Fungal keratitis

MOTS CLÉS Kératite ; Mycose ; Fongique ; Levure ; Filamenteux ; Antimycotique Résumé Les kératites fongiques (ou kératomycoses) représentent en Europe une cause rare, mais souvent grave, d’infection cornéenne. Cependant, leur incidence semble constante, en rapport avec l’usage intensif des corticoïdes, des immunosuppresseurs et des lentilles de contact. Les champignons responsables sont souvent opportunistes et envahissent des cornées pathologiques mais des kératomycoses peuvent également survenir sur cornées normales après traumatisme le plus souvent par un végétal. Le mauvais pronostic de ces infections est dû à la virulence des champignons qui infectent souvent des cornées déjà pathologiques mais aussi à des retards diagnostiques. Néanmoins, les nouveaux traitements antifongiques apparus ces dernières années ont contribué à améliorer sensiblement la prise en charge et le pronostic des kératomycoses. © 2011 Elsevier Masson SAS. Tous droits réservés.


Introduction
The significance of corneal pathology as a major cause of blindness in the world remains second only to cataract 1,2 .The epidemiology of corneal disease is complicated, variable across the world, and dependent heavily on socio-economic factors, eye care services and availability of donor cornea 1,2,3,4,5 .India has 2 million corneal blind, as much as a quarter of total corneal blindness in the world.However, the Andhra Pradesh Eye Disease Study [APEDS], the largest ever population based study in India found that 0.12% of the population is corneally blind, which if extrapolated to the entire country gives a mind boggling figure of 6.5 million 7 .Corneal infection, often following trivial trauma is a major cause of corneal blindness 1,5,6 .In India and South East Asia,fungal infection accounts for 40-50% of microbial keratitis, unlike the Western world 1,6,7,8,9 .The lack of potent fungicidal agents and poor ocular penetration of existing antifungal agents, together with crunch of resources result in significant ocular morbidity and often,permanent visual loss.
Fungi are eukaryotic organisms, present almost everywhere on earth from deep sea to desert.They are genetically diverse and comprise about 1.5 million species.Like bacteria they are saprophytic and decompose organic matter.But unlike bacteria they have nuclei and other cell organelles and are closely related to animals.Fungi can reproduce asexually, by spore formation, budding, fission and fragmentation, but most commonly by the former.Most species can reproduce sexually also.The versatile mode of reproduction is also responsible for their vast occurrence 16 .
Williamson et al 12 isolated fungi from 2.9% of 1106 healthy conjunctival sacs, a higher incidence observed in older age groups, but could not recover the same organisms on repeat culture after few weeks suggesting that fungi are transient aerial contaminants.Ando N et al 13 also observed similar findings and isolated fungi from 6.6% of healthy conjunctiva, more so in diseased eyes.

Classification
An overwhelming number of fungi have been reported to cause damage to the eye, however a small number of fungi have been repeatedly isolated in ophthalmic mycoses.Thomas et al who has done extensive work on fungi causing ocular infectionclassified into 4 categories [1]  Hyaline filamentous fungi [2]Dematiaceous fungi [3] Yeasts and zygomycetes [4] Thermally dimorphic fungi [5] Fungi of uncertain classification.

[5] Of uncertain classification
Pythium.insidiosum-resembles zygomycetes; causes severe keratitis& orbital cellulitis Rhinosporidium -R.seeberi -not associated with keratitis Pneumocystis -P.carinii-not associated with keratitis Filamentous fungi are the predominant etiological agents throughout the world.The common organisms in the order of frequency in Asia and Africa are Fusarium, Aspergillus and Dematiaceous fungi.Candida is a rare fungal pathogen in India [0.7%] 8,14 .

Risk factors for keratitis
The local predisposing factors are [1] trauma, especially with vegetable matter [2] contact lens wear [3] topical steroid use [4] use of traditional eye remedies and [5] ocular surface disorders-dry eye, bullouskeratopathy and lagophthalmos.The systemic predisposing factors are [1] diabetes mellitus and [2] immunocompromised status.
Fungal keratitis tend to occur more frequently in young males, more so in winter and monsoon.Gopinathan U et al 8 in the largest series of fungal keratitis [1354 cases] found that trauma was a predominant risk factor, seen in 54.4% of cases.Males (962) were affected significantly more (p< 0.0001) than females and 853 (64.4%) were in the younger age group.
Bharati et al 14 in a similar series found higher incidence of corneal trauma [92%,1009 cases] and the correlation between trauma and fungal keratitis was highly significant (P<0.0001).671 (61.28%) patients had corneal injury with vegetative matter and this correlation was highly significant (P<0.0001).73 (6.67%) patients had co-existing ocular diseases responsible for development of fungal keratitis.Use of corticosteroids were found in 1.12% and systemic predisposing factors in 16 % of cases.Traditional eye medicines, often comprising of dried leaves or other vegetable matter dispensed in urine, saliva or breast milk offer excellent opportunity to the pathogenic organism in an eye which already has a breach in epithelial integrity.In Tanzania, as much as 25% of corneal ulcers are related to use of traditional eye medicines 1 .
It is interesting to note that keratitis due to filamentous fungi usually occur in young healthy males with no apparent predisposing factor other than trauma.Candida infection, in contrast appears more in predisposed eyes [eg-in dry eye, lagophthalmos, eyes with a pre-existing epithelial defect like HSV keratitis or contact lens wear or systemic conditions like diabetes mellitus orimmunocompromised status.

Clinical features
Typically symptoms are less compared to bacterial keratitis.Clinical picture, however depends on the type of fungus, virulence of the particular strain and host factors 14 .
Common findings in fungal keratitis are l Dry raised greyish white infiltrate l Feathery or hyphate margins l Satellite lesions l Pigmentation -only for dematiaceous fungi l Gritty feel on scraping l Convex cheesy hypopyon l Endothelial exudates with fibrinous extension The first three are characteristic features of fungal keratitis.
Bharati et al 14 in their series of 1095 culture proven cases of fungal keratitis from Tirunelveli, comprising predominantly of rural population, found dry thick raised infiltrate in 75% and hyphate edges in 72%, but satellite lesions only in 10% of fungal keratitiscases.Early fungal keratitis resembled dendritickeratitis in 4% of cases.Endothelial exudate often called posterior corneal abscess is an infrequent presenting feature [1%] of fungal keratitis.The sensitivity of clinical diagnosis by a trained ophthalmologist in this retrospective series was 94 %.
Though most cases of fungal keratitis exhibit these basic features, some etiological agent may show some unique characteristics.Fusariumsolani can cause severe keratitis with deep extension, endothelial exudates filling up the entire anterior chamber resulting in a fungal mass involving cornea, iris and angles.Malignant glaucoma and endophthalmitis can supervene in such cases and can result in loss of the eye 16 .In contrast, certain dematiaceous fungi [Curvalaria spp, Bipolaris spp] presents as a persistent, lowgrade, smolderingkeratitis with pigmentation on the surface of plaque on the cornea.Simple debridement may suffice for resolution in most cases.However, Lecytophora, another Anil Radhakrishnan -Fungal Keratitis Kerala Journal of Ophthalmology dematiaceous fungus results in severe keratitis unresponsive to medical therapy.The clinical picture of Candida spp resembles bacterial keratitiswith a discrete infiltrate and slow progressionusually in eyes with a pre-existing disease 14,16,17,19 .
Diagnosis-Though clinical presentation can be suggestive, treatment must be initiated after microbiological study.WHO guidelines [for South-East Asia region] 22 , advocate corneal scraping to be done for all cases of microbial keratitis presenting to an ophthalmologist and sent for at least 10% KOH preparation .The treatment is based on the presence or absence of fungal hyphae in smear.
Whenever possible, corneal scraping should be sent for [1]  KOH preparation [2] Gram's stain [3] Cultures -on blood agar and SDA.Corneal scraping is done with a No.15 blade or Kimura's spatula.If the infiltrate is deep seated corneal biopsy may be required to procure the material.If endothelial exudate is present, anterior chamber tap may be taken aseptically and sent for microbiological study.
Direct microscopic evaluation-is a very useful, inexpensive and a rapid diagnostic tool for the detection of fungal filaments in corneal scraping.10% KOH mount has a sensitivity of 90 -99% in well-experienced hands 8, 14.Gram's stain can identify fungi in 45 to 89% of cases, Giemsa stain in 66%,Methenamine silver staining in 89% and Calcoflour white staining in 90% 14,18 .
Fungal culture -Culture positivity for fungi in blood agar or Saboraud's dextrose agar ranges from 52 to 68% and growth occurs usually within 48 -72 hours.
Polymerase Chain Reaction [PCR]is emerging as a rapid, highly sensitive test for the diagnosis of fungal keratitis.Primer used commonly is 28S rRNAgene, common to all medically important fungi.It has a sensitivity of 70 -89% and a specificity of 57 -88% 18,20,21 .
Confocal microscopy -has a definite role in the early diagnosis and follow-up of fungal keratitis in a tertiary care institute, more so being a non-invasive test 10 .Fungal filaments are seen as interlocking white lines approximately 6 microns wide and 200 -400 microns long mostly arranged parallel to the corneal surface.Vadavalli PK et al 11

Management
The guidelines for management as suggested by WHO 22 for the South East Asian Region are as follows.
At the secondary level [patient seen by an ophthalmologist] -If the 10% KOH mount is positive for fungus, treatment is started with 5% Natamycin eye drops or 0.15% Amphotericin B hourly and followed up every 2 days.Antifungal drops are continued 3 hourly for 2 weeksafter healing of the ulcer.
If there is no improvement in 7 days, the case should be referred to a tertiary care centre.Immediate referral to tertiary care [at the first visit itself] is recommended if [1] one eyed [2]  child [3] impending/actual perforation [4] fungal ulcer, but KOH or fungal stain not available.
If the smear is negative for fungus, it is treated as a bacterial keratits with a combination of fortified Cefazolin [5%] and Gentamycin [1.4%] hourly.

At the tertiary care level
In a tertiary care centre it is recommended that a detailed proformaisfilled and photographic documentation done.If photographic equipment is not available, a detailed corneal diagram is made.
Corneal scrapings are taken and sent for [1] KOH preparation [2] Grams stain [3] Cultures in blood agar, SDA and if  possible Brain Heart Infusion agar.5% Natamycin or 0.15% Amphotericin B eye drops are started one hourly for all patients with fungal keratitis.Treatment modification is done based on response to initial treatment and microbiology.
In-patient care is recommended [1] if there is immediate threat to vision [2] if patient is a child [3] to ensure hourly treatment.

Natamycin Nystatin
The polyeneantimycotics bind with sterols in the fungal cell membrane, principally ergosterol.This changes the transition temperature (Tg) of the cell membrane, thereby placing the membrane in a less fluid, more crystalline state.As a result, the cell's contents leak and the cell dies.Animal cells contain cholesterol instead of ergosterol and so they are much less susceptible.
Natamycin -5% Natamycinsuspension is the drug of choice in any fungalkeratitis.It is effective only when applied locally.Due to poor corneal penetration it may be ineffective in deep stromal lesions.

• Terbinafine • Naftifine
They inhibit ergosterol synthesis and are active against dermatophytes and candida.
[4] Echinocandins [3]Anterior chamber wash -Infrequently while treating keratitis, one comes across some patients with a nonprogressing corneal infiltrate but with increasing endothelial exudate.Mechanical removal of endothelial exudates by anterior chamber wash followed by intracameralinjection of 0.1 ml of 0.15% Amphotericin B is seen to be effective in controlling the infection in this subset of patients 23,24,25 .
It is likely that, in such cases, there is good response to topical antifungal [natamycin] treatment, butprogression of endothelial exudateoccurs as adequate anti-fungal concentration was not achieved in the anterior chamber.
Conclusion -Fungal keratitis accounts for a substantial load of corneal blindness in a country like ours dependent heavily on agricultural income.Preventive need to be directed at the villagers and socioeconomically deprived, who are the usual hapless sufferers.Medical management of fungalkeratitis is expensive, time-consuming and only partially effective in limiting visual loss.Although newer drugs like voriconazole show promise, there is need for more efficacious drugs, which work well against filamentous fungi especially Fusarium spp.As fungal keratitis is too miniscule a problem in the developed world, the initiative needs to come from countries like India.

Figure- 1 Figure 2 -
Figure-1 Dry raised infiltrate with hyphate borders in a prospective double masked controlled trial observed high sensitivity [88.3%] and specificity of 91.1%in the diagnosis of fungal keratitis.

3] Yeasts and Zygomycetes
and lens should be peeled off and washed with Amphotericin-B[0.15%] to minimize the chances of recurrent infection.