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EQUINE VETERINARY EDUCATION / AE / JANUARY 2017
gamma camera from acquiring images in 2 planes and thus limiting a comprehensive evaluation of the distal limb vasculature. Doppler ultrasonography of the palmar digital arteries has been used in order to compare differences in vascular dynamics between horses with systemic inflammatory response syndrome and laminitis (Adair III et al. 2000; Aguirre et al. 2013). Doppler ultrasonography was performed in Case 1 for the assessment of vasculature. With this technique, no palmar digital arterial flow could be documented one day post injury; however, it was re-established 5 days post injury. This finding suggests that distal limb vasculature exhibits plasticity and prognostication should be based on serial examinations several days apart. To our knowledge, no report of serial CTA examination has been performed in the human literature. Conversely, studies are typically performed immediately after stabilisation of the traumatised patient (Soto et al. 1999, 2001; Miller-Thomas et al. 2005; Inaba et al. 2006; Peng et al. 2008). The vascular response to traumatic injury in equids is poorly understood and the initiation of multiple CTA studies over a period of several days could potentially result in the identification of reperfusion of the affected extremities. Doppler ultrasonography was not performed in Case 2 due to the presence of subcutaneous emphysema. Computed tomography angiography has been described
in healthy horses where contrast material was injected into the median artery under general anaesthesia to assess distal limb vasculature. The technique was deemed safe and useful for the evaluation of distal limb blood supply in healthy mature horses (Collins et al. 2004). Arterial access is difficult to obtain in the limbs of horses and often requires the aid of ultrasonography (Collins et al. 2004). Access to the hindlimb arterial supply is generally acquired through the dorsal third metatarsal artery at the level of the proximal metatarsus. In cases of distal limb injury, access to this artery can be further complicated by the primary injury, oedema, or a secondary cellulitis. Intra-arterial injection of ionic iodinated contrast has been shown to have a 9% rate of minor cardiovascular or dermatological complications that resolve without treatment (Pollard and Puchalski 2011). Furthermore, selective arteriography could potentially lead to thrombosis, which would be catastrophic to the vasculature of the equine distal limb (Sole et al. 2012). Currently the gold standard for the assessment of vascular
integrity in human medicine is multi-slice CTA (Inaba et al. 2006; Peng et al. 2008). This is different to the traditional catheter based arteriography in that contrast is injected into a peripheral vein rather than the artery supplying the affected limb (i.e. nonselective). The advantages of nonselective CTA include decreased technicality and risk of iatrogenic damage when achieving vascular access, as well as a thorough examination of the contralateral limb vasculature, which can act as an internal control (Inaba et al. 2006; Peng et al. 2008). Multislice CTA boasts sensitivity and specificity in excess of 90% and 98%, respectively, as an initial diagnostic imaging modality in detecting clinically significant injuries to the traumatised extremities of human patients (Soto et al. 1999, 2001; Miller-Thomas et al. 2005; Inaba et al. 2006). The technical advantages of multislice CTA include
decreased time and expertise needed for the acquisition of vascular access, the ability to gain vascular access in a region remote to the lesion avoiding the complications of cellulitis and arteritis, and reducing the potential for iatrogenic thrombosis and haematoma to the vessel feeding the
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affected extremity (Miller-Thomas et al. 2005; Inaba et al. 2006; Peng et al. 2008). This decrease in morbidity would allow repeat examinations if the perfusion status is in doubt. An important diagnostic advantage of nonselective CTA is that the vasculature can be comprehensively evaluated in 3 planes to assess distal limb perfusion. The use of CT as a modality rather than radiography allows the added benefit of sensitively evaluating the osseous structures of the distal limb and the presence of gas within synovial structures or between the sensitive and insensitive lamina. Additionally, the use of nonselective CTA allows for the evaluation of both distal limbs in tandem. This permits evaluation of the nonaffected limb vasculature, which can act as a control if insufficient contrast enhancement to the extremity is in question or if aberrant vasculature is present. Although multislice CTA is the gold standard diagnostic
after traumatic extremity injury in human trauma centres, many technicalities limit its use in large animal veterinary medicine. The first obstacle is the availability and size of a CT scanner, which are becoming more common at referral centres. Small equids similar to the 2 cases in this report can be placed on the small animal table for image acquisition. Additionally, custom made large animal tables are now available at select specialty hospitals for application in larger animals. The biggest challenge is timing the image acquisition so that the contrast material is at peak concentration in the desired vessels. Power injectors can be used at a rate up to 10 ml/s and contrast injection should ideally be completed within 30 s. In Case 2, manual injection was performed simultaneously in both the left and right jugular veins within 30 s and image acquisition was performed immediately thereafter. The total volume of contrast that can be safely administered in equids is unknown. The dose for human patients is 100–150 ml i.v. irrespective of varying patient weights (Miller-Thomas et al. 2005; Inaba et al. 2006). We administered a dose of 1.9 ml/kg bwt twice within a 30 min period without any anaesthetic complications in Case 2. Nonselective CTA was deemed an accurate and sensitive
procedure for the evaluation of distal extremity vascular disruption based on long-term outcome and histology in the 2 small equids presented in this case series. Many variables need further investigation prior to validation of this technique. The ideal dose, volume and injection rate of contrast material for CTA are not yet known. Both power injector and manual injection techniques were used in this report; however, controlled experiments would be necessary to determine whether one method is superior. Furthermore, the cases in this report could be scanned in dorsal recumbency due to their small size. If this technique is to be used in larger animals, the effects of gravity and vascular pooling after peripheral injection in lateral recumbency might be confounding variables. The plasticity of the vascular system and response to healing also need further assessment. It is plausible that multiple CTA examinations several days apart are necessary to determine definitively if permanent vascular disruption is present.
Authors’ declaration of interests No conflicts of interest have been declared.
Manufacturers’ addresses
1Toshiba American Medical Systems, Tustin, California, USA. 2GE Healthcare, Princeton, New Jersey, USA.
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