Radiographs usually are the first images used to evaluate Charcot foot, although the findings can be misleading. The early stages may appear normal or may show only very subtle fractures and dislocations. Clinical suspicion should prevail if a patient who has neuropathy with a red, hot, swollen foot shows no radiographic changes.
Because early diagnosis and treatment are crucial, it is appropriate to consider other imaging methods to differentiate Charcot foot from cellulitis, osteomyelitis, and other conditions if the initial x-ray films are difficult to interpret or the results are negative.
MRI has been shown to provide the highest diagnostic accuracy, detecting subtle changes in the early stages of active Charcot neuroarthropathy before they are visible on plain radiographs.6
On MRI, both Charcot foot and osteomyelitis show bone marrow edema, joint effusion, and surrounding soft tissue edema. During the acute setting of Charcot, the presence of increased signal intensity within the bone marrow on T2-weighted images mimics osteomyelitis.
The distribution of bone marrow edema can help the distinction: In osteomyelitis, bone marrow edema generally tends to affect a single bone, with diffuse intraosseous involvement and adjacent soft tissue changes. In acute Charcot, bone marrow edema typically is periarticular and subchondral, involving multiple bones and joints.
T1-weighted and T2-weighted MRI scans (Figure 2) depict diffuse intermedullary changes to the midtarsal bones with consolidated fragments, subluxations, and dislocations, resulting in a gross deformity throughout the midfoot.
Chronic Charcot appears similar to the acute Charcot foot, but a deformity is very likely to develop. Similar changes are observed in osteomyelitis when there is a concomitant Charcot foot.6
If foot ulceration is present, determining whether there is underlying osteomyelitis can be challenging. Most cases of osteomyelitis involve a direct source of inoculation through an opening in the skin and are highly associated with ulceration, abscess, or sinus tract. Therefore, osteomyelitis should be suspected in the setting of a skin defect with abnormal bone marrow signal on MRI.
Three-phase technetium scans are highly sensitive (93%) for active bone pathology.7 Three-phase technetium-99 followed by Indium-111-labeled leukocyte scintigraphy is useful in differentiating between osteomyelitis and Charcot foot because they have relatively high sensitivity (92%) and speciﬁcity (110%).8
Bone scans are sensitive for any bone disorder but are not very specific. A simple probe-to-bone test has been accepted as indicative of osteomyelitis, but this has been called into question because the sensitivity and specificity have not been reproduced in outpatient populations.9 Positron emission tomography has shown potential for differentiating Charcot foot from osteomyelitis but has not been fully validated for this purpose.10
The Charcot foot may mimic deep vein thrombosis, which can be ruled out by duplex venous scan. However, simply observing the erythema also offers clues. Elevating the affected leg can help determine whether erythema is associated with infection. Dependent rubor related to Charcot foot usually resolves within 10 minutes of elevation; the erythema would remain unchanged in an infected limb. (In cellulitis, the margins of the erythema are more well-defined than in Charcot neuroarthropathy.)
The diagnosis of Charcot neuroarthropathy is highly dependent on a high index of suspicion and is based primarily on the history and clinical findings with help from imaging studies. A profoundly erythematous, edematous, warm foot in a neuropathic patient with unknown causes should be considered Charcot neuroarthropathy until suggested otherwise.
The diagnosis often becomes complicated when patients ﬁrst present with a concomitant foot ulcer, raising suspicion for cellulitis and possibly osteomyelitis. However, even with consideration of more common infectious processes in these patients, the presence of a Charcot foot must still be investigated because both pathologies can have severe complications and may present simultaneously.
The clinician must first recognize the condition and provide appropriate imaging and referrals. Plain radiographs should be the initial imaging study ordered with possible reliance on MRI to aid in confirming clinical suspicion in the presence of normal appearing radiographs. When in doubt, a bone biopsy also may be obtained to rule out osteomyelitis.
The Charcot foot, if unmanaged, can lead to a severe, limb-threatening condition. It must be distinguished from the similarly presenting and more common infectious processes so that proper treatment can be instituted immediately, limiting the resultant deformities and other complications.