Fractional anisotropy values detect pyramidal tract involvement in multiple system atrophy

Objective: Pathological studies have shown remarkable pyramidal tract involvement in multiple system atrophy (MSA), while clinical pyramidal signs are relatively rare. We investigated the fractional anisotropy (FA) values to assess the degree of pyramidal tract involvement in MSA, in comparison with amyotrophic lateral sclerosis (ALS) and controls. Furthermore, we compared FA values between MSA patients with or without clinical pyramidal signs and controls, and between MSA patients with or without positive conventional MRI findings and controls. Methods: We evaluated FA values in the internal capsule, corona radiate and whole pyramidal tract using visualized tractography of 65 subjects (20 probable MSA patients, 28 age-matched ALS patients, and 17 age-matched healthy controls) using a 3.0T magnetic resonance system. Results: The FA values in the internal capsule, corona radiate, and whole pyramidal tract were significantly lower in MSA patients than in controls and were at a level similar to those of ALS patients. In addition, low FA values were prominent in MSA patients, even in those with short duration of illness, lacking precentral gyrus hyperintensity in FLAIR images, and without pyramidal signs. Conclusion: FA values could identify pyramidal tract degeneration even in patients with early phase MSA and those without clinical pyramidal signs or abnormal MRI findings. More extensive degeneration of the pyramidal tract occurs in MSA than so far believed.