Thymoma associated with exfoliative dermatitis in a cat

Abstract

A 7-year-old, castrated male, domestic shorthair cat presented with generalized exfoliative dermatitis, lethargy, anorexia and weight loss. Multiple skin scrapings taken at the time did not reveal any abnormalities. Skin histopathological examination was consistent with sebaceous adenitis or exfoliative dermatitis caused by an underlying thymoma (thymoma-associated feline exfoliative dermatitis). Thoracic radiographs revealed a cranial mediastinal mass, which was removed surgically. Histopathological examinations indicated that it was a thymoma. Within 90 days of surgery, the cutaneous signs had resolved, suggesting a causal relationship between the thymoma and the skin disease. Recurrence of thymoma was detected 24 months after surgery.

Case Report

A 7-year-old, castrated male, domestic shorthair cat was presented with generalized and severe exfoliative dermatitis. In addition, lethargy, anorexia and weight loss of a few weeks’ duration had been reported. Clinically, the cat showed generalized scale and erythema with desquamation, alopecia of the body and multifocal crusts (Figure 1a, b). Skin scrapings and wood’s lamp examination were negative. Previous empirical therapies, such as antifungal and antibiotic drugs, did not improve the clinical appearance. Complete blood count, serum biochemical profile, and tests for evidence of infection with feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) were performed. Three punch biopsy samples of skin lesions localized in the neck, limb and back were taken for fungal culture and histopathological examination. Blood tests did not reveal any abnormalities; FeLV, FIV and fungal culture tests were negative. Histopathology revealed that all fragments exhibited marked orthokeratosis or parakeratosis and ostial keratosis with areas of follicular hyperkeratosis. There was mild-to-moderate inflammatory infiltrate at the dermis, consisting of lymphocytes and mast cells. The inflammatory infiltrate compromised the wall of hair follicles. In addition, a numerical decrease in sebaceous glands, even after serial cuts (identified only in one gland) was observed. Periodic acid-Schiff staining did not demonstrate the presence of fungi in the tissues. These findings could be consistent with sebaceous adenitis or exfoliative dermatitis caused by an underlying thymoma (thymoma-associated feline exfoliative dermatitis).

Figure 1.

(a,b) Generalized desquamation, alopecia and multifocal crusts

After 20 days, a worsening of dermatitis was observed (Figure 2). Thoracic radiography was performed (Figure 3), which showed a cranial mediastinal mass with caudal displacement of the cardiac silhouette and cranial displacement of the lung lobes. There was no evidence of megaesophagus or metastatic disease. Abdominal ultrasonography and echocardiography did not reveal any abnormalities.

Figure 2.

Worsening of dermatitis symptoms

Figure 3.

Laterolateral chest radiograph showing a cranial mediastinal mass

The presumptive diagnosis was thymoma-associated feline exfoliative dermatitis; thus, the animal was submitted for left thoracotomy with rib transection for resection of the mass. On surgical exploration, an 85 × 52 × 58 mm (height × width × length) irregular and well-encapsulated mass was detected, occupying much of the cranial thorax and connected to the right middle pleural lobe and the surrounding vessels (Figure 4). It was completely removed and hemostasis was achieved using bipolar electrocautery. Prior to closure of the thoracotomy, a thoracostomy tube was placed. The remaining steps for the closure of the surgical site were routine. Postoperative analgesia was provided by intravenous tramadol chlorhydrate (Tramal; Pfizer), dipyrone (Novalgina; Sanofi-Aventis) and meloxicam (Maxicam; Ourofino), as well as interpleural infiltration of bupivacaine (Cloridrato de Bupivacaína; Hypofarma). The thoracostomy tube was removed 48 h after surgery. The cat recovered well and was discharged from the hospital 72 h after surgery. Upon re-evaluation 15 days later, there was improvement of the exfoliative dermatitis, and it disappeared approximately 90 days later.

Figure 4.

Intraoperative photograph showing the mass

Histopathological examination of the mass revealed neoplasia containing numerous small lymphocytes surrounded by a substantial connective tissue capsule that extended into the tumor, giving a lobular appearance. The microscopic fields examined showed varying numbers (a few to several) cells with large round nuclei containing prominent nucleoli. A definitive diagnosis of thymoma was then established.

Thoracic radiographs were conducted 3, 12 and 18 months after surgery, and these did not show any abnormalities; 24 months postoperatively the result of the physical examination was normal and the previously detected dermatopathy had resolved (Figure 5). However, thoracic radiographs revealed a mass in the cranial mediastinum. Blood tests, abdominal ultrasonography and echocardiography did not reveal any abnormalities.

Figure 5.

The cat 24 months after surgery

Exploratory thoracotomy was performed. A 30 × 35 × 42 mm irregular mass was detected adhering to the pericardium and chest wall. Partial pericardectomy and partial tumor resection were performed. The cat died 48 h after surgery. Histopathological examination revealed that the mass was a thymoma.

Discussion

Although thymoma is the most common primary tumor of the thymus gland, ¹ it is rare in dogs and cats.^2,3

Thymoma is usually an encapsulated or locally invasive tumor, although metastases to the lungs, pericardium, local nodes and diaphragm have been reported, as well as rare distant metastases to the liver, spleen and kidneys.^1,4,5 A staging system has been previously described, in which stage 1 is growth within the thymic capsule (non-invasive thymoma); stage 2 is the pericapsular spread to mediastinal fat, pleura or pericardium; stage 3 is the infiltration of surrounding organs or intrathoracic metastasis; and stage 4 involves extrathoracic metastasis. ⁶ In this report, the thymoma was connected to the visceral pleura and the surrounding vessels, and thus characterized as a stage 2 invasive tumor.

Thymoma has also been associated with a number of paraneoplastic syndromes, including myasthenia gravis, autoimmune diseases such as granulocytopenia and polymyositis, and feline exfoliative dermatitis.^1,7

Feline exfoliative dermatitis is a rare paraneoplastic syndrome characterized by generalized severe desquamation with or without erythema.^1,8 In cats, the differential diagnosis of this rare condition includes systemic lupus erythematosus, drug eruptions, epitheliotropic T-cell lymphoma, erythema multiforme, cheyletiellosis, demodicosis, Malassezia species dermatitis, dermatophytosis, FIV or FeLV dermatitis, parapsoriasis, sebaceous adenitis and paraneoplastic syndrome caused by thymoma. ¹

Although the pathomechanism by which thymoma causes cutaneous lesions is unclear,^4,8 a number of mechanisms have been suggested. These include production or depletion of a specific substance by the tumor or an immune-mediated mechanism that leads to the activation of autoreactive cytotoxic T-cells. Another theory implicates an aberrant antitumor response with the production of autoantibodies that cross-react with epithelial antigens that might have been induced by a dysregulation of cytokine production in tumor cells. ⁴

Paraneoplastic syndromes may not always resolve following removal of the mass. In some instances, they may develop after surgery. ⁹ In cats with thymoma-associated exfoliative dermatitis that survive surgery, as was the case with the cat described in this report, removal of the tumor resulted in improvement and eventual normalization of the dermatopathy.^8,9

In dogs and cats, the approach of lateral thoracotomy is generally associated with a minimal complication rate. ¹⁰ In cats, resection of a thymoma can be accomplished by lateral thoracotomy, but rib transection may be required if the mass is large. Median sternotomy also can be performed, but the narrow sternum is often difficult to split precisely without fragmentation of the sternebrae. ² In humans, cats and dogs, iatrogenic neuropathic pain is probably the most important cause of long-term postsurgical pain; consequently, surgical techniques that avoid nerve damage should be applied whenever possible. Studies have shown the transcostal technique appears to be associated with less pain than the pericostal technique.^11–13 In dogs and cats, sternotomy might be more painful, ¹⁴ although there are few veterinary studies that compare the degree of pain following sternotomy and thoracotomy in order to discover which method can potentially lead to a lower level of postoperative pain.

The histological appearance of thymomas does not correlate well with neoplastic behavior, so the terms invasive and non-invasive are more descriptive than ‘malignant’ and ‘benign’.^1,3,4,8,9 Thus, exploratory thoracotomy is often necessary to determine the character of the tumor.^2,9

The prognosis for stage 1 thymoma without paraneoplastic disease is usually favorable, although excision of invasive thymoma is believed to be unrewarding, as vital surrounding structures are involved and prognosis is not well defined. ⁶ Recurrence of thymoma or myasthenia gravis after resection of the tumor is rare.^1,3 In humans, recurrence and metastasis are associated with invasive or non-invasive thymoma. ¹⁵

Conclusions

The cat described in this report suffered from a rare paraneoplastic disease that involved a thymoma, named thymoma-associated exfoliative dermatitis. Successful recovery, weight gain and resolution of dermatitis were observed following surgery, although a recurrence of thymoma was observed 24 months after surgery.