Monoclonal Gammopathy of Undetermined Significance with Associated Necrotizing Myopathy: A Case Report and Review of the Literature

Abstract

Background: Monoclonal gammopathies encompass many types of plasma cell proliferative disorders ranging from benign to malignant. Monoclonal gammopathies that meet diagnostic criteria for monoclonal gammopathies of undetermined significance (MGUS) but with clinical manifestations are now being referred to as monoclonal gammopathies of clinical significance (MGCS). MGUS associated myopathies are a rare form of MGCS.

Case description: We present a case of MGUS associated necrotizing myositis. The patient had been previously diagnosed with MGUS and myositis around the same time, but prior testing had not revealed the etiology of the myopathy. Repeat biopsy and work-up revealed a necrotizing myopathy.

Discussion: MGUS associated myopathies are rare, with the most common being sporadic late-onset nemaline myopathy (SLONM-MGUS) and amyloid light chain (AL) amyloidosis-associated myopathy. MGUS associated necrotizing myopathy is even rarer. Because this condition is so uncommon, there are no standardized guidelines on how to treat this condition. Some case reports or studies suggest treating the myositis with standard of care for the myositis type without considering the monoclonal gammopathy while other studies have suggested that treating the monoclonal gammopathy would be beneficial in treating the associated clinical syndrome.

Conclusion: Our case report of MGUS associated necrotizing myopathy encourages internists to broaden their differential diagnosis of myopathy, increasing awareness of a condition that is still not well understood.

Learning points: Monoclonal gammopathy of clinical significance is a new term that refers to nonmalignant monoclonal gammopathies that exhibit significant clinical manifestations.Monoclonal gammopathy associated myopathies are rare but should be considered as part of the differential diagnosis for myopathy.

Keywords: Monoclonal gammopathy; myopathy.

Figure 1

Magnetic resonance imaging showing bicipital myositis.

Figure 2

A) Left biceps muscle tissue with H&E stain showing necrosis and regeneration of muscle fibers; B) Magnified image of left biceps tissue showing necrotic fiber surrounded by endomysial infiltrates; C) Immunohistochemical stain showing occasional endomysial infiltrates around necrotic fibers. Per University of New Mexico pathology report findings were most consistent with necrotizing myositis.

Figure 3

A) The figure shows the amount of rheumatology (outpatient visits), emergency room visits, and hospital admissions that patient had in the ascribed time period with a chief complaint related to his myositis symptoms. Overall, the patient had less total visits on RTX + prednisone compared to prednisone only and MTX + prednisone. He had the same number of total visits on AZA + prednisone compared to RTX + prednisone, but in a much shorter time period (2 months compared to 1 year). B) shows a decreased M-protein while on RTX compared to other medications. Abbreviations: RTX, rituximab; MTX, methotrexate; AZA, azathioprine.