32PPerimysial inflammation with abundant macrophages

The field of inflammatory myopathies (IM) is constantly evolving, with new disease entities emerging. A subset of patients have been noticed to exhibit an unusually abundant macrophage infiltration in muscle tissue, suggesting a distinct IM subgroup. Here, we describe another case with numerous perimysial macrophages and a multisystemic clinical picture. A male patient with disease onset at age 48 presented cognitive disturbances and epileptic seizures, leading to a diagnosis of seronegative autoimmune encephalitis with brain MRI showing signs of limbic encephalitis in the mesiotemporal region. He later developed persistent fever, increased CRP, hepatomegaly, arthralgias, joint edema, and progressive proximal muscle weakness and atrophy. The CK levels were normal, and several panels of autoantibodies, including myositis- and encephalitis-associated, tested negative. MRI of the thighs showed muscle edema and subcutaneous fat infiltration. Muscle biopsy from the left thigh revealed marked perimysial inflammation with abundant macrophages, perimysial alkaline phosphatase positivity, MHC class I and II upregulation on the muscle fibers, and sarcolemmal perifascicular complement deposition. Extensive testing did not reveal any malignancies or infections. Treatment with steroids, Methotrexate, and Rituximab improved muscle and cognitive symptoms. One differential diagnosis, macrophagic myofasciitis, linked to vaccine-related aluminum deposits, was ruled out as the macrophages in our case did not contain diastase-resistant material. Inflammatory myopathy with abundant macrophages (IMAM) shares features with dermatomyositis but is histopathologically distinct with predominant CD68+ and few CD4+ cells. Unlike a previously reported IMAM case with absent MHC class I and II expression, our patient exhibited upregulation on the muscle fibers. IMAM has been associated with hemophagocytosis, cutaneous panniculitis, Sweets syndrome, and macrophage activation syndrome. However, autoimmune encephalitis has not been described before. In summary, we presented another case of macrophage-predominant myositis with novel features.