Cognitive impairment in probable multiple sclerosis
Achiron A and Barak Y;
Commented by , 26 May 2003
Background
In the past decade there has been an increasing focus on cognitive dysfunction in MS. Neuroimaging studies have shown, that the correlation between cerebral MS pathology and cognitive decline, are stronger than the correlation with measurements of physical disability. In this way cognitive testing seem to reflect the impact of MS pathology more directly.
Several reports have demonstrated irreversible tissue abnormalities, axonal loss and brain atrophy early in the course of MS, but little is still known about the consequences with regard to cognitive impairment in the initial stage of the disease.
Aim
To evaluate the frequency of cognitive impairment in patients with probable MS (in accordance with criteria by Poser et al.), and correlate these findings with demographic, clinical, and brain MRI parameters.
Methods
Patients with a single episode of new neurological symptoms within three months preceding the study, clinical examination revealing evidence of at least one CNS lesion, and paraclinical evidence (MRI) of demyelinating lesions (in accordance with criteria by Fazekas et al.) were recruited. The neuropsychological test battery was based on the Brief Repeatable Battery – Neuropsychological (BRB-N).
Abnormal performance was defined as one standard deviation below mean for normal matched subjects. Neurological disability was assessed using the Expanded Disability Status Scale (EDSS). Brain MRI was conducted on a 2.0 Tesla imager.
Results
Sixty seven patients completed the BRB-N within a mean of 29 days from disease onset. The majority of patients (94%) had an abnormal score in at least one of the tests. The largest group of patients (53.7%) consisted of patients who scored abnormally on one or two tests. A composite analysis showed that verbal abilities and attention span was the most frequently affected domain.
There were no correlation between cognitive dysfunction and MRI measurements (Gd, T2, PD and T1), nor did the authors find correlation with neurological disability. In the statistical analysis p<0,05 was considered significant.
Discussion
The authors report a very high incidence of cognitive dysfunction in probable MS disease. It is emphasized in the discussion that the BRB-N is a screening test and therefore sensitive at the cost of specificity. Furthermore, it should be taken into account, that the statistical approach is not very conservative. Still, the study provides evidence of substantial cognitive dysfunction in the initial phase of the disease.
The results are thus in line with numerous other studies, showing widespread pathology and cerebral dysfunction (changes of brain activation patterns) in the beginning of MS. Although none of the patients in the study showed dysfunction of social, personal or professional abilities, it is likely that the functional redundancy of the CNS, to a very large extent compensate for the focal and diffuse tissue loss at this stage.
Concerning the more qualitative aspects of the study, the authors show that verbal abilities and attention is most commonly affected, although other studies have shown, that information processing/memory is more sensitive in the later stages of the disease. It is proposed that this shift is caused by a transition from early inflammatory predominance to substantial axonal loss in MS, which is an interesting hypothesis.