PHOTODYNAMIC ACTION 707 



In experiments with white mice injected with hematoporphyrin, Haus- 

 mann (1910, 1914) divided the effects he observed into acute, subacute, 

 and chronic classes. In the acute form, produced when larger amounts 

 of porphyrin were injected and the irradiation was intense, convulsions 

 occurred, and the animals died within a few minutes. The acute reac- 

 tion was evoked also by smaller dosage of light or sensitizer, but then 

 the animals showed pruritus, erythema, and excitement, followed by 

 weakness and coma, and died within a few hours. Subacute reactions 

 consisted of erythema and edema, but the animal survived exposure for a 

 considerable period, so that the aftereffects such as necrosis of the skin 

 were observed. The chronic signs, consisting of necrosis and loss of hair 

 some time after exposure, with only minor reactions during exposure, 

 were the result of still lower dosage of light or porphyrin. Gray mice, 

 for example, showed only the chronic effects. 



Photosensitivity in human beings and agricultural animals is commonly 

 of the subacute or chronic type under Hausmann's classification. Effects 

 consistent with the less acute form can be produced experimentally in 

 sheep, and such effects and even death after prolonged exposure can be 

 seen in some cases of diseases such as facial eczema and hypericism. 

 Death in these animals may frequently be the result of other lesions in 

 the disease or of inability to graze normally rather than the effect of 

 photodynamic action alone. 



CLASSIFICATION OF DISEASES INVOLVING PHOTOSENSITIVITY 



The existing knowledge of the nature and mechanism of photodynamic 

 action, incomplete as it is, has led to considerable progress in the eluci- 

 dation of a number of pathological conditions involving abnormal reac- 

 tions on exposure to light. In few of these diseases is the photosensitivity 

 the only pathological manifestation; more commonly it is a consequence 

 of other disturbances of bodily function, and frequently it is a minor 

 aspect of the disease. The following classification of diseases involving 

 photosensitization can be made, using as a basis the mode by which the 

 photodynamic agent reaches the systemic circulation (Clare, 1952) : 



Type I, Primary Photosensitivity . In this type the photosensitizing 

 agent is either a substance, not normally encountered in the diet, which 

 is absorbed directly from the digestive tract and not completely excreted 

 by the hver, or a substance introduced into the tissues of the skin by 

 injection or contact. In animals there are two well-established diseases 

 of this type due to poisoning by Hypericum species and buckwheat, and 

 a number of other ill-defined photosensitivities caused by plants probably 

 belong to this group. In human beings primary photosensitization has 

 generally arisen through medication with fluorescent substances. 



Type II, Photosensitivity Due to Aberrant Pigment Synthesis. The 

 photosensitizing substance is a pigment, not normally found in animals, 



