Prospective memory in bipolar disorder: A meta-analysis

Highlights

• Prospective memory (PM) is vulnerable to change in patients with psychiatric disorders including bipolar disorder.

• To date, studies on PM in bipolar disorder have been published with inconsistent findings.

• This meta-analysis showed that both time-based PM and event-based PM appeared to be impaired in patients with bipolar disorder.

Abstract

There are conflicting findings about prospective memory (PM) performance in bipolar disorder. This meta-analysis systematically examined PM in patients with bipolar disorders. Articles were systematically searched in both English and Chinese databases, from their inception to Nov 15, 2016. Only case-control studies on PM in bipolar patients were included for analyses. The random effect model was used in all meta-analytic outcomes. Four studies (n = 390) comparing PM performance between patients with bipolar disorder (n = 208) and healthy controls (n = 182) were included. Three studies were rated as "high quality", while the quality of evidence in 3 meta-analyzable outcomes ranged from "moderate" (67%) to “high” (33%). Compared to healthy controls, bipolar disorder patients showed impairments in overall PM [2 studies, n = 196; SMD: − 1.08 (95%CI: − 1.61, − 0.55), P < 0.0001; I² = 65%], event-based PM [4 studies, n = 367; SMD: − 0.51 (95%CI: − 0.78, − 0.23), P = 0.0003; I² = 37%] and time-based PM performance [4 studies, n = 367; SMD: − 0.82 (95%CI: − 1.11, − 0.52), P < 0.0001; I² = 41%]. In this meta-analysis, both time-based PM and event-based PM deficits appeared to be evident in bipolar disorder.

Introduction

A wide range of cognitive components, such as attention, executive functions, and memory, may be impaired in different stages of bipolar disorder (Bearden et al., 2001, Martinezaran et al., 2004). Most studies on memory impairment in bipolar disorders have only focused on retrospective memory (RM); i.e., the ability to remember or recall past events. Recent evidence has found that up to “50–80%” of daily activities are largely influenced by “prospective memory (PM)” (Kliegel and Martin, 2010). PM refers to the ability to remember to do something in the future (Einstein and Mcdaniel, 1990), which is more complex and important for daily activities than RM. As PM is essential to maintain a structured daily routine (Ceci and Bronfenbrenner, 1985, Woods et al., 2011), its impairment could result in compromised personal and social functions (Altgassen et al., 2008), therefore PM has a major impact on the management of bipolar disorder. Impaired PM, such as forgetting to take medication or meet appointments as scheduled, could lead to poor outcome.

The process of PM includes four stages: encoding of an intention, retaining the information, executing the intention, and evaluating the outcome (Green et al., 2000). PM is characterized by the two features: 1) a delay between forming and performing of the prospective intention, and 2) the execution of the previously formed intention that is often self-initiated, since ongoing activities involved can distract the individual from staying on the prospective task (May et al., 1985; Tohen et al., 1992).

PM is typically classified as time-based PM, event-based PM and activity-based PM (Einstein and Mcdaniel, 1990). In time-based PM tasks, the individual needs to remember to perform an action at a specific time in the future, even when there is basically no explicit prompts. In event-based PM tasks, the execution of the prospective intention is triggered by an external event. Activity-based PM is linked to event-based PM as both are carried out when an external cue appears. However, the person does not have to interrupt the ongoing activity when carrying out the activity-based PM, because the external cue is just the “ending” of an ongoing task (Kvavilashvili and Ellis, 1996). Therefore, activity-based PM is considered to be the least demanding, while time-based PM is the most demanding PM task (Shum et al., 2004, Ungvari et al., 2008). Nonetheless, it has been argued that the cognitive processes underlying activity-based PM are different from those of event-based PM in many aspects (Brewer et al., 2011).

In psychiatry, research on PM have mainly focused on schizophrenia. Studies have consistently found PM deficits in chronic and first-episode schizophrenia patients and their first-degree relatives; therefore, making PM an endophenotype of schizophrenia (Liu et al., 2017, Lui et al., 2011, Wang et al., 2010, Zhou et al., 2012, Zhuo et al., 2013). A meta-analysis concluded that schizophrenia patients exhibited impairments in all subtypes of PM with time-based PM being the most impaired (Wang et al., 2009).

In recent years PM function in bipolar disorder has gained increasing attention. To date there are conflicting results from published case-control studies (Au et al., 2016, Au, 2013, Lee et al., 2010, Xiang et al., 2014), which are limited due to relatively small sample sizes. To the best of our knowledge, no meta-analysis on PM in bipolar disorder has been published which gave the impetus to conduct this meta-analysis.