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Definition of Myositis
What are Inflammatory Myopathies?
Inflammatory myopathies are a group of muscle diseases involving the inflammation
and degeneration of skeletal muscle tissues. They are thought to be autoimmune
disorders. In inflammatory myopathies, inflammatory cells surround, invade,
and destroy normal muscle fibers as though they were defective or foreign
to the body.
This eventually results in discernible muscle weakness. This muscle weakness
is usually symmetrical and develops slowly over weeks to months or even years.
Early signs of inflammatory myopathies may include difficulty rising from
a chair, climbing stairs, or lifting the arms. A patient may become exceedingly
fatigued after
prolonged standing or walking. In some cases, early signs may include difficulty
in swallowing or breathing. Your physician may run a number of tests, including
a
physical examination, blood tests, electromyography (EMG) and a muscle biopsy.
The diagnosis is confirmed by the muscle biopsy using special stains. Electron
microscopy of the muscle may be necessary. If a polymyositis patient does
not respond to treatment, it is important that he/she see a specialist to
rule out IBM. At
times a repeat muscle biopsy may be necessary.
Dermatomyositis
Dermatomyositis (DM) is the most easily recognized of the inflammatory myopathies
due to its distinctive rash. This rash occurs as a patchy, dusky, reddish
or lilac
rash on the eyelids, cheeks, and bridge of the nose, and on the back or upper
chest, elbows, knees and knuckles. Some people with dermatomyositis develop
calcified nodules or hardened bumps under the skin. The rash often precedes
muscle weakness. Muscle weakness usually develops over a period of weeks
but may develop over months or even days. The weakness initially affects
those muscles closest to and within the trunk of the body, including neck,
hip, trunk, and shoulder muscles. Difficulty swallowing occurs in at
least one third of DM patients. Whereas less than 25% of adults report muscle
pain, more than 50% of children with DM complain of muscle pain and tenderness.
Dermatomyositis can occur at any age from childhood to adulthood and is more
common in females than males. High dose prednisone (an immunosuppressant)
has been an effective treatment for many patients. Other nonsteroidal immunosuppressants
such as azathioprine and methotrexate are often used. Unfortunately, these
drugs have adverse side effects, especially after prolonged use. For patients
who do not respond well to prednisone, intravenous administration of immunoglobulins
(IVIg) has also proven effective.
Polymyositis
Polymyositis (PM) does not have the characteristic rash of dermatomyositis
(DM). Onset of muscle weakness usually progresses slower than DM. Proximal
(nearest
to the trunk of the body) limb and neck muscles are weakened; involvement
of distal (farthest from the trunk of the body) muscles varies. Difficulty
in swallowing is
common in PM. Inability to breathe due to muscle failure is uncommon but
occurs more often in PM than in dermatomyositis or inclusion body myositis.
As many as
one third of PM patients have muscle pain, but it is rarely the chief complaint.
PM rarely affects people under the age of 20 but cases of childhood and infant
polymyositis have been reported. More women than men are affected with PM.
High doses of prednisone (an immunosuppressant) have been an effective treatment
for many patients. Other nonsteroidal immunosuppressants such as azathioprine
and methotrexate are also prescribed. Unfortunately, these drugs have adverse
side
affects, especially after prolonged use. For patients who do not respond well
to prednisone, intravenous administration of immunoglobulin (IVIg) might
be effective.
Inclusion Body Myositis
Inclusion body myositis (IBM) is very similar to polymyositis. In fact, many
doctors believe patients diagnosed with PM that do not respond to treatment
actually may have IBM. The only definitive test for IBM is a muscle biopsy.
Onset of muscle weakness in IBM is usually very gradual, taking place over
months or years. It is different from PM in that both proximal and distal
muscles are affected. Typical findings include weakness of the wrist flexors
and finger flexors. Atrophy, or
shrinking, of the forearms is characteristic. In the legs, atrophy of the
quadriceps muscle is common with varying degrees of weakness in other muscles.
Difficulty
swallowing occurs in about half the patients afflicted with IBM. Facial muscle
weakness is present in a minority of patients. Falling is often the first
noticeable
symptom of IBM. Symptoms of IBM usually begin after age 50, although no age
group is excluded. IBM occurs more frequently in men than women. About one
in ten cases of IBM may be hereditary. Unfortunately, there is as yet no
known treatment for people with IBM. Intravenous immunoglobulin (IVIg) has
shown some
preliminary evidence for a slight beneficial effect in a small number of
cases. New drugs and other avenues are being explored. Prescribed physical
therapy may be
helpful to maintain mobility.
Juvenile Myositis
Juvenile idiopathic inflammatory myopathy (JIIM) or juvenile myositis (JM)
most often presents itself as dermatomyositis (JDM) with its typical rashes
and muscle
weakness. There are fewer cases of juvenile polymyositis (JPM), inclusion
body myositis (JIBM) and other clinical forms of myositis reported. The JDM
rash
precedes muscle weakness greater than 50% of the time. In both cases,(JPM
and JDM) muscle weakness usually develops over a period of months, weeks or
days.
The weakness being proximal (closest to and within the trunk of the body)
primarily involves neck, hip, trunk and shoulder muscles, but may also include
distal
muscles. Dysphagia (difficulty swallowing), dysphonia (hoarseness), abdominal
pain and arthritis can also occur with this disorder. Muscle pain is seen
in
approximately 50% of children with myositis. Corticosteroids are effective
in the majority of patients and remission with complete withdrawal of medication
can be
anticipated in a large percentage of patients. Patients with severe weakness
may need longer periods of treatment. For those patients who experience dose-limiting
side effects of corticosteroids or are unresponsive, there are other treatments
available, including intravenous immune globulin infusions. It is important
to diagnose these patients and start treatment as soon as possible. The parents
and the doctor of a child with myositis will also want to consider a rehabilitation
program with a team of professional experts in this field.
Diagnostic Criteria for the Inflammatory Myopathies
Classification Criteria for Polymyositis and Dermatomyositis
Patients presenting at least one item from the first criterion and four items
from the second through ninth criteria are said to have dermatomyositis. Patients
presenting
no items from the first criterion and at least four items from the second
through ninth criteria are said to have polymyositis.
1.Skin lesions
Heliotrope rash (red
purple edematous erythema on the upper palpebra)
Gottron's sign (red
purple keratotic, atrophic erythema, or macules on the extensor surface of
finger joints)
Erythema on the extensor
surface of extremity joints: slightly raised red purple erythema over elbows
or knees
2.Proximal muscle weakness (upper or lower extremity and trunk)
3.Elevated serum CK (creatine kinase) or aldolase level
4.Muscle pain on grasping or spontaneous pain
5.Myogenic changes on EMG (short-duration, polyphasic motor
unit potentials with spontaneous fibrillation potentials)
6.Positive anti-Jo-1 (histadyl tRNA synthetase) antibody
7.Nondestructive arthritis or arthralgias
8.Systemic inflammatory signs (fever: more than 37° C at
axilla, elevated serum CRP level or accelerated ESR of more than 20 mm/h
by the Westergren
method)
9.Pathological findings compatible with inflammatory myositis
(inflammatory infiltration of skeletal evidence of active regeneration may
be seen.)
At least 1 item from 1 and at least 4 items from 2 to 9 = DM. Sensitivity is 94.1% (127/135), and specificity of skin lesions against SLE and SSc is 90.3% (214/237). At least 4 items from 2 to 9 = PM. Sensitivity is 98.9% (180/182) and specificity of PM and DM against all control diseases combined is 95.2% (373/392).
Credits: This material is reprinted by permission from "Classification
Criteria for Polymyositis and Dermatomyositis" by Kiyoaki Tanimoto, Keiichiro
Nakano, Shogo
Kano, Shunji Mori, Hiroaki Ueki, Hiroshi Nishitani, Takeshi Sato, Takahiro
Kiuchi, and Yasuo Ohashi as published in The Journal of Rheumatology 1995;
22:4.
Classification and Diagnostic Criteria for Inclusion Body Myositis
I. Characteristic Features - Inclusion Criteria
A. Clinical features
1. Duration of illness greater than 6 months
2. Age of onset greater than 30 years old
3. Muscle weakness
Must affect proximal and distal muscles of arms and legs
and Patient must exhibit at least one of the following features:
1. Finger flexor weakness
2. Wrist flexor greater than wrist extensor weakness
3. Quadriceps muscle weakness (equal to or less than
grade 4 MRC)
B. Laboratory features
1.Serum creatine kinase less than 12 times normal
2.Muscle biopsy
1.Inflammatory myopathy characterized
by mononuclear cell invasion of nonnecrotic muscle fibers
2.Vacuolated muscle fibers
3.Either
Intracellular amyloid deposits (must use fluorescent method of identification
before excluding the presence of amyloid) or
15-18-nm tubulofilaments by electron microscopy
3.Electromyography must be consistent with features of an inflammatory
myopathy (however, long-duration potentials are commonly observed and do
not exclude diagnosis of sporadic inclusion body
myositis).
C. Family History
Rarely, inclusion body myositis may be observed in families. This condition
is different from hereditary inclusion body myopathy without inflammation.
The diagnosis
of familial inclusion body myositis requires specific documentation of the
inflammatory component by muscle biopsy in addition to vacuolated muscle
fibers,
intracellular (within muscle fibers) amyloid, and 15-18-nm tubulofilaments.
This information was developed by the Myositis Association of America, Inc. and is herewith used with permission.
Myositis Association of America, Inc. Definition of Myositis. Available at: http://www.myositis.org/about-im2.html. Accessed November 19, 1999.
Myositis Association of America, Inc. Definition of Myositis. Available at: http://www.myositis.org/criteria2.html. Accessed December 3, 1999
The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, Arbor Publishing Corp, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.