Diagnosis of Erythema Multiforme Associated with Thymoma in a Dog and Treated with Thymectomy

Abstract

A 7 yr old Labrador retriever initially presented for severe halitosis, mild ptylism, and depigmentation of the nasal planum. Erythema multiforme was diagnosed based on clinical signs and dermatopathology. Treatment was initiated but the condition did not resolve. Six months later, the dog was diagnosed with a mediastinal mass. Trucut biopsy was performed and histopathologic diagnosis was consistent with a thymoma. A median sternotomy was performed, the thymoma was excised, and the dog recovered well. Four months postoperatively, there were no longer any obvious erythema multiforme lesions and the skin condition was controlled without medication. Erythema multiforme may be a paraneoplastic disorder associated with thymoma in the dog, similar to thymoma-associated exfoliative dermatitis in the cat. Clinical signs of erythema multiforme may warrant performing thoracic radiographs or thoracic CT to rule out thymoma as an underlying cause.

CT imaging features of canine thymomas

“…This supports a previous ultrasound study, in which 57.14% of thymomas were cystic . This is also consistent with previous case reports of canine thymomas, in which they have been noted to be heterogeneous and cystic on CT . This study confirms the cystic and heterogeneous attenuation of thymomas precontrast and also has shown a correlation between size and development of internal cysts.…”

“…Canine thymomas typically affect middle‐aged to older dogs and have been reported to be more common in Labrador Retrievers and German Shepherds . Common clinical presenting signs may include vomiting, regurgitation, anorexia, cough, dyspnea, tachypnea, and lethargy . Thymomas are commonly associated with paraneoplastic syndromes, including myasthenia gravis, cranial vena cava syndrome, and hypercalcemia of malignancy …”

Nonthymoma‐associated exfoliative dermatitis in 18 cats

“…The histological pattern with apoptotic cells throughout the epidermis shows similarities to erythema multiforme. There is one case report of a dog in which erythema multiforme is described in association with thymoma . In the present case series, an underlying neoplasia that could have led to the development of autoreactive T cells was excluded as far as possible by imaging techniques (radiography of the thorax in all cases, abdominal ultrasound in 14 and postmortem examination in two cats), but we cannot completely exclude the presence of small neoplastic foci, which are still undetectable by these methods.…”

Erythema multiforme, Stevens–Johnson syndrome and toxic epidermal necrolysis: a comparative review

“…Rather than an alteration of epidermal antigenicity causing cytotoxic T cells (CTLs) to target and destroy these altered cells, changes in maintenance of active tolerance in the abnormal thymic environment could allow normal keratinocytes to be identified erroneously as ‘foreign’. There are insufficient data to confirm a putative case of canine thymoma‐associated EM …”