Characteristics of Induced Amnesia for Surgery
General anesthesia involves the use of anesthetic agents to suppress the central nervous system, causing amnesia, analgesia, unconsciousness, and loss of reflexes. Induced amnesia is crucial for preventing intraoperative awareness by temporarily blocking memory formation during surgery given the discomforting and invasive nature of surgery. However, it is equally important for memory suppression to resolve quickly after the procedure to restore normal recall and minimize the risk of postoperative cognitive dysfunction (POCD). These two matched characteristics of amnesia induced by anesthetics make modern surgery possible.
Anterograde amnesia, the inability to remember events after anesthesia takes effect, occurs because the drugs disrupt memory processes. This effect depends on anesthetic concentration in the bloodstream, making it adjustable, reversible, and controllable. Although the precise mechanism remains unclear, general anesthetics likely interfere with the brain’s ability to consolidate new memories1, 2.
Memory formation begins in the hippocampus, where information is encoded before being transferred to the cortex for long-term storage—a process known as memory consolidation. This relies on synaptic plasticity, which strengthens neuronal connections based on activity. General anesthetics act on molecular targets, particularly ion channels, and modulate ligand-gated ionotropic receptors linked to neurotransmitter systems. General anesthetics produce their effects by suppressing excitatory synaptic transmission (e.g., glutamate) and amplifying inhibitory synaptic transmission (e.g., γ-aminobutyric acid) via acting on specific ion channels. This modulation of synaptic transmission ultimately impacts synaptic plasticity1.
The degree of amnesia induced during surgery and the potential for long-term memory effects depend on the type and dosage of anesthetic used. Agents like propofol, thiopental, midazolam, and dexmedetomidine have varying impacts on memory and can impair it even at subclinical concentrations. Research also shows that patient age plays a critical role, with elderly patients being more susceptible to long-term memory loss associated with anesthesia3.
To better tailor anesthetic regimens and reduce the risk of cognitive decline, recent studies have examined the effects of anesthetics on electroencephalography (EEG) rhythms. These rhythms, which reflect the synchronized activity of large neuronal populations, include delta, theta, and gamma oscillations, spindles, and sharp wave ripples. These oscillations coordinate synaptic plasticity through specific frequencies. During surgery, general anesthetics disrupt neural oscillations, even at subanesthetic doses, causing memory impairment1.
Understanding the electrophysiological mechanisms underlying anesthesia-induced amnesia can provide theoretical support for improving clinical anesthesia practices. For example, EEG-guided anesthesia management could help customize anesthetic protocols to minimize cognitive risks. Since different anesthetics affect neural oscillations in distinct ways, EEG biomarkers could guide anesthetic selection. However, further research is needed to map perioperative EEG markers of memory function and their role in anesthesia-induced amnesia1, 4.
References
1. Liu H, Yang Z, Chen Y, Yang F, Cao X, Zhou G, Zhang Y. Neural oscillations and memory: unraveling the mechanisms of anesthesia-induced amnesia. Front Neurosci. 2024;18:1492103. Epub 20241114. doi: 10.3389/fnins.2024.1492103. PubMed PMID: 39610865; PMCID: PMC11602479.
2. Dobson GP. Trauma of major surgery: A global problem that is not going away. Int J Surg. 2020;81:47-54. Epub 20200729. doi: 10.1016/j.ijsu.2020.07.017. PubMed PMID: 32738546; PMCID: PMC7388795.
3. Pryor KO, Reinsel RA, Mehta M, Li Y, Wixted JT, Veselis RA. Visual P2-N2 complex and arousal at the time of encoding predict the time domain characteristics of amnesia for multiple intravenous anesthetic drugs in humans. Anesthesiology. 2010;113(2):313-26. doi: 10.1097/ALN.0b013e3181dfd401. PubMed PMID: 20613477; PMCID: PMC2910207.
4. Pollak M, Leroy S, Röhr V, Brown EN, Spies C, Koch S. Electroencephalogram Biomarkers from Anesthesia Induction to Identify Vulnerable Patients at Risk for Postoperative Delirium. Anesthesiology. 2024;140(5):979-89. doi: 10.1097/aln.0000000000004929. PubMed PMID: 38295384.