Factors of influence on postoperative cognitive function in the elderly
Lars S. Rasmussen, MD, PhD
Department of Anaesthesia, Centre of Head and
Orthopaedics
Copenhagen University Hospital, Rigshospitalet,
Copenhagen, Denmark
Elderly patients
frequently experience a deterioration in cognitive function after surgery and
anaesthesia. The two most common cognitive disorders are delirium and
PostOperative Cognitive Dysfunction (POCD).
Delirium usually
occurs during the first postoperative days as an acute, fluctuating
condition. The patient has lost
orientation and has impairment of attention and memory. The incidence of delirium
is approximately 10% in elderly patients but much higher following cardiac
procedures and surgery for hip fractures (1). Delirium is associated with
increased hospital stay and mortality. Age and pre-existing brain disease are
important risk factors. It is essential to be aware of eliciting factors that
may be correctable such as drug side effects, electrolyte or fluid deficits and
withdrawal symptoms.
PostOperative Cognitive Dysfunction (POCD)
POCD is much more
subtle and therefore neuropsychological testing is necessary for the detection.
It is present for weeks or months postoperatively and it resembles dementia but
in most cases the deficits are not so pronounced. Accordingly, this disorder is
easily overlooked and it is difficult to evaluate.
Most studies of
postoperative cognitive dysfunction have focused on cardiac surgery. The
incidence of cognitive deficits is 30-80% after 1 week (2) and POCD is still
common after several months (3). The aetiology behind POCD is thought to be related
to the use of cardiopulmonary bypass (CPB) because the duration of CPB is an
important risk factor. Other risk factors for POCD after cardiac surgery are
increasing age and poor cardiac function. Management of pH during hypothermic
CPB using alpha-stat is associated with less POCD than pH-stat, presumably
because cerebral hyperperfusion occurs during pH-stat. Aortic arteriosclerosis
is a source of emboli, especially at cannulation and should therefore be
identified (4). Hypothermia probably offers effective protection while
pharmacological neuroprotective agents generally have been disappointing.
Coronary artery bypass can now be performed without CPB in a high proportion of
patients in many institutions and this may reduce the incidence of POCD.
Non-cardiac surgery
Generally, it has not
been possible to detect POCD beyond the first postoperative week after
non-cardiac surgery. Numerous studies
have used cognitive function as end-point in comparison of surgical or
anaesthetic regimens. A common comparison is regional versus general
anaesthesia for suitable procedures such as hip or knee replacement or
gynaecological procedures. Williams-Russo et al. found no difference between
epidural and general anaesthesia after knee replacement but a deterioration of
cognitive function was found in 5% after 6 months (5). One methodological
problem was the lack of a control group who did not undergo surgery. This is an
important limitation and a common weakness in studies within this area. Without
a control group it is impossible to compensate for the practice effect and
estimate the variation related to the specific neuropsychological tests.
This study was the
first one that included a correction for both the practice effect and for the
variability in neurospsychological test results. The ISPOCD study included 1218
elderly patients who underwent major non-cardiac surgery with general
anaesthesia (6). Arterial hypotension or hypoxaemia were suspected as risk
factors and extensive monitoring was carried out using oscillometric blood
pressure monitoring and continuous pulse oximetry on three postoperative
nights. POCD was found in 25.8% after 1 week and in 9.9% after 3 months. Age
was found to be an important risk factor, the incidence of POCD after 3 months
was 7% in those aged 60-69 and 14% in those above 69 years. Other risk factors
after 1 week were duration of anaesthesia, respiratory complications,
infectious complications and second operation. Level of education was also
important because well-educated patients experienced less POCD after 1 week. No
significant correlation was found with hypoxaemia nor hypotensive episodes. The
ISPOCD study confirmed that POCD existed after non-cardiac surgery with general
anaesthesia and also that age was a significant risk factor.
Etiological considerations
Several etiologic
mechanisms behind POCD have been suggested. One possibility is brain hypoxia
caused by arterial hypoxaemia or low flow. Another plausible cause for POCD
could be residual concentrations of general anaesthetics such as
benzodiazepines that have also active metabolites. However, even though the
drugs were detected a week after surgery, no significant correlation was found with POCD in a study
evaluating this hypothesis (7). Another explanation could be a long-lasting
effect of general anaesthestics on cholinergic or glutaminergic
neurotransmission. Finally, psychological factors related to illness and
environment during hospitalisation may be important.
Conclusion
Delirium and
postoperative cognitive dysfunction detected with neuropsychological testing
are common in elderly patients. Age is the most important risk factor.
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