Point-of-care ultrasound use was pivotal in this case, enabling the treating team to further characterize the extent of the patient’s soft tissue infection and reach the diagnosis of osteomyelitis. Bidirectional flow of fluid through a disruption in the bone cortex was visualized on greyscale imaging and confirmed with color and spectral Doppler. The pulsatile movement of fluid within an abscess has not been previously described in the literature. The presence of Doppler signal in any fluid other than blood is known as pseudoflow [8]. Additional examples include amniotic fluid, ascites, and ureteral jets [9]. The presence of pulsatility in this case, which could represent either blood flow or pseudoflow, drew the ultrasound operator’s eye to the cortical defect, raising the concern for bony involvement and the need for CT imaging and surgical consultation. Generalized hyperemia of infected tissues or direct compression of larger vessels within the medullary cavity could both be responsible for the presence of this pulsatility. The central arteries that supply the medullary cavity sustain a relatively high blood pressure to support flow to their many distant branches [10].

Previous studies have examined the utility of color and power Doppler to evaluate osteomyelitis, primarily in children [7, 11]. Doppler imaging typically reveals increased vascularization within or around the periosteum best visualized with power Doppler [12, 13]. Additionally, color and power Doppler have detected advanced osteomyelitis and demonstrated progression/regression of inflammation in response to antibiotic treatment [11]. Hyperemic hyper-vascular flow is usually described at the margins of periosteal abscesses, while internal flow without external compression would be considered atypical [13]. The internal flow observed in this abscess communicating with the medullary cavity may be unique to deep space infections that develop in confined compartments adjacent to vascular beds.

Several studies have demonstrated sonography’s ability to detect early inflammatory changes in acute osteomyelitis when correlated with clinical findings [6, 7, 11,12,13,14,15]. Soft tissue edema near the affected bone, tendon inflammation, synovitis, and joint effusion may indirectly indicate early osteomyelitis [12]. Sonographic changes suggesting osteomyelitis can be visualized within 1–3 days from the onset of symptoms [13]. Studies also suggest that sonography can demonstrate signs of inflammation from osteomyelitis earlier than plain radiography [6, 7]. These early sonographic findings progress to periosteal thickening, periosteal elevation with fluid adjacent to infected bone, and subperiosteal abscess. Cortical erosion is a late sonographic finding [7]. In the first 1–3 days of symptoms ultrasound is typically normal or shows mild soft tissue swelling adjacent to bone. Periosteal reactions develop within 4–7 days and later findings such as subperiosteal abscess or cortical erosion occur after 8–15 days of symptoms [16]. The previously derived test characteristics of ultrasound for osteomyelitis are variable, with sensitivities ranging from 55 to 76% and a specificity of 47% [5, 7, 16]. Ultrasound has been shown to have a higher sensitivity (74%) and specificity (95–100%) for complications of osteomyelitis, such as periosteal erosion or abscess formation [5, 7]. Ultrasound findings correlated with clinical findings and subsequent radiographic studies increase the accuracy of acute osteomyelitis diagnosis [5] and aid in monitoring the progression/resolution of disease [11].

Subperiosteal abscesses present as hypo- or anechoic lenticular-shaped fluid collections adjacent to bone cortex [9]. When performing point-of-care soft tissue ultrasound, the discovery of any fluid collections adjacent to bony surfaces should prompt further evaluation for osteomyelitis. Emergency medicine practitioners should avoid evacuating an abscess eroding through bone at the bedside and point-of-care ultrasound can help to avoid this pitfall. Fluid or tissue sampling from the affected region under sonographic guidance may eventually be needed to confirm the diagnosis of osteomyelitis and guide antibiotic treatment, but this should not be attempted without surgical consultation [6].