Myositis is not a common manifestation of bacterial infection, but, when it occurs, the consequences to the patient may be severe or even fatal. Staphylococcus aureus and group A streptococci are the most likely causative organisms. Myositis also has been associated with several other infectious agents, including viruses, fungi, and parasites. These pathogens are listed in Box 57.1 ; they are discussed briefly in this chapter and more thoroughly in the chapters on the specific microorganisms. This chapter focuses on two forms of pyogenic myositis, sometimes differentiated as acute bacterial myositis and classical (or tropical ) pyomyositis . The former is caused primarily by group A streptococci and the latter by S. aureus , including methicillin-resistant S. aureus (MRSA). Treatment regimens are included from the United States and India.
Pyomyositis
The pathologic entity termed spontaneous acute myositis was recognized by Virchow in the mid-19th century, but the first clinical description of suppurative myositis generally is attributed to the Japanese surgeon Scriba. In 1904, another Japanese surgeon, Miyake, extensively reviewed the subject of skeletal muscle abscesses and added 33 more cases. As the British and French expanded their colonial empires at the transition to the 20th century, the disease was recognized with increasing frequency in the native populations and in the soldiers who lived in the tropical areas of Asia and Africa. It acquired the name by which it now is known widely— tropical pyomyositis .
The suitability of this designation was confirmed by epidemiologic studies conducted in East Africa, which confirmed that the disease was found commonly in regions with a truly tropical climate (i.e., a fairly constant high temperature and high relative humidity) at an altitude below 4000 feet. Pyomyositis has been described, however, in children from geographic regions of the United States as diverse as New England, California, Iowa, and Texas. Numerous reported cases within the southern half of the United States have occurred in and around San Antonio, Texas. In a 10-year chart review, one or two cases of pyomyositis per 4000 pediatric admissions occurred annually. In contrast, a review of consultations at the University of Iowa Children’s Hospital in the northern United States disclosed fewer cases of pyomyositis. In a children’s hospital in Chandigarh, India, pyomyositis was the admission diagnosis for 40 patients over the course of a decade, whereas in a children’s center in Brisbane, Australia, 34 patients with the diagnosis of pyomyositis were found in a 10-year retrospective review.
Pathophysiology
The etiologic agent of the skeletal muscle abscesses in more than 90% of cases is S. aureus . An increasing percentage are caused by community-associated MRSA. A distant second bacteriologic isolate is Streptococcus, including group A and nonhemolytic strains.
Miyake studied extensively the experimental conditions under which Staphylococcus spp. cause muscle abscesses. When healthy rabbits were given boluses of Staphylococcus intravenously, they occasionally developed small abscesses in the kidney, liver, or spleen but never in the skeletal muscles. When specific muscles were damaged by mechanical pinching or electric current 24 or 48 hours before the intravenous injection of bacteria was administered, small abscesses developed within 2 to 28 days at some of the injured sites in nearly half of the animals. Abscesses were not found in healthy muscle tissue.
The role of trauma was supported further by a study of pyomyositis in 1963 in the British Army. After physicians found this disease to be a common problem in army recruits serving in Asia, they investigated 32 cases and made several observations. Two-thirds of the men recalled having experienced trauma at the affected site, the incidence of abscesses increased as the severity of physical training increased, and the abscesses occurred three times more commonly on the dominant (right) side of the body. In an analysis of 78 cases in Uganda, abscesses also were found more commonly on the right side of the body.
From experimental evidence and clinical observations, two conditions commonly are found when pyomyositis occurs: muscle injury and bacteremia, usually staphylococcal. A reported case is illustrative. A 12-year-old girl caught her left foot in the wheel of a moving bicycle and tumbled to the ground. One week later, she developed a furuncle of the foot, and, within the next 2 weeks, she developed painful lumps in muscles of the thigh, shoulder, and chest wall (which had been injured during the original accident). Cultures from the furuncle and blood and from the incised muscle abscesses grew S. aureus . The initial episode of trauma resulted in a staphylococcal skin lesion and, presumably, a bacteremia that seeded sites of previously bruised muscle.
Seven children with pyomyositis are described in Table 57.1 . An analysis of all seven cases illustrates the consistent association of pyomyositis with trauma. Sources of muscle trauma have ranged from bicycle accidents to strenuous aerobic exercises. Many of these cases explain the predilection of the disease to occur in warmer climates or warmer seasons; concomitant skin infections and muscle trauma are more likely to occur in a climate in which children can play or work outside wearing fewer clothes. An unusual cause of muscle trauma was an intramuscular inactivated influenza vaccination (case 6). One predisposing disease not listed in Table 57.1 is varicella (chickenpox), in which muscle abscesses are secondary to bacterial invasion via the skin vesicles.
| Case a | Sex | Age (y) | Source of Trauma | Circumstances of Trauma | Extent of Disease |
|---|---|---|---|---|---|
| 1 | F | 12 | Bicycle accident | Thrown from bicycle onto street after foot was caught in the wheel | Right deltoid, right chest wall, left thigh, right groin |
| 2 | M | 3 | Fall while running | Fell while running on street | Left calf, right scapula, right buttock |
| 3 | M | 11 | Hay bale accident | Struck in abdomen by bale of hay thrown from a hay baler | Abdominal wall musculature |
| 4 | M | 6 | Blunt trauma to abdomen | Struck in abdomen during mock fistfight with sibling | Abdominal wall musculature |
| 5 | F | 17 | Aerobic exercises | Injured while instructing others in aerobic exercises | Left thigh |
| 6 | M | 1 | Injection | Influenza vaccination into right thigh muscle | Right thigh and septicemia |
| 7 | F | 13 | Volleyball accident | Fell several times diving for volleyball during training exercises | Left iliopsoas |
a Cases 1 and 2 from , cases 3 and 4 from , cases 5 from , case 6 previously unpublished, and case 7 from .
In older textbooks, pyomyositis is said to occur in individuals who are malnourished and who have multiple parasitic infections. However, this association has not been confirmed in children with pyomyositis seen in the United States, Australia, or India. Furthermore extensive immunologic evaluations also have been normal; the tests included quantitative immunoglobulins, enumeration of T-lymphocyte subpopulations, total hemolytic complement levels, and leukocyte function as tested by reduction of nitroblue tetrazolium.
Clinical Presentation
Pyomyositis often is considered a disease of adolescents and young adults, even though it occurs in individuals of all ages, including infants and young children. Boys are affected more often than girls. As more girls enter competitive sporting activities, however, pyomyositis is being reported in female athletes. Many children with pyomyositis have a solitary lesion, but multiple lesions are common findings, especially when the latest imaging technology is used (see the section on Diagnosis ). The most common site of abscess formation is the thigh, followed by the calf, buttock, arm, scapula, and chest wall. The muscle lesions are firm or “woody” to palpation, with a well-defined border. The sign of fluctuation may be difficult to elicit. Erythema and warmth often are not apparent because of the deep location of the masses, although diffuse tenderness usually occurs. When a muscle in an extremity is involved, the entire limb may be swollen. Occasionally pyomyositis also can occur in muscles of the pelvis, in which case pain may be transferred to the hip.
In case reports with a clinical history, children with pyomyositis often had similar presenting complaints. Many had incurred a recent accidental injury (often involving a leg) that usually was not considered serious. After a few days, the children developed low-grade fever, muscle pain, and, occasionally, an impaired gait. These symptoms persisted a few days to a few weeks until a mass appeared. When first examined, many of the patients were considered to have only a contusion or a hematoma; occasionally a child was diagnosed as having a rhabdomyosarcoma. Although the disease usually occurs in individuals who are otherwise healthy, pyomyositis has been reported in patients with malignancy. Pyomyositis also may develop in children with acquired immunodeficiency syndrome or other immunodeficiency. The pathophysiology of pyogenic muscle abscess may not be the same, however, in immunodeficient individuals with increased susceptibility to bacterial infection. Most children with pyomyositis have no definable immunologic abnormalities.
An unusual clinical presentation is acute abdominal pain. Beck and Grose described two children with pyomyositis whose initial complaints were confined to the abdominal wall. One patient, a 6-year-old child, had been struck in the abdomen in a mock fistfight with an older sibling. One week later, he developed a low-grade fever and began to walk with a stoop; after another week, his mother detected a “knot” in his right mid-abdominal wall. The second case involved the 11-year-old son of a rancher; the boy was struck in the abdomen by a bale of hay tossed from a hay baler. When he subsequently developed symptoms of abdominal pain, the diagnosis of appendicitis was entertained. When a mass later became palpable in his abdominal wall, rhabdomyosarcoma was suspected. A correct diagnosis was made after the use of scintigraphy and sonography ( Fig. 57.1 ). Reviews from Africa and India also included patients with muscle abscesses in the anterior abdominal wall.
