Discussion
Cognitive dysfunction is common in SLE and causes significant disability, but interpretation of the current literature has been limited by methodological issues. We sought to clarify the clinical associations in SLE with cognitive endpoints and found that among disease and treatment-related clinical factors, organ damage was the most consistent clinical variable associated with cognitive dysfunction. We used matched healthy controls for comparison, unlike previous studies that have used published normative data that did not take into account education or premorbid IQ, which can have a large impact on cognitive test results.32 In our study, we also examined for the presence of cognitive dysfunction using a variety of cognitive tests to evaluate different cognitive domains and used two thresholds of cognitive dysfunction to provide a thorough assessment. This study was performed on a real-world cohort, and unlike previous studies did not have exclusions such as patients with previous anxiety or depression or past discrete neurological events. We also found that cognitive dysfunction was vastly under-detected in clinical practice, being under-recognised by clinician assessment on SDI scoring.
While the correlation between organ damage and cognitive dysfunction has been reported in previous studies,33 34 findings regarding associations with disease activity measures have been conflicting. In our study, the association of organ damage and cognitive dysfunction was consistent across all cognitive test domains, and remained after adjustment for age, premorbid IQ or education. Despite our comprehensive cognitive testing, we did not observe an association between current or prior disease activity and cognitive dysfunction. Some previous studies shown a relationship between disease activity and cognitive dysfunction, but these were generally limited by narrow patient inclusion criteria,12 limited cognitive testing35 or small sample size.33 A lack of association between disease activity and cognitive dysfunction was observed in other studies.36–40 In our study, using matched controls and a larger sample size, we found that increased time-adjusted SLEDAI-2K correlated with improved verbal memory, and no other association with the other cognitive endpoints was found.
The effects of disease activity on cognition are complex. Although patients with severe acute neuropsychiatric manifestations can experience delirium and can present with acute cognitive dysfunction, the more common scenario is exemplified by what we studied, which is the presence of cognitive dysfunction in patients who were recruited from ambulatory care. At the time of recruitment, no patient was experiencing acute neuropsychiatric manifestations. Our cohort reflects real-world cross-sectional clinical experience, and thus participating patients had a range of disease activity, with median SLEDAI-2K of 3 (IQR 4, range 0 to 12), at the time of cognitive assessment. We also included an additional covariate of time-adjusted mean SLEDAI that captures information about disease activity across a period of observation. The range of disease activity both at the time of testing and over the duration of observation allows us to comment on relevant associations. Our findings suggest that pathogenic mechanisms that mediate contemporary disease activity in SLE may not be the same as those causing cumulative cognitive dysfunction.
High interferon-gene signature is known to be associated with serological and clinical activity in SLE.41 In our study, we found univariate associations between high interferon-gene signature and better cognitive performance. Anti-dsDNA positivity and hypocomplementemia also had a negative association with severe cognitive dysfunction. In murine lupus models, type 1 interferons stimulate microglia activation,42 suggesting a possible pathogenetic basis for interferon-driven central nervous system injury that is not supported by our clinical findings. Previous studies have found no correlation between both serum and cerebrospinal fluid interferon alpha levels and cognitive test performance in SLE.12 38 Ethnicity may play a role in this finding, as Asian ethnicity has been associated with better cognitive performance particularly on IQ testing43 and higher disease activity in SLE as previously demonstrated in our multiethnic lupus cohort.44
We explored relationships between cognitive dysfunction and medication use, as some previous studies have suggested that prednisolone use is associated with declining cognitive function,36 although a small randomised controlled trial (n=10) suggested some benefit from glucocorticoid therapy for cognitive performance in SLE.45 We did not observe any associations between glucocorticoid use or immunosuppressant use (both at the time of cognitive assessment and past exposure) and cognitive endpoints. Not unexpectedly, substance use correlated with worse cognitive test performance specifically in psychomotor speed. This finding is consistent with previous literature on the effect of substance abuse on cognition,46 although this is the first study reporting these effects in patients with SLE.
An important finding of this study is that cognitive dysfunction was vastly under-detected in clinical practice, even in a centre where annual SDI assessment in the clinic is routine. Of the 49 patients who had cognitive dysfunction on objective cognitive testing, only 4 (8%) had cognitive dysfunction recognised in the SDI domain which captures either cognitive dysfunction or psychosis that persists for longer than 6 months. Screening is not performed in the usual clinic setting, and formal neuropsychological assessment is not part of routine care and is only done in patients who present to their treating physician with cognitive complaints.
In terms of SDI domains, some appear to be more prominent in patients with cognitive dysfunction. The presence of damage in the ocular and neuropsychiatric domains were both significantly associated with cognitive dysfunction. Previous studies have similarly found higher incidence of cognitive dysfunction in patients with SLE with some of the neuropsychiatric domain conditions such as seizures and strokes.7 In our study, cardiovascular domain damage was also associated with severe cognitive dysfunction. The relationship of metabolic syndrome and cognitive function in SLE has been explored in other studies, with some risk association with type 2 diabetes36 and increased BMI,38 whereas hypertension was not associated with cognitive dysfunction36 38 and aspirin may in fact be protective.36 Findings on the relationship between hypercholesterolemia and cognition in SLE have been inconsistent.36–38 In our study, we found no associations between metabolic syndrome and cognitive performance.
Surprisingly, history of malignancy appeared to be associated with reduced cognitive dysfunction in our cohort. The effect of malignancy on cognition in lupus has not been previously studied. However, given that patients with cancer are known to have increased risk of both short-term and long-term cognitive dysfunction,47 this finding was unexpected. However, a previous study from our centre noted a possible association between malignancy and low interferon signature in SLE.31 It should also be noted that the cancer type, stage and treatments experienced by patients varied substantially, so a more detailed investigation of this in a larger cohort would be of interest.
There are some limitations to this study. Although the sample size is reasonable for a study that uses formal cognitive testing in the evaluation of cognitive dysfunction in patients with SLE, it is still relatively small when considering all the potential confounding covariates. Analysis of the SDI domain categories was limited by sample size and low frequency of individual organ domain damage events. Further exploration of the relationship between specific SDI domain categories and cognitive function would be of interest. As a cross-sectional observational study, we studied association of exposures of interest and the outcome of cognitive dysfunction, and causal inference should be interpreted carefully. Longitudinal analysis of the relationship between cognitive dysfunction and disease activity and organ damage would be of interest; a previous study suggests that cognitive dysfunction improves over time in the majority of patients.37
The relationship between mood disorders and cognitive performance is important to consider, as they may contribute to cognitive impairment seen in patients with SLE. The aim of this study was to focus on the relationship between cognitive function and clinical factors such as disease activity, damage, biomarkers and commonly used medications in SLE. We specifically chose not to exclude patients with common comorbidities in order to assess a ‘real world’ SLE cohort, in which there is high prevalence of depression and anxiety.48 49 In a separate study focusing on comorbidities, the results of which we have published,4 we found that there was a clear relationship between mood disorders and cognitive endpoints, in particular cognitive symptoms, in our SLE cohort.
Lastly, it is possible for some potential selection bias of patients who agreed to have the comprehensive neuropsychological assessment. We are conscious of this but have only excluded patients who do not have adequate proficiency in English to complete the tasks and recruited consecutive consenting patients. Healthy control group participants were recruited predominantly from family and friends of the SLE participants and via advertisement in the local community; their health status was self-reported. We believe that our study is generalisable for our English-speaking patients.
In conclusion, organ damage has a consistent association with worse cognitive performance in patients with SLE, whereas disease activity, serological activity and interferon-gene signature status did not. These findings highlight that cognitive dysfunction can occur in patients with SLE who do not exhibit high disease activity at a given point in time, and patients with more accumulated damage are particularly at risk. Screening is important in this population, as evidenced by the finding that SDI cognitive domain scoring vastly underestimated the prevalence of cognitive dysfunction identified by formal testing. Future studies should explore the potential mechanism of higher rates of cognitive dysfunction in patients without serological activity or high interferon-gene signature, as this may represent a specific subgroup of patients with SLE at higher risk that is currently unexplained.