Although prospective studies are likely to generate more useful and scientifically strong data on the correlation between US and disease categories, retrospective studies, such as this one, are also helpful and provide valuable information on which future prospective studies can be based.
In accordance with the purpose of this study, the results indicate that for some disease categories it is possible to find positive correlations between preoperative US examination and the intraoperative or post mortem findings.
In the horse population included in this study, several SI problems were identified, such as strangulating lipoma, mesenteric volvulus, herniation in mesenteric tears, inguinal herniation and epiploic foramen entrapment. For each specific condition, there were relatively few cases; we therefore concentrated on strangulated obstructions, nonstrangulated obstructions and intussusceptions. Since the last 2 groups were represented by a small number of cases, they were excluded from the statistical analysis.
In agreement with other studies (Scharner et al. 2002; Klohnen 2008; Busoni et al. 2010), the presence of completely distended SI loops was highly related to definitive diagnosis involving SI as to strangulating obstruction. However, in the authors' experience, it is not uncommon to observe partial distension of SI loops in horses with LI disorders. This US finding might be explained by the compression exerted by the distended and/or displaced LC on the duodenum and by tension on the mesentery (Hardy 2008). However, there is a distinct difference in the US appearance of SI loops between surgical colics involving the LC and the appearance of the SI for nonstrangulated obstruction of the SI and strangulated obstruction of the SI; in nonstrangulating obstruction, prestenotic loops may appear to be dilated and with absent contractility, whereas post stenotic loops appear of normal aspect and contractility (Reef 1998a). In strangulated obstruction, SI loops appear completely distended with lack of motility and, finally, in cases of LC disease, SI loops appear partially distended with reduced or normal contractility. However, although values obtained could be considered acceptable, they are lower than those obtained by Klohnen et al. (1996). The lower sensitivity could be attributed to an early referral from the onset of clinical signs when a complete SI distension has not yet manifested. In contrast, lower PPV may be due to a relatively high number of false positive results, as reported by Busoni et al. (2010), in these cases evaluating the number of completely distended SI loops could help to reduce the number of false positive horses. In agreement with previous studies in strangulated obstruction, completely distended SI loops are associated with absent contractility of SI loops (Klohnen et al. 1996; Busoni et al. 2010), this is because intestinal distension and ischaemia induce congestion and inflammation of the bowel wall (Blikslager 2008). The absence of contractility could also be mediated by mechanoceptor or nociceptor stimulation of extrinsic afferent neurons decreasing gastrointestinal progressive motility (Rakestraw 2008). Strangulated obstruction generally causes a more rapid and serious alteration of perfusion, distension and contractility of the SI loops (Blikslager 2008). As reported by Klohnen et al. (1996) and Freeman (2002a), a lack of motility and thickened wall loops should be used to discriminate between surgical and nonsurgical cases in horses with SI diseases that show dilated and turgid loops. In our study, based only on surgical or post mortem cases, the results suggest that surgical treatment must not be excluded in the presence of dilated loops with reduced motility, but this should be interpreted in conjunction with clinical signs and clinical pathology, because, in the authors' experience, this US finding could represent an early stage of strangulated obstruction of SI. In agreement with previous studies (Klohnen et al. 1996; Fischer 1997), increased wall thickness has been associated with strangulated obstruction as well, as a consequence of the development of mural oedema or haemorrhage (Rowe and White 2008) so that the 5 US layers are lost (Reef 1998a), but the increase is related to the onset of the condition (Reef 1998a,b; Klohnen et al. 1996). This US finding has to be differentiated from horses with intussusception that may have thickened intestinal wall due to hypertrophy of the muscular wall (Freeman 2008) and from horses with infiltrative diseases, in which cellular infiltration of all layers is responsible for increased wall thickness (Reef 1998a; Scott et al. 1999; Dechant et al. 2008).
Reduced motility of the duodenum was also significantly associated with a definitive diagnosis involving SI in a multivariable analysis model. However, the duodenum, except in cases of proximal enteritis (Busoni et al. 2010), is rarely involved in simple or strangulated obstruction. When there is any other disease of the SI, the duodenum becomes involved later.
Finally, increased peritoneal free fluid was statistically associated with a definitive diagnosis involving SI as well as strangulating obstruction of SI. The presence of increased free peritoneal fluid is a consequence of vascular congestion, which results in increased endothelial permeability. If bowel wall ischaemia occurs, an exudative process develops and large quantities of protein and white blood cells move into the peritoneal cavity (Mair and Edwards 2003; Ross 2010). Peritoneal fluid changes associated with strangulating obstruction occur after approximately 1–2 h from the onset, depending on the type and severity of the lesion. Moreover, compartmentalisation can make identification of peritoneal fluid difficult (Mair 2002). Again, only horses in which the amount of free peritoneal fluid was subjectively considered to be increased were included in this study, without comparison with abdominocentesis. To our knowledge, the amount of free peritoneal fluid detectable in horses and the sensitivity and specificity of abdominal US for the detection of increased free peritoneal fluid have not been investigated in horses so far.
Interestingly, in this study SI loops were identified in 88.8% of horses, in agreement with Busoni et al. (2010), who identified SI loops in 75% of horses, when both medical and surgical cases were considered. In equine abdominal US examinations, the visualisation of the bowel in normal horses is considered to be limited by gas and ingesta in the LC (Norman et al. 2010). The frequency of visualisation of SI loops in the normal equine population has not been investigated yet, and it could be useful to investigate normal vs. colic-affected horses, as well as medical vs. surgical colic horses. Fasting, a consequence of colic (reduced appetite/withheld feed), could improve the ability to obtain images of the SI, as reported in a previous study of normal fasted horses (Norman et al. 2010).
The results of multivariable analysis of renosplenic entrapment demonstrate that failure to visualise the left kidney was highly significantly associated with this condition. In agreement with other authors (Santschi et al. 1993), in horses with renosplenic entrapment the presence of gas-filled colon dorsal to the spleen precluded imaging of the kidney. There are no published data on the sensitivity, specificity, PPV and NPV of this US finding, but in this study the values were 87, 83, 42 and 98%, respectively. The low PPV is a consequence of the number of false positives (18/143), and a similar finding was reported in a previous study (Busoni et al. 2010). In fact, a recent rectal examination often obscures visualisation of the left kidney (Reef 1998a); in our practice, rectal examination precedes abdominal US examination. Other authors recommend assessment of several ultrasonographic features for the diagnosis of left dorsal displacement of the LC (Santschi et al. 1993; Reef 1998a). Failure to visualise the left kidney was the only parameter assessed and analysed in this study. Even if increased free peritoneal fluid was also related to both renosplenic entrapment and right dorsal displacement, the OR was low in both conditions compared with the OR for SI. In fact, the sensitivity, specificity and PPV of increased free peritoneal fluid for these diseases were very low.
In the multivariable model for right dorsal displacement, a distended LC was significantly associated with this condition. In LC displacements, which are nonstrangulating conditions, because of the impaired flow of ingesta over time, gaseous distension becomes more significant and the distension of the LC is a common feature. With regard to renosplenic entrapment, it could be very interesting to evaluate whether there is a difference in US appearance of the LC in medical vs. surgical cases, since there are no published data. In agreement with Pease et al. (2004), a thickened LC was highly sensitive and specific, with good PPV and NPV. In contrast, the appearance of the LC was not correlated to nonstrangulating volvulus. An indicative alteration of strangulated LC volvulus is complete obstruction of venous drainage, which causes the LC wall to become oedematous and thickened (Snyder et al. 1989). In nonstrangulating obstruction (and in displacement) of the LC, the viscus could become distended with gas and ingesta but, because the vasculature is not compromised, the wall does not become thickened as in LC torsion (Hackett 1983). Absent motility of the LC had good sensitivity, specificity and high NPV for both strangulated and nonstrangulated volvulus, with better PPV for strangulated volvulus.
Limitation of the study
This study had some limitations. Only one trained veterinarian performed the US evaluation in each case and, because US findings are operator-dependent, this factor could have influenced the results. The localisation and the number of SI distended loops were not evaluated (Klohnen et al. 1996). On the standardised form, the presence of completely distended loops together with normal loops was not recorded, but these findings could be relevant in differentiating between simple and strangulating obstruction or intussusception.
The localisation of free peritoneal fluid was not recorded and this should be further investigated, since the fluid could be constrained in specific areas, especially in cases of LI diseases. Finally, with respect to the motility of both large and small intestine, sedation could have influenced the results; although sedation is not performed routinely, it was necessary in a few cases, and it was not considered in this study.