Memory Deficits Persist in Multiple Sclerosis Despite Preserved Processing Speed

Evolving Cognitive Phenotypes in Multiple Sclerosis

Cognitive impairment remains a debilitating manifestation of multiple sclerosis that frequently leads to unemployment and reduced quality of life even when physical disability is minimal [1, 2]. For decades, clinical consensus has identified slowed information processing speed as the primary cognitive deficit, often utilizing the Symbol Digit Modalities Test (a brief, timed task requiring patients to match numbers to geometric symbols) to screen for decline [3, 4]. Meta-analytic data involving 136 studies show that thalamic volume correlates significantly with processing speed (r = 0.47, p < 0.001) [5], while molecular biomarkers such as neurofilament light chain (a protein released into the blood and cerebrospinal fluid during axonal injury) also show consistent associations with cognitive slowing (serum r = -0.137, p = 0.001) [3, 6]. To address these symptoms, network meta-analyses of over 3,000 patients suggest that exercise and mindfulness interventions offer the highest probability of improving information processing speed [7, 8]. However, as diagnostic criteria and disease-modifying therapies have evolved, the relevance of historical cognitive models to contemporary patients requires re-examination. A new study now provides a comprehensive analysis of cognitive performance in the modern era to determine if traditional assumptions about processing speed still apply.

Challenging the Speed-Centric Model

For more than 30 years, the clinical understanding of cognitive impairment in multiple sclerosis has been dominated by a speed-centric model. Established before the widespread availability of highly effective disease-modifying therapies, this framework identifies slowed information processing speed as the principal cognitive deficit. Under this historical model, clinicians presumed that slowed cognitive speed was the primary driver responsible for downstream deficits in other domains, including memory and executive function. This perspective suggested that if a patient could not process information quickly enough, their ability to encode and retrieve that information would naturally suffer, making speed the logical target for both screening and intervention. To re-evaluate this long-standing model, researchers analyzed patients diagnosed and treated within the modern diagnostic and treatment era, specifically those cared for between 2001 and 2025. By comparing contemporary patient cohorts against historical data and healthy controls, the study aimed to determine if the cognitive profile of the disease has shifted alongside advancements in pharmaceutical intervention and earlier diagnosis. Based on a comprehensive analysis of both case-control and large clinical cohorts, the researchers concluded that the speed-centric model of cognitive dysfunction is inaccurate for the modern diagnostic and treatment era. Instead, memory deficits remain highly prevalent even when processing speed is completely preserved, indicating that clinicians must rethink how they screen for cognitive decline in everyday practice.

Longitudinal Stability of Cognitive Speed

To investigate the trajectory of cognitive decline in the current treatment landscape, the researchers analyzed a case-control cohort consisting of 170 persons with early relapsing-remitting disease (defined as having a diagnosis for 5.0 years or less) and 45 neurologically healthy controls. The results indicated a clear divergence between cognitive domains. Patients with early relapsing-remitting disease performed worse than controls on memory assessments, yet they showed no such impairment in cognitive speed. This finding suggests that memory deficits emerge independently of processing speed, contradicting the traditional view that slowed processing is a prerequisite for other cognitive failures. The study further tracked these patients over a 6-year follow-up period to observe how these cognitive functions evolved. The researchers found that cognitive speed among patients remained normal and stable throughout the six years, even as the cohort experienced a subtle but measurable decline in memory performance. This longitudinal stability in processing speed highlights a significant shift in the clinical presentation of the disease compared to previous decades, where rapid slowing was often expected early in the disease course. When comparing these contemporary results to four historical studies of early relapsing-remitting disease diagnosed under older criteria, the researchers identified a narrowing gap in processing speed between patients and healthy individuals. The effect sizes for case-control differences in the current cohort were much lower for cognitive speed than those reported in historical data, meaning modern patients are performing much closer to healthy baselines. In contrast, the magnitude of memory deficits remained comparable to those seen in earlier eras. For the practicing neurologist, these data suggest that while modern disease management may be preserving processing speed, it has not yet achieved the same protective effect for memory functions.

Prevalence of Memory Impairment in Clinical Practice

To validate this shift in cognitive presentations within a real-world setting, the researchers analyzed an independent clinical cohort of 1004 consecutive patients aged 18 to 65 years with relapse-onset multiple sclerosis. These individuals completed standard-of-care cognitive screenings between 2018 and 2025, with data collected across three independent periods to ensure consistency. During the first period, which included 642 patients, the rates of poor performance (defined as scoring at or below the 7.5th percentile) were 8.4% for cognitive speed and 9.0% for attention. These rates are nearly identical to normative expectations for the general population, suggesting that these domains are well-preserved in many modern patients. In stark contrast, the same cohort exhibited significantly higher rates of impairment in memory functions, with 23.8% demonstrating poor verbal memory and 14.8% showing poor visuospatial memory. This dissociation between processing speed and memory was replicated in subsequent groups. Patients evaluated during the second period (n = 123) and third period (n = 239) demonstrated persistent memory deficits despite maintaining normal cognitive speed on co-normed tasks (standardized assessments where different cognitive functions are measured against the exact same reference population). The data revealed that objective cognitive speed among current patients showed a z-score mean (standard deviation) of 0.03 (1.14) and a median (interquartile range) of 0.00 (-0.67, 0.67). Because a z-score of 0.00 represents the exact average of the healthy normative population, these results indicate that the typical patient in the modern era does not exhibit the cognitive slowing once considered universal to the disease. Furthermore, objective cognitive speed in the modern cohort was much better than across several historical comparisons from 20 to 25 years ago. This improvement suggests that contemporary management strategies may be effectively protecting white matter integrity or compensatory mechanisms related to processing speed. However, the stability of memory deficits across eras indicates that memory impairment operates through different pathological mechanisms that current treatments may not adequately address. For clinicians, these findings emphasize that a patient's ability to perform quickly on a processing speed test does not rule out significant deficits in verbal or visuospatial recall, which remain prevalent and clinically relevant.

Patient-Reported Outcomes and Mood Influences

Beyond objective neuropsychological testing, the researchers analyzed subjective experiences within the total clinical cohort to understand how cognitive changes impact daily life. Self-reported cognitive deficits indicated that the most severe disease-related difficulties occurred in expressive language, specifically word-finding. These linguistic challenges were followed in severity by deficits in working memory and episodic memory (the clinical term for the ability to recall specific personal experiences and events). This hierarchy of patient-reported symptoms aligns with the objective findings of preserved processing speed, as patients prioritize communication and memory retrieval as their primary functional hurdles in the modern treatment era. The study also examined the intersection of cognitive performance and psychological factors to determine if perceived slowing was a primary neurobiological symptom or a secondary effect. While some patients reported minor issues with executive function and processing speed, the researchers found that a small difference in executive function and speed was fully explained by mood in patients with relapsing-remitting disease. This finding suggests that when clinicians encounter reports of cognitive slowing in this population, the underlying driver may be comorbid depression or anxiety rather than primary neurocognitive decline. For the practicing physician, this distinction is critical for treatment planning, as it may shift the clinical focus toward psychiatric management to alleviate the perceived cognitive burden. When comparing these modern subjective reports to historical data, a clear shift in the patient experience emerges. Current patient-reported attention and executive deficits were lower than those reported 35 years ago, despite comparable memory difficulties. This longitudinal trend suggests that while modern disease-modifying therapies and earlier diagnostic criteria may have mitigated the attention and executive impairments once common in multiple sclerosis, memory deficits remain a persistent and relatively unchanged challenge.

Clinical Implications for Progressive Disease and Future Targets

While the shift toward preserved processing speed is evident in relapsing-remitting populations, the researchers identified a distinct clinical profile for those with more advanced disease. Deficits in attention, executive function, and cognitive speed were observed as an exception in patients with secondary-progressive disease, suggesting that the speed-centric model of cognitive decline still applies to this specific subgroup. This divergence indicates that while modern disease-modifying therapies may protect cognitive velocity during the earlier relapsing-remitting stages, the neurodegenerative processes characteristic of secondary-progressive multiple sclerosis continue to impact the fundamental efficiency of information processing. For the clinician, this necessitates a stratified approach to cognitive screening, where the preservation of speed in early disease should not lead to a false sense of security regarding long-term cognitive stability. The persistence of cognitive challenges despite improved treatment outcomes underscores that memory deficits remain prevalent in the modern multiple sclerosis population, independent of improvements in processing speed. The study found that even when patients achieved objective cognitive speed scores remarkably similar to healthy normative expectations, they continued to exhibit significant impairments in verbal and visuospatial recall. This decoupling of speed and memory suggests that the pathological mechanisms driving memory loss are not merely downstream effects of slowed processing, but rather primary manifestations of the disease. Looking toward future clinical interventions, the researchers identified working memory maintenance (the ability to hold and manipulate information over short periods) as an important target for further investigation. Because current patient-reported attention and executive deficits are lower than they were 35 years ago, the focus of neuropsychological rehabilitation and drug development must shift toward these specific memory components. The study concludes that the field requires new, testable models of memory dysfunction informed by contemporary cognitive neuroscience. For practicing physicians, these findings emphasize the immediate need for memory-specific screening tools in the clinic, as traditional tests focused primarily on processing speed may overlook the most significant cognitive burdens facing patients today.

References

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