MRI for Evaluation of Flexor Digitorum Profundus Muscle Involvement in Inclusion Body Myositis.

A 77-year-old Japanese woman developed gait disturbance during her 50s. The gait disturbance and a muscle weakness in her lower limbs gradually worsened, and she consulted our hospital at the age of 76. Neurological examination showed proximaldominant muscle weakness and atrophy, particularly in the quadriceps muscles. In her left hand, the muscle strength during finger flexion at the distal interphalangeal joint was lower than that at the proximal interphalangeal joint and metacarpophalangeal joint. Needle electromyography showed low-amplitude and short-duration unit potentials and early recruitment of the muscles examined. MRI of the forearm showed hyperintensities in the flexor digitorum profundus (FDP) muscle on T1and T2weighted images; these hyperintensities relatively spared the muscles of the extensor forearm region and flexor digitorum superficialis (FDS) muscle (Figure 1A, B). MRI of the thigh showed hyperintensities in the quadriceps muscles on T1and T2weighted images; these hyperintensities relatively spared the muscles of the posterior compartment of the thigh (Figure 1C, D). A biopsy of the right biceps muscle was performed. Hematoxylineosin staining showed muscle fibers of various sizes, small and angulated fibers, necrotic and regenerating fibers, and inflammatory cell infiltration. Muscle fibers with vacuoles rimmed by basophilic granules, which stained red with modified Gomori-trichrome staining, were frequently observed (Figure 1E, F), consistent with the diagnosis of inclusion body myositis (IBM).