Sporadic late-onset nemaline myopathy

Pathophysiology

The cause of SLONM remains unknown. Some cases have occurred alongside HIV infection or immune disorders, suggesting a viral trigger or autoimmune etiology.

The presence of monoclonal gammopathy of unknown significance, which worsens prognosis, further supports an immune-mediated process. On electron microscopy, nemaline bodies within affected muscle fibers may be crisply rod-shaped or irregular and punctate, sometimes arising from thickened Z-discs. Affected fibers may appear vacuolated or lobulated.

Diagnosis

Electromyography shows a myopathic pattern with fibrillation potentials. Serum creatine kinase levels are normal or low normal.

Muscle biopsy reveals nemaline rods, which are less than 1 μm in length and easily overlooked; trichromatic staining of 2 to 4 μm sections aids visualization. Immunostains for myotilin and α-actinin strongly support the diagnosis. Nemaline rods may also appear post-mortem in neurosarcoidosis, which remains a differential consideration.

Did You Know?

• Head drop may be a presenting symptom.
• Improvement has been noted in two HIV-negative individuals treated with intravenous immunoglobulin.
• Autologous stem cell transplantation and chemotherapy with melphalan have also shown benefit.