Adult-onset nemaline myopathy

Adult-onset nemaline myopathy (also known as late-onset nemaline myopathy or sporadic late-onset nemaline myopathy, SLONM) is a rare acquired or genetic muscle disorder characterized by progressive muscle weakness that begins in adulthood, typically after the age of 40. The hallmark of this condition is the presence of nemaline rods (rod-shaped structures) within muscle fibers, which are visible on muscle biopsy. Unlike the congenital forms of nemaline myopathy that present in infancy or childhood, the adult-onset form frequently occurs sporadically without a clear family history, and in many cases is associated with an underlying monoclonal gammopathy (MGUS) or HIV infection, suggesting an acquired immune-mediated mechanism rather than a purely genetic cause. The disease primarily affects the skeletal muscular system, leading to progressive proximal and axial muscle weakness. Patients commonly experience difficulty with activities such as climbing stairs, rising from chairs, and lifting objects overhead. Respiratory muscles can be significantly affected, and respiratory insufficiency is a major cause of morbidity and mortality. Dysphagia (difficulty swallowing) and head drop due to neck extensor weakness are also frequently reported. Some patients develop limb weakness that may progress to involve distal muscles over time. There is no definitive cure for adult-onset nemaline myopathy. Treatment is largely supportive and may include physical therapy, respiratory support (including non-invasive ventilation), and management of swallowing difficulties. In cases associated with monoclonal gammopathy, treatment directed at the underlying hematologic condition — such as autologous stem cell transplantation, immunosuppressive therapy, or chemotherapy targeting the plasma cell clone — has shown benefit in some patients, with reports of clinical improvement or stabilization. Early identification of an associated monoclonal gammopathy is therefore critical, as it may open a window for targeted treatment.

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