Ultrasonography is the mainstay of the diagnosis of soft tissue injuries of the equine distal limb. The purpose of this critically-appraised topic was to determine whether there is scientific evidence for its use alongside b-mode ultrasonography in horses.
The search terms “(horse* OR equi*) AND (tendon* OR tendin* OR ligament* OR desm*) AND doppler” were generated from the question and inputted into the PubMed search engine. Seventeen results were yielded, of which eight were deemed to contain information potentially relevant to the PICO question.
Four studies investigated the use of power1 or colour2-4 doppler, and in four studies colour doppler was a dependant variable5-8.
A small case-control, semi-blind, prospective study1 documented B-mode parameters (lesion length and cross-sectional area [CSA], tendon CSA) and grade of power doppler signal in the suspensory ligament branches of lame (n=8) and sound (n=5) horses. Doppler signal was present in 11/20 symptomatic branches, with higher doppler grades (3/4, 4/4) seen in severe lesions. Mild (23) or moderate (2) B-mode abnormalities were identified in 25/48 asymptomatic branches, of which 12/25 had mild doppler signal. No doppler signal was seen in branches without B-mode abnormalities. Power doppler may help to differentiate between acute and healed lesions seen on B-mode ultrasonography, however there is a potential for false-positive results.
In 10 thoroughbred racehorses with naturally-occurring superficial digital flexor tendon (SDFT) lesions, tendon CSA (p<0.001) more than measures of doppler flow (p<0.05) were positively correlated with lesion CSA.2
Colour doppler signal was identified in 6.4% transverse images of asymptomatic SDFTs in young racehorses acquired over a 5-month training period.3 Of the 864 transverse images most positive results were in April (n=35), then February (n=14) and December (n=7), indicating a possible physiological response to increasing workload. Although no horses developed SDFT lesions in B-mode, a lack of follow-up data means that conclusions regarding whether increased doppler signal indicated preclinical injury cannot be made.
Four horses with surgically-induced bilateral forelimb SDFT lesions, were evaluated using doppler ultrasonography before and after sclerosing therapy.4 However, the reported data were incomplete and meaningful conclusions cannot be drawn.
One study reported differences in colour doppler signal in healing surgical lesions in the SDFTs between forelimbs and hindlimbs.5 However no correlation with differences in b-mode ultrasonography, biomechanical properties, clinical outcome, or histological grading of healing were identified and so the clinical relevance of the differences in doppler signal is unclear.
Significant differences in doppler signal between surgically-induced SDFT lesions treated with adipose tissue-derived mesenchymal stem cells (AT-MSC) or a control substance two weeks post-treatment were reported.6 The authors concluded that AT-MSC promoted neovascularisation during the proliferative phase of healing, however the data do not justify this conclusion because at 22 weeks there was a significant difference (p<0.0001) in histological vessel count between the two groups, yet no significant difference in doppler signal.
The prospective part of one study identified 2/6 true-positive, 3/6 true-negative and 1/6 false-negative results for determination of clinical significance of lesions in association with mineralisation within the DDFT.7
Colour doppler was used inconsistently to evaluate a thickened mesotenon in horses with manica flexoria tears and so it was impossible to ascertain the significance of the results.8
In conclusion, there is currently a lack of correlation between doppler signal in equine tendons or ligaments with clinical, biomechanical, or histological parameters. The relative usefulness of power versus colour doppler, and the effect of workload need to be ascertained. Large-scale, prospective studies are required to determine whether doppler can provide additional diagnostic or prognostic information to B-mode ultrasonography. Currently, scientific support is weak and any evidence for its usefulness in equine musculoskeletal imaging is anecdotal.
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