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So let's talk a little bit about infection,

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because this is one of the common reasons

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that somebody may present to the emergency department.

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So we'll consider the clinical and laboratory evaluation.

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We'll talk a little bit about imaging of infection.

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According to the anatomic structure, involved

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a little consideration

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of superficial infections and deep infections.

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Deep infections may involve the fascia, muscle, joint

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and bursa or the bone.

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So in terms of clinical

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and laboratory presentation, infection is characterized

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by inflammation, which may be calor, dolo, rubor.

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And two more. Patients that present with advanced infection

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may display signs of sepsis, such as with fever,

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hypotension, or tachycardia.

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And then things that are specific

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to musculoskeletal infections would be an inability

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to bear weight and reduced range of motion.

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With regards to laboratory evaluation, you might expect

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to detect increased white blood cells,

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increased inflammatory markers such as CRP and SED rate

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and bacteremia if blood cultures are obtained.

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So with superficial infections,

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this can produce a cellulitis.

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This is typically limited to the subcutaneous tissue,

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the hypodermis and superficial fascia.

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Without muscular and deep fascial involvement, staph

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and strep are the most common organisms in terms of imaging.

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There may be a non-specific soft tissue swelling

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identified on radiography

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and CT may show subcutaneous fat infiltration

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and potential underlying abscess if that's present.

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Now, when you have a soft tissue infection, it may become

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gas producing, and this is known as a necrotizing fasciitis.

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So with necrotizing fasciitis,

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you have a rapidly progressive infection

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of the deeper soft tissues.

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This can be associated with a higher mortality rate.

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The most common sites are the extremities,

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and that occurs in about 50% of the cases

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of necrotizing fasciitis.

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So these may be polymicrobial or mono bacterial,

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and as the name implies, since it has or necrotizing

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or gas producing component, these are related

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to gas producing organisms such as Clostridium.

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The pain is definitely out of proportion

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to the degree of skin involvement.

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So here radiography can show soft tissue swelling

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and emphysema as we see on this projection radiograph

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of really extensive gas involving the left side

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of the pelvis and proximal femur.

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CT can be an extension of this showing fluid with gas

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dissecting along these non enhancing fascial planes.

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Another type of deep infection is polymyositis,

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usually caused by hematogenous spread

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and transient bacteria rather than a direct extension

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from an adjacent infection.

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Here the organism typically is gram-positive.

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Staph aureus usually involves a single muscle,

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however, multiple side involvement can be present

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and up to 40% lower extremity muscles are typically more

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likely to be involved than the upper extremity.

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So with Polymyositis, we now have fluid collections

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within the muscles, and so this is not something

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that we would typically identify in radiography,

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but more of a CT or MRI diagnosis.

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Another form of deep infection is septic arthritis.

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So septic arthritis occurs in either native joints

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or joint replacements,

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but in up to half of the cases,

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they've had some prior joint disease.

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Typical organism here is staph ous with about 10%

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of them being MRSA.

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It's usually a hematogenous spread in adults, typically

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monoarticular, but in a small percentage

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can be polyarticular.

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And whether it's either a native joint

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or a joint replacement, this is one condition

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that requires prompt attention.

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So when we're investigating the joints,

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we may see manifestations of septic arthritis

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on radiography, ultrasound,

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CT and MRI.

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So here on radiography we see a series of knee x-rays.

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Initial X-ray shows normal appearing joint.

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The middle X-ray shows joint space narrowing.

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And then the final x-ray on the right now shows erosion

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and joint destruction related to septic arthritis.

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One particular topic is a granulomatous type of infection

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such as tuberculosis.

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So tuberculosis arthropathy occurs in about

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2% of joint infections.

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It may be from direct spread or hematogenous.

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They may have a normal chest x-ray about 50% of the time.

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So the fact that the person doesn't have active pulmonary

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tuberculosis does not exclude a diagnosis of

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septic arthritis from tuberculosis.

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They often have an indolent course with symptom duration

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of greater than a year, resulting in a delayed diagnosis.

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They can have large fluid collections, osteomyelitis,

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joint damage, go on to fibrous non-unions,

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and also generate soft tissue calcifications.

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There is a classic triad known as femur triad that describes

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perticular osteoporosis, that is regional demineralization,

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peripherally located osseous lesions

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and late gradual joint space narrowing,

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reflecting slow destruction of the cartilage.

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Infections of the bone are known as osteomyelitis.

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So osteomyelitis is a form of deep infection.

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It may be from hematogenous spread

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or direct contiguous inoculation.

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In young adults, it may be associated with an open fracture.

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In pediatric

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and elderly patients, it's often related to bacteremia.

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It most frequently involves bones such as the tibia, wrist,

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femur, ribs, and also the spine.

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Here again, the most common pathogen is staph aureus.

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So on radiography, the findings

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of osteomyelitis may not be evident for 14 to 21 days

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after the onset of infection.

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Initially, you may see soft tissue swelling

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as we see here about the lateral malleolus.

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There may be loss of their trabecular architecture.

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As you can see over the fourth metatarsal phlange joint

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compared to the other joints,

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you may have periostial reaction

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as seen along the distal radius,

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and then may result in osteolytic destruction

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as shown here in the proximal tibia.

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So the schematic here shows

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the different manifestations of osteomyelitis.

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As mentioned, it typically is hematogenous coming in

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through a nutrient vessel.

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It then may set up as an intraosseous lesion

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where you can have an lucrum

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with a sequestered bone fragment.

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Another manifestation is to have a subperiosteal collection,

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which then generates a defect in the bone known as a cloaca,

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and generates an interosseous abscess.

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So osteomyelitis may present

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in acute or chronic forms.

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So with acute osteomyelitis as shown here,

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you can have focal bone destruction

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where the arrow is pointing to an A

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and the radiograph on MRI.

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This will be associated with bone marrow edema pattern,

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and this can extend from the metaphysis into the epiphysis.

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As shown in image C.

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In subacute osteomyelitis,

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you now can generate an intraosseous abscess, which is known

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as the Brodys abscess,

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and can present as a lytic lesion

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shown on the radiograph in a,

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which contains fluid signal intensity

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and material demonstrated on the MRI.

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In DA more chronic osteomyelitis can generate

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an in lucrum as shown in the image on the left

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where the arrow is pointing to the sclerotic bone

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surrounding a sequestrum

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or sequestered piece of necrotic infected bone.

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In image A of the calcaneus, we can see

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sclerosis as one of the manifestations

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of chronic osteomyelitis

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And and in the lower panel

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there is a sinus tract with a fistula.

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Here in the first metatarsal,

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we see a laminated periostial reaction

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with underlying sclerosis.

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And the MRI shows us this multilayer appearance.