Sex hormones would explain why women are less sensitive to anesthesia.
- Anesthesia is less effective on women.
- Scientists have just discovered that sex hormones could be involved in this difference with men.
- The researchers conclude that their results “demonstrate that sex differences in sensitivity to anesthesia are conserved across evolution.”
Although awareness under general anesthesia is, fortunately, quite rare, it is a phenomenon that seems to affect one group more than others: women. Until now, scientists didn’t know exactly why this happened, but a new study suggests that sex hormones could be a contributing factor.
General anesthesia works in part by altering the activity of the hypothalamus, the part of the brain that regulates sleep and wakefulness, putting us into a state of controlled unconsciousness. Previous research has shown that neural circuits in this region differ between the sexes and are modulated by sex hormones, but the impact of this on anesthesia has not been studied until now.
Anesthesia: a behavioral analysis of sensitivity
Researchers at the University of Pennsylvania attempted to remedy this situation, first by carrying out a behavioral analysis of sensitivity to anesthesia in mice and humans. In mice, this involved detecting and removing a sticker from their snouts to indicate that they had emerged from anesthesia. Human data comes from a trial involving 30 healthy volunteers. Researchers assessed their ability to respond to sound cues and perform tests of awareness and cognition.
In both cases, the results revealed that females took longer to be anesthetized and emerged from the anesthetic state more quickly. They also discovered that males’ lower resistance was modulated by testosterone. The castrated male mice indeed became more resistant, then became less so when they were injected with testosterone.
Anesthesia: another analysis via electroencephalograms
The next step in the study was to monitor the brain under anesthesia. The first step was to perform electroencephalograms (EEGs), which can be used clinically to measure the depth of the anesthetic state. Somewhat unexpectedly, these electroencephalograms revealed no sex-related differences in either mice or humans. However, mapping whole-brain activity in mice revealed differences, with higher activity levels in the hypothalamus in male mice.
In their article, the researchers conclude that their results “demonstrate that sex differences in sensitivity to anesthesia are conserved across evolution and are not reflected in conventional measurements of depth of anesthesia based on[EEG]“.
