Laparoscopic Ultrasound in the Evaluation of Abdominal Structures in the Horse

Laparoscopic examination and treatment in horses has achieved considerable acceptance in recent years [1-9]. While many advances have been made in the routine applications of this modality, there remain many unanswered questions as a result of the anatomic limitations inherent to the species.

The utilization of transcutaneous ultrasonography, while remaining a very valuable tool, has certain restrictions and cannot be used to its full potential due, at least in part, to the inherent limitations of size of the structure to be examined, location of specific structures within the abdominal cavity, presence of gas filled intestinal structures, lungs and ribs, which all interfere with ultrasonographic evaluation using the transcutaneous approach. Based on the restrictions described above, it is logical to consider the possibility of combining these two modalities to maximize their potential in veterinary medicine. The concept of laparoscopic ultrasonography (LUS) has been well established in human medicine for a considerable time. Intraoperative ultrasonography (IOUS or IUS) refers to the utilization of ultrasonic examination during the course of a conventional operation and implies the direct application of an ultrasound probe head to an intra-abdominal organ or structure while maintaining a sterile field. Laparoscopic ultrasonography involves the utilization of the principle of minimal invasive surgery combined with ultrasonography whereby the ultrasound head in the form of a laparoscopic probe is inserted through a laparoscopic trochar and placed directly on the organ to be examined. The application of these methods in this manner was named invasive ultrasound by Hölscher in 1995 [10].

This report documents the utilization of an ultrasound probe currently used in human medicine using minimal invasive methods to examine abdominal structures in the horse. Specific information is presented relative to operative positioning and examination of horses of various sizes, an aspect of particular interest to practitioners.

Method

Whenever possible, feed should be withheld from patients for 24 hours in preparation for the examination, water may be offered ad libitum. The operative site is prepared in a standard manner as for conventional surgery. The procedure may be performed in either the standing or recumbent patient. In standing patients, sedation with an alpha-2 agonist (detomidine 0.02 mg/kg body weight, or with Sedivet® romifidine 0.08 mg/kg body weight). Levomethadon (Polamivet® 0.1 mg/kg body weight) is used as analgesic medication.

The two trochar sites in the center of the flank on the upper limit of the internal abdominal oblique muscle and 10 cm directly ventral to this site are infused with Lidocaine 2% at subcutaneous and intramuscular levels to desensitize the animal, allowing placement a laparoscopic trochar. Following placement of the rigid optic and insufflation of the peritoneal cavity with CO2 to a pressure of 8 - 10 mm Hg, inspection of the abdominal structures is performed in a systematic manner. Investigations on recumbent animals are accomplished under general anesthesia by means of isoflurane and O2 using intermittent positive pressure ventilation. The intra-abdominal pressure in recumbent animals may be raised to as high as 15 mm Hg. Following insufflation, the abdomen is explored according to the method of Scharner et al., [11]. Using this technique in decumbency allows for placement of the optic in the midline and the ultrasound probe in either left and/or right flank locations thus optimizing the placement of the ultrasound probe on a wider spectrum of organs.

Positioning the body in a head-down or head-up position may be used to improve the visibility of organs in selected abdominal and pelvic regions [12] and may also facilitate sonographic probe placement and evaluation.

The initial laparoscopic examination of all visible organs is followed by insertion of the ultrasonic probe through a separate port and direct placement on all available organs under laparoscopic visualization. The endoscopic tower (monitor on the top) should be positioned on the opposite side of the horse and oriented in the same axis as the optic channel (Fig. 1).

Figure 1. Laparoscopic ultrasonographic investigation on the left side.

An additional person operating the ultrasound equipment and associated instruments greatly facilitates the collection and optimal documentation of images. The coordination of laparoscopic and ultrasound images, enhancement of ultrasound images (focus, penetration, contrast) and video-endoscopic images can thus be managed to maximize and optimize all data collection.

Equipment

The ultrasound machine was combined with a laparoscopic linear transducer settings of 5.0, 7.5 and 8.0 MHz (Toshiba, Eccocee SSA-34A/3E, working length 40 cm). The laparoscopic sonography requires a twin video system for mixing two pictures on the monitor with a port for the telescope and a second for the ultrasound machine (Twin video, Storz).

The authors worked with a three-chip camera (Storz) and a light source (300 watt xenon light source). Standard endoscopic / laparoscopic technique was utilized in combination with a rigid endoscope 57 cm in length with a viewing angle of 30°. Standard surgical equipment was used to facilitate minimal invasive approaches and closures of the abdomen. Video images were stored on SVHS videotape, on black / white photographs (Sony) or on a still recorder (Sony).

Results

It was possible to complete all abdominal investigations in the standing horse without difficulty. Following a short initial learning phase, it was possible to easily apply the ultrasound probe to the desired abdominal organ which is considered to be an advantage over the transcutaneous method, which requires constant vigilance to obtain ultrasound (US) imaging of the desired organ or organ part. This is particularly so in those organs which must be visualized between the ribs using the larger transcutaneous probes which often provide limited visual fields. In addition, the relatively large image field obtained provides improved tissue details and better orientation relative to the complete organ thus further facilitating the examination. A further advantage of LUS is related to the reduced effect of movement which normally interferes with constant, prolonged imaging of a specific organ or tissue using the transcutaneous method. Using direct application of the probe head to the selected surface under direct vision establishes controlled imaging despite moderate animal body, abdominal wall or organ movements relative to body positioning, respiratory or intra-abdominal organ position changes. The ability to examine organs in multiple planes under visualization significantly increases the amount of information collected due to the utilization of the previously described three dimensional effect of internal organs which now become possible using the addition of the laparoscopic modality but which are not feasible using the transcutaneous technique.

Liver

The liver is completely contained within the intra-thoracic part of the cranial abdominal region with its cranial surface approximating the diaphragm. Its caudal aspect lies in close association with the stomach, duodenum, diaphragmatic flexure of the ascending colon, the right dorsal aspect of the ascending colon and the basis of the cecum. The left diaphragmatic lobe of the liver is comprised of a left medial lobe and a left lateral lobe [13]. It is possible to visualize laparoscopically, that part of the left lobe of the liver which is associated with the left triangular ligament in the standing horse (Fig. 2). The triangular ligament extends from the left lobes of the liver to the central tendinous portion of the diaphragm. In warmblood horses weighing over 500 kg, the 40 cm long ultrasound probe was not long enough to allow contact with the left liver lobe.

Figure 2. Laparoscopic ultrasound of the left liver in a standing pony.

The right abdominal view shows the left lateral aspect of the liver, the quadrate lobe, the right liver lobes and the caudate process of the liver. Here one can see the triangular ligament blends with the hepatorenal ligament to attach the dorsal border of the right liver lobe to the costal portion of the diaphragm [12,13]. In the standing horse it was possible to examine the visceral surfaces of the right hepatic (Fig. 3) and the caudate lobes which are connected by the hepatorenal ligament to the right kidney via a right flank approach (including one 18 year old patient over 1.70 meters at the withers). In older patients, the right liver is often smaller [14] probably due to pressure from the ascending colon causing atrophy of the right liver [15]. Thus, the ability to reach the various portions of the liver with the ultrasound probe varies depending upon the size and age of the horse despite the fact that the laparoscopic camera provides visual access to most regions.

Figure 3. Laparoscopic ultrasound of the right liver in a standing horse.

In dorsally recumbent patients it is possible to visualize the left lateral lobe, the left medial lobe, the quadrate lobe and the right liver lobe by laparoscopic means [12]. The falciform ligament which runs along the medial lobe of the liver provides an excellent means of orientation within the abdominal cavity. It is advantageous to use the ventral midline as an entry port for the optic as this allows visualization of most parts of the liver and supports laparoscopic ultrasound evaluation of all organs within 40 cm. Reduction of intra-abdominal pressure allows examination of deeper structures whereby the ultrasound probe may be pushed deeper into the abdomen due to reduced tension on the abdominal wall (Fig. 4). Using this technique the ultrasound head may be applied to the visceral and diaphragmatic surfaces of the liver by careful manipulation of bands and mesentery within anatomical limits. It is possible to examine more of the liver in the recumbent horse than in the standing horse.

Transcutaneous ultrasound evaluation of the cranial abdomen has been reported by a number of authors. Reef [14] reported that it is possible to visualize the left parts of the liver below the left lung between the ribs from the 6th to 9th intercostal spaces. Rantanen [16] reported that it was possible to examine the right portion of the liver from the right 6th to 15th intercostal spaces although the amount of the liver which could be evaluated was highly variable.

Figure 4. Laparoscopic ultrasound of the left liver and the spleen in a recumbent horse.

Using transcutaneous ultrasound, the parenchyma of the liver is seen as an homogeneous tissue of medium echogenicity and contains fine and coarse grained structures. Tubular structures are readily observed using the LUS technique throughout most of the liver tissue. In comparison to the spleen, the liver appears to have a slightly reduced echogenicity. In every portion of the liver examined, the quality of the ultrasound images were consistently considered to be very good.

Qualitative laparoscopic ultrasound examination of the liver provides images of even the smallest vessels as deep as the central regions of the organ.

Spleen

The spleen is readily identified in the left abdomen by its characteristic structure and location. It extends obliquely in a curved direction, corresponding to the left part of the greater curvature of the stomach, from the left crus of the diaphragm to the ventral third of the tenth or eleventh rib. Two surfaces, two borders and two extremities are available for evaluation [17].

The parietal surface of the spleen is easily identified just caudal and lateral to the stomach [12], the visceral surface is identified from medial.

The hilus of the spleen runs along the cranial border and gives rise to the gastrosplenic ligament and contains the splenic artery and vein. The nephrosplenic and phrenicosplenic ligaments may be seen in their respective locations [13]. The spleen is readily examined using transcutaneous ultrasound methods through the 8th intercostal to the 17th intercostal spaces as well as through the paralumbar fossa [14]. In the standing horse, laparoscopic examination of the spleen is readily performed through the left flank, and even those areas of the spleen which are difficult to visualize laparoscopically, may be documented by passing the ultrasound probe blindly between the spleen and abdominal wall. The presence of pre-existing abdominal fluid facilitates the examination by reducing the contact problem of the ultrasound head. The examination of the visceral surface is easier in the region of the nephrosplenic ligament, while the more caudal and ventral regions of the spleen are easier to examine from the parietal surface because the probe is easier to advance from this position.

The central vein is readily visualized in close proximity to the stomach and provides a valuable landmark which is readily confirmed by use of the color-doppler mode.

The spleen appears as a very homogenous, fine granular, parenchymatous organ of medium echodensity.

The spleen is acknowledged as being, for the most part, readily examined transcutaneously and the advantage of laparoscopic sonographic examination lies in the obviously improved images and the ability to follow the course of the central splenic vein. This can be an advantage when splenic biopsies are required in the region of the larger splenic vessels.

Kidneys

The bean shaped left kidney lies between the last rib and the first two or three lumbar transverse processes. It can be readily localized by using the nephrosplenic ligament as a landmark. The heart shaped right kidney lies between last two or three ribs and the first lumbar transverse process [18].

The renal capsule appears as a echogenic, non-homogeneous, smooth structure with a thin, well defined structure. The cortex is seen as an hypoechogenic, homogeneous, fine granular structure in contrast to the medulla which has a thin external zone and a wider internal zone. The external zone is less echogenic than the cortex, and the internal zone has the same echogenicity as the cortex. Renal papillae appear as round to oval homogeneous, hypoechogenic areas. The blood vessels have thin walls and an hypoechogenic lumen (Fig. 5). It is possible to follow the arteries and veins to the level of the arcuate vessels.

Figure 5. Laparoscopic ultrasound of the left kidney in a standing horse.

Both kidneys are easily reached and may be examined by port placement from both sides. The right kidney is more difficult to reach in those animals having considerable retroperitoneal, perirenal fat due to its retroperitoneal location. These factors may result in difficulty in obtaining optimal organ-to-ultrasound head contact. In contrast to the transcutaneous results reported by Rahlenbeck [19], intraoperative ultrasonic differentiation between renal capsule, cortex and medulla (with its two zones) is readily made. It is not always possible to differentiate the renal pelves and terminal recesses transcutaneously by using sonographic, high quality ultrasound heads in horses without renal disease, since all echodense tubular structures have images resembling those of blood vessels. Rahlenbeck [19] grouped all such echodense, irregular formed structures in the center of the kidney (which included the renal pelves, peripelvic fat, blood and lymphatic vessels) as "Central Reflex Complexes". In contrast, these "Complexes" of echodense tissue and blood vessels are readily differentiable using laparosonographic methods.

The ureters require very little practice to localize. They are readily differentiated from the renal artery and vein using the colour-doppler. The ureters are easily recognized by their relatively large hypoechogenic lumen and the thick wall with several layers (Fig. 6). The positioning of the ultrasound head from the medial aspect of the left kidney provides optimal visualization of the ureter.

Figure 6. Laparoscopic ultrasound of the right kidney in a standing horse.

Gastrointestinal Tract

The parietal surface of the stomach in the region between the spleen and liver can be visualized in the left cranial quadrant of the abdominal cavity but is only reachable by ultrasound probe in smaller horses. When the stomach is filled with fluid, it is possible to obtain good images of both stomach walls. The utilization of the spleen as a "tissue offset" facilitates the acquisition of very good images. In specific instances it is possible to obtain the five-layer ultrasonographic gastric wall structure not reported using the transcutaneous technique (Fig. 7). The layers may be separated into: an echodense line, a broad hyperechogenic band, a second echodense line, a line of very low echodensity and a narrow echodense outline. In human medicine and in larger small animals, the presence of a five-layered stomach image is often seen and is correlated with the histologic structures composing the stomach wall. The first echodense structure is the entry echo, the next hypoechogenic structure is the muscular propria, the echodense structure is the submucosa, two hypoechogenic regions are the mucosa and muscularis mucosa [20,21].

Figure 7. Laparoscopic ultrasound of the stomach on the left side with spleen as an "offset pad".

The serosa and submucosa are less echodense in man based on less connective tissue content.

The examination may be extended by pressing the ultrasound head against adjacent tissue or organs which results in high quality images of the opposite stomach wall.

A detailed characterization of the large intestine and colon is not readily obtained without the use of an offset to facilitate detailed structural images. The use of a commercial offset system (laparoscopic cover kit ®) to better define the small intestinal structures proved to be unsatisfactory in the examination of the large intestine and colon.

The results of our initial study using the combination of laparoscopy and ultrasound modalities for the examination of the equine abdominal structures suggest a significant potential for the use of this modality for diagnosis and therapy in equine medicine and surgery. With increased experience and an open attitude towards incorporation of these methods in veterinary medicine, advances paralleling those in human medicine [22,23] can be expected. The ability to facilitate differential diagnoses, and assist with surgical planning relative to localizing and defining the extent of pathological processes will improve our surgical and diagnostic capabilities. The potential for future applications of this modality in equine medicine and surgery is significant and must be explored for the benefit of the animals and our profession.