Ectopic Intrathoracic Hepatic Tissue and Accessory Lung Lobe Aplasia in a Dog

Introduction

Ectopic liver is defined as histologically normal liver tissue in an abnormal location.¹ Ectopic hepatic tissue, unlike ectopic splenic and pancreatic tissue, is rare and generally has a subdiaphragmatic distribution.² Intra-abdominal ectopic hepatocellular carcinoma has been reported in one dog; however, that diagnosis is consistent with the subdiaphragmatic location of ectopic hepatic parenchyma.³ One case of an ectopic lobe of liver connected to normal diaphragm was reported in a cat.⁴ This report describes a case of ectopic intrathoracic supradiaphragmatic hepatic tissue located in a site normally occupied by lung tissue that caused clinical signs necessitating veterinary consultation and surgery.

Case Report

A 6 yr old male Yorkshire terrier was presented to his primary veterinarian for a chronic cough that had been present since he was a puppy. His coughing had recently become more frequent. At that time, he was not currently experiencing any other clinical signs and had not been treated with any medications.

At the time of presentation, the patient ausculted within normal limits despite experiencing a persistent cough, increased respiratory rate, and decreased activity level. Physical examination was unremarkable. Thoracic radiographs taken by the primary veterinarian were reviewed by a board-certified radiologist (Figure 1). Six images of the thorax were available for interpretation. The cardiac silhouette was normal in size and measured less than three intercostal spaces wide. No left atrial enlargement was seen, and the pulmonary vasculature was normal. There was a large focal soft-tissue mass effect identified within the right caudal thorax, which measured ~6 cm in diameter. This mass effect was causing a partial silhouette sign with the diaphragm and appeared to be displacing the heart slightly to the left. The right side of the liver appeared to be diminished in size within the abdominal cavity. No pleural effusion was seen, and the remaining lung fields were normal. The cranial mediastinum was normal. The trachea was not well seen at the thoracic inlet due to overlying shoulders but the intrathoracic trachea was normal. No comments could be made in the assessment of the extrathoracic trachea. Final radiographic interpretation was a soft-tissue mass effect present within the caudoventral thorax. The mass could not be definitively localized based solely on radiographs. A caudal mediastinal mass and diaphragmatic hernia were considered most likely, but a pulmonary mass could not be excluded. Caudal thoracic ultrasound revealed a mass with ultrasonographic features of liver. A computerized tomographic scan was recommended but declined by the owner due to financial constraints.

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Due to the respiratory clinical signs, an abdominal exploratory was recommended for evaluation and repair of a suspected chronic diaphragmatic hernia. The patient was premedicated with a combination of IV acepromazine^a (0.05 mg/kg) and hydromorphone^b (0.05 mg/kg). Anesthesia was induced with propofol^c (4 mg/kg IV to effect). Cefazolin^d (22 mg/kg IV) was administered prior to the start of surgery and again 2 hr later. Intraoperative pain was managed with a continuous rate infusion of hydromorphone (0.05 mg/kg/hr), lidocaine^e (20 µg/kg/min), and ketamine^f (0.6 mg/kg/hr) at 10 mL/hr throughout the procedure. IV fluid therapy^g was administered at a rate of 39 mL/hr during the procedure. Hypotension was experienced during surgery, which was treated medically with an IV fluid bolus, two boluses of hetastarch^h (5 mL/kg), and 3 mL/hr hetastarch (maintenance). The patient was also started on a dobutamineⁱ continuous rate infusion at 2.5 µg/kg/min that was increased intraoperatively to 5 µg/kg/min.

A routine ventral celiotomy was performed with the patient in dorsal recumbency. After thorough examination, no diaphragmatic hernia was identified. No evidence of prior diaphragmatic trauma was seen. All six liver lobes were found to be anatomically normal as well as normal in gross appearance. No other abnormalities were identified in the abdominal cavity. A caudal median sternotomy was then performed, which revealed that the accessory lung lobe was particularly hyperemic and consolidated in appearance. The consolidated accessory lung lobe was interpreted as the mass identified on thoracic imaging. The accessory lung lobe was removed using a stapling device^j near its base. The consolidated mass had the gross appearance of liver, but the pedicle did appear grossly to have a bronchus and normal pulmonary vasculature. The thoracic and abdominal cavities were thoroughly lavaged with saline. A transdiaphragmatic thoracostomy tube was placed to evacuate thoracic air upon closure of the chest cavity. A local lidocaine block (2%) was applied at the level of the intercostal nerves. The diaphragm was sutured using 2-0 suture material^k in a simple continuous pattern. The sternotomy site was apposed with cruciate sutures placed around the sternal bodies. The external abdominal oblique fascia and the ventral thoracic fascia were closed in a simple continuous pattern, and the subcutaneous tissue was closed with a simple continuous pattern. A subcuticular pattern was performed with 3-0 monocryl^l. The transabdominal/transdiaphragmatic thoracostomy tube was secured with three simple interrupted friction sutures using 2-0 nylon^m. The excised lung lobe was submitted for histopathological evaluation.

Postoperatively, the dog recovered routinely from anesthesia. Precautionary and intense postoperative monitoring was implemented, the respiratory and heart rates were monitored q 1 hr, and continuous electrocardiogram was performed. Aspiration and quantification of air and fluid was measured from the chest tube. The patient's pain was managed postoperatively with a fentanyl patchⁿ (12 µg transdermally), a hydromorphone/lidocaine/ketamine continuous rate infusion at 10 mL/hr (which was decreased to 5mL/hr, then 3mL/hr, and ultimately discontinued 2 days postoperatively), as well as oral (PO) administration of gabapentin^o (10 mg/kg q 8 hr). IV fluid therapy was maintained at 8 mL/hr then decreased to 5 mL/hr prior to discontinuation 2 days postoperatively.

Microscopically, the mass consisted of hepatocytes arranged in variably sized lobules containing terminal hepatic venule-like structures and occasional portal regions (Figure 2A). The distance separating one portal region from an adjacent portal region was very small. Similarly, some regions contained increased numbers of terminal hepatic venules. Hepatic lobules were occasionally delineated by minimally increased quantities of fibrous connective tissue. Hepatocytes were frequently swollen and contained vacuolated eosinophilic cytoplasm. Kupffer cells containing occasional hemosidern pigment were present (Figure 2B). No pulmonary tissue was seen. Based on gross and microscopic features and the clinical scenario, the dog was diagnosed with ectopic intrathoracic hepatic tissue with accessory lung lobe aplasia.

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A follow-up conversation with the owner 6 mo postoperatively revealed that the patient had returned to normal activity and no longer demonstrated any clinical signs.

Discussion

There are four main types of ectopic liver in human medicine (1) considerably sized accessory liver lobe that remains attached to the liver via a stalk; (2) small accessory liver lobe; (3) macroscopic ectopic liver, often associated with the gallbladder or other intra-abdominal organs; and (4) microscopic ectopic liver.¹

No ectopic liver classifications exist in veterinary medicine. Ectopic hepatic tissue, unlike ectopic splenic and pancreatic tissue, is rare and generally has a subdiaphragmatic distribution.² Intra-abdominal ectopic hepatocellular carcinoma has been reported in one dog; however, that diagnosis is consistent with the subdiaphragmatic location of ectopic hepatic parenchyma.³ The human medical literature also indicates that ectopic hepatic parenchyma can occur secondary to the migration of the liver during embryologic development, during which time the development of an accessory lobe of the liver occurs with either atrophy or regression of the original connection to the main liver.^2,5–7 This case report describes a case of ectopic intrathoracic supradiaphragmatic hepatic tissue located in a site normally occupied by lung tissue. One case of an ectopic lobe of liver connected to normal diaphragm was reported in a cat.⁴ In the present case, given the age of the patient and the close embryologic origin of the liver and lung, the authors suspect that this was a congenital abnormality.

Embryologically, there are three primary germ layers involved in the development of tissues: the ectoderm, mesoderm, and endoderm. The ectoderm layer primarily forms neurologic tissue as well as the surface structures and their derivatives. The endoderm layer primarily forms the gut tube, the yolk sac and allantois, as well as other derivatives of the embryonic gut. The mesoderm is responsible for the formation of most of the other embryologic structures.⁸

In this case, it is presumed that the development of ectopic hepatic tissue in the thoracic cavity at the location normally occupied by the accessory lung parenchyma is the result of an abnormality of embryologic development. The liver is a derivative of the embryonic gut, arising from the arrangement of endodermal cells that differentiate into hepatocytes, and is comprised of hematopoietic stem cells derived from mesenchyme of the septum transversum.⁹ The lungs develop, starting at the level of the fourth pharyngeal pouches, as a longitudinal diverticulum of endodermal epithelial cells growing ventrally from the foregut. The blind caudal end of the respiratory diverticulum is the lung bud.¹⁰

Experimental analyses have shown that the endodermal epithelial component and the mesenchyme that surrounds it are both essential for normal lung morphogenesis. Lung bud epithelium stripped of its mesenchyme fails to continue growing and branching; however, if the mesenchyme is replaced, even with mesenchyme from a different organ, typical pulmonary-type growth and branching resumes.¹⁰

Hepatic herniation with diaphragmatic hernia or peritoneal pericardial diaphragmatic hernia has been documented.¹¹ Diaphragmatic hernia resembling a pulmonary mass has been reported in a cat.⁷ However, in this case report, no evidence of hernia was seen. Although initially a diaphragmatic hernia was suspected, the suspicion of a diaphragmatic hernia was considered less likely following an exploratory laparotomy.

Based on that information, it is plausible to conclude that pulmonary epithelial cells stripped of their mesenchyme were replaced with ectopic hepatic tissue. Ultimately, this transformation likely resulted in ectopic liver contained within the thoracic cavity associated with the lungs.

Conclusion

This report describes a case of ectopic intrathoracic supradiaphragmatic hepatic tissue located in a site normally occupied by lung tissue. To the best of the authors' knowledge, this is the first reported case of ectopic intrathoracic hepatic tissue in a dog. This solitary case report demonstrates that ectopic intrathoracic hepatic tissue should be considered as a possible differential diagnosis for a caudal mediastinal mass. Although it is possible that the ectopic hepatic tissue was an incidental finding, the clinical presenting signs of cough and dyspnea resolved with excision of the intrathoracic mass, making it the likely source of symptoms.