The immediate and short‐term effects of bilateral intrahippocampal depth electrodes on verbal memory

Abstract

In contrast to previous studies, Ljung et al. provide evidence of permanent cognitive consequences of bilateral intrahippocampal depth electrodes for verbal memory in patients who were not operated or operated in the right temporal lobe. Stimulated by this, we provide historical confirmatory and supplementary evidence of the detrimental effect of bilateral depth electrodes implanted along the longitudinal axis of the hippocampus on verbal learning and especially on delayed verbal memory and recognition performance. This is demonstrated in 31 patients with memory assessments before implantation, after explantation, and 3 months later after left/right temporal lobe surgery. After surgery, significant recovery from postimplantation impairment is found in right temporal patients. Left temporal resection patients stay on the level seen after implantation and do not recover. Surgery, however, has its own effects in addition to the implantation. Intracranial electrodes for electroencephalographic monitoring or electrical stimulation are commonly and increasingly used for diagnosis or treatment in pharmacoresistant epilepsies. Thus, the monitoring of invasive stereotactic approaches is recommended to find safe procedures for the patients. In response to the findings, we restricted indications and used different implantation schemes, different trajectories, and targets to minimize the risk of additional damage.

Cognitive safety of intracranial electrodes for epilepsy

“…Helmstaedter et al from Bonn University in Germany conducted a retrospective study from data collected in the late 1990s to assess the immediate and short‐term effects of bilateral hippocampal depth electrodes on verbal memory. Electrodes were implanted along the longitudinal axis of the hippocampus in 31 patients, and memory assessments were obtained before implantation, shortly after explantation, and 3 months after temporal lobe epilepsy (TLE) surgery (left = 21, right = 10).…”

“…Visual memory was not reported. The authors concluded that their data “provide historic confirmatory and supplementary evidence of the detrimental effect of bilateral depth electrodes implanted along the longitudinal axis of the hippocampus on verbal learning.” Based on their findings, the Bonn group “restricted implantations to one hippocampus if ever possible” and later switched to “stereotactic implantations with lateral depth‐electrodes, placed orthogonally to the long axis of the hippocampus.”…”

“…Helmstaedter et al acknowledge several limitations of their study including “(1) its retrospective design, (2) the small sample size, (3) lack of matched nonimplanted control subjects, and (4) the limited follow‐up interval of 3 months after surgery.” Because their article was a Brief Communication, this may have limited their ability to fully detail their investigation. Nevertheless, many important concerns were not addressed including the following: (1) Seventy percent of the right temporal lobe patients had subdural strips in addition to depths, although no information on location of strips is provided, and analysis of possible effects of the strip electrode implantation was not conducted.…”

“…Helmstaedter et al included histopathology from a recent implant patient who was not one of their study patients. However, the brain tissue response to implanted electrodes is a well‐established phenomenon .…”

“…The first study in this regard was conducted by the Bonn group in 1997, which reported memory outcomes in 115 patients with longitudinally intrahippocampal depth electrodes (IDEs) inserted into the hippocampi bilaterally for an average of 15.9 days. Verbal learning and memory performance were tested preoperatively and 3 months after amygdalo‐hippocampectomy using the same measure as the recent Helmstaedter et al study (ie, VLMT, a German version of the Rey Auditory Verbal Learning Test). Pertinent to the present discussion, 21 patients were tested who underwent right amygdalohippocampectomy (10 patients were assessed preoperatively by scalp/sphenoidal EEG, and 11 patients were assessed by IDE/subdurals).…”