Discussion:
There are two types of ADR based on expectedness of adverse drug reactions, Type A ADRs which are pharmacologically predictable and Type B ADRs which are idiosyncratic[8]. They are described and classified by the percentage of body surface area (BSA) involvement with less than 10% in SJS, 10% to 30% in SJS/TEN overlap and more than 30% in TEN[12]. This case with 10-30% BSA involvement is an SJS/TEN overlap due to oxcarbazepine and a Type B ADR and also defined as a severe cutaneous adverse drug reaction (SCAR). The pathophysiological mechanism behind SJS/TEN is conceptualised to be a type IV hypersensitivity reaction due to binding of drug to Major Histocompatibility complex I that results in cytotoxic CD8+ T cells mediated destruction of lower layer epidermis[13]. The activated antigen driven CD8+ T cells in the epidermis produce cytolytic peptides such as granulysin, a cytolytic peptide that leads to keratinocyte death[13,14]. There is a strong genetic association with HLA-B*1502 allele and carbamazepine SJS/TEN, which is widely seen in Southeast Asia population led to recommending the individuals of Asian ethnicity to be genotyped for the presence of the risk allele before receiving carbamazepine by The Food and Drug Administration. However, HLA-B*15:02 is less strongly associated with oxcarbazepine SJS/TEN with a significantly lower Positive Predictive Value of 0.73 and >5000 patients would need to be tested to prevent one case of SJS/TEN[13].
There had been lesser incidence of SJS reported with Oxcarbazepine but there is reportedly more incidence of drug rashes and SJS in patients who are hypersensitive to CBZ pointing towards cross-sensitivity. This cross sensitivity may be due to the dibenzazepine ring which is the common structure in CBZ and OXC[15]. In our patient there was no previous recorded hypersensitivity to any drugs. Here it is also interesting to note that our patient had SJS within one week of starting the incriminated drug which may be due to her older age.
In conclusion, Steven Johnsons syndrome due to oxcarbazepine is rare but still happens nevertheless. SJS being a life-threatening adverse effect, clinicians should be aware and vigilant about the same while starting a patient on OXC and should be prompt in withdrawing the agent at first sight of any cutaneous reaction. Patients should also be informed in detail about the same and encouraged to report back in case of adverse effects. Thus, studying and reporting of ADRs associated with any drug is also vital for estimating their incidence, expectedness, severity, morbidity and mortality in clinical practice and further determining the predisposing risk factors.