Multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD) are chronic immune-mediated demyelinating conditions of the central nervous system (CNS) (Rosenthal et al., 2020). MS affects approximately 2.8 million people worldwide, most often women aged 20-50 years, and is characterized by demyelination, inflammation, neuroaxonal loss, and the accumulation of progressive disability (Batran et al., 2024, Koriem, 2016). NMOSD, while less prevalent, typically begins in adulthood between ages 35 and 45, shows a strong female predominance, and is often associated with pathogenic aquaporin-4 antibodies (AQP4-Ab), leading to astrocyte damage (Borisow et al., 2018, Bagherieh et al., 2023). Both diseases can cause a wide range of neuropsychiatric manifestations (Mirmosayyeb et al., 2026a, Mirmosayyeb et al., 2026b, Shaygannejad et al., 2025), such as depression, anxiety, irritability, apathy, and cognitive impairment (CI), which has emerged as a leading feature accompanied by motor and sensory impairment (Chiaravalloti and DeLuca, 2008, Silveira et al., 2019, Blanc et al., 2008, Oertel et al., 2019; Yazdan Panah et al., 2025).

CI affects up to 65% of MS patients and is regarded as one of the major manifestations of the disease (Benedict et al., 2020). CI in MS patients primarily affects information processing speed (IPS), memory, and executive function (Chiaravalloti and DeLuca, 2008), and has been linked to increased unemployment (Strober et al., 2014), lower quality of life (Baumstarck-Barrau et al., 2011), and disability progression (Pitteri et al., 2017). MS presents with different disease courses, and progressive forms are commonly associated with more pronounced and widespread CI than relapsing–remitting MS (RRMS), underscoring the importance of disease course in evaluating cognitive outcomes (Benedict et al., 2020). Among NMOSD patients, CI has been underestimated, but today it is recognized as a significant burden (Oertel et al., 2019). Near 45% of NMOSD patients have CI (Panah et al., 2025), with impairments typically affecting attention, IPS, and memory (Saji et al., 2013, Czarnecka et al., 2020).

Personality traits are described as stable patterns of thought, feeling, and behavior, encompassing Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism, which are increasingly recognized as relevant psychological features in both MS and NMOSD (McCrae and John, 1992, Hampson, 2012; Vaheb et al., 2024). Moreover, it seems that personality traits are not different between MS and NMOSD (Shin et al., 2019). Personality traits are most often evaluated using the NEO–Five Factor Inventory (NEO-FFI), which is a 60-item self-report questionnaire in which responses are given on a five-point Likert scale (Schwartz et al., 2011).

CI can manifest during the early phases of MS and even among patients with minimal physical disability, which reflects substantial interindividual variability in cognitive outcomes (Portaccio and Amato, 2022). Personality traits can influence cognitive function, psychosocial outcome, quality of life, and treatment adherence in CNS demyelinating disorders such as MS and NMOSD (Maggio et al., 2020, Kazzi et al., 2024, Bustos et al., 2022, Song et al., 2023, Ko et al., 2020). Prior research has demonstrated that personality traits are associated with cognitive performance in MS patients (Chu et al., 2022, Akbar et al., 2011, Eijlers et al., 2018). Among personality traits, Conscientiousness and Neuroticism have shown the most associations with cognitive outcomes, such as IPS and global cognitive functioning in MS (Roy et al., 2018, Mirmosayyeb et al., 2025). Moreover, changes in personality traits over time, particularly decrease in Conscientiousness and increase in Neuroticism, are linked to cognitive decline in MS (Kazzi et al., 2024).

The existing literature has considerable limitations. Most of them focused on selected personality traits and included small sample sizes (Maggio et al., 2020, Mirmosayyeb et al., 2025). Thus, the relationship between the full spectrum of personality traits and cognitive function in MS remains incompletely understood. Furthermore, the role of personality traits in cognitive functioning has not yet been comprehensively investigated in NMOSD. Given the distinct immunopathology, clinical manifestations, and psychosocial impact of MS and NMOSD, examining the relationship between personality traits and cognitive function separately within each disease group is warranted. Therefore, the aim of this cross-sectional study was to assess personality traits and their associations with cognitive performance in two distinct group of MS and NMOSD patients.