Anatomic Distribution of MS Lesions in the Posterior Fossa

Introduction The brainstem and the cerebellum are frequently the sites of damage of clinically isolated syndromes suggestive of multiple sclerosis (MS) (l), and the sites of the most disabling lesions in clinically definite MS (2). This is why the presence of infratentorial lesions is included in various magnetic resonance imaging (MRI) criteria designed to assist in the differential diagnosis of MS (2-4). This study evaluated the anatomical distribution of lesions in the posterior fossa of MS patients to increase confidence when using MRI to confirm a diagnosis of MS. Material and Methods Twenty-nine patients (17 women, 12 men) affected by clinically definite MS (relapsing-remitting or secondary progressive) were studied. Using a 1.5 T scanner, the following pulse sequences were acquired in each patient: a) axial dual-echo fast-spin echo (FSE): TR/TE=3400/15-105; b) saga c) axial Tl-weighted FSE: TR/TE=760/14. For all the scans, 19 slices, with a thickness of 3 mm and an interslice gap of 0.3 mm were acquired. Dual-echo, T2-weighted and Tl-weighted hardcopies from all patients were reviewed by agreement by two experienced observers. Hyperintense lesions on dual-echo and T2weighted scans and hypointense lesions on Tl-weighted scans were identified and marked on the hardcopies; they were also classified according to their site. The following sites were evaluated: Cerebellum: cerebellar white matter, dentate nucleus, lobules; Brainstem: mesencephalon (tectum, tegmentum, cerebral peduncles); pons (tegmentum, pontine base); medulla oblongota (olive, paraolive, pyramid); Peduncles: superior, middle, inferior. Results We detected a total of 463 lesions. Thirty-five percent (n=162) of them were located in the pons, 23.5% (n=109) in the cerebellum, 20.7% (n=96) in the cerebellar peduncle, 11.9% (n=55) were at level of the mesencephalon, while the remaining 8.9% (n=41) involved the medulla oblongota. Only 17 lesions (3.7%) were hypointense on Tl-weighted images. Six of these lesions were located in the cerebellum, 1 in the mesencephalon, 4 in the pons, 1 in the medulla oblongota and 5 in the middle cerebellar peduncle. In the Table, the total number of lesions detected in each site and the total number and percentage of patients with lesions in these sites are reported. Of the 29 patients, 79.3% had lesions in the pontine tegmentum, 79.3% had periventricular lesions and 69% had lesions in the middle cerebellar peduncles. Conclusions To increase the diagnostic confidence of MS, various lesion patterns had been noticed and different criteria established (24). The criteria by Fazekas et al. (3) include at least 3 lesions, 2 of which have to be either >5mm, infratentorial or border the lateral ventricle. This study showed that the use of infratentorial lesions to make a diagnosis of MS has a sensitivity of 66% and a specificity of 98%. The criteria by Barkhof et al. (4) include 2 1 enhancing lesion, 2 1 juxtacortical lesion, 2 3 periventricular lesions and z 1 infratentorial lesions. It was shown that infratentorial lesions have a positive predictive value of 68%, sensitivity of 58%, specificity of 78% and an accuracy of 69%. Our study demonstrates that lesions in the cerebellum and brainstem are a common feature in MS patients (about 90% in our sample) and they involve mainly the cerebellum, the pons, the periventricular regions and less often the midbrain and the medulla oblongota. Interestingly, MS lesions in the posterior fossa have different signal characteristics than those located in the supratentorial white matter, since they are rarely associated with the presence of black holes on Tlweighted images.