Response to rituximab in an anti-muscle-specific receptor tyrosine kinase- and anti-acetylcholine recepto-double-positive myasthenia gravis patient concomitant with follicular dendritic cell sarcoma

M gravis (MG) is an antibody-mediated autoimmune disorder affecting neuromuscular junctions, which is strongly associated with thymoma. Usually, the target of the autoimmune attack is the acetylcholine receptor (AChR) of the postsynaptic skeletal muscle endplate. However, non-AChR components, such as muscle-specific receptor tyrosine kinase (MuSK) and low-density lipoprotein receptor related protein 4 (LRP4), both of which are involved in the endplate maturation, may also serve as targets for autoantibodies.1 Myasthenia gravis may develop secondary to follicular dendritic cell sarcoma (FDCS), a rare malignant neoplasm of follicular dendritic cells (FDCs).2 However, therapeutic options for these MG patients are unclear. We herein illustrated the therapeutic effect of Rituximab on an anti-MuSKand anti-AChRpositive MG patient concomitant with FDCS. A 60-year-old woman was admitted to the in-patient department with 20 days symptoms of exertion dysphagia and dyspnea. She has a 6-month history of fatigable limb weakness, fluctuating ptosis, and diplopia, which worsened with activity and ameliorated with rest. Her fluctuating symptoms were responsive to intramuscular neostigmine. She had been diagnosed with recurrent FDCS during the past 15 years, and lymphadenectomy had been performed for 4 times. Upon physical examination, she was fully alert and oriented. Bilateral ptosis, weakness of eyelid closure without lower facial involvement, weakened lifting of bilateral soft palates, and head drop were noted while the other cranial nerves were intact. The muscle strength grade was Medical Research Council (MRC) Grade-4 in the limbs and tendon reflexes were normal. The Babinski signs were negative bilaterally. Lymph nodes were palpable in her left cervical and axillary regions. The laboratory investigations including routine blood test, liver functions, and renal, serum cancer marker (carcinoembryonic antigen, CA199, CA153, CA724, CA242, neuron-specific enolase, free beta-human chorionic gonadotrnpin, squamous cell carcinoma antigen, alpha fetoprotein) and paraneoplastic neurological syndrome (PNS) screening were unremarkable. The antibodies to AChR (optical density [OD] value=0.442, reference value: OD value <0.367) and MuSK (OD=0.846, reference value: OD <0.512) were positive. Thoracic and abdominal CT revealed no thymic abnormality, or any mediastinal or distant sites of mass suggestive of possible extranodal lesion. Instead, lymphadenectasis of her left cervical and axillary region was reported. Needle biopsy of the lymph node and pathology confirmed a diagnosis of FDCS (Figure 1). The Hematoxylin and Eosin staining revealed diffuse proliferation of spindled cells with nuclear atypia (Figure 1A) and eosinophilic cytoplasm (Figure 1B). Scattered small lymphoid cells and perivascular (arrow) clusters of small lymphocytes (Figure 1C) are also marked. Immunohistochemistry demonstrates positivity of tumor cells for CD21 (Figure 1D), CD35 (Figure 1E), vimentin (Figure 1F), S-100 (Figure 1G), CD3 (Figure 1H) and CD1a (not shown) while negativity for EMA, CK, CD20, lysozyme,