EEG-derived technologies such as the Bispectral Index (BIS) and Density Spectral Array (DSA) have revolutionized perioperative neurophysiology, allowing anesthesiologists to visualize brain states in real time. In the elderly demented brain, however, interpretation of BIS alone can be misleading due to baseline EEG slowing and cortical atrophy. DSA, in contrast, provides a continuous color-coded representation of EEG frequency power, revealing distinct cortical transitions during induction, maintenance, and emergence phases.
This article analyzes in detail the three-phase DSA evolution in a 90-year-old female with Alzheimer’s dementia who underwent left femur PFN fixation under general anesthesia with a Left PENG (Pericapsular Nerve Group) block. Particular focus is placed on the Spectral Edge Frequency (SEF)—a numerical EEG parameter derived from the DSA—and how it reflected this patient’s cortical physiology through the anesthetic continuum.
Anesthetic brain monitoring using EEG is particularly valuable in geriatric anesthesia, where cortical electrophysiology is profoundly altered. Patients with Alzheimer’s disease demonstrate:
Loss of beta and alpha coherence, leading to reduced high-frequency activity.
Predominance of low-frequency delta and theta rhythms, even when awake.
Diminished cortical connectivity, making the EEG less reactive to stimuli.
The BIS algorithm, originally calibrated on young, healthy subjects, can underestimate arousal in the elderly. Hence, the DSA and Spectral Edge Frequency (SEF) serve as complementary tools for interpreting the true depth of anesthesia.
References
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Leistedt SJ et al. EEG characteristics of Alzheimer’s disease. Clin Neurophysiol. 2009;120(10):1901–11.
Brown EN, Lydic R, Schiff ND. General anesthesia, sleep, and coma. N Engl J Med. 2010;363(27):2638–50.
Kreuzer M. EEG-based monitoring of general anesthesia: taking the next steps. Front Syst Neurosci. 2017;11:64.
Anesthetic agents: Sevoflurane (inhalational induction), fentanyl 100 µg, glycopyrrolate 0.2 mg, succinylcholine 50 mg.
No midazolam used (preserving baseline cortical dynamics).
Regional analgesia: Left PENG block pre-incision.
Observation (10–15 min post-induction):
BIS = 38–40
SEF = 6–8 Hz
DSA: Dense red–yellow base (delta–theta dominance)
Sevoflurane-induced GABA-A hyperpolarization led to synchronized oscillations between cortical and thalamic neurons. On DSA, this appeared as a broad red–orange layer (delta power, 0.5–4 Hz) and fading of blue beta activity(>13 Hz).
Because of Alzheimer’s-related cortical slowing, the SEF (the frequency below which 95% of EEG power resides) dropped rapidly from pre-induction values (~14 Hz) to 6–8...
