Introduction

There are several methods to obtain hepatic tissue for histopathological or cytological analysis. These include fine needle aspiration, ultrasound-guided needle biopsy, laparoscopy-guided biopsy and wedge biopsy at the time of laparotomy. All these methods have advantages and disadvantages. There is strong institutional bias regarding these, relating to the experience and expertise of the entire team, including the clinician, radiologist and pathologist. This lecture will review methods of obtaining hepatic tissue, and discuss the advantages and disadvantages of each.

Pre-Biopsy Considerations

It is questionable whether pre-biopsy coagulation studies will identify patients at risk for bleeding. Studies in humans and dogs/cats have shown poor correlation with coagulopathies and hepatic bleeding times. Thus, coagulation profiles are unreliable guides of the risk of bleeding after liver biopsy, and hence, are of limited value in determining contraindications to this procedure. In other words, patients with coagulopathies are no more likely to bleed than patients without coagulopathies. In most cases of significant bleeding following hepatic biopsy, there are technical problems. The rapidity of bleeding and/or necropsy examination suggests that in most patients a large vessel was damaged rather than there being persistent oozing from needle biopsy sites. Controlled studies in veterinary patients will be necessary to make final conclusions regarding post-biopsy haemorrhage in the patient with a coagulopathy.

Fine Needle Aspiration

Fine needle aspiration (FNA) involves obtaining a small amount of hepatic tissue for cytological analysis. The technique is usually performed using ultrasound guidance. Often multiple sites are chosen to represent different lobes, and in the case of focal lesions, to sample more than one area of abnormal tissue and sample seemingly normal tissue. The sites are also chosen based on accessibility. Usually a 22 gauge, 1.5 inch needle is used.

Advantages of FNA

There are several advantages of FNA. Firstly, little to no sedation is usually required. Because the size of the needle is so small, there is little risk of haemorrhage and therefore multiple sites can easily be sampled. The procedure is rapid and can usually be performed on an outpatient basis. There is also less cost to the client.

Disadvantages of FNA

The main disadvantage of FNA is the questionable accuracy. Multiple studies have shown poor correlation with FNA cytology and histopathology of the liver. In many types of hepatic diseases, there are several important elements in interpreting pathological information. These include lobular architecture, presence and location of inflammation within a lobule, presence and severity of fibrosis, metal accumulation, vascular abnormalities and heterogeneity within a lobule. None of these criteria can be accurately defined with cytology of FNA samples.

Summary

Although FNA is easy to perform, involves little risk and little to no sedation, the information is of little value if it is inaccurate as often as it is accurate. There is institutional bias regarding its accuracy, which may relate to the experience and expertise of the cytologists. If FNA is performed, it should be considered an adjunctive diagnostic modality and used in conjunction with other techniques. Clearly, hepatic cytology does not replace histopathology.

Ultrasound-Guided Biopsy

Ultrasound-guided biopsy uses a cutting-type needle to sample the liver. Newer automated needles are preferred. The author generally uses a 16 gauge needle. Smaller (18 gauge) needles are safer but collect less tissue, whereas larger (14 gauge) needles obtain more tissue but induce a greater risk of bleeding. In most dogs, biopsy of the liver can be performed using local anaesthesia and minimal sedation. Most cats require general anaesthesia to safely obtain tissue. The degree of sedation must be tailored to each individual patient.

A careful ultrasound examination is performed prior to biopsy. This allows planning of the procedure based on echo pattern, lesion size, proximity to other organs, proximity to blood vessels, determination of cystic or solid tissue and optimal approach of the needle path. The needle is directed so the trajectory will avoid other structures when it is fired. After completion of the procedure, an ultrasound examination is performed to check for excessive haemorrhage.

Advantages of Ultrasound-Guided Biopsy

Ultrasound-guided biopsy has many of the advantages of FNA, including the need for minimal sedation in some patients; multiple sites can be sampled and low to moderate cost to the client. Additionally, tissue is obtained for histopathology.

Disadvantages of Ultrasound-Guided Biopsy

Disadvantages include: the risk of bleeding (especially when multiple sites are sampled and larger-gauge needles are used; in the author's experience, the risk of bleeding following hepatic biopsy is greatest with ultrasound-guided sampling); the need for sedation or anaesthesia in some patients; the difficulty of imaging small livers; the difficulty of obtaining liver tissue in patients with fibrosis; and the question of whether needle biopsy samples accurately represent the pathology of the liver. The diagnostic accuracy of needle biopsy has been questioned by many clinicians, observing that results of needle biopsy analysis often do not adequately reflect the clinical and laboratory features of the patient. This question was addressed by Cole et al. (2002) when they studied the diagnostic accuracy of Tru-Cut needle biopsy compared to the 'gold standard' of surgical wedge biopsy of the liver in 124 patients. The overall discordance between needle and wedge biopsy was 52%, with the greatest discordance (>60% discrepancy) occurring with chronic hepatitis/cirrhosis, cholangitis/cholangiohepatitis complex, portosystemic vascular anomalies, microvascular dysplasia, fibrosis and miscellaneous disorders. As with FNA, inflammatory disease, vascular abnormalities and fibrotic disorders had the least accuracy. The greatest accuracy was with neoplasia.

Summary

Ultrasound-guided biopsy is easy to perform but involves more risk to the patient (primarily bleeding). As with FNA, the information is of little value if it is inaccurate as often as it is accurate. There is institutional bias regarding its accuracy, which may relate to the experience and expertise of the pathologist and the individuals obtaining the biopsy specimens. The accuracy can be improved by using large-gauge needles, but this increases the risk of bleeding. The clinician must choose which animals are candidates for this method. If the patient is suspected of having inflammatory disease, vascular abnormalities or significant fibrosis, laparoscopy or laparotomy should be considered. Furthermore, if the patient is considered at risk for bleeding, laparoscopy should be considered.

Laparoscopy-Guided Biopsy

The technique of laparoscopy is beyond the scope of this manuscript; the reader is referred to the further reading list for details of this procedure. When the liver is directly visualised, directed biopsy specimens of multiple lobes are obtained. Specimens are obtained with a 'spoon' or 'clam-shell' forceps. The author typically obtains at least seven specimens for histopathology, metal analysis and culture.

Advantages of Laparoscopy

Perhaps the biggest advantage of laparoscopy is the large sample size of the biopsy specimens. They are large enough to give similar information to surgical wedge biopsies. In addition, since biopsy is performed under direct visualisation, appropriate sites are sampled. This is especially important when there are focal lesions or with heterogenous areas. Since multiple biopsy specimens can be obtained, sampling artefact in cases of geographical diversity is less of a problem. By directly visualising the hepatic parenchyma, the clinician can better correlate the histopathology findings with the clinical data and gross appearance to render the most accurate diagnosis. Laparoscopy also gives the clinician an excellent view of the liver, regardless of hepatic size or conformation of the patient, making it easy to sample the liver in patients that are difficult to image with ultrasound. Another advantage is the minimal bleeding encountered during laparoscopy and the ability to assess and control unexpected haemorrhage.

Disadvantages of Laparoscopy

Disadvantages of laparoscopy include the need for expensive equipment, the need for extensive training, the need for general anaesthesia in most cases and higher cost to the client.

Summary

Laparoscopy gives the clinician the advantages of a laparotomy (large sample size, ability to best direct sampling and ability to take multiple samples, thus resulting in the highest accuracy) with a relatively minimally invasive procedure. The complication rate (especially haemorrhage) is far less than with ultrasound-guided biopsy in the author's practice. For these reasons, it is the author's method of choice for obtaining hepatic biopsy specimens in most cases.

References

1.  Bigge LA, Brown DJ, Penninck DG. Correlation between coagulation profile findings and bleeding complications after ultrasound-guided biopsies: 434 cases (1993-1996). Journal of the American Animal Hospital Association 2001; 37: 228-233.

2.  Cole T, Flood S, et al. Diagnostic comparison of needle and wedge biopsy specimens of the liver in dogs and cats (n = 124). Journal of the American Veterinary Medical Association 2002; 220: 1483.

3.  Ewe K. Bleeding after liver biopsy does not correlate with indices of peripheral coagulation. Digestive Diseases and Science 1981; 26: 388-393.

4.  Richter KP. Laparoscopy in dogs and cats. Veterinary Clinics of North America Small Animal Practice 2001; 31: 707-727.

5.  Wang KY, Panciera DL, et al. Accuracy of ultrasound-guided fine-needle aspiration of the liver and cytologic findings in dogs and cats: 97 Cases (1990-2000). Journal of the American Veterinary Medical Association 2004; 224: 75-78.