In this study, 36.1% of patients were exposed to topical steroids before diagnosis of fungal keratitis. Previous OSD and previous ocular surgery history were higher in the PS group likely because steroids were used for treating their underlying causes. A previous Korean study reported that 14.1% of fungal keratitis cases had used topical steroids prior to diagnosis , while studies in other countries have reported a range from 13 to 44% [11-13].
The types and distribution of the microbiological profile in fungal keratitis varies according to geography, climate, and socioeconomic characteristics. In this study, the most commonly identified organism in both groups was Candida species (20% in PS and 11.3% in NPS) followed by Fusarium species. Our findings in relation to Candida species is similar to the results reported across studies from London (60.6%) , Paris (58%) , Denmark (52%) , and Pennsylvania (45.8%) . In contrast, studies in north China (73.3%) , Florida (41%) , Mexico City (37.2%) , south India (37.2%) , central China (30.6%) , and Korea (29%)  reported that the Fusarium was the most commonly identified organism. In addition, some reports from north India (41%) and Saudi Arabia (27.2%) showed that Aspergillus was the most commonly identified organism [22, 23]. With regard to Aspergillus, our study showed that it was found only in the PS group. This result can be supported by the study of Tony et al. who had reported that corticosteroids promote the growth of Aspergillus .
We expected that the PS group would have more severe initial clinical characteristics than the NPS group. However, our study found no significant differences in initial clinical characteristics between the two groups except in terms of depth of infiltration. We speculate that this finding may be related to the inflammation-masking effect of previous topical steroids in early clinical characteristics of keratitis. This finding may also make clinical suspicion and early diagnosis of fungal keratitis difficult. Deep infiltration at the initial presentation was higher in the PS group. In a study by Panda et al., it was reported that hyphae are located more vertically in the steroid-used group . Fungi are characterized by penetration into the deep corneal stroma, and vertically located hyphae are more involved in penetration and more virulent . A study by Lixin et al., found that the vertically growing hyphae had higher recurrence rate after lamellar keratoplasty than horizontally growing hyphae . Therefore, it is important to evaluate the detailed characteristics of the lesion and to take detailed history at the initial visit.
In this study, only microbiologically proven fungal keratitis was included, and microbiological evidences of fungus were made through potassium hydroxide smear, culture, PCR, or biopsy. The percentage of identified fungal isolates was higher in the PS group when compared with the NPS. One potential interpretation of this result is that steroid use can promote fungal proliferation thereby enhancing its identification. However, the relationship between the use of prior topical steroids and the positive rate of culture has rarely been reported and further studies are needed to investigate this. Furthermore, this study is not a prospective design and does not include the cases of negative microbiological tests, so there is a limit to evaluation and interpretation.
There was no significant difference in the proportion of most antifungal agents used between the two groups, but topical and systemic voriconazole use was significantly higher in the PS group. In our institute, we have added the use of topical and systemic voriconazole when there is no response to conventional antifungal therapy. The significantly higher use of topical and systemic voriconazole in the PS group indicates that the treatment response was worse than expected in this PS group.
The PS group had significantly higher surgical intervention and treatment failure than that of the NPS group. This is consistent with other studies which have suggested that the prior use of topical steroids in fungal keratitis may contribute to worse outcomes [27, 28]. These results highlight the side effects of prior topical steroid use in the setting of fungal keratitis. Evisceration/enucleation was performed in 13.3% of the overall patients, similar to the proportion reported in a multicenter study in Korea (10.6%) . We expected that there would be higher incidence of evisceration/enucleation in the PS group, but no significant difference was observed between the PS and the NPS group in this study. On the other hand, the proportion of evisceration/enucleation within 1 month was relatively higher in the NPS group than in the PS group (5/7, 71% vs. 2/4, 50%, p = 0.576). Therefore, we performed logistic regression analysis to determine the risk factor of evisceration/enucleation. As a result, univariate analysis revealed that hypopyon was the only significant risk factor of evisceration/enucleation (OR 4.88, 95% CI 1.28-18.56, p = 0.020). Therefore, we speculate that the evisceration/enucleation was more associated with initial clinical severity than the prior topical steroid use.
In this study, significant risk factors for treatment failure were hypopyon and deep infiltration. Prior topical steroid use and previous OSD were significant in univariate logistic regression analysis but their effects were attenuated in multivariate analysis. Hypopyon can be regarded as a marker of inflammation, and the study of Lalitha et al. reported that the presence of hypopyon was a significant predictor of treatment failure . The depth of infiltration can be a factor related to the progression of the lesion. Poor treatment results in deep fungal keratitis are thought to be associated with low corneal penetration of antifungal agents [3, 4]. Therefore, it is important to evaluate these features at the initial visit because fungal keratitis can penetrate deeply into the stroma early. Other studies have reported variable risk factors for treatment failure in fungal keratitis including severe initial clinical characteristics such as large epithelial defect size and prior topical steroid use [30, 31].
The role of steroids on fungal keratitis have been reported variously, include suppression of inflammation and subsequent growth promotion of the fungal genus. Moreover, vertically oriented hyphae are more commonly observed in the eyes of patients who used steroid . Furthermore, steroid use has been associated with decreased response to antifungal agents, and steroid treatment itself is a known risk factor for fungal infection [8, 28, 32]. Also, steroids aggravate infection with severe inflammatory side effects and delay epithelial regeneration [33-36]. Therefore, it should be emphasized that early steroid use is contraindicated when an infection is suspected, and clinicians should be cautious when prescribing steroids for suspected cases of infectious keratitis.
This study has some limitations. First, this study was confined to South Korea, which is temperate climate, and the cases included were from one tertiary hospital. Therefore, the results of this study cannot be generalized. Second, due to the study’s retrospective design, we could not accurately identify the potency and dose of topical steroids for patients referred from their primary eye clinic. Third, only the patients with microbiological evidence of fungal keratitis were enrolled in this study while cases without such evidence were excluded, even if fungal keratitis was highly suspected. Despite such limitations, this study has important clinical value by highlighting the risk and side effects of prior topical steroid use in clinical practice. Moreover, this study is a clinical analysis of fungal keratitis in South Korea, and it is thought that this will be a good reference for various regional differences in fungal keratitis.