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.