.8 months had worse categorical verbal fluency than preoperatively (t = 2.613, p = 0.016). After including patients with long follow-up (n = 28, 21.6 ± 11.4 months), the group scored better on the delayed recall of the ROCF (17.09 ± 8.84 to 20.65 ± 8.32 points, t(22) =  - 2.618, p = 0.016) and the Happé strange stories test (5.0 ± 2.6 to 6.1 ± 2.1 points, t(14) =  - 3.281, p = 0.005). No significant changes were observed in other cognitive domains (p > 0.05).

CONCLUSION

We suggest improvements in a task of social cognition and short-term visual memory after longer use of VNS therapy. Such findings should be confirmed in larger trials after controlling for changes in ictal or interictal activity.

The Effects of Transcutaneous Vagus Nerve Stimulation on Functional Connectivity Within Semantic and Hippocampal Networks in Mild Cognitive Impairment

Better treatments are needed to improve cognition and brain health in people with mild cognitive impairment (MCI) and Alzheimer's disease (AD). Transcutaneous vagus nerve stimulation (tVNS) may impact brain networks relevant to AD through multiple mechanisms including, but not limited to, projection to the locus coeruleus, the brain's primary source of norepinephrine, and reduction in inflammation. Neuropathological data suggest that the locus coeruleus may be an early site of tau pathology in AD. Thus, tVNS may modify the activity of networks that are impaired and progressively deteriorate in patients with MCI and AD. Fifty patients with MCI (28 women) confirmed via diagnostic consensus conference prior to MRI (sources of info: Montreal Cognitive Assessment Test (MOCA), Clinical Dementia Rating scale (CDR), Functional Activities Questionnaire (FAQ), Hopkins Verbal Learning Test - Revised (HVLT-R) and medical record review) underwent resting state functional magnetic resonance imaging (fMRI) on a Siemens 3 T scanner during tVNS (left tragus, n = 25) or sham control conditions (left ear lobe, n = 25). During unilateral left tVNS, compared with ear lobe stimulation, patients with MCI showed alterations in functional connectivity between regions of the brain that are important in semantic and salience functions including regions of the temporal and parietal lobes. Furthermore, connectivity from hippocampi to several cortical and subcortical clusters of ROIs also demonstrated change with tVNS compared with ear lobe stimulation. In conclusion, tVNS modified the activity of brain networks in which disruption correlates with deterioration in AD. These findings suggest afferent target engagement of tVNS, which carries implications for the development of noninvasive therapeutic intervention in the MCI population.

“The Wandering Nerve Linking Heart and Mind” – The Complementary Role of Transcutaneous Vagus Nerve Stimulation in Modulating Neuro-Cardiovascular and Cognitive Performance

The vagus nerve is the longest nerve in the human body, providing afferent information about visceral sensation, integrity and somatic sensations to the CNS via brainstem nuclei to subcortical and cortical structures. Its efferent arm influences GI motility and secretion, cardiac ionotropy, chonotropy and heart rate variability, blood pressure responses, bronchoconstriction and modulates gag and cough responses via palatine and pharyngeal innervation. Vagus nerve stimulation has been utilized as a successful treatment for intractable epilepsy and treatment-resistant depression, and new non-invasive transcutaneous (t-VNS) devices offer equivalent therapeutic potential as invasive devices without the surgical risks. t-VNS offers exciting potential as a therapeutic intervention in cognitive decline and aging populations, classically affected by reduced cerebral perfusion by modulating both limbic and frontal cortical structures, regulating cerebral perfusion and improving parasympathetic modulation of the cardiovascular system. In this narrative review we summarize the research to date investigating the cognitive effects of VNS therapy, and its effects on neurocardiovascular stability.

Brain Stimulation Using Responsive Neurostimulation Improves Verbal Memory: A Crossover Case-Control Study

BACKGROUND

The effects of brain stimulation on memory formation in humans have shown conflicting results in previous studies. We hypothesized that direct cortical stimulation using an implanted responsive neurostimulation (RNS) system will improve memory.

OBJECTIVE

To evaluate whether direct cortical stimulation using RNS improves memory as measured with recall scores of a list-learning task.

METHODS

During outpatient visits, a list-learning task (Hopkins Verbal Learning Test-Revised) was administered to 17 patients with RNS implants. Patients were read a list of 12 semantically related words and asked to recall the list after 3 different learning trials. True or sham stimulations were performed for every third word presented for immediate recall. Most patients had frontotemporal network stimulation-one patient each had insular and parietal stimulations. After a 20-min delay, they were asked to recall the list again, first freely and then through a "yes/no" recognition paradigm. A crossover design was used in which half the patients had true stimulation during the initial visit and half had sham stimulation-followed by crossover to the other group at the next visit.

RESULTS

The Hopkins Verbal Learning Test-Revised delayed recall raw score was higher for the stimulation condition compared with the nonstimulation condition (paired t -test, P = .04, effect size d = 0.627).

CONCLUSION

Verbal memory improves by direct cortical stimulation during a list-learning task. The RNS system can be effectively used in memory research using direct cortical stimulation. This study has implications in the development of neurostimulation devices for cognitive enhancement in conditions such as epilepsy, dementia, and traumatic brain injury.

Neuromodulation in epilepsy: state-of-the-art approved therapies

Three neuromodulation therapies have been appropriately tested and approved in refractory focal epilepsies: vagus nerve stimulation (VNS), deep brain stimulation of the anterior nucleus of the thalamus (ANT-DBS), and closed-loop responsive neurostimulation of the epileptogenic zone or zones. These therapies are primarily palliative. Only a few individuals have achieved complete freedom from seizures for more than 12 months with these therapies, whereas more than half have benefited from long-term reduction in seizure frequency of more than 50%. Implantation-related adverse events primarily include infection and pain at the implant site. Intracranial haemorrhage is a frequent adverse event for ANT-DBS and responsive neurostimulation. Other stimulation-specific side-effects are observed with VNS and ANT-DBS. Biomarkers to predict response to neuromodulation therapies are not available, and high-level evidence to aid decision making about when and for whom these therapies should be preferred over other antiepileptic treatments is scant. Future studies are thus needed to address these shortfalls in knowledge, approve other forms of neuromodulation, and develop personalised closed-loop therapies with embedded machine learning. Until then, neuromodulation could be considered for individuals with intractable seizures, ideally after the possibility of curative surgical treatment has been carefully assessed and ruled out or judged less appropriate.