Adults With SMA Type 2 Have Lower IQ Scores in Specific Areas
Adults with spinal muscular atrophy (SMA) type 2 have lower intelligence quotient (IQ) scores on assessments for working memory and perceptual reasoning compared with the general population, according to data from a new study.
The study, “Cognitive Impairment in Adult Patients with 5q-Associated Spinal Muscular Atrophy,” was published in Brain Sciences.
Spinal muscular atrophy (SMA) is a genetic condition mainly characterized by muscle weakness and atrophy, which affects motor function. Conflicting evidence suggests that children and adolescents with SMA have either a mean IQ above or below average compared with healthy children. Few data, however, are available on cognitive performance in adult patients with SMA.
“Most studies so far on cognitive performance and impairment in SMA include only children or adolescents with SMA, although the progression of disease continues even into adulthood,” the researchers wrote.
Thus, a team led by researchers in Germany analyzed IQ scores of adult SMA patients as compared with the general population, using the WAIS-IV intelligence test, a commonly used and validated IQ test.
The team analyzed 33 adults with SMA, 12 women and 21 men, with a mean age of 35 (age range 18–58). One patient had SMA type 1, a severe form of SMA with infant-onset, and was only included in the analysis for the total cohort. The rest of the cohort was composed of 15 patients with type 2 (an intermediate, infant-onset type of SMA) and 17 patients with type 3 (a milder, juvenile-onset type of SMA).
Among the 32 patients with SMA types 2 and 3, 69% (21 patients) graduated from high school and attained a university entrance qualification (a certificate that qualifies the holder to study at a German university). Eleven of those had SMA type 2, and 10 had SMA type 3. Of these 21 high school graduates, 17 (53%) currently study or studied in German universities — 10 with SMA type 2 and seven with SMA type 3.
In three intelligence domains, there was no significant difference in the average IQ scores of the 33 adult SMA patients when compared with the average IQ of the general population.
The three intelligence domains assessed were the verbal comprehension index (VCI, a measurement of verbal acquired knowledge and verbal reasoning), the perceptual reasoning index (PRI, a measurement of visual perception, organization, and reasoning using only visual material), and the working memory index (WMI, a measurement of a person’s ability to attend to information presented verbally and visually, manipulate that information in short-term memory, and then form a response).
Average scores for the entire SMA cohort were 96.21 for VCI, 95.61 for PRI, and 95.82 for WMI. The average IQ for the general population was 100 across the three domains.
However, when the results were broken down by SMA type, patients with SMA type 2 had lower average IQ scores in the PRI (mean of 90.73) and WMI (mean of 90.33) intelligence domains, as compared with 100 in the normal population. There was no difference in the VCI domain — 95.73 versus 100 in the normal population.
Those with SMA type 3 did not have significant differences in their mean scores across the three intelligence domains compared with the 100 score of the general population — 95.47 for VCI, 99.53 for PRI, and 97.94 for WMI.
“In SMA type-3 patients, the index scores for verbal comprehension, perceptual reasoning, and working memory did not differ from the normal population but showed a trend of IQ scores towards lower points,” the researchers wrote.
A comparison between SMA types resulted in no significant differences in IQ scores for those with SMA type 2 when compared with those with type 3.
The team then compared data across SMA types 2 and 3 from those who had university entrance qualifications with those who did not: 12 versus three for type 2 and 10 versus seven for type 3.
Results showed that the only significant difference was in the average PRI score between SMA type 2 and 3 with university entrance qualifications: It was lower (92.58) in those with type 2 versus type 3 (109.1). In patients with SMA type 2 and 3 without a university entrance qualification, the mean IQ of PRI scores did not differ from each other.
The team noted, however, that due to the small sample size of patients with and without university entrance qualification, a correlation between educational level and IQ subgroups could not be fully investigated.
Researchers also noted that all patients analyzed were receiving treatment with Spinraza (nusinersen), an approved SMA therapy, and thus “a nusinersen effect cannot be completely excluded,” they wrote. Nonetheless, the team does not expect Spinraza to have a significant impact in cognitive performance, given that the therapy’s concentration within brain cells has been reported to be generally low.
Overall, “in this cohort of adult SMA types 2 and 3 patients, the IQ index scores for Working Memory and Perceptual Reasoning were lower in the patients with SMA type 2 than the normal population. The IQ index scores of SMA type 3 patients were not different from the normal population, but there was a trend towards lower cognitive performance,” the researchers concluded.
According to the team, “this study provided further evidence that SMA is a multi-systemic disease and may refute the widespread hypothesis that SMA patients might improve their cognitive skills to compensate for their physical impairment.”