By Björn Falck (Turku, Finland), Erik Stålberg (Uppsala, Sweden)
● cell mediated autoimmune response against muscle fibers
● incidence 1-9/1000000
● more common in women than men
● usually begins after the age of 20 years
● muscle weakness develops subacutely or insidiously
● proximal limb and neck flexor muscles involved more than other muscles
● muscle pain is sometimes present, but it is not as prominent as one would expect
● dysphagia is common in polymyositis
● CK is usually moderately elevated
● demonstrate myopathy
● assess: severity (mild, moderate, severe) and activity (stationary, active)
EMG
● fibrillations in the acute stage, they disappear in remission
● small, brief MUPs in the acute stage
● later long duration polyphasic MUPs in the chronic stage
Neurography
● if distal muscles are severely involved MCS ampl are reduced
Neurography
● SCS
● MCS
EMG
● m.vastus lateralis/m.vastus medialis
● m.deltoideus/m.biceps
● m.interosseus dorsalis I/m.abductor digiti minimi
● m.tibialis anterior
● paravertebral muscles in the low thoracic region should be studied if limb muscles do not show distinct abnormalities
Neurography
● MCS: n.medianus and n.peroneus unilaterally
● SCS: n.radialis and n.suralis unilaterally
● in chronic PM, there may be a mixed pattern of small and large MUPs
● it is not unusual for patients with PM to have concurrent PNP, especially if they have malabsorption
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