Intrastromal antimicrobial injections in microbial keratitis: Current evidence and clinical perspectives

Abstract

Intrastromal antimicrobial injections are an emerging adjunctive treatment for severe, deep, or recalcitrant microbial keratitis when conventional topical or systemic therapies are insufficient. By delivering high local concentrations of antimicrobial agents directly into the corneal stroma, this approach may overcome limitations in tissue penetration and, in selected cases, delay or avoid the need for keratoplasty. Current evidence supporting intrastromal therapy is strongest in fungal keratitis, where it is increasingly used in deep stromal disease. In contrast, evidence for bacterial keratitis remains limited and is derived largely from case reports and small case series, typically in refractory or biofilm-associated infections. This review synthesizes the available literature on intrastromal antimicrobial injections, including indications, technique, efficacy, safety, and dosing considerations. A targeted literature review was performed using PubMed, Scopus, and Google Scholar. While intrastromal therapy shows promise, outcomes vary according to organism, antimicrobial selection, and study design. Further high-quality studies are needed to define optimal protocols, comparative efficacy, and long-term safety.

INTRODUCTION

Microbial keratitis is a vision-threatening ocular emergency that requires prompt and effective antimicrobial therapy to prevent corneal perforation and permanent vision loss. Topical antimicrobial therapy remains the standard of care, particularly for bacterial keratitis. However, treatment failure may occur because of inadequate drug penetration into the deep stroma, organism virulence, host factors, or biofilm formation. These limitations have led to growing interest in adjunctive treatment strategies.[1,5,9]

Intrastromal antimicrobial injections have emerged as a targeted approach for delivering high concentrations of drug directly into the site of infection. This technique has drawn particular attention in fungal keratitis, where antifungal penetration into the corneal stroma is often limited.[1,4,5] Although intrastromal therapy is not universally established as standard practice, it is increasingly recognized as a useful adjunct in deep or recalcitrant fungal keratitis.[1,4,5] In bacterial keratitis, its role is less clearly defined and is generally reserved for refractory cases. Infectious crystalline keratopathy represents a distinct clinical entity associated with biofilm formation and relative immune quiescence, in which intrastromal therapy has been reported.[6,7,10]

METHODS

A targeted literature review was conducted using PubMed, Scopus, and Google Scholar to identify relevant studies published between 2010 and 2025. Search terms included “intrastromal injection,” “intracorneal injection,” “fungal keratitis,” “bacterial keratitis,” “amphotericin B,” “voriconazole,” and “antimicrobial therapy.” Studies were screened for relevance to intrastromal antimicrobial therapy in microbial keratitis. Priority was given to randomized controlled trials, comparative studies, and larger case series, although case reports were included, given the limited evidence base, particularly in bacterial keratitis. Studies lacking clinical outcome data or not available in English were excluded. Owing to heterogeneity in study design, organisms, and treatment protocols, findings were synthesized narratively rather than through formal meta-analysis.

PROCEDURE FOR INTRASTROMAL INJECTION

Intrastromal injections are performed under topical anesthesia using strict aseptic technique. A fine-gauge needle, typically 27–30-gauge, is introduced tangentially into the mid-to-deep corneal stroma, usually at the periphery of the infiltrate. Care is taken to avoid full-thickness penetration and minimize the risk of perforation. Antimicrobial agents such as amphotericin B and voriconazole require appropriate dilution before administration, with commonly used concentrations of approximately 5–10 μg/0.1 mL for amphotericin B and 50 μg/0.1 mL for voriconazole.[1,2,3]

In fungal keratitis, a circumferential injection technique is often used, with multiple small-volume injections placed around the lesion to create a ring of stromal drug deposition. Individual injection volumes are typically small, and the total dose is distributed across several sites to optimize stromal coverage while minimizing local toxicity. Injections are generally placed just peripheral to the infiltrate margin. Repeat injections may be considered based on clinical response, and adjunctive topical therapy is typically continued throughout treatment.[1,2,3]

PATIENT SELECTION AND INDICATIONS

Intrastromal antimicrobial injections are most commonly considered in patients with deep stromal involvement, progressive disease despite appropriate topical therapy, or recalcitrant infections. While they may be particularly useful in cases where keratoplasty is not immediately feasible, they are also used in situations in which surgery may be deferred or supplemented. In selected cases, intrastromal therapy is used preoperatively to reduce microbial load or intraoperatively as an adjunct to keratoplasty.[1,4,5]

Contraindications include impending corneal perforation, severe stromal thinning, superficial infections, and viral keratitis. Careful patient selection is essential to balance potential benefits with procedural risks.

EFFECTIVENESS OF INTRASTROMAL INJECTION IN BACTERIAL VERSUS FUNGAL KERATITIS

The role of intrastromal injections differs substantially between bacterial and fungal keratitis. In bacterial keratitis, the available evidence is limited and consists primarily of case reports and small case series. Intrastromal antibiotics such as cefuroxime, moxifloxacin, and tobramycin have been used in refractory infections with variable success.[6,7,10,11] These interventions are most commonly described in the setting of infectious crystalline keratopathy, which behaves differently from typical bacterial keratitis because of its association with biofilm formation and reduced inflammatory response.[6,10] As such, outcomes reported in this context may not be generalizable to broader bacterial keratitis populations. Overall, intrastromal antibiotic injections should be regarded as a rescue strategy rather than a standard therapeutic approach.[7,9,11] Representative bacterial intrastromal injection regimens are summarized in Table 1.

Table 1.

Bacterial intrastromal injection doses

Study	Organism/diagnosis	Drug	Concentration	Volume	Notes/outcomes
Khan et al.[6]	Infectious crystalline keratopathy	Cefuroxime	250 µg/mL	Small aliquots	Clinical resolution reported
Liang and Lee[7]	Recalcitrant bacterial keratitis	Tobramycin	0.3%	0.02 mL	Used as adjunctive therapy
Martinez-Velazquez et al.[10]	Infectious crystalline keratopathy	Cefuroxime + Moxifloxacin	~1 mg/0.1 mL cefuroxime and 0.5 mg/0.1 mL moxifloxacin	0.1 mL	Report included combined intrastromal and intracameral use
Pak et al.[11]	Polymicrobial keratitis	Moxifloxacin	0.5%	0.2 mL	Case report

In contrast, the evidence supporting intrastromal antifungal therapy is more robust, particularly for deep stromal fungal keratitis. Several comparative studies and randomized controlled trials have evaluated intrastromal antifungal agents, most commonly amphotericin B and voriconazole.[2,3,8,15] Additional reports have described combined intrastromal voriconazole and amphotericin B therapy and adjunctive intrastromal voriconazole in deep recalcitrant fungal keratitis.[13,14] These studies suggest that intrastromal therapy may improve clinical resolution and reduce the need for therapeutic keratoplasty in selected cases, although outcomes are influenced by organism type, disease severity, and study design.[2,3,8,15]

Amphotericin B demonstrates strong activity against organisms such as Candida and Aspergillus, whereas voriconazole offers broader antifungal coverage but with variable efficacy depending on the pathogen.[3,4] Importantly, some randomized trials have shown no significant benefit of adjunctive intrastromal voriconazole in filamentous fungal keratitis, with no improvement in visual acuity and no reduction in perforation or keratoplasty rates.[8] Fusarium species often respond better to topical natamycin, and amphotericin B should not be viewed as uniformly superior across all fungal isolates.[3,4,8,15]

SAFETY, OUTCOMES, AND COMPARATIVE ANALYSIS

Intrastromal antimicrobial injections are associated with both procedural and pharmacologic risks. Procedure-related complications include corneal perforation, Descemet membrane detachment, and mechanical stromal injury. Drug-related toxicity varies by agent. Amphotericin B is associated with higher rates of epithelial and stromal toxicity, including necrosis and possible perforation, particularly at higher concentrations or with repeated dosing. Voriconazole is generally better tolerated but may have variable efficacy depending on the infecting organism. Data on antibiotic toxicity in the intrastromal setting remain limited but suggest acceptable tolerability in reported cases.[1,3,8,15]

Disease progression remains a concern, as some infections may continue to worsen despite intrastromal therapy and ultimately require surgical intervention. Although intrastromal injections may reduce the need for keratoplasty in selected patients, they do not replace surgery in advanced or nonresponsive disease.[1,6,7]

RECOMMENDED DOSES AND TOXICITY PROFILES OF DIFFERENT INTRASTROMAL MEDICATIONS

There is currently no standardized dosing protocol for intrastromal antimicrobial injections, and reported regimens vary across studies. Amphotericin B is commonly used at concentrations of 5–10 μg/0.1 mL, while voriconazole is typically administered at approximately 50 μg/0.1 mL. Antibiotic dosing varies according to the agent and clinical context. Toxicity appears to be dose-dependent, particularly for amphotericin B, underscoring the importance of careful preparation and administration.[1,2,3,15] Representative fungal intrastromal injection regimens are summarized in Table 2.

Table 2.

Fungal intrastromal injection doses

Study	Organism	Drug	Concentration	Technique	Notes
Singh et al.[3]	Mixed fungal isolates	Amphotericin B	5–10 µg/0.1 mL	Multiple intrastromal sites	Improved outcomes in comparative study
Singh et al.[3]	Mixed fungal isolates	Voriconazole	50 µg/0.1 mL	Multiple intrastromal sites	Comparator arm
Kalaiselvi et al.[2]	Filamentous fungi	Voriconazole	50 µg/0.1 mL	Five divided intrastromal injections	Case series
Nada et al.[12]	Keratomycosis	Amphotericin B	20–50 µg	Intrastromal with intracameral use in some cases	Retrospective study
Saluja et al.[15]	Recalcitrant fungal keratitis	Natamycin	10 µg/0.1 mL	Experimental water-soluble formulation	Investigational approach
Hu et al.[16]	Severe fungal keratitis	Amphotericin B	5 µg/0.1 mL	Combined intrastromal and intracameral approach	Used in severe cases

Intrastromal natamycin has been investigated using newly developed water-soluble formulations. Although early results are encouraging, this approach remains experimental and is not yet widely available in routine clinical practice.[15]

CONCLUSION

Intrastromal antimicrobial injections represent a valuable adjunctive strategy in the management of microbial keratitis, particularly in deep and recalcitrant fungal infections. Their role in bacterial keratitis is more limited and should be reserved for selected refractory cases. Clinical outcomes vary according to organism type, antimicrobial selection, and study design. While amphotericin B may be preferred in certain fungal infections, its superiority is not universal and should be interpreted in the context of pathogen-specific response patterns.[1,3,4,8,15]

The current evidence base remains limited by small sample sizes, heterogeneity in study design, and variability in treatment protocols. Further high-quality randomized studies are needed to establish standardized guidance for patient selection, dosing, technique, and long-term outcomes.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.