Soft Tissue & Bone Infections
Osteomyelitis (OM) is an infection of the bone and can occur from hematogenous spread, direct inoculation from trauma or surgery, or contiguously from adjacent soft tissue involvement. Osteomyelitis can also be acute or chronic (it’s often chronic in the setting of a diabetic foot infection).
Let’s break it down by pathophysiology:
1) Hematogenous seeding of bone accounts for ~20% of osteomyelitis cases, and the vertebrae are the most common location. These infections are usually monomicrobial and Staph aureus is the most common culprit (although gram-negative organisms can certainly cause OM—for example, Salmonella OM is classically seen in sickle cell patients). The source of infection is often distant, such as a skin/soft tissue source, intravascular catheter or endocarditis. Interestingly, in those who use injection drugs, bloodborne pathogens will tend to involve the sternoclavicular and sacroiliac bones.
2) Direct inoculation from trauma or surgery: This is usually polymicrobial; open trauma or post-surgical closed traumatic fractures may involve skin flora as well as soil organisms or hospital-acquired pathogens.
3) Contiguous spread: Think diabetic foot wounds, decubitus ulcers and vascular insufficiency… So as you’d expect, OM associated with contiguous spread of infection is also often polymicrobial.
Despite the name, acute OM typically presents gradually, with patient reporting a dull pain at the site followed by local erythema, warmth and tenderness. Chronic OM will present similarly to acute OM but if you see a draining sinus tract, that is pathognomonic for chronic OM. (Boards love to test this.) Fever and other constitutional symptoms are uncommon in both!
Definitive diagnosis of OM is made by bone biopsy and culture, NOT imaging, although most clinicians order imaging first. In terms of imaging, MRI is king—not only does it have high sensitivity for bone infections, it can show the surrounding tissues at high resolution (it’s usually better than nuclear medicine imaging!) Unfortunately, because bone marrow edema on MRI can result from other causes (Charcot arthropathy or recent fracture/surgery), the results should be interpreted with these in mind.
Practical Pearl
What about follow-up imaging after treatment for OM?
MRI changes will persist for an extended amount of time even after appropriate therapy, so per IDSA guidelines, there is no role for repeat imaging unless a patient does not experience clinical improvement, if complications develop, or if inflammatory markers remains elevated without explanation.
The same steps for diabetic OM apply again here. Let’s say you have a patient in whom you’re worried about OM:
Step 1) Before you start any antibiotics, get blood cultures. (As we discussed above, hematogenous seeding accounts for ~20% of OM, and you may be able to avoid more invasive testing if a blood culture is positive!) A pre-treatment CRP and ESR are also useful to have.
Step 2) Get a deep culture, preferably of bone and culture if possible. (Bone biopsy is not absolutely required). Remember, most patients with OM are not septic, so you have time to gather data. Superficial cultures or cultures from sinus tracts correlate poorly with deep cultures from bone.
Step 3) NOW you can consider starting empiric antibiotics if the infection is severe (you want to make sure to cover Staph as well as possible GPCs and GNRs, so this is typically vancomycin and ceftriaxone) while you wait on your cultures to result… and then narrow!
Treatment of OM usually requires prolonged IV antibiotic treatment, usually 6 weeks after debridement in most cases (this is believed to be how long it takes for bone to completely remodel, hence why we treat for 6 weeks). Successful treatment usually involves adequate surgical debridement and removal of any associated hardware. We tend to monitor CRP and ESR weekly to ensure that patients are being treated adequately.
Practical Pearl
Do we really need to use IV antibiotics for OM?
The recent OVIVA trial (Li et al, NEJM 2019) showed that in patients without Staph aureus bacteremia, after initial therapy with IV antibiotics, 6 weeks of oral therapy was non-inferior to IV therapy for the treatment of various forms of OM (vertebral OM, prosthetic joint and native joint infections were included). Keep in mind that most of the patients in this study had a known causative organism using culture data and the most commonly used oral antibiotic was a quinolone (~40%). So the answer is: not necessarily!
A quick note about vertebral OM: Vertebral OM is most often due to hematogenous spread so blood cultures are typically positive in >50% of patients! Vertebral OM is also often treated for a bit longer than most other forms of OM, usually 6-8 weeks.
Think about vertebral OM in a patient with worsening back or neck pain without an alternative explanation, local tenderness, radicular pain or neurologic changes. The biggest fear is extension of the infection to the epidural space as this can cause cord compression and sometimes, paraplegia or quadriplegia if not addressed early. Get that imaging!
Congrats! Another module done.