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Myositis

The myositides (inflammatory myopathies) are a heterogeneous group of disorders, causing three distinct clinical syndromes: polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM).

■ Pathogenesis

Most myositides found in the temperate zones are autoimmune diseases of unknown cause, characterized histologically by muscle inflammation and fibrosis and loss of muscle fibers. In PM, cytotoxic CD8+ T cells penetrate and damage muscle fibers (^ intramuscular cellular infiltrates). CD8+ T cell activation is induced by abnormal expression of class IHLA antigens on the surface of the muscle fibers, which are normally HLA-negative. DM is thought to be largely due to antibodies against blood vessels within muscle, which activate the complement system (membrane attack complex). Vascular endothelial damage ultimately leads to ischemia and death of muscle tissue (^ perifascicular atrophy). Inflammatory T cells and macrophages migrate into muscle and cause further damage. IBM is of unknown pathogenesis. Infectious myositis may be due to bacteria, viruses, parasites, or fungi.

Polymyositis (PM) begins with weakness of the proximal muscles of the lower limbs, which then progresses and slowly spreads to the upper limbs. The deltoid and neck flexor muscles are commonly involved. Dysphagia may be present. The involved muscles eventually become atrophic. In overlap syndrome, myositis appears together with another autoimmune disease, e. g., progressive systemic sclerosis, systemic lupus erythematosus, rheumatoid arthritis, pol-yarteritis nodosa, polymyalgia rheumatica, or Sjögren syndrome. Myalgia is often the major symptoms in patients with PM, as also in patients with hypereosinophilia syndrome (Churg-Strauss syndrome) or eosinophilic fasciitis (Shulman disease). Dermatomyositis (DM) progresses more rapidly than PM and is distinguished from it mainly by the bluish-red or purple (heliotrope) rash found on exposed areas of the skin (eyelids, cheeks, neck, chest, knuckles, and extensor surfaces of the limbs). Small hemorrhages and telangiectasias are found in the nailbeds; affected children may have subcutaneous calcium deposits. Cancer accompanies DM six times more frequently than PM; DM is also associated with scleroderma and mixed connective tissue disease.

Inclusion body myositis (IBM) is characterized by distal (sometimes asymmetric) weakness and muscle atrophy, mainly in the lower limbs (plantar flexors), with early loss of the quadriceps reflexes. There are both sporadic and hereditary forms of IBM (see also p. 252).

The myositides are diagnosed by history and physical examination, elevated serum concentration of sarcoplasmic enzymes (particularly CK-MM), and characteristic findings on EMG and muscle biopsy. Muscle atrophy can also be assessed with various imaging techniques (CT, MRI, ultrasonography). The presence of antibodies in association with a connective tissue disease may be relevant to the diagnosis (p. 180).

PM and DM are treated by immune suppression, e.g., with corticosteroids, azathioprine, or intravenous gammaglobulin (ivig). Physical therapy is begun once the patient's condition has stabilized. IBM may respond to intravenous immunoglobulin therapy.

Cervical muscle weakness

-Lymphomonocytic infiltrate in muscle, vessel (PM, cross section of muscle fiber)

Cervical muscle weakness

Proximal muscle weakness and atrophy

Proximal muscle weakness and atrophy

-Lymphomonocytic infiltrate in muscle, vessel (PM, cross section of muscle fiber)

Ischemic lesion of muscle fiber

Perifascicular atrophy (DM; cross section of muscle fiber)

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Polymyositis (PM)

Polymyositis (PM)

Polymyositis Atrophy

Telangiectasis, hemorrhage (nailbed)

Erythema in joint region (extensor side)

Proximal muscle weakness

Distal muscle atrophy

Telangiectasis, hemorrhage (nailbed)

Erythema in joint region (extensor side)

Dermatomyositis (DM)

Distal muscle atrophy

Bleeding -

Bleeding -

Inclusion body myositis (IBM)

Butterfly rash (lupus erythematosus)

Muscle Pain (Myalgia)

Myalgia is an aching, cramping, or piercing pain in muscle. It is triggered by stimulation of nociceptors (p. 108). Pressure or traction on a muscle causes myalgia that subsides once the mechanical stimulus is removed, while inflammatory and other lesions in muscle cause persistent and gradually increasing myalgia. Muscle ischemia and/or metabolic dysfunction are reflected by myalgia occurring only during muscle activity. Myalgia includes allodynia, which is defined as pain induced by normally nonpainful stimuli and is explained by the sensitization of nociceptors by pain-related substances such as brady-kinin, serotonin, and prostaglandin. A "charley-horse" is a type of myalgia that normally begins 8-24 hours after muscle overuse (simultaneous stretching and contraction) and lasts 5-7 days. It is caused by an inflammatory reaction to muscle fiber damage. Myalgia can be triggered by disorders whose primary pathology lies anywhere in the nervous system (peripheral nerve, spinal cord, brain).

I Causes of Myalgia

Type of Myalgia

Selected Causes

Localized myalgia

• Toxic-metabolic

• Overactivity

• Exercise-induced

• Parkinsonian

Generalized myalgia

• Toxic-metabolic

• Trauma, coagulopathy

• Infectious: Streptococcal infection, trichinosis, influenza, epidemic pleurodynia. Noninfectious: Nodular focal myositis, eosinophilic fasciitis, sarcoidosis, myositis ossifi-cans

• Arteriosclerosis (intermittent claudication), embolism

• Acute alcoholic myopathy, metabolic myopathy (pp. 402, 405)

• Stiff-man syndrome, neurogenic myotonia, tetanus, strychnine poisoning, amyotrophic lateral sclerosis, tetany

• Metabolic myopathy, arteriosclerosis, physical exertion

• Polyneuropathy, metabolic disorder (electrolyte imbalance, uremia, thyroid dysfunction)

• Restless legs syndrome, painful legs and moving toes syndrome

Polymyositis/dermatomyositis (p. 344)

Hypothyroidism, medications2, mitochondrial myopathy (pp. 340, 402, 405) Polymyalgia rheumatica, amyloidosis, osteomalacia, Guillain-Barre syndrome, porphyria, hypothyroidism, corticosteroid withdrawal, fibromyalgia

1 E.g., emetine, lovastatin, and e-aminocaproic acid.

Rhabdomyolysis

Local or generalized damage to skeletal muscle can cause myoglobinuria and an elevated serum concentration of creatine kinase, usually accompanied by the acute onset of proximal or diffuse weakness, with myalgia, muscle swelling, and general manifestations including nausea, vomiting, headache, and sometimes fever. The urine may be discolored at the onset of symptoms or several hours later. Rhabdomyolysis can be caused by certain types of myopathy (e. g., poly-myositis, central core disease, metabolic myopathies; pp.402, 405), by muscle strain or trauma (long-distance walking or running, heat stroke, delirium tremens, status epilepticus), by toxic substances (see below), and by infectious disease (bacterial sepsis, influenza, coxsack-ievirus or echovirus infection).

Malignant Hyperthermia (MH)

This life-threatening disorder of skeletal muscle function is characterized by hyperthermia, muscle rigidity, hyperhidrosis, tachycardia, cyanosis, lactic acidosis, hyperkalemia, massive elevation of the serum creatine kinase concentration, and myoglobinuria. It is induced by anesthetic agents such as halothane and succinyl-choline. The predisposition to MH is inherited as an autosomal dominant trait (gene loci: 19q13.1, 17q11-24,7q12.1, 5p, 3q13.1,1q32). The creatine kinase level may be chronically elevated in susceptible individuals, who can be identified with an in vitro contracture test performed in specialized laboratories. Persons suffering from central core disease, multicore disease, and King-Denborough syndrome (dwarfism, skeletal anomalies, ptosis, high palate) are also at risk for MH. Treatment: dantrolene. Malignant neuroleptic syndrome clinically resembles MH; unlike MH, however, it is usually of subacute onset (days to weeks), it is not hereditary, and it is triggered by psychotropic drugs (haloperidol, phenothiazines, lithium). Malignant neuroleptic syndrome can also be induced by abrupt withdrawal of dopaminergic agents in patients with Parkinson disease.

Paraneoplastic Syndromes

Distant neoplasms can affect not only the CNS (see p. 388) but also the PNS and skeletal muscle. Remarkably, paraneoplastic syndromes sometimes appear months or years before the underlying malignancy becomes clinically manifest. Paraneoplastic neuromuscular syndromes typically present with marked weakness of subacute onset (i.e., developing over several days or weeks).

Toxic Neuromuscular Syndromes

The muscle fiber lesions regress if the responsible substance is eliminated in timely fashion (Table 75, p. 405).

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