Imaging of Musculoskeletal Infections Related to Recreational Drug Use

Key Points

Imaging evaluation of musculoskeletal infections in recreational drug use is usually performed using radiographs, which are frequently nonspecific but may guide early treatment and can be used for subsequent monitoring of treatment algorithms.
Magnetic resonance imaging (MRI) is often the imaging modality of choice for evaluating musculoskeletal infections, manifesting as increased T2 signal, correspondingly low T1 signal, and enhancement with intravenous contrast.
In necrotizing fasciitis, early recognition of fascial edema and interfascial fluid on MRI is essential. These signs are detected before soft tissue gas and will facilitate prompt treatment and dramatically impact prognosis.
Prompt diagnosis of osteomyelitis is essential to the initial early administration of antibiotics to prevent osteonecrosis, which leads to significant long-term pain and disability.

Introduction

Recreational drug abuse is a significant problem worldwide, resulting in substantial morbidity and mortality, as well as economic impact. The United Nations Office on Drugs and Crime estimates that, in 2017, 271 million people, constituting 5.5% of the global population aged 15 to 64 years, had used drugs in the previous year. This was similar to recent years but represented a 30% increase from 2009, when an estimated 210 million people had used drugs in the previous year. In addition, the economic impacts of recreational drug use are significant, with 585,000 deaths and 42 million years of “healthy” life lost in 2017.

Injected drug abuse is particularly problematic: of those who abused drugs, an estimated 11.3 million worldwide injected them, with a large proportion of those people residing in the United States, China, and Russia. Injected drug abuse leads to significant health consequences. For instance, human immunodeficiency virus (HIV) and hepatitis C infections are highly prevalent among people who inject drugs (PWID), affecting 1.4 million and 5.6 million people, respectively. More acute infectious sequelae of injected drug use are numerous, including localized soft tissue infections such as cellulitis, abscess, and necrotizing fasciitis; local and distant bone and joint infections such as osteomyelitis and septic arthritis; and distant vascular pathologies such as septic thrombophlebitis and mycotic aneurysm.

Because the clinical presentation of musculoskeletal infections may overlap, histories are often unreliable with PWID, and prompt treatment of pathologies such as necrotizing fasciitis and septic arthritis are critical, imaging plays a crucial role in the diagnosis of infections. Diagnosis often starts with radiographs, which, aside from being inexpensive and readily available, provide important anatomic information and may guide subsequent imaging and treatment decisions.

Ultrasound (US) is helpful in evaluating fluid collections such as abscesses and joint effusions. If a collection is identified, it may be used to guide aspiration. It is especially beneficial in the pediatric population, as it produces no ionizing radiation and does not require sedation. However, it is heavily user-dependent and provides little additional value in older patients.

Computed tomography (CT) is fast, relatively inexpensive, and widely available in emergency departments, and provides high spatial resolution and bony detail. CT with intravenous (IV) contrast can be used to evaluate for any enhancing lesions or areas of inflammation and can detect changes of osteomyelitis earlier than conventional radiography, but not as early as magnetic resonance imaging (MRI). The most significant drawback of CT is its use of ionizing radiation.

MRI is the modality of choice in diagnosing most musculoskeletal infections because of its superior soft tissue resolution and sensitivity for pathologic fluid. It is the most sensitive modality for acute osteomyelitis. However, MRI is relatively costly and time-consuming, may not always be available, and can possibly be contraindicated due to incompatible implanted devices/hardware.

Soft Tissue Infections

Cellulitis

Cellulitis is defined as an infection of the dermis and subcutaneous soft tissues. Diagnosis is typically clinical; however, imaging may be beneficial in PWID due to the increased risk of complications, such as an abscess or venous thrombosis. Clinically, cellulitis presents as a painful, poorly defined area of inflammatory skin changes, such as edema, erythema, and warmth. Systemic symptoms such as fever and chills may be present.

Radiographic findings of cellulitis are nonspecific and include indirect signs of infection, such as generalized swelling and obliteration of fascial planes ( Fig. 10.1 ). Radiography may reveal retained radiopaque foreign bodies such as needles in drug abusers, which may serve as a nidus for infection ( Fig. 10.2 ). Cellulitis demonstrates soft tissue edema, resulting in a “cobblestone” appearance of subcutaneous fat on US ( Fig. 10.3 ), as well as increased echogenicity of the soft tissues. This is a nonspecific appearance on grayscale US and may be present with other causes of subcutaneous edema, such as congestive heart failure or fluid overload. One distinguishing feature of edema due to infection is the presence of hyperemia on Doppler imaging ( Fig. 10.4 ). On CT, cellulitis demonstrates increased attenuation of the subcutaneous fat with inflammatory changes, such as stranding and skin thickening ( Figs. 10.5, 10.6 ).

Fig. 10.1, Lateral radiograph of the right foot (A) demonstrates an area of soft tissue swelling (blue shaded area) along the dorsum of the foot, without periosteal bone reaction, representing cellulitis. Lateral radiograph of the left foot (B) in a different patient demonstrates more extensive soft tissue swelling (green shaded area) along the dorsum of the foot.

Fig. 10.2, Anteroposterior (AP) (A) and lateral (B) radiographs of the right forearm in a patient with history of heroin abuse demonstrates a thin, linear, radiodense object in the proximal volar soft tissues (red arrows), representing a retained needle. AP (C) and lateral (D) radiographs of the left elbow in a separate patient, also with history of heroin abuse, demonstrate a similar thin, linear, radiodense object in the antecubital fossa (yellow arrows), representing a retained needle.

Fig. 10.3, Greyscale ultrasound image of the palmar surface of the hand demonstrates the characteristic “cobblestone” appearance of subcutaneous edema (blue shaded area), consistent with cellulitis.

Fig. 10.4, Greyscale (A) and color Doppler (B) ultrasound of the wrist demonstrates complex, edematous subcutaneous tissue (blue shaded area) and hyperemia, consistent with cellulitis.

Fig. 10.5, Contrast-enhanced axial (A) and sagittal (B) computed tomography of the right hand in soft tissue window demonstrate diffuse soft tissue swelling and subcutaneous fat stranding of the dorsum of the hand (blue shaded area) representing cellulitis.

Fig. 10.6, Contrast-enhanced axial (A), coronal (B), and sagittal (C) computed tomography of the left lower extremity in soft tissue window demonstrate diffuse severe superficial soft tissue swelling and subcutaneous fat stranding (blue shaded area) without drainable collection or deep fascial involvement, consistent with cellulitis.

MRI investigation of cellulitis typically includes pre- and postcontrast T1-weighted images, T2-weighted images, and fluid-sensitive sequences such as short tau inversion recovery (STIR). Diffusion-weighted imaging (DWI) may be added to assess for abscess formation, which will restrict diffusion. Cellulitis appears as a high T2 and STIR signal within the subcutaneous tissues with corresponding low T1 signal and contrast enhancement ( Fig. 10.7 ).

Fig. 10.7, Axial (A) and sagittal (B) T2-weighted magnetic resonance imaging (MRI) with fat saturation of the right foot demonstrate diffuse T2 hyperintensity along the dorsal cutaneous and subcutaneous soft tissues. Axial (C) and sagittal (D) T1-weighted MRI of the same region demonstrate a smaller region of T1 hypointensity along the dorsal soft tissues. Findings are consistent with cellulitis.

Treatment of cellulitis involves antibiotics and supportive measures.

Abscess

A potential complication of cellulitis is the formation of an abscess, an encapsulated purulent fluid collection, which may be located in superficial or deep soft tissues. Abscesses may form via direct extension from injection sites or due to hematogenous seeding from distant sites. Abscesses are more likely to form around retained foreign bodies and in immunocompromised states, both of which are more prevalent among PWID. Clinical presentation of abscesses depends somewhat on location and depth. Systemic signs and symptoms such as fever, chills, leukocytosis, and elevated inflammatory markers are common.

Abscesses are difficult to visualize on conventional radiographs because, aside from soft tissue swelling, the sequelae of abscesses may be the only indicator, such as adjacent joint effusions or periosteal reactions ( Fig. 10.8 ). On US, abscesses appear as well-defined anechoic cavitary collections with peripheral hyperemia ( Fig. 10.9 ). Mobile internal debris or septae are often seen within abscesses. US is also a useful tool to guide abscess aspiration and drainage.

Fig. 10.8, Lateral radiograph of the wrist demonstrates focal soft tissue swelling of the dorsum of the wrist (blue shaded area) representing an abscess.

Fig. 10.9, Grayscale (A) and color Doppler (B) ultrasound of the antecubital fossa demonstrate a superficial complex fluid collection (red outline) representing an abscess and increased regional blood flow representing hyperemia. Mild increased echogenicity of the deeper musculature (yellow outline) suggests a degree of myositis.

When abscesses are located in deep soft tissues, US may be inadequate for evaluation, and CT may be required. Abscesses appear on CT as fluid collections with rim enhancement and adjacent inflammatory changes ( Fig. 10.10 ). Like US, CT may be used to guide aspiration and drainage.

Fig. 10.10, Anteroposterior radiograph (A) of the humerus demonstrate diffuse soft tissue swelling and a focal, linear, radiodense object (yellow arrow) representing a retained needle. Contrast-enhanced axial (B) and coronal (C) computed tomography in soft tissue window of the same patient show a heterogeneous area of mass-like density (blue shaded area) and a small focus of soft tissue gas (red arrow). The linear density (yellow arrow) representing the retained needle is redemonstrated.

On MRI, abscesses exhibit increased T2 and STIR signal with corresponding low T1 signal and peripheral enhancement ( Figs. 10.11 and 10.12 ). DWI will show restricted diffusion internally. Apparent diffusion coefficients of 0.6 to 1.1 × 10 ⁻³ mm ² /s generally indicate true restricted diffusion seen with abscesses.

Fig. 10.11, Axial T2-weighted image with fat saturation (A), axial T1-weighted postcontrast (B), sagittal T1-weighted postcontrast with fat saturation (C), and short tau inversion recovery (D) magnetic resonance imaging of the wrist demonstrate diffuse T2 hyperintensity and enhancement along the dorsum of the wrist (yellow outline) surrounding a focal area of T1 hypointensity/T2 hyperintensity with peripheral enhancement (blue outline), consistent with focal abscess formation.

Fig. 10.12, Axial and sagittal T1-weighted (A, B), T2-weighted (C, D), and T1-weighted postcontrast (E, F) magnetic resonance imaging of the lower thoracic spine demonstrates a T1 hypointense, T2 hyperintense, and peripherally enhancing fluid collection (blue shaded area) anterior to the T11 and T12 vertebral bodies, consistent with a paraspinal abscess.

In addition to antibiotics and supportive measures, drainage of abscesses may be required. Simple abscesses may be drained percutaneously via imaging guidance. However, complex or multiloculated abscesses may require surgical drainage.

Inclusion Cyst

Epidermoid or implantation dermoid is an encapsulation of keratin- and lipid-rich debris from inoculation of the hair follicle or epithelial cells to the underlying dermis, often secondary to penetrating trauma, such as IV drug abuse. Over time, PWID lose access to superficial veins and introduce drugs and toxins via subcutaneous or intradermal routes, or through a practice referred to as “skin popping,” which may lead to depressed lesions with central pallor and occasional surrounding ecchymosis. Epidermoid cysts are usually asymptomatic but can become a nidus for infection, especially with repeated localized trauma and the introduction of bacteria. Superficial lesions may be evaluated with US, which will show a lipid-rich cyst. CT may be employed for deeper cysts, which will show an encapsulated lesion without peripheral enhancement in the absence of superimposed infection.

Treatment is relatively simple, with no intervention needed unless the patient develops an abscess or localized infection.

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