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.
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.
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.
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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