In 1987, Amlot et al. proposed OAS as a phenomenon in which allergic symptoms may appear in the oral mucosa and may spread to the whole body after ingesting food positive on the skin prick test [1].
OAS is a relatively new concept in food allergies. Food allergies are thought to have the same sensitizers and inducers as those of other allergies. However, in recent years, food allergies have exhibited cross-reactivity toward the pollen antigen owing to differences between sensitizers and inducers.
Food allergens that cause conventional intestinal sensitization are termed class I food allergens, and those proteins in fruits and vegetables causing allergy owing to cross-reactivity with the pollen antigen are termed class Ⅱ food allergens [2]. Class I food allergens are resistant to heat and digestive enzymes; thus, the symptoms appear systemically. Class II antigens are not resistant to heat and digestive enzymes; thus, their symptoms are often localized to the oropharynx. OAS is a class II food allergy. Most sensitizers causing OAS include pollen and latex, with many of these agents being key components of food. OAS is a new concept in food allergy and is classified as a special type of immediate allergy [3]. Being a new class, there are no unified diagnostic criteria for OAS.
The diagnostic criteria for OAS proposed by Muluk and Cingi are based on history taking and positive skin prick test result triggered by fresh food extract [4]. Oral challenge results are normally positive for raw food and negative for the cooked version of the same food item. In our case, oral lesions and positive skin prick test results were present and conformed to the aforementioned diagnostic criteria for OAS; thus, a definitive diagnosis of OAS caused by Japanese radish was made.
Since 1974, a total of eight cases of radish allergy have been reported in the literature, including this case (Table 2) [5-11]. Oral mucositis was observed in three cases, and oral cavity involvement was not mentioned in five other cases. Despite this being a food allergy, there appears to be a little interest in studying the changes occurring in the oral cavity. According to the diagnostic criteria proposed by Muluk and Cingi [4], only cases 7 and 8 (this case) were diagnosed with radish-induced OAS. In addition, this is the first case report containing a detailed description of oral symptoms and supporting photographs.
In our patient, the allergic symptoms appeared not only in the oropharynx but also in the abdomen. However, consuming cooked radish did not induce any symptoms. Therefore, this was classified as a class II food allergy. Mustard, which belongs to the same Brassica family as radish, reportedly cross-reacts with mugwort [12]. However, RIST investigations were negative, including those for mugwort, and the sensitizing allergen could not be detected.
Ausukua et al. mentioned “viral infections (herpetic)” as a differential diagnosis for OAS [13]. In our case, we initially suspected herpetic gingivostomatitis because of fever, general malaise, and findings of the oral mucosa. Vegetables of the Brassica family, such as radish, mustard, and wasabi, contain a volatile substance called isothiocyanate, a component that gives them their spicy taste. Glucosinolate is a precursor of isothiocyanate. While glucosinolate itself is not volatile, it chemically reacts with an enzyme called myrosinase that is released after the plant cells are ruptured, yielding isothiocyanate [14]. While the prick test result using grated raw radish was positive for our patient, that of the test using non-grated radish was negative. Grating raw radish destroys cells, causing glucosinolate and myrosinase to chemically react with each other over a sufficient period to form isothiocyanate. It was speculated that the non-grated raw radish did not contain enough isothiocyanate to induce allergy. Thus, isothiocyanate was considered an allergen in our case.
OAS can be prevented by avoiding ingestion of foods containing culprit allergens. In addition, consuming antihistaminic drugs before ingestion of the causative food may alleviate symptoms [4]. In our case, antihistamine and injectable epinephrine were prescribed prophylactically to help the patient control any accidental onset in the future.
OAS can be readily diagnosed if the causative food is identified. However, guiding the diagnosis can be challenging unless the patient is aware of food allergies. When encountering widespread erosion in the oral cavity, it is essential to keep OAS in mind.