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Inclusion body myositis
Inclusion body myositis (IBM) is a slowly progressive degenerative inflammatory disorder of skeletal muscles characterized by late onset weakness of specific muscles and distinctive histopathological features.
ORPHA:611Classification level: Disorder
IBM has a highly variable prevalence according to geographic, ethnic and age criteria. Prevalence in the general population ranges from 1:1,000,000 to 1:14,000 but a three-fold increase is observed when considering only a population over 50 years. Underdiagnosis may be an explanation for the high ethno-geographic variation. Male-to-female ratio is 2:1 on average (0.5 to 6.5:1).
IBM onset is over 50 years but may also occur earlier, in the 5th decade. First signs are weakness or even atrophy of the quadriceps or of the finger flexors, leading to difficulties in rising from chairs or ground, walking stairs, gripping, lifting and using tools, and provoking falls. Flexor digitorum profundus and flexor pollicis longus are more severely involved than the forearm extensor muscles, especially duing early stages of the disease. With disease progression, other groups of muscles are also involved, such as the elbow flexors, the hip, knee or neck flexors, and the ankle dorsiflexors, leading to footdrop. Patients frequently have a mild weakness of facial muscles, except for the extraocular muscles. Dysphagia may occur in approximately 66% of patients in advanced stages of the disease and can be severe in some cases.
Etiology of IBM is poorly understood. No causal gene has been identified but HLA-DR3 and 8-1 MHC genotypes have been shown to correlate with IBM susceptibility. Ageing and environmental factors are also surmised to play a triggering role. Whether IBM is primarily an immuno-inflammatory disorder leading to muscle degeneration or a degenerative disorder leading to muscle inflammation is still under debate: as is the pathogenic role of anti-IBM-43, an autoantibody directed against the muscle protein cytosolic 5'-nucleotidase 1A, and which is detected in approximately half of patients.
Diagnosis is based on physical examination (especially the detection of finger flexor weakness), patient's age and the duration of symptoms over 6 months being additional diagnostic clues. Muscle biopsy allows identification of endomysial inflammatory cells surrounding myofibers and rimmed vacuoles and, occasionally, an abnormally elevated number of COX-negative fibers. Laboratory findings are not specific as serum creatine kinase is only slightly raised in some cases. Electromyography can only help confirming the myopathic origin of the weakness or atrophy. Magnetic resonance imaging (MRI) helps delineating the characteristic pattern of muscle involvement.
Differential diagnosis may include polymyositis (see this term) and, in early stages of the disease, arthritis or any motor neuron disease (see this term).
IBM occurs sporadically, but very rare familial cases (called familial-IBM), almost always siblings, with an unknown inheritance pattern have been observed.
IBM occurs sporadically, but very rare familial cases (called familial IBM) with an unknown inheritance pattern, almost always of siblings, have been observed.
Management and treatment
There is no curative treatment for IBM, and patients usually do not respond to anti-inflammatory or immuno-modulatory therapies. Symptomatic treatments include exercise therapy, ergotherapy, and orthotic appliance.
After 5 years, most patients require a walking aid and after 10, a wheelchair. No change in mean life expectancy has been observed.