Multiple sensory evoked potential techniques

SEPs, VEPs and BAEPs show different sensitivities in revealing lesions in the central sensory pathways. This can mostly be explained according to the different length of the pathways concerned and the tendency of the MS plaques to develop in some areas more frequently than in others. In many studies, the incidence of VEP and SEP abnormalities was similar; in some investigations VEPs proved to be more

sensitive than SEPs, whereas in others the opposite was found. In a series of 29 definite and suspected MS cases, Kjaer (1980b) recorded abnormal VEPs in 62% and abnormal

SEPs in 55%. In another series of 220 patients, Javidan et al. (1986) recorded abnormal VEPs in 84% of definite, 55% probable and 39% of possible MS cases, whereas the in­

cidence of abnormal SEPs was 79%, 65% and 26%, respec­ tively. In an investigation of 123 patients with estab­ lished or possible MS, Bartel et al. (1983) detected abnor­ mal VEPs in 58% of cases and abnormal SEPs in 61%. The in­ cidence of BAEP abnormalities is consistently lower than that of VEPs or SEPs, especially in cases in whom the diag­ nosis of MS is uncertain. In the above quoted studies, the incidence of BAEP abnormalities was 52% (Kjaer, 1980), 48%

(Bartel et al., 1983) and 66%, 39% and 17% in definite, probable and possible cases (Javidan et al., 1986). All in­ vestigators agreed that the combined use of VEPs, SEPs and BAEPs is very useful for diagnostic purposes, as it in­ creases the overall yield of abnormalities.

Some studies have considered the relative prognostic value of VEPs, SEPs and BAEPs, that is their sensitivity in detecting abnormalities in the very early stages of MS and how this predicts the subsequent course of the disease. Deltenre et al. (1982) studied 133 cases during a four-year follow-up. In this period, 44 patients developed clinically definite MS, 27 were found to have another neurological disease and 62 still had an uncertain diagnosis at final examination. EPs had revealed one or more clinically silent lesion in 81.8% of patients of the first group (VEPs being more often affected than SEPs and BAEPs), in 18.5% of cases

of the second and in 19.4% of subjects of the third. EPs had been normal in only one patient (2.2%) who developed definite MS, in 35% of subjects who were found to have another neurological disease and in 34% of cases who

remained undiagnosed at the end of the study period. The findings of Matthews et al. (1982) were similar. They

studied 71 patients during a variable follow-up period not exceeding 38 months. Of 49 initially probable or possible MS cases, 24 were re-classified as definite MS. EPs had revealed clinically silent lesions in 13 of these (54%), while false negative results were obtained in eight cases

(33%). In three patients progressing to definite MS, EP findings had only confirmed sensory deficits which were ap­ parent on clinical examination. In 21 patients the diagnos­ tic category was unchanged: EPs had revealed asymptomatic lesions in two of these (10%), had been normal in 17 (81%) and had confirmed clinical deficits in two others. Two out of the 49 patients were not followed-up and two others progressed from "possible" to "probable" category. In the latter, EPs had not shown sub-clinical abnormalities. In a third category of 22 patients with a single acute non- diagnosed neurological episode, EPs had revealed sub- clinical lesions in one out of four patients who then developed definite MS, but had been normal in four who progressed to the probable or possible category. EPs had also been abnormal in 3/14 acute non-diagnosed patients who did not show any change in the clinical picture during the

study period. Hume and Waxman (1988) studied 204 patients during a period not exceeding 33 months. EPs had revealed clinically silent lesions in 65% of patients who developed definite MS, in 42% of those who progressed to a less un­ certain diagnostic category, in 14% of patients whose diag­ nostic category was unchanged. The incidence of normal

responses was 10% in the first group, 37% in the second and 75% in the third. These findings demonstrate that sensory EP techniques have a predictive value in the diagnosis of MS suspects, when the clinical picture is uncertain. Be­ cause of the occurrence of false negatives, however, the absence of EP abnormalities at this stage does not exclude the diagnosis of MS. The fact that subclinical EP abnor­ malities can be recorded in patients who subsequently

develop another neurological disease (19% in the series by Deltenre et al., 1982) underlines that EP findings are not specific to MS and, therefore, must be interpreted "in the whole clinical context".

In the majority of patients with active MS, EPs show a trend to worsen with time, despite the fact that symptoms and signs may improve. Walsh et al. (1982) recorded median nerve SEPs and VEPs in 56 patients with definite MS at the beginning and end of a 30 months follow up study. At the beginning of the investigation, VEPs were abnormal in 84% of cases, the cervical SEP in 49% and the cortical SEP in 52%. At the end of the follow up period, none of the

changed or more abnormal. This trend was found in patients whose clinical condition had improved, worsened or shown no

significant change during the study period. The EP evolu­ tion was thought to reflect the progressive dissemination of the disease, which was not always reflected in the clinical picture.

In recent years, some studies have compared the rela­ tive sensitivity of MRI and VEPs, SEPs and BAEPs in the in­ vestigation of MS. It is now well established that, in general terms, MRI is more sensitive than EPs and this dif­ ference is greater in the early stages of the disease. In the series published by Cutler et al. (1985), MRI revealed abnormalities in 16/19 (84%) definite MS cases and in 5/8

(63%) probable ones. Sensory EPs were abnormal in 13/19 (68%) and 1/8 (13%), respectively. In another study of 30 patients with suspected MS (Gebarski et al., 1985), MRI

showed abnormalities in 26 cases (87%) and EPs in 13 (43%). In two out of the four cases with no MRI lesions, EPs were either normal or showed doubtful abnormalities. However, in another investigation of 23 suspected MS cases (Giesser et al., 1987), EPs were found to be abnormal in 18 patients and MRI in 15. In 19/23 cases MRI and EPs were either both abnormal or both normal, whereas in four cases in whom MRI showed no lesions EPs were abnormal.

Thus, EP techniques are still useful in the investiga­ tion of MS, as they may reveal abnormalities when MRI fails to do so. This happens especially in areas of the CNS which

are relatively inaccessible to MRI, such as the thoracic and lumbar spinal cord and the optic nerves (see also paragraph 1.4.2.).