Inclusion body myositis (IBM) is a rare disease that is part of a group of muscle diseases known as idiopathic inflammatory myopathies, which are characterised by chronic excessive muscle inflammation associated with muscle weakness. IBM has a variable prevalence depending on geographic, ethnic and age criteria. Prevalence in the general population ranges varies between 0.3 and 13.9 per 100,000.
IBM onset is usually over 50 years but may also occur earlier, in the 5th decade and it is more prevalent in males (2:1 over female).
First signs of the disease are usually weakness of the long finger flexors (typically flexor digitorum profundus and flexor pollicis longus) or the quadriceps muscle. Alternative presenting phenotypes include dysphagia, foot drop or respiratory weakness. With the progression of the disease, more groups of muscles are involved and the decrease of muscle strength is usually slowly progressive (occurs gradually over months or years). Dysphagia occurs in around 50 % and may be severe, rarely requiring enteral feeding. Cardiac function appears to be spared in IBM. There are reports of sleep-disordered breathing (SDB) being identified primarily later as the disease progresses, although not necessarily correlated to the severity of peripheral muscle weakness. The involvement of the respiratory muscles is characterised by a restrictive ventilatory syndrome of progressive evolution. Respiratory decline and dysphagia predisposing to pneumonia was the most common cause of death in a long-term follow-up of patients with IBM.
The aetiopathogenesis of IBM remains uncertain, but muscle damage is considered to involve both an abnormal immune response reflected by excessive inflammatory infiltrates and a degenerative process resulting in rimmed vacuoles, mitochondrial impairment and protein aggregates. No causal gene has been identified but HLA-DR3 and 8-1 MHC genotypes have been shown to correlate with IBM susceptibility. Antibody cytosolic 5'-nucleotidase 1A, an autoantibody against the muscle protein cytosolic 5'-nucleotidase 1A, was detected in approximately half of the patients, but is also commonly seen in Sjogren’s syndrome and SLE.
The European Neuromuscular Centre published the ENMC IBM Research Diagnostic Criteria 2011. These include combined clinical and laboratory features: duration >12 months, age at onset >45 years, sCK no greater than 15 ULN and either knee extension weakness ≥ hip flexion weakness and/or finger flexion weakness > shoulder abduction weakness or pathological features: endomysial inflammatory infiltrate, rimmed vacuoles, protein accumulation or 15–18nm filaments.
Differential diagnosis may include polymyositis and, in early stages of the disease, arthritis or motor neuron disease. There is no curative treatment for IBM and patients usually do not respond to anti-inflammatory or immunomodulatory therapies. Symptomatic treatments include exercise therapy and orthotic appliance. As the disease progresses, weakness can lead to frequent falls, walking aids and wheelchair use around 15 years after diagnosis. Bone fractures and other complications may occur as a result of falls. A change in mean life expectancy has not been observed.