Introduction
Idiopathic inflammatory myopathies (IIMs) are commonly referred to as myositis. It is an uncommon medical condition marked by inflammation and weakness in the skeletal muscles. These conditions may extend to internal organs like the lungs, heart, and esophagus. Polymyositis (PM) and dermatomyositis (DM) are the most prevalent myopathies. The categorization of these diseases has expanded in recent years. The presently accepted diagnostic classifications include PM, DM, necrotizing autoimmune myopathy (NAM, also recognized as immune-mediated necrotizing myopathy or IMNM), antisynthetase syndrome, overlap myositis, inclusion body myositis, and juvenile myositis. The frequency and occurrence of dermatomyositis (DM) are 1.4 and 5.8 cases per 100,000 individuals in the United States, respectively. There is a higher prevalence among females and an increased incidence in older individuals. In the case of juvenile DM (JDM), it can manifest in children, with a prevalence of 3.2 cases per one million children in the UK but is more common among girls.
What Are Uncommon Myositis Syndromes?
Patients typically display symmetrical proximal muscle weakness that develops over weeks or months, accompanied by characteristic erythematous changes. These skin alterations can precede or follow the myopathy and include features such as a heliotrope rash, eyelid edema, mechanic’s hands, Gottron papules at extensor surfaces, and subcutaneous calcification. While myalgia is not a typical symptom, it can occur. Severe cases of DM may lead to dysphagia and dysarthria, with other significant complications, including the identification of interstitial lung disease (ILD) or tumors. A subtype known as clinically amyopathic DM (CADM) is characterized by typical skin changes with minimal or no signs of myopathy. It constitutes up to 20 % of all DM cases and may also be associated with ILD. The presence of anti-CADM-140 antibodies has been linked to DM/CADM and the prediction of outcomes, particularly in rapid-progressive ILD cases.
What Causes Uncommon Myositis Syndromes?
The pathology of DM involves the binding of immune complexes to endothelial cells, triggering complement system activation and cell lysis through the membrane-attack complex (MAC). This process results in cell necrosis and a reduced number of capillaries in the muscle, leading to perifascicular atrophy. Recent challenges to this classical concept propose a type-I-interferon-mediated cascade as a predominant element in the pathology, with overexpression of type-I-interferon-induced genes correlating with disease activity. Within the inflammatory tissue, pro-inflammatory mediators such as TGF-β, MHC-I, IL-1β, CCL-3, and CCL-4 are overexpressed. Immune cell extravasation into the muscle tissue is facilitated by the upregulation of vascular cell adhesion molecule (VCAM)-1 and intercellular adhesion molecule (ICAM)-1 on endothelial cells, binding to their receptors VLA-4 and LFA-1 on immune cells. Perimysial and perivascular areas contain T-cells, B-cells, macrophages, and plasmacytoid dendritic cells as cellular components of the immune system. In the skin, characteristic findings include vacuolar degeneration of epidermal basal cells with epidermal atrophy and dermatitis featuring lymphocytes and macrophages. MAC depositions are evident along the dermoepidermal junction and vessels.
What Are the Clinical Features Of Myositis?
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Muscle Manifestation - Around 80 % of individuals diagnosed with dermatomyositis (DM) exhibit myopathy, indicating an impact on muscle tissue. The remaining 20 %, identified by dermatomyositis-consistent skin symptoms but lacking myopathy, are diagnosed with clinically amyopathic dermatomyositis (CADM). Polymyositis (PM) is inherently myopathic, implying that 100 % of PM patients undergo myopathy at some stage of their illness. In DM, the characteristic muscle manifestation involves a sudden or subacute onset of symmetrical, proximal muscle weakness. Patients with PM typically display symmetric weakness in the upper and lower extremities, primarily in a proximal distribution, although distal muscle involvement is also observed to a lesser extent. The myopathy associated with DM is typically devoid of pain. Muscle tenderness and myalgia may also be documented.
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Skin Manifestation - A common display of skin symptoms in adults diagnosed with dermatomyositis (DM) could include a bluish-purple rash encircling the eyes accompanied by swelling; a red rash on the face, knees, elbows, ankles, neck, front chest forming a V-sign, and on the back and shoulders creating a shawl sign; and violet rashes on the knuckles known as Gottron's rash, which progresses into scaly discolorations. Additionally, distinctive features encompass expanded capillary loops at the nail bases, irregular and thickened cuticles, and fissured fingertips on the palms.
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Systemic Effects - DM and PM are linked to various potentially incapacitating complications affecting multiple bodily systems, encompassing the pulmonary, cardiac, gastrointestinal (GI), endocrine, and vascular systems. Interstitial lung disease (ILD) stands out as the most prevalent complication of idiopathic inflammatory myopathies (IIM), occurring in about 23 percent of Americans diagnosed with DM or PM. ILD typically manifests with a dry cough, shortness of breath, fine inspiratory crackles during lung examination, and a restrictive pattern in pulmonary function tests. Myositis-related ILD manifests through two primary clinical patterns contingent on the disease course: chronic interstitial lung disease and rapidly progressive (RP-ILD). RP-ILD is linked to an unfavorable prognosis, marked by a swift decline in lung function and mortality rates ranging from 70 to 90 percent.
How to Diagnose and Treat Myositis?
A precise medical history and thorough medical examination are essential prerequisites for accurately diagnosing myositis. The pattern of paresis distribution can offer insights, distinguishing IBM through a specific pattern compared to the proximal weakness shared by DM, PM, and NM. The standard laboratory examination typically reveals elevated creatine kinase (CK) levels and occasionally other enzymes like LDH, AST, or ALT, with the possibility of detecting myositis-specific autoantibodies. Needle electromyography (EMG) of affected muscles commonly exhibits a myopathic pattern, often revealing signs of acute damage. Magnetic resonance imaging (MRI) can detect alterations in signal intensity within the muscle, showcasing edema in early myositis stages and later stages reflecting fatty transformation or muscle atrophy. While these changes are not specific to myositis and may appear in injuries or other conditions like muscle infarction, subacute denervation, or rhabdomyolysis, they remain valuable in identifying affected muscle groups and aiding in an accurate diagnosis. Notably, MRI can reveal subclinical muscle involvement, potentially indicating another condition, such as muscular dystrophy.
A biopsy represents the most crucial and invasive step in reaching a correct diagnosis, providing the only means to differentiate between various myositis subtypes and ruling out muscular dystrophy or other hereditary myopathies. Selecting a representative muscle for the biopsy, usually one demonstrating moderate paresis, is critical. Chest X-rays are recommended for detecting interstitial lung disease (ILD), and in patients with antisynthetase autoantibodies and anti-CADM, high-resolution CT scans may be performed to identify pulmonary changes before clinical symptoms emerge. Patients at high risk of cardiac involvement should undergo regular cardiological examinations. For those on prednisone therapy exceeding 5 mg daily, assessing bone density is advisable.
While dysphagia is frequently reported in IBM, patients with DM and PM also commonly exhibit signs of swallowing difficulties. Studies indicate a high incidence of self-reported dysphagia in myositis patients, emphasizing the importance of including inquiries about this topic in every examination. When dysphagia is reported, additional diagnostics may involve procedures such as video fluoroscopy or flexible endoscopic evaluation of swallowing (FEES).
Conclusion
Uncommon Myositis Syndrome is an inflammatory rare disease. It is marked by inflammation and weakness in the skeletal muscle. Although proper medical diagnosis and treatment is required. It is a curable and rare disease.
