History

A 15-year-old castrated male domestic shorthair cat was referred for evaluation of uncontrolled hyperthyroidism and for possible treatment with radioactive iodine. The owner reported that the diagnosis of hyperthyroidism had been made approximately 2 years earlier and that the cat had been treated, although irregularly, with methimazole since that time. The referring veterinarian was previously suspicious of a thoracic mass on the basis of findings on thoracic radiographs that were not available for review at the time of referral. The cat was otherwise active with a good appetite and had a history of polydipsia, mild weight loss, and occasional vomiting. On physical examination, the cat had a grade 3/6 heart murmur and was of thin body condition. The rest of the findings on physical examination were unremarkable. Two-view thoracic radiographs were obtained to investigate the suspected mass (Figure 1).

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Right lateral (A) and ventrodorsal (B) radiographic views of the thorax of a 15-year-old castrated male domestic shorthair cat with hyperthyroidism and a grade 3/6 heart murmur.

Citation: Journal of the American Veterinary Medical Association 244, 2; 10.2460/javma.244.2.157

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Radiographic Findings and Interpretation

A large, predominantly soft tissue–opaque mass in the caudal ventral aspect of the thorax displaces the trachea and caudal lung lobes dorsally. The cranial aspect of the mass is rounded and discretely marginated, with a slender focus of fat-opaque tissue separating the mass from the caudal border of the cardiac silhouette. The caudal border of the mass obscures the ventral aspect of the diaphragm. A reflection of the diaphragm is continuous with the dorsal aspect of the mass just ventral to the caudal vena cava, consistent with a dorsal peritoneopericardial mesothelial remnant. On the ventrodorsal view, the mass is caudal to the cardiac silhouette and does not contribute to an overall increase in size of the cardiac silhouette. A well-marginated, ovoid, mineral-rimmed nodule is seen within the mass on midline, consistent with nodular fat necrosis (ie, Bate's body; Figure 2). A mild cranial positioning of the pylorus (gastric axis shift) suggests a decrease in size of the liver or could be secondary to cranial displacement of the liver. There is also generalized cardiomegaly.

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Same radiographic images as in Figure 1. Notice the large fat to soft tissue–opaque mass in the caudal ventral aspect of the thorax (asterisk) that obscures the ventral margin of the diaphragm. On the lateral view, there is a reflection of the diaphragm that is continuous with the dorsal aspect of the mass at the level of the caudal vena cava (white arrow). There is generalized cardiomegaly, the cardiac silhouette is upright, and the trachea is displaced dorsally. On both views, a well-marginated mineral nodule is present within the mass at midline (black arrow).

Citation: Journal of the American Veterinary Medical Association 244, 2; 10.2460/javma.244.2.157

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On the basis of these findings, a preliminary diagnosis of a peritoneopericardial diaphragmatic hernia (PPDH) with herniation of falciform fat, liver, and nodular fat necrosis was made. Herniation of other abdominal contents could not be ruled out. Other, less likely, differential diagnoses included a caudal mediastinal, pleural, or diaphragmatic neoplasm; cyst; abscess; or granuloma.

Treatment and Outcome

For further evaluation, thoracic ultrasonography was performed. A large amount of fat was observed around the heart that was caudally continuous with a cranially displaced but otherwise normal-appearing liver. A focal, round hyperechogenicity with distal shadowing was present within the fat, consistent with the radiographic diagnosis of nodular fat necrosis.

Surgery to correct the apparent PPDH was not pursued because the cat had other concurrent medical conditions and was otherwise only subclinically affected by the PPDH. Although the occasional vomiting could have been related to the PPDH, the definitive etiology was not identified. The murmur, weight loss, and generalized cardiomegaly were suspected to be due to poor control of the cat's hyperthyroidism, considering that the owner was not giving the methimazole on a regular basis. The owner elected to continue medical management of hyperthyroidism. No additional diagnostic testing or treatment was performed, and the cat was subsequently lost to follow-up.

Comments

A PPDH occurs when there is abnormal communication between the pericardial sac and peritoneal cavity.^1–5 In a retrospective study¹ of 66 cats with PPDH, it was an incidental finding in 40% of cases, with Himalayans and domestic longhair cats being overrepresented; another retrospective study² of 31 cats also found longhaired breeds to be overrepresented. In dogs and cats, PPDH is always caused by a congenital anomaly. This is in contrast to PPDH in humans, in which it can be caused by trauma as the human diaphragm forms one of the walls of the pericardial sac.^1–3 Multiple theories exist regarding the pathogenesis of PPDH, with most implicating the septum transversum, which forms the ventral portion of the diaphragm during embryological development. Some argue that PPDH is caused by failure of closure of the septum transversum itself, whereas others believe it is formed by failure of fusion of the septum transversum with the pleuroperitoneal folds.^1–4

A PPDH can be diagnosed on the basis of imaging findings, including by means of radiography and positive-contrast radiography, ultrasonography, CT, and MRI. A radiographic diagnosis of PPDH is made on the basis of characteristic signs including enlargement of the cardiac silhouette with dorsal displacement of the trachea, overlapping borders of the diaphragm and caudal cardiac silhouette, presence of abdominal viscera in the thorax, cranial displacement of abdominal viscera, and the presence of a dorsal peritoneopericardial mesothelial remnant in cats.^1–5 In the cat of the present report, instead of the herniated contents enlarging the cardiac silhouette, the herniated contents remained caudal to the heart, giving it an hourglass appearance with the cardiac silhouette on the ventrodorsal radiographic view. Possible reasons for this include the presence of a restrictive band of tissue within the peritoneopericardium or a more normal formation of the pericardium relative to the diaphragm during development preventing herniation of contents further cranially. Magnetic resonance imaging or CT would have helped to better characterize this finding.

Nodular fat necrosis, or Bate's body, is a known radiographic finding in dogs and cats and is usually seen in obese animals.⁶ It is thought to occur secondary to pressure necrosis of fat secondary to obesity or entrapment of fat in hernias.⁶ Radiographically, it appears as a focal, circular-to-ovoid, mineralized soft tissue mass embedded within intra-abdominal fat and usually has an eggshell-like calcified rim.⁶ Ultrasonographically, it appears as a hyperechoic focus with distal acoustic shadowing.⁶ In the case reported here, the nodular fat necrosis may have been secondary to ischemia of falciform fat secondary to entrapment within the hernia. Alternatively, the fat necrosis may have formed in the peritoneum and then herniated.

A PPDH can be treated either surgically or conservatively. Conservative treatment is often chosen when it is an incidental finding or when age or concurrent disease makes surgery an impractical choice.¹ Surgery has a reported mortality rate of 3.2% to 14% in cats.^1,2 Like identifying nodular fat necrosis, the presence of a longstanding PPDH can be an unexpected finding in many cases, given that clinical signs are generally not referable to either radiographic diagnosis.