Sporadic inclusion body myositis was identified in the 1990s and its onset usually occurs after the age of 50 years. It is one of the inflammatory myopathies, but stands out because of its resistance to immunosuppressants. Arguments that would tend to favour diagnosis of this condition include the presence of quadriceps and finger flexor muscle weakness, the observation of endomysial inflammation and rimmed vacuoles on muscle biopsy and, in almost half of all cases, the presence of anti-cN1A (anti-cytosolic 5′-nucleotidase 1A) autoantibodies. However, these antibodies are not specific to this condition and their pathogenic role has not, as yet, been established.
A not-so-effective biomarker
In the United States, a centre of expertise has conducted a retrospective study on a cohort of 92 patients with inclusion body myositis:
- 51% of them had anti-cN1A autoantibodies;
- patients who were anti-cN1A seropositive and seronegative did not show any significant differences in clinical terms (including in terms of oropharyngeal involvement), in terms of disease severity and and in terms of muscle biopsy, except for a higher incidence (100% versus 87%) of inflammation in those patients who were anti-cN1A positive;
- the anti-cN1A levels did not correlate with phenotype, or creatine kinase levels or presence of myotonic discharges on electromyogram (EMG), which means that, for the authors, these levels are of limited value as an evaluation tool.