Much confusion and debate has centered on how to clinically differentiate between the members of this "family" of diseases. From a practical standpoint, it is sufficient to recognize a few key points.
- Lesions may be difficult to appreciate in long-haired animals without careful clipping of the hair coat. They consist of some combination of the following, which may appear on the skin and/or mucous membranes:
- Flat to raised "target" lesions (central healing, peripherally spreading)
- Erythematous or purpuric macular to patchy eruptions
- Ulcerations
- Epidermal detachment-in severe cases of toxic epidermal necrolysis (TEN), one third or more of the epidermal surface can become detached, resulting in sudden "sloughing".
- Systemic signs may be present and can vary from mild to severe, usually relating to the severity of the skin disease.
- In some cases these diseases are clearly related to a drug reaction, but it must be emphasized that they may also be secondary to infection or neoplastic disease or be idiopathic in origin.
Although they are uncommon, these diseases are among the most serious CADRs; they are characterized by possible progression and potentially fatal outcomes. The clinical course varies-although patients with erythema multiforme have a rather dramatic appearance, it is considered a relatively mild disease because if the underlying precipitating cause can be found and eliminated, most cases recover with supportive care within 1 to 3 weeks. On the other hand, patients with TEN constitute the rare "dermatologic emergency" and have a worse prognosis.
Skin biopsy with histopathologic evaluation is always the diagnostic test of choice. Assuming that relatively early (nonulcerated) skin biopsy specimens can be obtained, the histopathologic findings are unique and often diagnostic. A histopathology report mentioning any of these diseases should prompt immediate concern, search for a drug-related cause, and consultation with a dermatologist on current treatment recommendations.
Despite the presumed immunopathology of these diseases, most authorities state that therapy with glucocorticoids or other immunosuppressive therapies is not helpful.