When Charcot described multiple sclerosis in 1877, he observed that "conceptions are formed slowly" in many patients, presciently identifying cognitive processing speed as a core MS symptom.
Nearly 150 years later, his observation underpins a fundamental challenge in MS clinical trials: how to measure cognitive changes that determine whether someone can work, manage complex tasks, or maintain independence, yet often escape detection by physical disability scales.
The disconnect between physical and cognitive function in MS creates hidden disability affecting 40 to 70% of patients. Someone with minimal physical impairment, measured by the Expanded Disability Status Scale, may struggle with employment due to slowed information processing, impaired attention, or working memory deficits.
Current evidence demonstrates that cognitive impairment predicts occupational disability more strongly than physical disability in early-stage MS, yet cognitive assessment remains inconsistently applied in both clinical practice and trial design.
Progressive MS trials face particular challenges. The Expanded Disability Status Scale (EDSS), for decades the primary disability outcome measure, emphasizes walking ability in its upper ranges while essentially ignoring cognitive and upper extremity function. When trials test treatments intended to slow neurodegeneration across the brain, relying solely on a measure weighted toward ambulatory function misses critical domains of disability.
This limitation drove the creation of the Multiple Sclerosis Outcome Assessments Consortium (MSOAC), formed specifically to qualify outcome measures that are valid, reliable, sensitive to change, and meaningful to patients. Among MSOAC's priorities: establishing a cognitive performance outcome that captures information-processing speed deficits. Charcot recognized deficits that affect daily functioning but that physical measures fail to detect.
The Symbol Digit Modalities Test (SDMT) emerged from MSOAC's comprehensive psychometric analyses of 14 MS disease-modifying registration trials as the cognitive measure of choice. This recognition validated decades of MS research demonstrating SDMT's sensitivity to cognitive impairment, correlation with brain pathology on MRI, and ability to detect clinically meaningful change. The Multiple Sclerosis Functional Composite, which originally included the Paced Auditory Serial Addition Test (PASAT), increasingly replaces PASAT with SDMT due to the latter's superior psychometric properties and patient acceptance.
Yet SDMT, while validated and widely adopted, measures a specific cognitive construct information processing speed. Research demonstrates that SDMT performance reflects not only processing speed but also lexical access speed, working memory, and sustained attention. Multiple cognitive processes contribute independently to SDMT scores, making it sensitive but not specific. For comprehensive cognitive profiling across the domains affected in MS, additional assessment tools become necessary.
MS-related cognitive impairment follows characteristic patterns. Processing speed deficits appear most frequently, affecting approximately 52% of patients on SDMT and 27% on PASAT. Executive dysfunction manifests in 15 to 25% of patients, compromising planning, concept formation, and abstract reasoning. Working memory, sustained attention, and episodic memory show variable impairment depending on disease course and lesion burden. Visuospatial processing and verbal fluency affect smaller but significant patient subsets.
Recent large-scale online cognitive assessment in 4,526 UK MS Registry members revealed a prevalent MS subtype exhibiting significant cognitive deficits with minimal motor impairment, a hidden disability that current clinical evaluation often misses entirely. Clustering analysis identified this cognitive-predominant phenotype across the patient population, underscoring the importance of systematic cognitive screening beyond those presenting obvious physical disability.
This heterogeneity demands domain-specific assessment. Treatments targeting specific pathological mechanisms remyelination, neuroprotection, inflammatory modulation may produce selective cognitive benefits. Trials testing clemastine fumarate for remyelination, BTK inhibitors for progressive disease, or immunomodulatory approaches measure cognitive outcomes as secondary or exploratory endpoints. Detecting domain-specific treatment effects requires assessment batteries capable of profiling attention, memory, processing speed, and executive function independently.
In Part II, we'll explore how computerized cognitive assessment addresses these measurement challenges and what this means for the future of MS drug development.
About the author
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Helen Brooker is a specialist in cognitive test development and aging, with over 18 years of experience in academic and clinical research. She leverages her expertise in neuropsychological assessment, clinical trial delivery, and digital health solutions as a Senior Product Manager at Signant, where she oversees the company's proprietary computerized cognitive test solution. |