Oct 5, 2026·Special Population & Related Conditions
Multiple Sclerosis and IQ: How MS Affects Thinking Speed and Intelligence
Multiple sclerosis and IQ: MS rarely causes a large IQ drop, but about 34% to 65% of adults show cognitive impairment, most often slowed processing speed.
Dr. Russell T. WarneChief Scientist
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Multiple sclerosis rarely causes a large drop in overall IQ, but it often affects thinking: between about a third and two-thirds of adults with MS meet research criteria for cognitive impairment, most often in processing speed and memory. Vocabulary and general knowledge tend to hold up, so a full-scale IQ score can look normal while a person is noticeably slower than before.
Multiple sclerosis (MS) is a disease of the central nervous system in which the immune system damages "myelin", the insulating layer around nerve fibres, leaving scattered lesions in the brain and spinal cord. This page covers how common cognitive problems are in MS, why processing speed is usually affected first, why standard IQ scores can mislead, how premorbid IQ and cognitive reserve shape outcomes, and what is known about children and teenagers with MS.
How common cognitive problems are in MS
The classic community study is Rao and colleagues' 1991 comparison of 100 people with MS and 100 matched healthy controls. Using a battery of 31 test scores, they classed 48 people with MS and 5 controls as impaired, an estimated rate of 43% in the MS group. Earlier clinic-based studies had reported 54% to 65%, which the authors suspected overstated the rate because clinic patients tend to be more affected.
A 2020 review in The Lancet Neurology by Benedict and colleagues puts the range for adults at 34% to 65%, depending on the research setting and disease course. The variation between people is large. Some have marked problems early, while others show little change after many years, and cognitive symptoms can appear without other neurological signs.
Cognitive impairment matters for daily life. In the companion paper to Rao's study, people in the impaired group were less likely to be working and took part in fewer social activities than people with MS whose cognition was intact, even though the two groups did not differ in physical disability or illness duration.
Processing speed comes first
The most consistent finding in MS is slowing. Denney and colleagues gave a battery of executive, memory and speeded tests to people with relapsing and primary progressive MS and to healthy controls. Once differences in fatigue and depression were statistically controlled, the only differences that remained involved the speed of information processing, which is why the paper's subtitle is "mostly a matter of speed." Benedict's review likewise names "cognitive processing speed" and episodic memory (memory for events and new information) as the domains most often affected.
Rao's study also found which abilities were least affected: language and immediate and remote memory were impaired less often. That pattern matters for IQ. Vocabulary and accumulated knowledge, which make up much of crystallized intelligence, tend to hold up better than timed tasks.
The standard screening test reflects this. In the Symbol Digit Modalities Test (SDMT), a person sees a key pairing symbols with digits and says aloud the matching digit for as many symbols as possible in 90 seconds. An international expert panel recommended a brief battery for MS clinics (BICAMS) built around the SDMT plus a verbal and a visual memory test. Benedict's review also notes that cognition can decline abruptly during a relapse, sometimes as the main sign of the flare, with the drop visible on the SDMT.
Why a full-scale IQ can mislead in MS
A full-scale IQ blends many abilities into one number. In a person with MS, strong verbal and reasoning scores can offset a slow processing speed index, producing an average-looking total that hides the change the person has noticed. For this reason, Benedict's review notes that clinical neuropsychologists have largely moved from lengthy batteries to targeted tests of speed and memory. Research on COVID-19 often converts cognitive composites into IQ-point equivalents, while MS research mostly reports SDMT scores and impairment rates.
The disease can also interfere with how a test is taken. MS can cause arm weakness, poor coordination and visual problems, which slow written or hands-on tasks for reasons unrelated to reasoning. Rao dropped the written version of the SDMT in favour of an oral response for exactly this reason, to avoid confounding by arm weakness and "ataxia" (loss of coordination). Fatigue and depression add further noise, as Denney's results show.
The key comparison in MS is with the person's own earlier ability, which is rarely on record. Clinicians therefore estimate premorbid IQ, usually from vocabulary or word-reading tests, because those skills are relatively resistant to brain disease. A current score well below that estimate, especially on timed tasks, suggests decline even when the total is in the average range. These assessments belong to a neuropsychologist working with the person's neurologist.
Premorbid IQ, cognitive reserve, and the years before diagnosis
People with the same amount of brain damage can show very different cognitive outcomes, and premorbid ability helps explain why. In a study of 38 people with MS, Sumowski and colleagues measured brain atrophy on MRI and estimated premorbid intelligence from a Wechsler vocabulary test. Atrophy predicted slower information processing, but the effect was weaker in people with higher premorbid scores. A follow-up study of 44 people found the same buffering pattern for learning and memory, using a vocabulary-based estimate of lifetime intellectual enrichment.
This is the "cognitive reserve" idea, also studied in ageing and dementia. Our article on whether IQ is correlated with dementia explains the concept in more detail. Sumowski and Leavitt's review adds that early-life reading and hobbies, as well as larger maximal brain growth, are linked to better resistance, and that the benefit may come from more efficient patterns of brain activity.
The disease may also start affecting cognition before diagnosis. Cortese and colleagues linked the conscription test scores of Norwegian men, taken at ages 18 to 19, to the national MS registry. Men whose first MS symptoms appeared within two years of testing scored lower than controls, a gap the authors equated to about 6 IQ points. Men who later developed primary progressive MS scored 4.6 to 6.9 IQ points lower as much as 20 years before their first progressive symptoms. This suggests a lower score can be an early sign of disease activity, so a lower premorbid IQ is not necessarily a cause of MS.
Children and teenagers with MS
Paediatric-onset MS, with onset before age 18, accounts for about 2% to 10% of cases. Because it strikes while the brain is still developing, it can affect IQ itself as well as speed and memory. In an Italian study of 63 children and adolescents with MS and 57 healthy controls, Amato and colleagues found that 5 patients (8%) had an IQ below 70 and 31% met criteria for cognitive impairment. Younger age at onset predicted lower IQ, and an IQ below 90 was the strongest predictor of wider cognitive impairment.
The effects can extend into adult life. Using the Swedish MS Registry, McKay and colleagues compared 300 adults with paediatric-onset MS to 5,404 with adult-onset MS. After adjusting for age and disease duration, the paediatric-onset group scored lower on the SDMT, declined faster over time and had 1.44 times the odds of cognitive impairment. For children and teenagers with MS, a neuropsychological assessment arranged through their paediatric neurology team can guide school support.
Frequently asked questions
Does multiple sclerosis lower IQ?
In adults, MS more often slows processing speed and memory than it lowers overall IQ, because verbal knowledge tends to be preserved. Children with MS, especially those with earlier onset, are more likely to show lower IQ scores.
What percentage of people with MS have cognitive problems?
Studies put the figure for adults at about 34% to 65%, depending on the setting and disease course. A community study found 43%.
Why is processing speed affected first in MS?
MS damages myelin, which speeds signals along nerve fibres, and lesions are spread across brain networks. Slowed processing is the most consistent finding across studies, even after controlling for fatigue and depression.
Does a high IQ protect against MS?
It does not prevent MS. People with higher premorbid ability or more intellectual enrichment tend to show fewer cognitive problems for the same amount of brain damage, which is called cognitive reserve.
Should someone with MS take an IQ test?
A full IQ test is not the usual first step. Neurologists typically screen with the SDMT, and a neuropsychologist can then compare current scores with an estimate of earlier ability.
The takeaway
Multiple sclerosis and IQ have a more specific relationship than the headline number suggests. Adults with MS usually keep their vocabulary and knowledge, while processing speed and new learning are where change tends to appear, in roughly a third to two-thirds of people. Premorbid ability and cognitive reserve help explain why outcomes vary so much, and paediatric-onset MS carries a higher risk of lower IQ. Concerns about thinking in MS are best raised with a neurologist, who can arrange a neuropsychological assessment. For adults without a neurological condition who want to see where their reasoning stands, an online IQ test built by psychometricians gives a normed starting point.
References
1. Rao, S. M., Leo, G. J., Bernardin, L., & Unverzagt, F. (1991). Cognitive dysfunction in multiple sclerosis. I. Frequency, patterns, and prediction. Neurology, 41(5), 685-691. doi.org
2. Benedict, R. H. B., Amato, M. P., DeLuca, J., & Geurts, J. J. G. (2020). Cognitive impairment in multiple sclerosis: Clinical management, MRI, and therapeutic avenues. The Lancet Neurology, 19(10), 860-871. doi.org
3. Rao, S. M., Leo, G. J., Ellington, L., Nauertz, T., Bernardin, L., & Unverzagt, F. (1991). Cognitive dysfunction in multiple sclerosis. II. Impact on employment and social functioning. Neurology, 41(5), 692-696. doi.org
4. Denney, D. R., Lynch, S. G., Parmenter, B. A., & Horne, N. (2004). Cognitive impairment in relapsing and primary progressive multiple sclerosis: Mostly a matter of speed. Journal of the International Neuropsychological Society, 10(7), 948-956. doi.org
5. Langdon, D. W., Amato, M. P., Boringa, J., Brochet, B., Foley, F., Fredrikson, S., ... Benedict, R. H. B. (2012). Recommendations for a Brief International Cognitive Assessment for Multiple Sclerosis (BICAMS). Multiple Sclerosis Journal, 18(6), 891-898. doi.org
6. Sumowski, J. F., Chiaravalloti, N., Wylie, G., & DeLuca, J. (2009). Cognitive reserve moderates the negative effect of brain atrophy on cognitive efficiency in multiple sclerosis. Journal of the International Neuropsychological Society, 15(4), 606-612. doi.org
7. Sumowski, J. F., Wylie, G. R., Chiaravalloti, N., & DeLuca, J. (2010). Intellectual enrichment lessens the effect of brain atrophy on learning and memory in multiple sclerosis. Neurology, 74(24), 1942-1945. doi.org
8. Sumowski, J. F., & Leavitt, V. M. (2013). Cognitive reserve in multiple sclerosis. Multiple Sclerosis Journal, 19(9), 1122-1127. doi.org
9. Cortese, M., Riise, T., Bjørnevik, K., Bhan, A., Farbu, E., Grytten, N., ... Myhr, K.-M. (2016). Preclinical disease activity in multiple sclerosis: A prospective study of cognitive performance prior to first symptom. Annals of Neurology, 80(4), 616-624. doi.org
10. Amato, M. P., Goretti, B., Ghezzi, A., Lori, S., Zipoli, V., Portaccio, E., ... Trojano, M. (2008). Cognitive and psychosocial features of childhood and juvenile MS. Neurology, 70(20), 1891-1897. doi.org
11. McKay, K. A., Manouchehrinia, A., Berrigan, L., Fisk, J. D., Olsson, T., & Hillert, J. (2019). Long-term cognitive outcomes in patients with pediatric-onset vs adult-onset multiple sclerosis. JAMA Neurology, 76(9), 1028-1034. doi.org
Hero image: Stopwatch, 1810201155, ako, by Ansgar Koreng, licensed CC BY-SA 4.0 (creativecommons.org/licenses/by-sa/4.0). Via Wikimedia Commons.
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