idiopathic inflammatory myopathies

Question 1

definition of idiopathic inflammatory myopathies

Answer 1

Idiopathic inflammatory myopathies constitute a heterogeneous group of sub-acute, chronic, and, rarely, acute diseases of skeletal muscle that have in common the presence of moderate-to-severe proximal muscle weakness and inflammation on muscle biopsy.[1] Based on distinct features, they are divided into 3 groups: polymyositis, dermatomyositis, and inclusion body myositis. Polymyositis and dermatomyositis seem to have an autoimmune pathogenesis, whereas inclusion body myositis seems to have an autoimmune and degenerative pathogenesis.

Question 2

epidemiology

Answer 2

Polymyositis occurs most commonly between the ages of 40 and 60 years, and inclusion body myositis usually in people >50 years of age. Dermatomyositis incidence has two age peaks: one between the age of 40 and 70 years and the other one in childhood at around 9 years.

Specific questioning concerning risk factors known to be associated with the development of an inflammatory myopathy (e.g., history of exposure to high intensity of global UV radiation; genetic predisposition; treatment with lipid-lowering agents, D-penicillamine, or other drugs or toxins known to be risk factors; presence of HIV infection; history of preceding other viral or non-viral infection; and preceding vaccination)

History may reveal increased difficulties in performing motor tasks predominantly requiring proximal muscles, such as getting up from a chair, climbing steps, and combing hair.

Fine motor tasks that depend on distal muscles, such as sewing, knitting, or writing, are affected late in the course of dermatomyositis and polymyositis, but earlier in inclusion body myositis. Falling is commonly reported in patients with inclusion body myositis, because of early quadriceps involvement.

Other extramuscular symptoms include arthralgia, dysphagia, shortness of breath, palpitations, syncope, and MI symptoms.

IIM is also associated with malignancy. The most frequent malignancies for dermactomyositis are ovarian cancer, pancreatic cancer, and non-Hodgkin’s lymphoma.
Patients with polymyositis have a 1.4 to 2 times increased risk of developing cancer compared with the general population. The most frequent malignancies observed with polymyositis are non-Hodgkin’s lymphoma, lung cancer, and bladder cancer.

Question 3

signs and symptoms

Answer 3

• difficulty with motor tasks
• muscle weakness
• muscle atrophy
• frequent falls (in inclusion body moyositis)
• fatigue and generalised malaise
• mild fever
• cardiac +/- resp involvement (not in ibm) -> SOB, abnormal breath sounds

Question 4

skin changes in dermatomyositis

Answer 4

in dermatomyositis, the following skin lesions are characteristic:

Heliotrope rash with eyelid oedema
Facial rash
Gottron’s papules, which are erythema of knuckles accompanied by a raised violaceous scaly eruption
Erythematous rash over the knees, elbows, malleoli, and at the base of the neck and upper chest, forming a V sign
Nail fold changes such as dilation of capillary loops of periungual area.

Question 5

risk factors

Answer 5

• age group (40-60 in polymyositis, <9 or 40-70 in dermamyositis, >50 in inclusion body myositis)
• exposure to high intensity of global UV radiation
• genetic predisposition
• female sex & black ethnicity – poly and derma
• male sex & white ethnicity – inclusion body

Question 6

investigations

Answer 6

1st investigations:
CK
EMG
muscle biopsy
aldolase (present in muscle and liver!)
LDH (muscle, liver, and erythrocytes)
alanine transaminases (Less specific for muscle injury than CK)
myoglobin (sensitive index of the integrity of muscle fibres)

others to consider:

• ESR
• ANA => +ve in derma and poly
• myositis-specific antibodies (for prognosis)