Muffled lung sounds often noted with thymoma

Signalment:

Canine, West Highland White Terrier, 10 years old, male neutered, 20 lbs.

Clinical history:

The dog was diagnosed with ascites and pleural effusion by the family veterinarian. There has been some weight loss of about 5 lbs. noted for the last month. The dog is eating and drinking fine.

Physical examination:

The findings include a rectal temperature of 101.5° F, heart rate of 120 beats/min, respiratory rate of 80 breaths/min, slightly depressed attitude, severe dental disease, distended abdomen and muffled heart sounds. There was 750 ml of fluid removed from the thorax. Therapy has included enrofloxacin, furosemide and spirolactone. The ECG shows a heart rate of 110 bpm and a normal sinus rhythm.

Table 1: Results of laboratory tests

Laboratory results:

A complete blood count, serum chemistry profile and urinalysis were performed and are outlined in Table 1.

Ancillary tests:

The thoracic and abdominal fluid analysis shows the fluid to be a transudate. The Snap 3Dx test is negative.

Radiograph examination:

The thoracic and abdominal radiographs were done.

My comments:

The thoracic radiographs show evidence of pleural effusion, dilated intrathoracic trachea and possible cranial mediastinal mass. The abdominal radiographs show thickened gastric and intestinal walls.

Image 1, 2, and 3.

Ultrasound examination:

Thorough thoracic and abdominal ultrasonography was performed.

My comments:

The liver shows an increased mixed echogenicity in its parenchyma. The caudate liver lobe is prominent. There are hypoechoic irregular-shaped lesions in the left medial and left lateral liver lobes. No masses noted within the liver parenchyma. The gall bladder is mildly distended, and its walls are not thickened or hyperechoic. The gall bladder is filled with sludge material. The spleen shows an inhomogeneous texture in its parenchyma. No masses noted. The left and right kidneys are similar in size, shape and echotexture. Each kidney shows an inhomogeneous texture in the renal cortex. No masses or calculi were noted in either kidney.

The urinary bladder is distended with urine and contains some urine sediment material. No masses or calculi noted. The left and right adrenal glands are similar in size and shape. The stomach and intestinal walls are slightly thickened. The colon is normal. The pancreas shows an inhomogeneous texture in its parenchyma. There is a mild to moderate amount of pleural effusion noted. The echocardiogram is basically normal except for mild thickening of the mitral valve. There is an irregular-shaped, mass-like structure seen attached to the external wall of the right heart.

Case management:

In this case, cranial mediastinal mass and chronic protein-losing enteropathy is the clinical diagnosis. This cranial mediastinal mass is usually a lymphoma or thymoma. Thymoma is most likely in this case and surgical removal of this intrathoracic mass may not be possible. Fine needle aspirations of the cranial mediastinal mass for cytologic examination are warranted to confirm the diagnosis of thymoma. The chronic protein-losing enteropathy is most likely because of chronic inflammatory bowel disease and secondary lymphangiectasia.

Review of thymoma

Most dogs with a cranial mediastinal mass will present with signs of dyspnea (usually rapid, shallow breathing), coughing and exercise intolerance. Other signs may include regurgitation, vomiting or gagging secondary to esophageal compression or paraneoplastic myasthenia gravis. Generalized myasthenia gravis may also occur with a primary complaint by the owner of recurrent weakness or collapse. Precaval syndrome (swelling of the head, neck and/or thoracic limbs) is possible if the mediastinal mass causes compression of or invades the cranial vena cava. On physical examination, muffled lung sounds are often noted.

Image 4, 5, and 6.

Cranial mediastinal masses are usually thymoma or lymphosarcoma. Other causes may include ectopic thyroid tissue, branchial cyst, chemodectoma or thoracic wall tumor. Lymphadenopathy due to infectious or inflammatory causes can also be found in the cranial mediastinum. Fluid within the cranial mediastinum (transudate, exudate, hemorrhage) can occasionally mimic a mediastinal mass.

Hypercalcemia may occur in both thymoma and lymphoma. Non-specific azotemia secondary to pre-renal and renal causes may be found. Animals with lymphoma and liver involvement may have variable increases in serum ALP, ALT and total bilirubin. Hyperphosphatemia can be seen with renal failure, and hypophosphatemia is usually associated with hypercalcemia of malignancy.

Two or three thoracic view radiographs (ventrodorsal or dorsoventral and one or two lateral views) are the preferred way to diagnose an intrathoracic mass versus pulmonary, airway or pleural diseases causing respiratory signs.

Tracheal elevation is a consistent sign of a mediastinal mass on the lateral image. Differentiating a pulmonary or thoracic wall mass from a mediastinal mass may be done with the ventrodorsal view. The mediastinum should be twice the width of the spine in the dog. Fat in obese dogs can widen the mediastinum in the absence of a true mass. Pulmonary masses will usually be positioned lateral to the mediastinum, and thoracic wall masses will be peripheral and often cause rib lysis or spreading.

Animals with thymoma and paraneoplastic myasthenia gravis can have signs of megaesophagus and possibly aspiration pneumonia. Pleural effusion can also be present in mediastinal masses, usually containing either neoplastic cells or chyle.

Ultrasound of the cranial mediastinum can be useful in differentiating a cranial mediastinal mass from pleural fluid and may be helpful in determining an aspiration or biopsy site.

Ultrasound of the abdomen is indicated in staging of lymphoma, to determine intra-abdominal organ involvement. Because thymomas are rarely metastatic, abdominal ultrasound is not routinely performed in these dogs, except when attempting to differentiate thymoma from lymphoma or in the case of potential intra-abdominal organ dysfunction, based on hematologic or serum chemistry profile abnormalities.

Image 7, 8, and 9.

Aspiration cytology

Aspiration cytology can differentiate thymoma from lymphoma in many instances. Thymomas contain mature lymphocytes, neoplastic epithelial cells and often mast cells. Lymphomas are usually lymphoblastic with large, immature lymphocytes. A lymphocytic lymphoma or a cystic thymoma may cause cytology to be misleading, and tissue samples may be required. Cytology of pleural effusion can also be diagnostic for lymphoma if there are exfoliated lymphoblasts present.

Thymomas are invasive or non-invasive. Staging should include thoracic radiography to rule out obvious metastatic disease and further testing based on clinical signs, physical examination or hematologic and serologic findings.

Imaging of the thoracic cavity may not indicate the invasiveness of disease, and the presence of effusions should not rule out exploratory surgery. Exploratory surgery is still the best staging procedure to determine resectability.

Surgical treatment

Surgery is the preferred treatment for thymomas, but some will not be surgically resectable. About 70 percent of these masses are resectable. In the face of unresectable disease, radiation therapy may be useful, either as solitary therapy or as an adjunct to incomplete removal. About 75 percent of thymomas appear to have a response to radiation, although complete remissions are rarely achieved. Partial remission and relief of clinical signs for variable lengths of time is common. Megavoltage radiation appears to be well-tolerated in most dogs, with hair loss and dermatitis being the most common complications.

Pneumonitis can be a potentially life-threatening complication. The optimal course of radiation has not been determined. Chemotherapy may also play a role in the treatment of thymomas.

Prednisone alone has caused remission in some animals. Prednisone is also useful in the management of myasthenia gravis along with anticholinesterase drugs.

Other chemotherapeutic drugs have not been reported as useful in thymomas. However, most treatment courses have been under the presumption of lymphoma. Lymphoma protocols may have some utility in lymphocyte-rich tumors. Cisplatin is the drug used most frequently.

Dogs without signs of megaesophagus that have their mass completely resected usually have prolonged survival times (>80 percent one-year survival). Animals with megaesophagus often have very short survival times, and surgery may be contraindicated due to the morbidity and mortality associated with the procedure, usually related to aspiration pneumonia.

Paraneoplastic syndromes associated with thymoma may or may not resolve with therapy and may occur later in life despite successful therapy. There have been reports of prolonged survivals in some animals with no therapy for their thymomas, which may indicate the slow-growing nature of some tumors.