Delirium is well defined and is described in the Diagnostic and Statistical Manual of Mental Disorders fourth edition
). The key characteristics are a change in mental status characterized by a reduced awareness of the environment and a disturbance in attention. This may be accompanied by other, more florid, perceptual symptoms (hallucinations) or cognitive symptoms including disorientation or temporary memory dysfunction. The patient may express hypoactive, hyperactive, or mixed psychomotor behaviours. Several tests have been developed and validated for use in diagnosis and grading of delirium. These include the Confusion Assessment Method (CAM), the Delirium Rating Scale Revised-98, and the Delirium Symptom Interview.9,21
A recent study from Japan found that the NEECHAM Confusion Scale and the Estimation of Physiologic Ability and Surgical Stress (E-PASS) are useful in diagnosis as well.17
Severity may vary, can be graded, and may have prognostic value.52
By definition, although the disorder develops acutely, the condition will wax and wane during the course of a day. These symptoms are not exclusive to delirium. Patients who have baseline dementia, psychosis, or anxiety/depressive disorder may present diagnostic challenges.
There are many subtypes of delirium, including those attributable to an underlying medical condition (delirium due to a general medical condition
), medications (substance-induced delirium, substance intoxication delirium
), or withdrawal from medications (substance withdrawal delirium
). Sometimes delirium may be multifactorial (delirium due to multiple aetiologies
) or of unclear aetiology (delirium not otherwise specified—NOS
). Emergence agitation or delirium might be thought of as a subset of substance-induced
delirium. It has predominance in paediatric patients, has been correlated with general anaesthesia, and provided the patient is guarded from harming themselves, usually resolves without sequelae.24
Emergence delirium in the paediatric population has been demonstrated to be associated with preoperative anxiety and responds to behavioural preparation and preoperative sedation.25
For the purpose of this review, we are interested in delirium that occurs after a relatively normal emergence and that occurs at some interval after surgery and anaesthesia. This entity, which is more closely associated with older age, is referred to as postoperative (interval) delirium.
Postoperative delirium (POD) is not temporally related to emergence from anaesthesia. By definition, patients with POD do not have an identifiable aetiology, although there can be other contributing factors. These patients often emerge smoothly, and may be lucid in the post-anaesthesia care unit. However, after this initial lucid interval, the patients develop the classic fluctuating mental status, most commonly between postoperative days 1 and 3. Some postoperative patients may reside in the ICU; however, the term ICU delirium
(previously known as ICU psychosis
) may include both medical and surgical patients. POD can differ from delirium in medical patients because the admission characteristics of the two groups can be different. By definition, patients hospitalized for medical indications are either acutely ill or have exacerbations of chronic diseases. Most surgical operations are elective and patients have been managed to ensure optimal physical status before entering the hospital. Surgery and the associated anaesthetics and analgesics are generally absent in medical patients, but can contribute to POD. An important reason to distinguish POD from delirium seen in medical patients is the report by Brauer and colleagues,8
suggesting that patients with POD are more likely to result in initial complete recovery than other forms of delirium. However, POD is far from benign. In several recent 2 yr-plus cohort studies of elderly patients, hip fracture patients who develop POD are more likely to die, be diagnosed with dementia or mild cognitive impairment (MCI), and require institutionalization.6,26
In contrast, postoperative cognitive dysfunction (POCD) is more difficult to define. Broadly, POCD refers to deterioration in cognition temporally associated with surgery. While the diagnosis of delirium requires a detection of symptoms, the diagnosis of POCD requires preoperative neuropsychological testing (baseline) and a determination that defines how much of a decline is called cognitive dysfunction. The spectrum of abilities referred to as cognition is diverse, including learning and memory, verbal abilities, perception, attention, executive functions, and abstract thinking. It is possible to have a decrement in one area without a deficit in another. Self-reporting of cognitive symptoms has been shown to correlate poorly with objective testing, so valid pre- and postoperative testing is essential to the diagnosis of POCD.23
Many elderly patients have pre-existing MCI that has gone undiagnosed.
Unfortunately, there has not been a standard methodology used in the multiple studies within the POCD literature.36
Selection of neuropsychological test instruments and the amount of change considered to be significant, timing of testing, and inclusion and exclusion criteria have all varied.38
Furthermore, the batteries used, while relevant, have had floor effects and we have not incorporated batteries that are somewhat different from those used by dementia researchers. Hence, it is difficult to define the presence and therefore incidence of POCD or to clearly understand the relationship between POCD and other dementing illnesses. Some commonly used testing instruments include the Logical Memory Test, the CERAD word list memory, the Boston Naming test, Category Fluency test, Digit Span Test, Trail making test, and Digit symbol substitution test. Interestingly, POCD test batteries tend to be a compilation of tests which have shown differences among subjects in previous studies of POCD. The domains that were most sensitive include verbal learning and working memory, episodic memory, processing speed, and set shifting.
The method of scoring the testing batteries and determining how much dysfunction is clinically significant remains an open subject. One method is the percentage change method, that is, postoperative score−preoperative score/preoperative score. Averaging across groups is discouraged, because while some patients will decline, others improve over time and this difference can be masked. Another method defines a number of standard deviations outside of which a score will be called a decline. However, this method is flawed for patients with low baseline scores (floor effect). By necessity, the absolute magnitude of the change required for significance will vary between studies, since the norm is determined from the preoperative baseline test scores. Finally, some studies have used per cent change (e.g. 20%) to define decline. The limitation of this method is that the baseline low scoring patients require a smaller change in their raw score to meet POCD criterion.
The timing of testing is important as well. It is possible that patients who undergo baseline testing on the morning of their procedure might not score and also patients tested days before, secondary, preprocedural anxiety. After operation, patients who are testing shortly after surgery can test worse than those who are tested weeks to months later possibly due to pain, residual drugs, and health status. However, long-term follow-up and testing is confounded by attrition, that is, patients who experience the greatest decline are the least likely to follow-up with their postoperative cognitive testing and drop out of the study. This may be a significant cause for underestimating the true incidence of POCD. Additionally, there can be significant variability between testing sessions due to learning and examiner bias.29
Although variability in neuropsychological test data contributes to a low consistency between postoperative test sessions, the differences detected suggest that this does not fully explain the detection of cognitive dysfunction after major surgery.39
It is clear that deterioration is not random variation between testing sessions.
The current literature is also diverse with respect to inclusion and exclusion criteria of patients with MCI. MCI is described as the prodromal state, a heterogeneous group of conditions including Alzheimer's dementia, cerebral vascular disease, and other dementia. Most of the major studies have excluded this group due to limitations of the test battery. This is true even though this group may be the most significant risk for POCD by virtue of having less cognitive reserve.45
By not differentiating this patient population, it is possible that the incidence of cognitive decline has been ‘washed out’ by the larger sample.