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Soft Tissue Infection: Cellulitis, Pyomyositis, Abscess, Septic Arthritis
Infections of the soft tissues range clinically from indolent, low-grade conditions to fulminant disease that may be life-threatening within a matter of hours. A wide range of organisms can produce an infection, although there are common culprits. Clinical confusion may occur because the presentation may mimic tumor or degenerative disease and vice versa. Infection should always be in the mind of those involved in the diagnosis of musculoskeletal disorders. Antibiotics provide an important part of the treatment, but, in many cases, drugs alone are inadequate to deal with the disease. It may be necessary to perform surgery to remove dead and necrotic tissue and allow drainage of deep cavities. Resistance to antibiotic regimens is an increasing problem, and the recognition of failure of response to antibiotic therapy is an important part of the diagnostic process.
Etiology
Organisms may be introduced into the body by a variety of means, depending on environmental, organism, and host factors. These routes include inhalation, direct inoculation (often by trauma or surgery), and ingestion. These can be followed by further local invasion, hematogenous or lymphatic spread, and thereby invasion of organisms into regions of the body where their presence is damaging. As soon as the organism has reached the location where infection might develop, the chance of establishing infection depends on organism factors and the host's local and systemic immunity. Poor vascular supply, dead or damaged tissue, collections of blood or lymph, and foreign implanted material, including endoprostheses, increase the risk of infection. Tumors are also destructive and produce necrotic areas within them that may be the center of secondary sepsis.
Systemic disorders and immune suppression not only change the individual's susceptibility but also affect the way in which infection manifests and which organisms are the likely cause. Imaging plays a fundamental role in the diagnosis, assessment, treatment, management, and follow-up of patients with soft tissue and bone infection.
Prevalence and Epidemiology
The Staphylococcus aureus infection is the most common cause of soft tissue infection throughout the world. However, there are regional variations in the incidence and causative organism. For example, abscesses due to melioidosis are typically seen in rice field workers, and Mycobacterium marinum is a hazard to those who keep tropical fish.
Clinical Presentation
Given the wide variety of organisms and means of infections, there are many ways in which soft tissue infection may present. The clinical entities described here are not really separate diseases but more an emphasis of one part of the spectrum. One may lead to the next, and two or more may be present in the same case.
Pathology
In the early stages of infection, when the organism is establishing itself, there will be an acute associated inflammatory response leading to soft tissue edema and opening of normal vascular channels, with increased blood flow to the affected area. The patient will complain of swelling, pain, and heat if the site is superficial. There is likely to be a systemic response with a febrile illness. Subsequent progress depends on the location and nature of the organisms involved. Some infections lead to rapid tissue destruction and necrosis. This, in turn, leads to systemic toxicity and severe ill health. Septicemic shock, hypotension, and tachycardia may result. In other infections in which the growth rate is slower, there may be local destruction of tissue with abscess formation. At first, microcavities containing pus will occur, but these may coalesce into larger collections. These abscesses, in turn, may spread and penetrate adjacent structures. They may perforate the skin and discharge through a sinus track. They may enter joint spaces or abdominal viscera. When soft tissue infections are located adjacent to bone, they may excite a periosteal reaction and cause underlying bone edema. Those that are particularly destructive may cause erosion of the bone cortex. Infections that begin within or enter joints can cause early damage to the articular cartilage, which, intrinsically, has a poor blood supply. This destruction is likely to be irreversible. Joint effusion will occur early, either due to pus within the joint or, more commonly, to a reactive effusion. Joint space narrowing occurs late because the early manifestation of cartilage involvement will be edema. Secondary osteomyelitis may be due to direct spread of the organism into the adjacent bone. This implies penetration of the articular cartilage and joint capsule if the infection arose primarily within the joint. Osteomyelitis is infection arising principally in bone. However, sometimes, it will present as a soft tissue infection due to cortical penetration and spread into the adjacent structures.
Chronic infection is when the disease process reaches a stable state or one that is changing very slowly. Here the combination of reactive changes to the infection will be seen with the acute structure or residual necrosis. Chronic abscess cavities may be associated with fibrous reaction in the adjacent soft tissue. Cloacae and sinus tracks may become lined by epithelium. In the very long term, areas of chronic infection may be complicated by amyloid formation and rarely by malignant change.
Manifestations of the Disease
Imaging is pivotal in the diagnosis and management of soft tissue and bone infections. The appropriate investigation depends on the clinical presentation.
Soft Tissue Swelling with Erythema
In acute cellulitis and in abscess formation, radiographs may show no abnormality. Later in the disease, there may be subtle periosteal reaction, and large abscesses may cause bone damage. The only circumstance in which early radiography is contributory is when gas-forming organisms are present (e.g., in clostridial myonecrosis) because the gas may be identified. However, these patients are critically ill, and clinical features will be more important in the initial assessment. Tissue crepitus is occasionally palpable. The radiologist is unlikely to add to the clinician's view of the severity of the case by finding gas in the immune-competent patient. Ultrasound examination is useful to discriminate cavitating abscess formation from diffuse cellulitis. Areas of fluid collection may be aspirated both to obtain a microbiologic diagnosis and to alleviate pressure from an abscess cavity. The ultrasound appearances will be of a hypoechoic area that may appear as clear fluid or alternatively could contain particulate matter. The adjacent soft tissues will have increased blood flow on Doppler imaging.
MRI is useful for assessing the extent of edema and the size of any abscess cavity. It is the definitive way of determining whether an abscess communicates with a joint. MRI is particularly useful in determining the extent of sinus tracks and the involvement of adjacent bones and has virtually replaced sinography. Although ultrasonography can be used as described earlier, the majority of patients suspected of having an infection should have MRI as the initial examination after radiography.
Unexplained Pain with or without Fever
In the assessment of acute osteomyelitis, radiographs may be normal or may show some patchy and localized osteopenia in the region of the infection. The finding of periosteal reaction is a specific one but is not often present in the early stages. Ultrasonography is of less value, unless periosteal edema is detected with increased blood flow on power Doppler imaging. This constellation of ultrasound findings is useful in this setting. Unfortunately, patients with acute osteomyelitis may not exhibit this finding. Therefore, the combination of ultrasonography and MRI is the best way to discriminate soft tissue infection from bone infection ( Fig. 64-1 ).
The combination of rapid and slow growth in the same patient is a feature that suggests a lesion is infectious rather than tumorous in origin.
Nuclear medicine studies have a minor role to play in the assessment of infection and would probably be of value only when MRI is not available. This technique is less specific than MRI and no more likely to show up areas of edema. The patients normally present with pain in a specific location, and so a limited examination of that area by MRI is appropriate. Furthermore, the epiphyseal regions of the younger child with active growth will show increased radionuclide accumulation on the nuclear medicine study, and this finding may be misleading or confusing. Bone scintigraphy may show areas of uptake in soft tissue infection due to the increased vascular supply and areas of necrosis. A central area of activity void suggests an abscess. More specific techniques show infected areas, including gallium citrate–labeled and indium chloride–labeled white blood cell scintigraphy. These studies are more specific and have an occasional role in complex cases.
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