INTRODUCTION
Monoclonal gammopathy of undetermined significance (MGUS) may be associated with pathologies with severe neuromuscular manifestations, namely POEMS Syndrome and AL Amyloidosis[1-4]. Another less-known association is with a rare disorder named sporadic late-onset nemaline myopathy (SLONM)[5-10] and happens in more than half of these patients[11, 12]. Diagnosis is confirmed by muscle biopsy with detection of intracytoplasmic nemaline rod bodies, representing Z-disc disorganization products that immunoreact with alfa-actinin and myotilin, accompanied by progressive atrophy of the muscle fiber[5, 11]. Necrotic or regenerating fibers and inflammatory infiltration are usually absent[13]. Clinical­ly, SLONM appears mostly after the fourth de­cade of life as a rapidly progressing tetrapa­resis, mostly by proximal/axial muscle weakness with features such as dropped head syndrome, facial, bulbar involvement (dysphagia) and respiratory insufficiency leading to severe disability or death[5, 11, 12]. Head drop, neck muscle weakness, or neck muscle pain are features described in more than half of the patients with SLONM associated with MGUS (SLONM-MGUS)[13]. Cardiac involvement is less common, but there are echocardiographic and electric conduction cardiac abnormalities in some patients[3, 11-16]. The first cases published suggested an autoimmune etiology, and it was uncertain whether monoclonal gammopathy was associated or coincidental[6]. Treatment of first cases using immunosuppressive strategies showed better results with intravenous immunoglobulin[11, 17], mostly on non-MGUS associated cases[12]. Nonetheless, other immunosuppressive treatments (prednisone, cyclophosphamide, rituximab) or even plasmapheresis showed disappointing and inconstant outcomes which led the autoimmune etiology to be questioned, at least for some patients[3, 5, 6, 10, 14, 18, 19]. The hypothesis that SLONM-MGUS is a continuum of light-chain deposition disease (LCDD) is corroborated by the presence of light chain deposition in skeletal muscles observed in some cases described [20]. However, LCDD is a multiple organ disease while SLONM attainment is restricted to muscles, which could be an argument in favor of autoimmune abnormalities in its pathogenesis[13]. AL amyloidosis and POEMS are usually treated targeting the plasma cell clone with clinical improvement, as any manifestation associated with a plasma cell dyscrasia as postulated to SLONM-MGUS. [21-24]. The fast progression and severity of SLONM-MGUS (mortality of 70% in 1-5 years) led clinicians to use the AL Amyloidosis treatment approach by using the autologous stem cell transplant (ASCT) with good results and low mortality rate (2-3%) [5, 12, 25]. There are successful cases with clinical and histological improvement, as well as an increment in survival after conditioning with melphalan and ASCT, suggesting a possible toxic action of monoclonal protein as a degenerative mechanism for neurologic symptoms[14, 19, 20, 26-28]. Some propose light and heavy chains, rather than nemaline bodies, as responsible for myotoxic effects, and there is a report of coexistence of nemaline myopathy and amyloid myopathy[20, 29]. Furthermore, the evidence is growing about the association between disease severity and the size of the monoclonal protein supporting the rationale to use therapeutics targeting the clonal plasma cells[11, 12, 14, 28]. Other effective treatment options beyond ASCT, used in plasma cell dyscrasias can also be efficient, such as bortezomib[3, 14, 20, 30], lenalidomide[28, 31], cyclophosphamide or thalidomide[32]. Hematologic therapy seems to be the one that allows a lower risk of progression of the disease 24 months after treatment[15].