By Michael Rubin, MD

Professor of Clinical Neurology, Weill Cornell Medical College

Dr. Rubin reports no financial relationships relevant to this field of study.

SYNOPSIS: Inclusion body myositis has remained a clinical diagnosis assisted by a muscle biopsy, but now muscle ultrasound and biomarker identification may aid diagnosis.

SOURCE: Nodera H, et al. Intramuscular dissociation of echogenicity in the triceps surae characterizes sporadic inclusion body myositis. Eur J Neurol 2015 Dec. 26; DOI: 10.1111/ene.12899 [Epub ahead of print].

Sporadic inclusion body myositis (sIBM), the most common acquired idiopathic myopathy in patients > 50 years of age, remains a diagnosis that, when suspected, requires muscle biopsy, which itself is not always confirmatory. Creatine kinase is mildly elevated at most, acute phase reactants are usually normal, and myositis-specific antibodies are typically absent. Aside from the characteristic history of slowly progressive weakness with early involvement of knee extensors and forearm flexors, might muscle sonography distinguish sIBM and facilitate its diagnosis?

Patients with sIBM, polymyositis (PM), dermatomyositis (DM), or myositis associated with a connective tissue disorder, were recruited from the Vihara Hananosato Hospital, Tokushima University, Tokyo, Japan, and prospectively evaluated, using normal controls for comparison. Clinical diagnoses were based on guidelines and criteria from the European Neuromuscular Center. Sonography of the right arm and leg, in the supine position, was performed on all subjects by a single, blinded technician, using a LOGIQ7 with a fixed 11-MHz linear array transducer, applying the transducer to the medial head of the gastrocnemius, and 5 cm distal to the olecranon, visualizing the medial head of gastrocnemius, soleus, flexor digitorum profundus (FDP), and flexor carpi ulnaris (FCU). Muscle echo intensity (EI) was interpreted by three experienced, blinded examiners, who routinely performed muscle sonography and rated them as normal or slightly, moderately, or severely increased EI. Statistical analysis comprised one-way ANOVA with Games-Howell post-hoc test, intraclass correlation coefficients (ICCs), Cronbach’s alpha, and, where applicable, Spearman’s correlation coefficient, with P = 0.05 being statistically significant.

Among 11 patients each with sIBM, PM/DM, and controls, sIBM patients showed selectively and significantly increased EI in the FDP and gastrocnemius, with relatively scant EI in the adjacent FCU and soleus, respectively, in contrast to PM/DM patients where these neighboring muscles demonstrated equivalent EI. EI was greatest in sIBM gastrocnemius muscle, followed by sIBM FDP, with no correlation found between EI and patient age, disease duration, muscle strength, or creatine kinase level. In sIBM, there is sonographic dissociation between adjacent muscles in forearm muscles and in the triceps surae.

COMMENTARY

Two important findings have recently come to light regarding sIBM. First, anti-cN-1A autoantibodies targeting cytosolic 5’-nucleotidase 1A (cN-1A), a 44-kDa skeletal muscle protein, have been identified in the serum of sIBM patients, representing the only biomarker diagnostically useful in differentiating sIBM from other forms of inflammatory myopathy.1-3 Sera from patients with Sjogren’s syndrome and systemic lupus erythematosus often demonstrate these antibodies as well. Second, hepatitis C virus (HCV) infection appears to be statistically significantly associated with sIBM, suggesting a causative link between these two diseases.4 Among 114 sIBM patients, anti-HCV antibodies were found in 28%, compared to only 4.5% of 44 age-matched polymyositis patients. Further research is warranted in understanding this incurable disease, but the initial steps may have already begun.

REFERENCES

  1. Salajegheh M, et al. Autoantibodies against a 43 KDa muscle protein in inclusion body myositis. PLoS One 2011;6:e20266.
  2. Pluk H, et al. Autoantibodies to cytosolic 5’-nucleotidase 1A in inclusion body myositis. Ann Neurol 2013;73:397-407.
  3. Larman HB, et al. Cytosolic 5’nucleotidase 1A autoimmunity in sporadic inclusion body myositis. Ann Neurol 2013;73:408-418.
  4. Uruha A, et al. Hepatitis C virus infection in inclusion body myositis: A case-control study. Neurology 2016;86:211-217.