How does anaesthesia affect the brain in older adults and children?
Recent studies by Massachusetts General Hospital (MGH) looked at the differences in the way common anaesthetics affect the brains of older patients and children, findings that could lead to ways of improving monitoring technology and the safety of general anaesthesia for these patients.
“Anaesthetists know that the management of patients age 60 or older requires different approaches than for younger patients,” says Emery Brown, of the MGH department of anaesthesia, critical care and pain medicine. “The doses required to achieve the same anaesthetic state in older patients can be as little as half what is needed for younger patients. Explanations for that difference have focused on age-related declines in cardiovascular, respiratory, liver and kidney function, but the primary sites of anaesthetic effects are the brain and central nervous system.”
Patrick Purdon also of the MGH department of anaesthesia, critical care and pain medicine, adds: “We know even less about how anaesthetic drugs influence brain activity in children, and the current standard of care for assessing the brain state of children under anaesthesia calls only for monitoring vital signs like heart rate and blood pressure.
“This lack of knowledge is especially troubling, given recent studies suggesting an association between early childhood surgery requiring general anaesthesia and later cognitive problems.”
Two papers from the MGH team recently published in the British Journal of Anæsthesia are the first to take a detailed look at anaesthesia-induced brain changes in older patients. Purdon and Brown are co-corresponding authors of one study that analysed detailed EEG recordings of 155 patients aged 18 to 90 receiving either propofol or sevoflurane. The study found that the EEG oscillations of older patients were two to three times smaller than those of younger adults with reduced occurrence of frontal alpha waves. The synchronisation between the cortex and thalamus occurred at slightly lower frequencies in older patients, who were more likely than younger patients to experience a state called burst suppression that reflects profoundly deep anaesthesia at lower doses. The other BJA study, led by MGH anaesthetist Ken Solt found that older animals took two to five times longer than younger animals to recover from equal anaesthetic doses and observed similar age-related differences in EEG patterns as seen in the patients.
In another study appearing in the same issue of BJA, Purdon and his co-author Oluwaseun Akeju analysed EEG patterns of 54 patients ranging from infancy through to age 28 during anaesthesia with sevoflurane. They found that anaesthesia-induced EEG signals tripled in power from infancy until around age 6 and then dropped off to the typical young adult level at around age 20. Frontal alpha waves were not observed in children under the age of one, suggesting that the brain circuits required for cortical/thalamic synchronisation had not yet developed.
“It appears as though the structure of anaesthesia-induced brain dynamics mirrors brain development in children, with different brain wave patterns ‘turning on’ at ages that coincide with known developmental milestones,” says Purdon. “In older patients we see a similar effect but in reverse, with certain brain waves decaying in a manner consistent with brain aging. It’s been known that commercially available EEG-based anaesthesia monitors were developed for young adults, and while they are limited for that population – reducing brain activity to a single number – they are even more inaccurate for children and the elderly. These studies illustrate why this is the case and suggest a new, age-specific monitoring paradigm that – along with monitors that track a broader range of EEG signals – could help avoid both anaesthesia-induced neurotoxicity in children and post-operative delirium and cognitive dysfunction in elderly patients.”
Brown adds, “Understanding how the brain’s responses to anaesthesia change with age allows us to provide personalised, patient-specific strategies for monitoring the brain and dosing the anaesthetics, thereby moving us closer to side-effect-free anaesthesia care.”
Source: Massachusetts General Hospital