antifungal therapy for keratomycoses

Review

2004 © Ashley Publications Ltd ISSN 1465-6566 865

Ashley Publicationswww.ashley-pub.com

1. Introduction

2. Epidemiology

3. Diagnosis of keratomycoses

4. The fungal organism

5. Pathobiology of keratomycoses

6. Antifungal agents

7. Cost-effectiveness of antifungal

therapies

8. Future directions

9. Expert opinion

Antifungal therapy for keratomycosesNihal Ganegoda & Srinivas K Rao†

†Medical Research Foundation, 18, College Road, Chennai 600006, Tamil Nadu, India

Keratomycoses have recently emerged as an important cause of ocular mor-bidity, especially in third-world countries. Available antifungal agents are lim-ited in their efficacy, due to limited penetration into the cornea, thefungistatic nature and the development of drug resistance. Effective usage ofthe available drugs is hampered by the inefficiency of currently availableantibiotic sensitivity tests for fungal organisms. There is also limited knowl-edge regarding the ideal combination(s) of antifungal agents, includingissues of synergism and antagonism. Despite these problems, recent publica-tions indicate encouraging outcomes in the treatment of a large series offungal keratitis. Advances include better drug formulations, new agents andnovel methods of drug delivery into the eye. As our ability to deal withadvanced fungal keratitis remains limited, the importance of early diagnosishas been stressed and molecular biological techniques may play an importantrole in the future. This article summarises the important new advances inthese areas in the past 2 years and provides guidelines for the managementof these serious corneal infections.

Keywords: antibiotic sensitivity, azoles, chlorhexidine, corticosteroids, culture, ergosterol, filamentous fungi, intracameral injection, keratomycoses, polyene, polymerase chain reaction, smear examination, yeast

Expert Opin. Pharmacother. (2004) 5(4):865-874

1. Introduction

Although keratomycosis is a sight-threatening infection of the cornea that causes sig-nificant morbidity in some parts of the world, the number of antifungal agents avail-able for therapy remains limited. Delay in diagnosis worsens the outcome inkeratomycosis as many of the agents available for topical therapy penetrate the cor-nea poorly. In advanced infections, medical treatment is quite expensive and in∼ 50% of cases, blinding sequelae can result, such as perforation, phthisis and severescarring [1]. Large corneal grafts are often required to rid the eye of infections and inthis situation, there can be serious complications, such as recurrence of infection,rejection and glaucoma, which compromise the surgical outcome. Previous articlesdealing with fungal infections of the eye have stressed the slow progress in the devel-opment of new antifungal agents and have attributed this to the small perceivedmarket for such drugs, which is a disincentive for major pharmaceuticals companiesto invest in this field [2]. It may, therefore, be important to better understand the epi-demiology and clinical presentations of keratomycoses in order to prevent and treatsuch infections effectively. Advances in these areas of keratomycoses in the past2 years are reviewed in this article, which also highlights emerging new therapies andoutlines the possible improvements in the future.

2. Epidemiology

More than 70 species, representing 40 genera of fungi, have been reported as patho-genic to the human cornea [3]. Since the first report of fungal keratitis in 1879 byLeber [4], there has been a steady increase in the number of reported cases, especially

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Antifungal therapy for keratomycoses

866 Expert Opin. Pharmacother. (2004) 5(4)

in the last four decades. At our tertiary care centre in Chen-nai, South India, 69 culture-proven cases of keratomycoseswere seen in 2003 (this constituted 39% of all infectious kera-titis seen that year at our centre). The epidemiology of fungalkeratitis is specific to the climate in the region, and filamen-tous fungi are the predominant pathogens in tropical areas,whereas yeast forms predominate in the temperate regions [5].A recent article describing the epidemiological features of1352 culture-proven cases of fungal keratitis was publishedfrom a tertiary care centre in South India [6]. The analysisindicated that keratomycosis was predominant in young adultmales, with trauma as the major predisposing factor. A higherincidence of infection was noted during the monsoon andwinter. Among the 1360 fungal isolates recovered from a totalof 1354 eyes, filamentous fungi and yeast were the aetiologicalagents in 1351 (99.3%) and 9 (0.7%) eyes, respectively. Ofthe filamentous fungi isolated, Fusarium (37.2%) andAspergillus (30.7%) spp. predominated in the hyaline fungalspectrum, whereas Curvularia spp. (2.8%) were more com-mon among the dematiaceous isolates. A similar spectrum ofpathogens has also been reported in other studies from southIndia [7], Florida [8] and Ghana [9]. In contrast, a study fromthe Wills Eye Hospital, Philadelphia, where there is a temper-ate climate, reported an occurrence of 24 cases in the periodfrom January 1991 to March 1999 [10]. A total of 50% of thecases in this series were caused by yeast, with Candida albicansbeing the most frequently isolated organism (45.8%). Apartfrom the variation in the spectrum of fungal pathogens withchanging climatic patterns, it has also been reported that fila-mentous fungi are often associated with post-traumatic infec-tions, especially with vegetable-contaminated matter, whereasyeast infections occur in eyes with pre-existing ocular surfacedisease [6,10]. Other factors that can predispose to fungal kera-titis include contact lens wear, atopic disease and topical ster-oid use. Recent articles have also documented the occurrenceof keratomycoses after corneal surgical procedures, such aslaser in situ keratomileusis [11], nonpenetrating glaucoma sur-gery [12] and in a compromised corneal surface as in vernalkeratoconjunctivitis [13]. Corneal infections by rare organismscontinue to be reported, including pathogens, such as Beauve-ria bassiana [14], Verticillium spp. [15] and Scedosporiumapiospermum [16]. Unusual presentations of keratomycosesincluding an epicorneal granuloma after penetrating kerato-plasty [17] and advanced Fusarium spp. keratitis progressing toendophthalmitis, have also been described [18].

3. Diagnosis of keratomycoses

Appropriate treatment of fungal keratitis is only possible if anaccurate diagnosis is first made. In the initial stages of micro-bial keratitis, typical clinical features of individual pathogensare often evident and the classic features of keratomycosesinclude a dry corneal infiltrate with an irregular epithelialdefect, the presence of hyphate edges and satellite lesions.There is often a pyramidal, dome-shaped hypopyon but the

eye is relatively quiet and patient symptomatology is often lessthan the clinical picture would suggest. The infection evolveswith an indolent course, as the infection gradually involvesdeeper corneal layers. However, in patients who present laterin the course of disease, these typical features are often miss-ing. A large corneal infiltrate is present which does not haveany characteristic morphology. The overlying epithelium mayoften be intact and in advanced disease, peripheral cornealguttering and corneal vascularisation are also seen. Variationsin presentation include a wet appearance of the ulcer at pres-entation and pigmentation of the infiltrate, both of which areindicative of a rapidly progressive infection, that is oftenresistant to antifungal therapy. Prior use of steroids can alsoalter the presentation and in this milieu, the spread of the fun-gus is believed to change from a superficial to a sagittal orien-tation, resulting in penetration of the Descemet’s membraneand proliferation in the anterior chamber.

3.1 Microbiological investigationsAs there can be variations in clinical presentation, resulting inmisdiagnosis, laboratory investigations form an importantadjunct in the diagnostic armamentarium of the cliniciantreating microbial keratitis. In a large series of fungal keratitis,direct microscopic examination of smears of corneal scrapingsalone gave a clue to the presence of fungal keratitis in 95.4%of 1352 cases with a culture-proven diagnosis [6]. The smearexamination technique included potassium hydroxide (KOH)preparation, Calcofluor white (CFW), Gram and Giemsastains. The positivity of the KOH preparation (91%) was onlyslightly less than that of the CFW preparation (91.4%). Aslarge fungal ulcers do not respond well to treatment, theauthors highlight the need for basic smear examination inmicrobial keratitis. The use of a KOH wet mount preparationand a Gram smear can aid in the distinction between a fungaland bacterial corneal infection in the majority of cases, allow-ing institution of appropriate therapy.

Some fungal corneal infections tend to involve the deeperlayers of the cornea with no associated superficial infection. Inthese cases, routine corneal scrapes are unlikely to provide suit-able material for analysis and anterior chamber tap as a diag-nostic procedure has been described [19]. In patients with alarge anterior chamber hypopyon, evacuation of the materialmay also serve a therapeutic purpose. Such material may besubjected to both smear and culture evaluation. As fungal cul-tures often require incubation for 10 days in order to isolatethe pathogen, there has been interest in more rapid techniquesfor the diagnosis of these infections. The use of a polymerasechain reaction (PCR)-based assay has been described in tworeports of patients with fungal keratitis [20,21]. Gaudio et al. [20]

developed an assay to amplify a portion of the fungal 18Sribosome gene. PCR and fungal culture reports matched in22 of 30 (74%) scrapings from infected corneas [20]. In sevensamples, the PCR assay was positive, although the fungal cul-tures were negative. In one sample, the PCR assay was nega-tive, whereas the culture was positive for fungal organisms.

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Expert Opin. Pharmacother. (2004) 5(4) 867

The authors concluded that PCR is a potentially valuable toolfor diagnosing keratomycoses, although there is scope forrefining the technique. The technique may also have thepotential to distinguish between filamentous fungal and yeastinfections, although further work is required in this area. It isalso suitable in situations where specimens are analysed at asite distant from that of collection. Obvious disadvantagesinclude the cost of setting up and maintaining a facility forsuch diagnosis, inability to distinguish between active and pastinfections (although quantitative PCR techniques may be ableto address this) and the inability to perform antibiotic sensitiv-ity studies. Nevertheless, these initial reports are encouragingand suggest that PCR may eventually complement the current‘gold standard’ diagnostic techniques in the diagnosis of kerat-omycoses. Another method described recently in this context,is the use of confocal microscopy for the in vivo diagnosis ofAspergillus fumigatus keratitis [22]. In a study in rabbits, theauthors experimentally induced A. fumigatus keratitis andcompared the efficacy of confocal microscopy and cultures indetecting the presence of fungal pathogens in the corneas ondays 2, 14, 20 and 22. They found that on days 14 and 22,confocal microscopy was more sensitive than the culture tech-nique, as not all cases of fungal keratitis could be cultured.They concluded that confocal microscopy is a fast, safe andsensitive diagnostic tool in the diagnosis, follow-up and treat-ment monitoring of keratomycoses.

4. The fungal organism

Fungi are a heterogeneous, heterotropic and ubiquitous groupof eukaryotic organisms that require complex organiccompounds for growth. Fungi are part of the normal eyelid

flora in up to 17% of the normal population, whereas theconjunctival sac may contain fungi as transient residents in upto 37% of healthy eyes [23]. The normal eye is protected fromfungal keratitis by the presence of an intact epithelium, theflushing action of eyelid blinking and normal tear flow, anoxygen radical-producing system in the mucin network of thetear film and the secretory antibodies in the tear film [24].Infections with fungi can occur if these defences are compro-mised by factors such as trauma, contact lens use, neurotro-phy, eyelid defects, prolonged use of topical steroids whichlower the ocular immune defence, if there is a compromise insystemic immunity or diabetes mellitus [25]. An increasingnumber of fungal corneal infections have been reported in thepast four decades and may reflect in part the increased clinicalawareness and improved laboratory techniques used, but alsothe widespread use of antibiotics, immunosuppression, chem-otherapy and ocular prosthetic devices [26].

5. Pathobiology of keratomycoses

The natural history of fungal keratitis depends on the out-come of the interaction between agent factors (e.g., virulence,drug resistance and toxicity) and host factors (e.g., predispos-ing factors, inflammatory response and hypersensitivity reac-tions). Antifungal agents attempt to alter the above outcome,in favour of the host by aiding in destruction of the fungalpathogens. In choosing an antifungal agent, it is important tonot only choose a drug with an appropriate antifungal spec-trum but to also ensure that it has favourable pharmacokineticproperties, which allow the drug to achieve suitable therapeu-tic concentrations in the cornea In addition, it is also impor-tant that the drug be introduced sufficiently early in the

Table 1. Dosages of antifungal agents described in the article.

Drug Route of administration

Dosage

PolyenesNatamycinAmphotericin B

TopicalTopicalIntracameral

5% suspension0.15% solution (prepared from powder for intravenous use, using distilled water as a diluent)5 – 10 µg (in 0.1 ml of distilled water or 5% dextrose)

TriazolesFluconazole

Itraconazole

Voriconazole

TopicalOralTopicalOralOral

1% Aqueous solution200 – 400 mg/day2% Suspension100 – 200 mg/day400 mg/day

ImidazolesKetoconazole

Miconazole

Econazole

TopicalOralTopicalIntravenousTopical

1% Suspension (prepared using crushed tablets in artificial tears)200 – 400 mg/day1% Solution (Intravenous preparation used as eye drops)600 – 3600 mg t.i.d.1 – 2% solution

Pyrimidines5-Flucytosine Topical

Oral1 – 1.5% Aqueous solution100 – 150 mg/kg/day



course of the disease and that the patient is compliant withthe therapeutic regimen prescribed. In this context the molec-ular weight of the antifungal agent assumes importance [27].Those with a weight > 500 Da do not penetrate the cornealepithelium well and in this instance, the diffusion is depend-ent on lipid solubility [28].

The fungal cell wall is composed of chitin, chitosan, glu-can, mannan and occasionally cellulose. Knowledge of thestructure of the composition and architecture of the fungalcell wall is important as most antifungal drugs work by affect-ing the integrity of this structure.

6. Antifungal agents

The main classes of antifungal agents in use today are – poly-enes, azoles and 5-flucytosine. The squalene epoxidase inhibi-tors, especially terbinafine, may have a synergistic role withthe azoles, although there is, as yet, little clinical data to sup-port the in vitro studies [29]. The use of the antiseptic chlo-rhexidine in the treatment of fungal keratitis has also beenexplored in a small pilot study in India [30]. In a recent report,the role of topical 0.5% cyclosporin eye drops (twice-daily)was suggested as an option in the treatment of therapeutickeratoplasties for fungal keratitis [31] and this is discussed inmore detail later.

6.1 The polyenesThese are the oldest group of antifungal agents and work bybinding directly to ergosterol, a sterol unique to fungal cellmembranes, thereby disrupting its integrity. Amphotericin Bwas introduced in 1956 and is isolated from Strepto-myces nodosus. The postulated mechanisms of action includethe formation of ion channels in the cell membrane increasingits permeability and the formation of a complex with ergos-terol which results in the removal of the latter from the lipidphase [32]. It may also cause direct oxidative damage to the cellmembrane [33]. It may also have an immunopotentiatingeffect on the humoral and cellular pathways, as seen inmurine studies [34]. In one study, an increased degree of killingwas noted in neutrophils pretreated with a very high concen-tration of amphotericin B [35]. It has a high molecular weight(924.11 Da) and hence, poorly penetrates the corneal epithe-lium. However, removal of the corneal epithelium results inhigh levels of the drug in the corneal stroma [28]. Natamycin,another polyene, was first isolated in 1958 from Strepto-myces natalemsis. Natamycin binds preferentially to ergosterolon the fungal plasma membrane and causes localised mem-brane disruptions by altering membrane permeability. Itexhibits antifungal activity through two mechanisms of action(depending on the concentration). At lower concentrations, itis fungistatic and inhibits sterol-4-α demethylase, a micro-somal P450 related enzyme, whereas at higher concentrations,it is fungicidal causing direct membrane damage to thephospholipids present in the fungal cell wall. Similar toamphotericin B, it requires removal of the epithelium to

achieve therapeutic levels in the cornea, as it has a molecularweight of 665.75 Da. Unlike amphotericin B, it is availablecommercially as a 5% suspension for topical use.

Amphotericin is available as a powder and is reconstitutedwith distilled water for use as a topical eye drop. It is used invarious concentrations (0.05 – 0.25%) with increasing toxicityif higher concentrations are used. Both amphotericin andnatamycin eye drops should be stored protected from light. Aloading dose concept is used in the treatment of fungal kerati-tis, with amphotericin B eye drops administered at 5-min inter-vals for 1 h, whereas natamycin is used every 30 min initially, asthe suspension ensures that it is retained longer in the eye.When used topically, as described above, amphotericin B iswell-tolerated in doses up to 0.15%, except for a stinging sensa-tion when instilled on the eye. In higher concentrations, how-ever, it can result in delayed corneal epithelial healing, stromalhaze, oedema and iritis. Natamycin eye drops have low levels oftoxicity and may cause punctate epithelial keratopathy in somepatients. Neither drug is recommended for subconjunctival useowing to the potential for toxic conjunctival necrosis.

Although amphotericin B can be administered intravit-really in a dose of 5 – 50 µg without apparent retinal toxicity[36], such use is reserved for endophthalmitis. Recently tworeports have described the use of intracameral amphotericin Binjections as safe and efficacious in the management of severekeratomycoses, in small clinical series [37,38]. In one report [37],four patients with clinical keratomycosis (Fusarium spp. intwo, unidentified fungus in one and no culture growth inone) failed medical therapy and hence, were treated withrepeated intracameral injections of amphotericin B 5 µg in5% dextrose 0.1 ml. The infection cleared in three eyes,whereas one eye perforated and had to be eviscerated. In theother report [38], three patients with culture-provenAspergillus flavus keratitis that were unresponsive to medicaltherapy were treated with intracameral injections ofamphotericin B 7.5 – 10 µg in 0.1 ml of distilled water. Therewere no toxic effects noted in the cornea or crystalline lensand the authors conclude that this may be a promising modal-ity for the treatment of advanced keratomycosis that is unre-sponsive to conventional medical therapy.

Systemic therapy with natamycin is not possible and withamphotericin B is associated with significant toxicity. In addi-tion to general side effects such as fever, chills, nausea andvomiting, specific effects such as thrombophlebitis and anae-mia have been described [39]. It is also a nephrotoxic drug andthose with a cumulative dose of amphotericin B > 4 – 5 g areparticularly at risk [40]. Intravenous amphotericin B therapy isseldom used in uncomplicated fungal keratitis and is reservedfor those with endophthalmitis or scleral extensions of theinfection. New lipid formulations of amphotericin B are nowavailable for clinical use. These are amphotericin B lipid com-plex (ABLC), amphotericin B colloidal dispersion andAmBisome®. These preparations have reduced nephrotoxicityand hence, can be used in higher concentrations. The efficacyof ABLC in treating a patient with Fusarium solani keratitis

Ganegoda & Rao


progressing to endophthalmitis was recently reported [41]. Thepatient had previously received intravenous amphotericin B(standard preparation) with no appreciable improvement intheir condition.

The use of amphotericin B with other drugs has been stud-ied to a limited extent. In in vitro studies and in an experi-mental rabbit Candida spp. keratitis model, a synergisticactivity was shown to exist between amphotericin B andrifampin. The alteration in the cell membrane induced byamphotericin B allows rifampin to enter the cell and alterRNA synthesis [42]. A similar synergistic effect has been pro-posed with 5-fluorocytosine (5-FC) [43], although as with theformer example, clinical proof is lacking. Even though theabove interactions are fairly well-defined, at least in vitro, theeffects of combining polyenes with the other major antifungalgroup – the azoles are not quite clear. Studies reveal conflict-ing data suggesting antagonism, no effect or synergism [44,45].Although it is common clinical practice to use a topical poly-ene in combination with systemic ketoconazole, especially insevere keratomycosis, validation of this protocol may need theuse of randomised clinical trials, better animal models ofkeratomycoses or more efficient in vitro systems in order toevaluate such interactions.

Amphotericin B has a fairly wide spectrum of action and isgenerally effective against most strains of C. albicans. It is alsoeffective against the filamentous fungi, although it has limitedactivity against the Fusarium spp. and has no significant activ-ity against Pseudallescheria boydii [46]. It can be fungistatic orfungicidal, depending on the tissue concentration and the sus-ceptibility of the pathogen. Natamycin on the other hand, hasbeen recommended as the drug of choice for the treatment ofFusarium spp.-induced ocular infections [47]. Antifungal resist-ance to the polyenes appears to be relatively rare and is unlikelyto be a significant clinical problem. The last member of thepolyene group, nystatin can be used topically in a suspensionof 100,000 U. It is, however, rarely used clinically owing to theready availability of the more potent agents in this group.

6.2 The azolesAzoles were developed as less toxic alternatives to amphotericinB. There are currently two major types in use – the imidazolesand the more recent addition, the triazoles. The azoles work bybinding to a cytochrome P450 (CYP) fungal enzyme, which isresponsible for the formation of ergosterol. Interference withthis pathway results in the accumulation of 14-methylatedsterols, which increase membrane permeability, inhibit growth,alter membrane enzymes and induce cell death [48]. They havealso been proposed to act by inhibiting cytochrome-C oxida-tive and peroxidative enzymes, and increasing the concentra-tions of peroxidase. Azoles have also been implicated in thecausation of decreased lymphocyte function, with the excep-tion for fluconazole [49]. These actions may affect the efficacyof these drugs in vivo and may promote tissue damage due toinflammation. In the treatment of ocular fungal infections,these agents are considered as being fungistatic.

Ketoconazole is used as an oral preparation (100 mg b.i.d.)and is effective against yeasts and some filamentous fungi suchas P. boydii and F. solani. It has variable activity against mem-bers of the Aspergillus family. A topical preparation is notavailable and it has not been used as an injection peri- orintraocularly. When used orally, hepatotoxicity must be mon-itored. Miconazole is available as a 1% preparation for intra-venous use and has been reported to have a relatively broadantifungal spectrum, mostly effective against the filamentousfungi. In prolonged intravenous treatment however, thepotential for cardiotoxicity must be remembered. The sameintravenous preparation has also been used topically, with var-iable success. A recent study compares the use of 2% econa-zole drops and 5% natamycin suspension in the treatment offilamentous fungal keratitis [50]. In a randomised, clinical trial,112 of 116 patients completed the study. There were no sig-nificant differences in the two arms either at baseline or intreatment success. Oral clotrimazole is used less often.

The triazoles are more popular in the treatment of kerato-mycoses today, owing to their improved pharmacokinetic pro-files. Fluconazole (200 – 400 mg/day) penetrates well into theocular fluids when used orally. Hepatoxicity can occur, as cangastrointestinal disturbances and rare instances of Ste-vens–Johnson syndrome and thrombocytopenia have beendescribed. It can also be used as a 1% solution. As its molecularweight is low (306.30 Da) it can penetrate the corneal epithe-lium when used topically. It is also able to diffuse into the ante-rior chamber owing to its hydrophillic nature [28]. Currentevidence indicates that its main spectrum of action is againstthe Candida spp., although it is now clear that there is poorcorrelation between in vitro and in vivo activity of this drug. Inan Indian study, the authors indicated that topical fluconazolewas ineffective in the treatment of filamentous fungal keratitiswhen used in a clinical trial comparing it to natamycin [51].Itraconazole has a broader spectrum of action, including somefilamentous fungi, such as Aspergillus spp. It is, however, notvery effective in Fusarium spp. infections. It is well absorbedwhen given orally at dosages of 100 – 200 mg/day, particularlywhen administered with a meal. It has a very high affinity fortissues and this may explain why it is as effective as fluconazole,although the latter produces a higher plasma concentration.

6.3 Newer azolesA newer triazole agent, voriconazole, has also been shown tobe effective in the treatment of S. apiospermum keratitis. Thismay be significant because this pathogen is usually resistant tocommonly used topical ophthalmic antifungal agents. In acase report, macrobroth dilution susceptibility testing wasperformed on five isolates of S. apiospermum obtained frompatients with keratomycosis to determine the minimuminhibitory concentrations (MIC) values for amphotericin B,natamycin, ketoconazole, itraconazole and voriconazole [52].Amphotericin B and natamycin were not active against thesefungal strains. Among the azoles, voriconazole was moreactive than ketoconazole for all five isolates and had a lower



MIC than itraconazole for three isolates. Another triazole,currently under investigation is posaconazole (SCH-56592).Posaconazole was used topically at 2-h intervals (oral suspen-sion 100 mg/ml) and 200 mg q.i.d. p.o., in a patient withF. solani keratitis resistant to conventional medical therapy[53]. Despite the fact that these new agents have proved effec-tive in the treatment of these severe ocular fungal infections,both the patients’ reported in these case studies had cornealperforation and required surgical treatment in addition to themedications used. Hence, although these agents are promisingnew options in the treatment of keratomycosis, they needfurther evaluation.

6.4 Flucytosine (5-fluorocytosine)First synthesised as a potential antineoplastic agent, 5-FC wasfound to be effective as an antifungal agent because it is con-verted by fungi into 5-fluorouracil, a toxic antimetabolite.This agent may be used topically at concentrations of1 – 1.5% or at a dosage of 150 mg/kg body weight p.o. every6 h [28]. It shows markedly selective activity against patho-genic yeasts, especially C. albicans and Cryptococcus spp. Oneof the major problems with flucytosine use has been the devel-opment of resistance among many strains of fungi and hence,it is often administered in combination with amphotericin B,with which it has synergistic effects.

6.5 Miscellaneous drugsAlthough chlorhexidine in a topical form was as effective aseconazole in the initial treatment of fungal corneal ulcers in asmall pilot study [30], it has not gained popularity as a first-lineantifungal agent. It also causes considerable toxicity to theocular surface and has been suggested as a treatment option insituations where other agents are not available.

6.6 CorticosteroidsEven though reasonably clear guidelines exist regarding the useof corticosteroids in the treatment of bacterial and viral kerati-tis, there is still considerable controversy concerning their usein keratomycoses. On the one hand, the use of these anti-inflammatory agents may have a role in reducing leukocyte-mediated damage in the cornea Conversely, the use of topicalsteroids is known to predispose to fungal infections of the cor-nea and when fungal infections are treated with steroids, thereis an increased propensity for the infection to progress throughthe deeper corneal layers and penetrate the Descemet’s mem-brane, resulting in fungal proliferation in the anterior cham-ber. As most of the antifungal agents in use today areconsidered fungistatic, interfering with the host defence mayadversely affect the outcome of fungal keratitis. Despite thesecontroversies, most authorities would be reluctant to recom-mend the use of steroids in keratomycoses, and in immuno-compromised patients, their use is contraindicated.

However, a recent experimental study in rabbits, indicatesthat there may be a role for topical steroid use in kerato-mycoses, provided the timing of their introduction is

appropriate [54]. The corneas of 105 rabbits were inoculatedwith C. albicans and topical fluconazole therapy was institutedafter 48 h. Topical steroid therapy with topical prednisolone(5 or 10 times per day; 3, 9 or 15 days after infection) was alsoinstituted. The authors found that eyes treated with pred-nisolone, 9 or 15 days after infection had significantly less cor-neal clouding and vascularisation and there was no differencein the recultivation rate of C. albicans compared to controls.They concluded that the delayed introduction of steroids in thetreatment of keratomycoses was not contraindicated and maybe beneficial. The use of this approach in humans, in a clinicalsituation, however, is yet to be determined.

This poses a dilemma in the treatment of patients undergo-ing therapeutic keratoplasty for fungal keratitis. The use ofsteroids would help control the inflammation following sur-gery and would also protect against graft rejection. However,if the surgical procedure does not extirpate the fungal ele-ments from the cornea, recrudescence of infection is possible.In this context, there has been interest in the role of topicalcyclosporin A as a substitute for corticosteroids. The results ofan in vitro study, in which fungi were grown in the presence ofvarying concentrations of methyl prednisolone, cyclosporin Aand vehicle controls, indicated that cyclosporin A had aninhibitory effect on fungal growth [55]. There were, however,concerns about the in vivo effects, as cyclosporin A could alsoaffect the host defence adversely, thereby potentiating the fun-gal infection. A more recent report describes the use of topical0.5% cyclosporin A twice-daily in three patients who under-went therapeutic penetrating keratoplasty for the treatment ofculture-proven fungal keratitis [31]. Two of the patients main-tained a clear graft, 15 and 42 months postoperatively. In thethird patient, the graft failed secondary to pre-existing ocularsurface disease. The authors conclude that topical 0.5%cyclosporin A may be a useful adjunct in the management oftherapeutic keratoplasties for mycotic keratitis.

6.7 Antibiotic sensitivityAlthough a few options are available for the medical treat-ment of fungal corneal infections, choosing the appropriatedrug for an individual patient has proved difficult over theyears, as the results of antibiotic sensitivity testing for fungiare considered to be of doubtful significance. Furthermore,the results obtained in vitro often correlate poorly with thein vivo effectiveness. A novel drug susceptibility test, Etest®

(AB Biodisk™) is now available. It uses antimicrobial gradi-ent strips for quantitative determination of the susceptibilityor resistance of microorganisms to the drugs being tested.Test strips are available for five antifungal drugs – amphoter-icin B, fluconazole, itraconazole, ketoconazole and 5-FC. Ina case series, the authors tested the drug sensitivities of iso-lates from the eyes of three patients with fungal keratitis [56].In all cases, drug sensitivity demonstrated by the Etestaccorded with clinical efficacy of the drugs. This mayimprove our ability to select the appropriate antifungal agentfor treatment of keratomycoses.

Ganegoda & Rao


6.8 Other optionsIn addition to the newer agents that have been described forthe treatment of mycotic keratitis, the use of techniques toimprove the penetration of existing antifungal agents has alsobeen described in a recent report. In a case report of Paecilo-myces spp. keratitis, the authors used iontophoresis to increasethe topical penetration of miconazole [57]. The authors indicatethat this is a safe adjunct in the treatment of fungal keratitis. Inpatients with thinning or perforation of the cornea secondaryto fungal keratitis, the use of n-butyl cyanoacrylate tissue adhe-sive is a useful modality to maintain the integrity of the cornea,until healing occurs [58]. It may also serve as an option for tec-tonic support in eyes with large corneal lesions, whilst patientswait for therapeutic keratoplasty. Amniotic membrane trans-plantation for the resolution of corneal inflammation intreated keratitis has been described [59]. In this situation theamniotic membrane is postulated to promote wound healingand reduce inflammation because it contains protease inhibi-tors and is able to exclude inflammatory cells. Amniotic mem-brane has also been used as a patch graft to reduce stromalmelting and promote re-epithelialisation in extensive infec-tious scleral and corneo–scleral ulcers [60]. Finally, althoughearlier reports have indicated that lamellar corneal surgery isunlikely to succeed in fungal keratitis, a recent report indicatesthat such a procedure was effective in eradicating the fungalcorneal infection in 92.7% of 55 surgeries [61]. In the other4 eyes, the infection recurred within 2 weeks of the surgery.Although more validation of this approach is required, thisreport is encouraging as the tissue requirements for lamellarkeratoplasty are less exacting than that for penetrating surgery.

6.9 Outcomes of treatmentDespite appropriate therapy, 20 – 25% of fungal keratitiscases require surgery in the form of therapeutic penetratingkeratoplasty [62,63]. In a histological study of 167 corneal but-tons from 148 patients of microbiologically-diagnosed and-treated cases of mycotic keratitis, factors necessitating earlykeratoplasty were heavy fungal load, deeper penetration of thefungus and insufficient inflammation to combat infection [64].In a series of 368 fungal corneal infections treated at our cen-tre in south India between 1990 and 1999, medical therapywas effective in eradicating the infection in 60.6% of eyes[unpublished data].

7. Cost-effectiveness of antifungal therapies

There have been no detailed cost-effectiveness analyses of fun-gal keratitis in the recent past. This is probably because suchanalyses are often performed in westernised countries wherekeratomycoses do not form an important segment of eye dis-eases. In the developing world, where the load of fungal kera-titis is more extensive, such analyses are often not performed.The drugs commonly available in these countries includenatamycin and amphotericin B among the polyenes and keto-conazole, fluconazole and itraconazole among the azoles.

Flucytosine is not commonly available. Although chlorhexi-dine has been suggested as an option, it is not routinely used.

As fungal keratitis often requires prolonged therapy, analy-sis of cost of therapy should also include the number of visitsthat the patient makes to the clinic, the cost of such travel aspatients in third world countries often travel long distances toreach medical care, the time spent by the ophthalmologist andthe cost of surgery and convalescence should such be required.Ultimately, the economic impact of keratomycoses shouldalso be computed in terms of the loss of work hours that thedisease causes.

8. Future directions

In addition to the newer agents, such as voriconazole andposaconazole, better formulations like amphotericin-lipidcomplexes have improved our ability to treat keratomycoses.Hopefully other strategies that are attempting to target othercomponents of the fungal cell wall interfere with the biosyn-thesis of amino acids in fungi and a better understanding ofthe molecular basis of fungal virulence will open up new ave-nues for the treatment of keratomycoses.

9. Expert opinion

Fungal keratitis is an important public health problem in thedeveloping countries, more so as it often affects young adultmales and can result in visual loss. In such situations, it isimportant that the primary care physician is familiar with thefeatures of early microbial keratitis. Once the presence of a cor-neal infection is recognised, it is important that a distinction ismade between bacterial and fungal keratitis. Although this ispossible if the clinician is experienced in the treatment of cor-neal infections, in other situations, it may be important to per-form basic microbiological investigations. The use of a KOHwet mount and Gram stain can be very effective in making thisdistinction. This would also help distinguish between filamen-tous and yeast infections which is important, as the choice ofantifungal agent may depend on this. In early fungal keratitis,natamycin for filamentous fungi and fluconazole for yeast maybe appropriate topical therapy. Repeated debridement of theepithelium will help in penetration of these agents in the cor-nea In advanced keratitis, oral azoles, such as itraconazole forfilamentous infections and fluconazole for yeast infections canbe tried. Intracameral amphotericin B may be a promising newmodality for advanced infections. When patients present latein the course of the disease and routine scrapes are negative,corneal biopsies and the use of techniques, such as PCR, canprovide a clue to the diagnosis. In patients with fulminantinfections and stromal inflammation, the use of amnioticpatch grafts and n-butyl cyanoacrylate glue may help resolvethe inflammation and promote healing, in conjunction withantifungals. If medical therapy fails to control the infection,recourse to surgery can help save the eye and visual function.As fungi which are recalcitrant to medical therapy often tend



to progress to the peripheral cornea fairly rapidly, early identifi-cation of such non-responders and timely surgical treatmentmay help improve the eye salvage rate in such infections.Although lamellar keratoplasty may be appropriate in selectedcases, the gold standard treatment is still penetrating kerato-plasty. Maintaining the clarity of the graft in the postoperativeperiod may be possible with the use of topical cyclosporin Aeye drops. Based on current evidence, there does not appear tobe a role for corticosteroids in the treatment of keratomycoses.

In conclusion, appropriate treatment of fungal keratitisrequires prompt diagnosis, early institution of appropriate

therapy, ensuring patient compliance with treatment, closemonitoring of clinical progress with appropriate interventionsuch as epithelial debridement, and using surgical proceduresto maintain the tectonic integrity of the cornea until medicaltreatment is completed. Corneal transplantation in the posttreatment phase can be performed to restore visual function.

Acknowledgements

We would like to thank the Vision Research Foundation,Chennai, Tamil Nadu, India for their financial support.

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AffiliationNihal Ganegoda DO, MD1,2 & Srinivas K Rao DO, DNB, FRCSEd†1

†Author for correspondence1Medical Research Foundation, 18, College Road, Chennai 600006, Tamil Nadu, IndiaTel: +91 44 28271616;Fax: +91 44 28254180;E-mail: [email protected] Eye Hospital, Colombo, Sri Lanka

antifungal therapy for keratomycoses

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