Infections of muscle

Genetic testing

NCV/EMG

Laboratory

Imaging

Biopsy

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Fig. 10. Pyomyositis. A Marked neutrophil inflammation (arrow). The muscle fibers are tex-tureless and have no nuclei, features consistent with rhab-domyolysis. B Neutrophil inflammatory response dispersed between several fibers

Fig. 9. HIV myopathy. Proximal arm atrophy and bilateral scapular winging in a patient with HIV myopathy

Fig. 10. Pyomyositis. A Marked neutrophil inflammation (arrow). The muscle fibers are tex-tureless and have no nuclei, features consistent with rhab-domyolysis. B Neutrophil inflammatory response dispersed between several fibers

Fig. 11. Trichinella spiralis.

Slide shows a calcified cyst within the muscle (arrow)

Distribution/anatomy The distribution is variable depending on the type of infection.

Time course

Is variable depending on the type of infection.

Onset/age

Any age.

Clinical syndrome

Viral myositis

Influenza virus myositis is characterized by severe pain, tenderness and swelling which usually affects the calf muscles but may also affect thigh muscles. Myalgia is the most common symptom, and starts approximately one week after the onset of the influenza infection, and then persists for another 2-3 weeks. The disorder is usually self-limiting, however in rare cases it may be severe with myoglobinuria and a risk of renal failure. Coxsackie virus infection is characterized by a wide spread acute myositis which may be severe and may be associated with myoglobinuria. Epidemics of Coxsackie virus infection tend to occur during the summer and fall. In children aged 5-15 years there may be a self-limiting acute inflammatory myopathy. Infection is usually caused by Coxsackie virus group B. Affected patients may complain of muscle aching, often exacerbated by exercise, and weakness if it occurs may be minimal. The symptoms usually resolve within 1-2 weeks. Bornholm's disease is associated with severe pain and tenderness in the muscles of the chest, back, shoulders, or abdomen and may be associated with a more severe Coxsackie B5 infection.

The human immunodeficiency virus (HIV), and human T-cell lymphotrophic virus (HTLV) may be associated with a variety of myopathic manifestations. HIV infected patients may develop one of the following manifestations: a) An HIV associated myopathy (Fig. 9) that resembles polymyositis. b) Zidovudine myopathy, which resembles mitochondrial myopathy. c) AIDS-associated cachexia with muscle wasting. d) Opportunistic infections and tumor formation within muscle. e) A myopathy resembling nemaline myopathy. f) An HIV associated vasculitis. With HIV associated nemaline rods, the CK is often very high and there may be evidence of muscle fiber necrosis. HIV may also be associated with a necrotizing myopathy with proximal weakness. Pyomyositis and lymphoma may also develop in the muscle, and may be associated with painful limb swelling. A variety of organisms have been associated with pyomyositis including cryptococcus, CMV, Mycobacterium avium intracellularae (MAI), and toxoplasma. With HIV wasting disease, which is more common in sub Saharan Africa, there is fatigue and evidence of type 2 muscle fiber atrophy.

HTLV1 may also be associated with polymyositis, as well as causing a tropical spastic paraparesis (TSP).

Pyomyositis associated with staphylococcus, streptococcal and clostridial in- Pyomyositis fections are the most common forms of bacterial myositis. Pyomyositis most commonly occurs in tropical areas and may occur without any antecedent illness or other predisposing factors. It may also be associated with trauma, malnutrition, diabetes mellitus, following an acute viral infection, associated with a suppurative arthritis or osteomyelitis, or from hematogenous spread from a bacterial source within the body. Non-tropical pyomysitis may occur in elderly bed ridden patients with bed sores, intravenous drug users, burn victims, in immunosuppressed patients, e.g. AIDS or underlying cancer. In the vast majority of cases, Staphylococcus aureus is cultured from the abscesses, however other organisms including Streptococcus pyogenes, salmonella, and pneu-mococcus may also be isolated from the abscess. Clinically there is painful swelling of the muscle, the pyomyositis often affects the quadriceps, glutei muscles, biceps or pectoral muscles. Although the swelling may initially be hard, it rapidly becomes fluctuant as the inflammation increases and muscle necrosis occurs. Clostridial myositis is due to infection with Clostridium welchii, and develops after wound or muscle contamination. The clinical features of clostridial myositis include local pain, swelling, production of serosanguinous fluid, and local brownish discoloration. Patients may develop systemic signs of septicemia. Necrotizing fasciitis and myonecrosis (a flesh eating infection) is a rare but life-threatening disease, most often caused by group A P-hemolytic Streptococcus pyogenes. The disorder may occur postoperatively, or following minor trauma. There is destruction of skin and muscle in response to streptococcal pyrogenic exotoxin A.

Fungal myositis is uncommon in man. In immunocompromised patients, fungal Fungal myositis myositis is becoming increasingly more common in those suffering from AIDS or with malignancies. Sporotricosis, histoplasmosis, mucormycosis, candidiasis, and cryptococcosis are all associated with myositis. In sporotricosis and histoplasmosis a single muscle or group of muscles is usually affected with formation of an abscess. Mucormycosis can spread into the orbit where it produces ophthalmoplegia, proptosis, and edema of the eyelid. In disseminated candidiasis, patients develop papular cutaneous rashes, and wide spread muscle weakness with myalgia. Toxoplasmosis may cause local inflammation within the muscle. In immunocompromised hosts it is often asymptomatic, however in other infected subjects, an acute infection may develop with lymphadenopathy which may remit spontaneously, and in some patients a polymyositis-like syndrome may develop.

American trypanosomiasis (Chagas' disease) caused by Trypanosoma cruzi can Parasitic myositis cause an inflammatory myopathy coupled with evidence of a neuropathy. In

African trypanosomiasis, there is malaise and fever along with myocarditis, polymyositis and encephalopathy. Microsporidiosis is caused by the zoonotic protozoa, microsporidium, and results in polymyositis in immunocompromised patients. In addition to causing the systemic illness malaria, plasmodium falciparum can also cause acute muscle fiber necrosis. Cysticercosis results from infection by Cysticercus cellulosae, the larval form of the pork tapeworm Taenia solium. The encysted parasite may be found in skeletal and heart muscle, as well as eye and brain. The clinical features vary according to the location and number of cysts, however myalgia, fever, and vomiting may occur as part of the overall syndrome. Trichinosis is caused by the larva of Trichinella spiralis and may be associated with periorbital and facial edema, fever, myalgia, and proximal muscle weakness. Occasionally the disorder may mimic mild dermatomyositis. Myositis is also reported with echinococcosis, visceral larva migrans, cutaneous larva migrans, coenurasis, sparganosis and dracunculosis.

Pathogenesis The specific mode of muscle injury depends on the particular pathogen. Several of the viral infections, including HIV may cause myositis by increasing release of cytokines and interferons. Viral infections may also cause perivascular, perimysial, or endomysial inflammation. In streptococcous pyogenes infections the pathogenic M-protein and associated proteases may prevent the normal host phagocytic response.

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