EDUCATION CORNER

Postoperative Cognitive Dysfunction: Basics Revisited

Ashish Bindra, MBBS, MD, DM
Associate Professor, Neuroanaesthesiology and Critical Care
All India Institute of Medical Sciences (AIIMS)
New Delhi, India

Surya Kumar Dube, MBBS, MD, DM
Assistant Professor, Neuroanaesthesiology and Critical Care
All India Institute of Medical Sciences (AIIMS)
New Delhi, India

Preet Mohinder Singh Rajpal, MBBS, MD, DNB
Instructor, Department of Anesthesia,
Washington University in
Saint Louis, Missouri

Post-operative cognitive dysfunction (POCD) is a disorder of thought processes, influencing isolated domains of cognition such as verbal or visual memory, language comprehension, visuospatial abstraction, attention, or concentration in absence of a better disease which explains the symptoms.¹ It can be subtle to disabling deterioration temporally associated with surgery and anesthesia. There is disagreement over definition of POCD since a formal definition has not been proposed. Neither Diagnostic and Statistical Manual of Mental Disorders (DSM) nor International Classification of Diseases (ICD) mentions POCD as a separate disease entity.  Patients think and speak incoherently, are disoriented and show impairment of memory and attention. Previously normal patients may have difficulty in memory, calculations and recall which results in difficulty in executing daily life activities. It may persist for days to months or longer. The symptoms are not only annoying for patients but also worrisome for relatives. A significant impact on long term quality of life, especially in the elderly is seen.^2,3 With increasing geriatric surgical procedures, recognition of special challenges and awareness of cognitive dysfunction is paramount for perioperative clinicians.

Incidence
There is a lack of consistency regarding definition and diagnosis of POCD. The reported incidence varies enormously depending on the definition, composition of the test battery, the percentage of decline considered significant and time of postoperative cognitive assessment.^4,5 In an international multicentre study of POCD (ISPOCD), a decline in memory function in more than one quarter of patients (over 60 years old) undergoing non-cardiac surgery was seen one week after surgery, 10% at three months and only 1% after one year. Studies are being done to see the long-term outcome of POCD but there is no evidence to prove its lasting or progressive nature.⁴

Diagnosis
The diagnosis is based on performance of battery of preoperative and postoperative neuropsychological tests. There is no fixed criteria/test to be used. Different types of tests used are depicted in Table 1. The tests are time consuming and lack sensitivity or specificity. Most of the tests are validated for detecting dementia and may miss mild cognitive impairment.  Montreal Cognitive Assessment Tool (MoCA), Addenbrooke’s Cognitive Exam (ACE-III) and the Quick MCI Screen (Qmci) are more suitable for being utilised in a preoperative clinic.5,6 There is no outright superiority of one test over the other. The neuropsychological tests used, hinge upon institutional or study protocol. Ideally POCD test batteries should be a collection of tests. The domains that were most sensitive include verbal learning and working memory, episodic memory, processing speed, and set shifting.¹ There has also been disagreement over how to classify patients who drop below the pre-set threshold in one domain, but who show cognitive improvement in other domains. Many other practical problems are seen with the execution of these tests. Application in patients with preexisting dementia and cerebral disease is challenging.^4-8 Test results can be affected by pain, anxiety and perioperative medicines so should not be performed on the day of surgery preoperatively. Postoperatively, the test should be done only after complete dissipation of acute effects of surgery and anaesthesia. Depending upon the clinical picture, postoperative testing should be performed at one week and three months post procedure. Ideally there is no role of testing before one week. No biomarkers or radiological studies are available for diagnosing POCD.

Differential Diagnosis
POCD is different from postoperative delirium which is a recognisable acute confusional state.6 Dementia is a progressive disease of the brain unrelated to surgery and anaesthesia.  The Diagnostic and Statistical Manual of Mental Disorders (DSM) V recognises delirium and dementia as clearly defined disease entities. Delirium occurs 24-96 hours after a procedure and manifests as a confusional state with disturbance in attention and reduced awareness of the environment; it is treatable. POCD develops later in the postoperative period. It is not clear whether delirium and POCD are a part of a continuum that culminates in dementia or a distinct entity in their own right.

Etiopathogenesis
What causes POCD is largely unknown. Broadly POCD is multifactorial in origin. Advancing age is considered to be the most defined risk factor for its development.⁸ Anesthesia related mechanisms like hyperventilation, hypotension, hypoxia leading to cerebral hypoperfusion have been proposed but largely remain unsupported by evidence.9 Change in the permeability of the blood brain barrier in response to anesthetics especially volatiles like isoflurane and sevoflurane is another likely mechanism.¹⁰ No correlation between the type of anesthetic technique (Regional versus General), agent (Total intravenous/Volatile agents) has been proven.^8,11,12 Patients undergoing cardiac surgery have a higher likelihood of developing POCD. Inflammatory reaction and cerebral micro emboli from the surgical site or cardiopulmonary bypass circuit are the likely mechanisms.¹³ POCD can be an immune response of the body to surgery resulting in systemic inflammation involving the brain. There is a weak correlation between pain and development of POCD. Vascular risk factors such as hypertension, obesity, diabetes mellitus and smoking are linked with cognitive decline in the general population.^14,15 Alcohol excess is strongly linked with delirium and long-term cognitive impairment and dementia via cerebral atrophy.¹⁵ Table 2 enlists the >risk factors involved in the development of POCD. Described risk factors include invasiveness of surgery, previous cerebral vascular accident, previous POCD, poor cognition, respiratory complications, infectious complications, alcohol abuse, anticholinergic, opiate medication etc. Inflammatory and genetic markers may also play an important role in pathogenesis of POCD.^9,16,17

Though there is no conclusive evidence to establish the relationship between POCD and anesthesia, anesthetics are still considered as one of the top reasons for development of POCD. Similarly, the duration of anesthesia is also considered a significant determinant of POCD.¹ The influence of a specific drug on outcome cannot be calculated due to non-uniform reporting across the available trials.^18,19 However, cutting down the total duration of anesthesia (both general or regional) can help alleviate the number of patients developing POCD. There is a growing evidence that titrating depth of anesthesia by monitoring processed EEG can help in decreasing incidence of POCD. Maintaining BIS values between 40-60 and avoiding BIS values less than 40 have found to benefit.^20,21 Similarly, use of near infrared spectroscopy (NIRS) in some studies was associated with the decrease in occurrence of POCD.²²

Treatment
There is no specific treatment for POCD and no definitive measures to prevent its occurrence. There are no major identifiable risk factors at to whom will decrease its incidence. However, the condition should be given due importance by the perioperative clinicians including surgeons and anesthesiologists as detection and management of postoperative cognitive disorders make a huge difference to an aging mind. Possibility of the cognitive dysfunction, especially in the elderly should be discussed and documented during preoperative evaluation.²³ Supportive therapy is recommended for the management of symptoms. Postoperative pain control is important. Non-pharmaceutical interventions include frequent communication of the health professional team with the patient. The visual and/or hearing impaired should be given appropriate aids in the perioperative period. Patients should be oriented to his/her environment. Clocks mounted in a prominent position, a calendar or television can help in reorientation.⁵ Few have suggested orientation of the patient to the hospital and its environment preoperatively, such as arrangements for adequate daylight in the room to maintain an adequate circadian rhythm. Perioperative medicine should be consulted early for caring for a geriatric patient with cognitive decline. It is important to maintain general health, correct anemia, electrolyte abnormalities, and continue chronic preoperative medications. The Royal College of Anaesthetists (2017) provide guidelines for provision of preoperative assessment services and recommends that old, frail patients have access to joint surgical, geriatric and anesthetic clinics. Perioperative care bundles developed from some of the above interventions can help develop strategies to cope with this important postoperative complication.²⁴

Conclusions
POCD may result in major life style changes, particularly in elderly patients.  Current evidence for diagnosis and management of POCD is patchy and incomplete. Patients in the perioperative period require support for diagnosis and management of this less apparent condition. Any connection of cognitive dysfunction with anesthesia and surgery is of concern. No individual anesthetic/surgery technique, drug or mode of monitoring has been proven to reduce the incidence of POCD.  Along with screening for other disorders, the preoperative assessment should also focus on counselling high-risk patients of POCD and adopt techniques to minimize perioperative insult. Lack of standardized definition, variable battery of diagnostic tests and different timing of diagnosis are major hinderances in management of the disease. The significance of cognitive dysfunction cannot be understated as it decides the ability of an elderly to live independently. Formulation of standard protocols for detection and management of postoperative cognitive disorders can make a difference to an aging mind.

TABLES

Table 1: Different types of tests used for the diagnosis of POCD

Table 2: Risk factors for the development of POCD

References

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