Introduction
Subscalp EEG recording is a technological advancement positioned to revolutionize diagnostics, treatment and understanding of epilepsy.
Over the past 7 years or so, several investigatory groups have developed subscalp EEG recording systems. These systems consist of electrodes that can be implanted under the skin of the scalp. Electrodes are attached to external data collection and analytical systems. These systems allow continuous EEG monitoring 24/7 for months and potentially for years.
Unmet Need
There is a unmet need for accurate long term measurements of seizures. To provide a correct diagnosis of epilepsy and develop optimal therapy, the physician needs information on exact seizure frequency, location, and brain wave activity prior to and after a seizure. As discussed in Blog 8 (Intracranial EEG recording), implantation of electrodes directly in the brain continually assesses brain wave activity and has provided significant insights in the cyclic nature of the disease. However, intracranial recordings necessitate major surgery and, therefore, are available for only a small number of patients with epilepsy. On the other hand, scalp EEG (electrodes placed on top of the scalp) coupled with video monitoring, although not invasive, supply continuous seizure assessment for no more than 14 days, at best.
Ultra-long term EEG recording lies between intracranial recording and scalp recording. It has potential to not only determine seizure frequency and location but to confirm the cyclic nature of epilepsy, thereby significantly adding to insights from intracranial EEG recordings. Furthermore, subscalp EEG systems appear capable of predicting future seizures, thus allowing for preemptive suppression.
However, keep in mind, this technology is in its initial stages but has great promise.
Validation of Subscalp EEG Recording
While there are many foreseeable uses of continuous subscalp EEG recordings, the first use is for long term acquisition of seizures and pre/post seizure activity in patients with epilepsy.
There are 6 systems in clinical validation phases (clinical trial completion, ongoing or about to begin). Two systems, the 24/7EEG SubQ (https:// www.uneeg. com/ en/ epilepsy/products/subq) and the Minder have completed small clinical safety/feasibility studies (see Weisdorf et al., 2018; 2019; Sterling et al., 2021).
The 24/7EEG SubQ system
The 24/7EEG SubQ system consists of a) subcutaneously implanted electrodes and b) a data collection system composed of a transceiver (transmitter/receiver combo), data storage device and power supply. The first study with this system (4 patients with temporal lobe epilepsy) was a feasibility (proof-of-concept) study. Results showed that SubQ EEG recordings were similar to standard scalp EEG recordings. Specifically, the SubQ system measured seizures, pre-seizure activity, sleep brain wave activity and some unrelated activity. These are all findings comparable to standard EEG recordings.
The most outstanding characteristic of the subscalp recording is, unlike standard scalp EEG measurements, its ability to gather data over long periods of time. The second clinical trial with the SubQ device implanted in 9 patients with temporal epilepsy and monitored EEG activity for the target time of 3 months (achieving a minimum of 9 weeks of recording for each patient). This is clearly a milestone since it is the first study to demonstrate “ultra-long” EEG monitoring with a minimally invasive device and without disruption of daily activities of living.
The Minder System
Another subscalp EEG system, the Minder completed an 8 months study with 5 patients with epilepsy. The components of the Minder system are electrodes capable of monitoring brain activity in both hemispheres and collecting data by telemetry to a processor placed behind the ear. A smart phone receives the data and stores it in the cloud.
Subscalp EEG data from the Minder compared favorably with standard video/scalp EEG data obtained at specified times during the study. These results confirm the feasibility of a minimally invasive implanted subscalp EEG system such as the Minder for long term monitoring of focal seizures.
Adverse Effects of Subscalp EEG Recording
Adverse effects from subscalp devices described above are mild. Expected pain and soreness at the site of surgical implantation lasting about a week occurred in most patients. However, few experienced pain and soreness beyond one week. Some experienced mild headaches unrelated to the surgery.
Limitations of Subscalp EEG Recordings
Firstly, the collection of thousands of hours of brain wave activity creates an analytical nightmare. In summarizing the data to reasonably handle it, some data may be lost. Efforts to develop intelligent algorithms are in development so that all data can be treated equally. Secondly, the position of the implanted electrodes may miss seizures originating deep in the brain. Thus some patients with epilepsy may not benefit from subscalp EEG devices. Thirdly, completed safety and feasibility studies necessarily involved a small number of select patients (those with temporal lobe epilepsy). Therefore, for confirmation of the generalized benefit of subscalp EEG for most patients with epilepsy, larger scale studies with patients with diverse types of epilepsy are required.
Conclusions
Subscalp EEG recording systems represent a relevant and dearly needed advancement over the short term EEG recordings of the standard scalp EEG. Subscalp EEG systems provide ultra-long term recordings. These recordings provide accurate determinations of seizure frequency and location, information essential for optimal drug therapy or future surgical decisions. Especially attractive is the fact that subscalp EEG systems require minimal surgery for implantation and once implanted do not disrupt everyday activities.
There are at present 6 subscalp EEG systems, 2 of which successfully demonstrated safety and feasibility in patients with temporal lobe epilepsy. This is a significant start. The limited results thus far are indeed very promising. Continued development and clinical assessment can promote and expand a transforming new technology to better serve patients with epilepsy. This blog will provide updates on subscalp EEG systems from future clinical trials.
References (http://pubmed)
Duun-Henriksen J et al., A new era in electroencephalographic monitoring? Subscalp devices for ultra–long-term recordings. Epilepsia.61: 1805–1817, 2020.
Stirling RE et al., Seizure forecasting using a novel sub-scalp ultra-long term EEG monitoring system. Front. Neurol. 12:713794, 2021.
Weisdort S et al., High similarity between EEG from subcutaneous and proximate scalp electrodes in patients with temporal lobe epilepsy. J Neurophysiol 120: 1451–1460, 2018.
Weisdort S. et al., Ultra‐long‐term subcutaneous home monitoring of epilepsy—490 days of EEG from nine patient. Epilepsia. 60: 2204–2214, 2019.