Aspergillus Keratitis

The cornea must be perfectly transparent to allow an individual to visualize his/her environment. Keratitis, an inflammation of the cornea that frequently arises due to infection, is a threat to corneal transparency. Species of Aspergillus may cause keratitis. The first reported case of Aspergillus keratitis (which is also believed to be the first reported case of fungal keratitis) involved a farmer who was struck in the eye by oat chaff with a resultant keratomycosis (Leber, 1879). By 1970, 118 cases of direct infection of the eye by Aspergillus species had been reported (Jones et al, 1974). In a review of data from 40 studies on microbial keratitis from 1980 to 2001, fungi were found to account for 0 to 58% of the isolates; Aspergillus species were the commonest fungal isolates in 17 of these studies and the second most common fungal isolate (after Fusarium species [8 studies] and Candida species [2 studies]) in another 10 studies (Leck et al, 2002). Thus Aspergillus keratitis is an important ophthalmological problem worldwide, but especially in outdoor workers in agricultural communities in the developing world and in tropical and subtropical areas, where it frequently occurs following traumatic inoculation of Aspergillus conidia into the cornea through injury.

 

In recent years, Aspergillus keratitis has been reported to occur following ocular surgical procedures, such as radial keratotomy, laser-assisted in situ keratomileusis and keratoplasty (Thomas, 2003). Other factors may also predispose to the infection. Aspergillus keratitis is a medical emergency, since the patient frequently presents with extreme pain and loss of vision, and needs to be recognized and treated promptly. A presumptive diagnosis ofAspergillus keratitis can be established by a careful clinical examination and by direct microscopic examination of stained smears of corneal scrape material; Aspergillus hyphae in corneal material need to be distinguished from hyphae of other hyaline filamentous fungi such as species of Fusarium and Acremonium (Aspergillus hyphae may exhibit dichotomous branching whereas the other hyaline filamentous fungi usually do not) and hyphae of phaeohyphomycetes such as Curvularia (the hyphae of phaeohyphomycetes exhibit brown coloration). The diagnosis can be confirmed by culture of corneal scrape or corneal biopsy material. Other, more rapid methods of diagnosis, such as confocal microscopy, in which fungal hyphae can be directly visualised, and molecular methods, have also been described. Of the 118 cases of Aspergillus ocular infection reported up to 1970, 73 were due to A. fumigatus, making this organism the commonest cause of Aspergillus keratomycosis at that point of time (Jones et al., 1974). In recent years, other species , particularly Aspergillus flavus, have been reported to predominate in Aspergillus keratitis. Of 76 Aspergillus isolates from keratitis in India, 59 were A. flavus, 15 A. fumigatus, and one each were A. niger andAspergillus sp.; of 19 Aspergillus isolates from keratitis in Ghana, 9 were A. flavus, 7 A. fumigatus and one each were A. niger, A. nidulans and Aspergillus species (Leck et al, 2002). Of 7 Aspergillus isolates from corneal ulceration in elderly Indian patients, 4 were A. flavus and one each were A. terreus, A. niger and Aspergillus species (Kunimoto et al, 2000). Bacterial co-infection (mixed infection) has been reported to occur in 5 % to 25% of patients with fungal keratitis, although a recent study suggested that bacterial co-infection occurred 3.16 times more frequently with yeast keratitis than with filamentous fungal keratitis (Pate et al, 2006).

Non-specific treatments

Cycloplegics are usually used to relieve the iridocyclitis (anterior uveitis) which typically accompanies fungal keratitis. Broad-spectrum antibacterials are usually given empirically before the diagnosis is established and continued to prevent secondary bacterial infection.

Certain antiseptics, such as chlorhexidine and silver sulfadiazine, have been tried for the therapy of Aspergillus keratitis. In a randomised trial of 0.2% chlorhexidine gluconate and 2.5% natamycin for fungal keratitis in Bangladesh, only one of 7 patients with Aspergilluskeratitis responded to primary treatment with chlorhexidine (the corneal ulcer healed at 10 days); marginally better results were obtained when chlorhexidine was used to treatFusarium keratitis (Rahman et al, 1998). In India, 66.6 % of patients with Aspergillus keratitis were found to respond to topical 1% silver sulphadiazine as primary treatment, in comparison to 60% of patients who responded to primary topical miconazole treatment; the 40% of patients who did not respond to primary miconazole therapy responded to subsequent therapy with topical silver sulphadiazine (Mohan et al, 1988). However, additional data on the efficacy of topical silver sulphadiazine for fungal keratitis have not been forthcoming.

Antifungal therapy

Natamycin (Pimaricin) was the first antifungal specifically developed for topical ophthalmic use. A 5% topical preparation is used as primary therapy for fungal keratitis, given hourly. Response to natamycin was noted in six (54.5%) of 11 patients receiving natamycin (compared with (50%) of 10 patients receiving itraconazole 1% ophthmalmic solution) (Kalavathy et al, 2005). In another study, 89 patients with culture-proven fungal keratitis received topical natamycin alone while another 19 patients received oral ketoconazole and topical natamycin (Lalitha et al, 2006). In the group of 89 patients, treatment success was achieved in 50 (56%), slow healing of the keratitis occurred in 19 (21%) while there was failure of primary treatment in 20 (23%); corresponding figures in the group of 19 patients were 2 (10%), 6 (32%) and 11 (58%). Aspergillus keratitis was diagnosed in 28 patients; treatment success was achieved in 4 (14%), slow healing of the keratitis occurred in 6 (22%) and there was failure of primary treatment in 18 (64%); corresponding figures in 28 patients with Fusarium keratitis were 12 (43%), 10 (36%) and 6 (21%), respectively, and corresponding figures in 24 patients with keratitis due to other filamentous fungi were 13 (54%), 5 (21%) and 6 (25%). Unfortunately, no data were provided about the specific antifungals received by the patients with Aspergillus keratitis (Lalitha et al, 2006).

Amphotericin B eye drops (0.15% (w/v)) prepared from the intravenous preparation (Fungizoneadministration of 2 doses of voriconazole (400 mg each dose, at an interval of 12 hours), concentrations of 2.13 mg/L have been noted in the plasma, 1.13 mg/L in the aqueous and 0.81 mg/L in the vitreous (Hariprasad et al, 2004). Voriconazole given intravenously (4 mg/kg twice daily) and topically led to concentrations of 3.2 mg/L in the anterior chamber and 2 mg/L in the plasma (Klont et al, 2005).

Surgical intervention

Data from 13 studies indicate that over 80% of patients with superficial keratitis due toAspergillus spp. responded to medical therapy with a variety of topical or systemic antifungals (Thomas, 2003). However, fungal keratitis is associated with a five-to-six fold higher risk of subsequent perforation than bacterial keratitis (Wong et al, 1997). Surgery appears to be advantageous in the presence of deep corneal lesions as almost 60% of patients did not respond to medical therapy alone, particularly if natamycin was not used. More recently, a prospective, nonrandomized, interventional, comparative study on 115 consecutive patients determined that the presence of an ulcer that exceeded 14 mm with hypopyon and infection with Aspergillus were predictors of a poor outcome, even if natamycin 5% treatment was used (Lalitha et al, 2006). Imminent perforation or descemetocele formation are clear indications for surgery. Surgical procedures used range from simple removal of the corneal epithelium (debridement) through anterior lamellar keratectomy, creation of conjunctival flaps and use of tissue adhesives to therapeutic penetrating keratoplasty. The objective of surgery is to augment medical therapy by removal of infected corneal tissue, increasing drug penetration when conjunctival flaps are formed, by stabilising the corneal epithelial surface itself or by providing tectonic support to the entire eye when its integrity is threatened by thinning or perforation of the cornea (Agrawal et al, 1994).

Debridement

Debridement is usually performed under topical anaesthesia, with a Bard-Parker blade No.15, ensuring that a margin of 1 to 2 mm. is left at the limbus (Agrawal et al, 1994). Regular debridement of necrotic material from the base of the corneal ulcer probably aids elimination of fungi. In a model of deep stromal Candida albicans keratitis in rabbits, a significant reduction in the number of fungal colony forming units occurred when daily debridement of the corneal epithelium and topical administration of amphotericin B (0.075% or 0.15%) or natamycin (5%) was performed; when the epithelium was left intact, this antifungal effect was much reduced (O'Day et al, 1984). It is not known whether a similar effect occurs in Aspergillus keratitis, but is likely given data from other localizations of aspergillosis.

The results of radiolabelling studies suggest that removal of the corneal epithelium by debridement facilitates corneal penetration of natamycin. Thirteen topical applications of natamycin (one every 5 min.) resulted in a drug concentration of approximately 2.5 mg/g of cornea in rabbit corneas debrided of epithelium, with levels peaking at approximately 10 minutes after administration. Far lower levels (7.0 μg/g) were attained in corneas where the epithelium was left intact (O'Day et al, 1986). It is unclear whether these levels are actually achieved during therapy of clinical fungal keratitis.

Lamellar keratectomy / keratoplasty

Lamellar keratectomy or keratoplasty helps to remove fungal hyphae on the surface of the cornea and facilitate visual improvement (Lin et al, 2005; Xie et al, 2008). In this procedure the superficial 50% of the corneal thickness is removed and replaced by a 50% thickness donor graft. The corneal tissue used in this procedure is more easily obtained than the healthy tissue used in therapeutic penetrating keratoplasty (Xie et al, 2002). If the procedure is done under good antifungal coverage, this procedure can provide useful vision with few complications In one study, six patients with culture-proven Aspergillus keratitis who had not responded to topical and oral antifungal medication underwent lamellar keratoplasty followed by topical antifungal treatment for two weeks with gradual tapering of the drugs (Xie et al, 2002). The visual results were good in five of the six patients, with recurrence of infection in one patient (Xie et al, 2002). Unrecognised or under-treated Aspergillus keratitis managed with lamellar keratoplasty was found to be one of the major risk factors for treatment failure (Xie et al, 2008).

Laser ablation

Use of the excimer laser for superficial stromal corneal infiltrates (involving less than half the corneal thickness) may also yield good results in Aspergillus keratitis. Recently, phototherapeutic keratectomy with a 193-nm excimer laser was successfully used to eradicate the infiltrates and facilitate antifungal therapy in a patient with culture-provenAspergillus keratitis (Lin et al, 2005).

Conjunctival flap

Fashioning a conjunctival flap aids the achievement of a stable conjunctival surface in patients who have persistent or recurrent epithelial defects and progressive ulceration. Such flaps have been found especially beneficial in patients with peripheral fungal corneal ulcers, since the flap will not encroach onto the visual axis and blood vessels present in the flap aid healing of peripheral fungal corneal ulcers (Agrawal et al, 1994). At the same procedure, a superficial lamellar keratectomy should be done initially to remove the necrotic stroma. Then a thin conjunctival flap should be anchored over the ulcerated site. There have been no studies evaluating the specific use of conjunctival flaps in Aspergillus keratitis.

Amniotic membrane transplantation

Recently, permanent or temporary amniotic membrane transplantation has been used as an adjunctive surgical procedure for the management of microbial (including Aspergillus) keratitis by promoting wound healing and reducing inflammation (Kim et al, 2001). Chen et al (2006) applied this procedure in the management of persistent corneal ulcers and perforations in 23 patients with acute fungal keratitis, including three patients with active, culture-provenAspergillus keratitis. Infection was controlled without recurrence of infection in two of the three patients with Aspergillus keratitis. A larger number of patients need to be studied to confirm the utility of amniotic membrane transplantation in management of Aspergilluskeratitis. It is also not clear whether this technique will be useful in patients who have extensive corneal epithelial ulceration and stromal infiltration.

Therapeutic penetrating keratoplasty

Therapeutic penetrating keratoplasty is corneal transplantation using entire thickness corneal grafts. This procedure may be the last resort to preserve or to restore useful vision in severely affected eyes or when there is a poor response to antifungal therapy. A disproportionately high percentage of patients with Aspergillus keratitis, compared to patients with other types of fungal keratitis, ultimately required therapeutic penetrating keratoplasty due to non-responsiveness to medical therapy (Rosa et al, 1994; Vemuganti et al, 2002). Galarreta et al (2007) used this procedure in over 50% of eyes with filamentous fungal keratitis. It is recommended that a margin of at least 0.5 mm all around the infected area should be excised, to reduce the rate of recurrence. Antifungal therapy should certainly be continued post-operatively. Topical corticosteroids should be used with caution in this setting, especially if there is evidence of any persistent infection post-operatively (corticosteroids act as a growth enhancer for A. fumigatus and A. flavus (Ng et al, 1994)). Smaller grafts (8 mm or less in diameter) yield a better outcome than when larger grafts are used. Topical cyclosporin A (5 mg/ml) has been used to prevent allograft rejection in patients with culture-proven fungal keratitis (Xie et al, 2001). Both successful and poor outcomes (Sridhar et al, 2000; Xie et al, 2001; Freda et al, 2006) have been reported following therapeutic penetrating keratoplasty for Aspergillus keratitis. Complications include allograft rejection, especially when the donor graft is greater than 8.5 mm in diameter, refractive errors and other visual problems (Xie et al, 2001).

Keratomycotic malignant glaucoma may be one of the complications of Aspergillus keratitis (Jain et al, 2007). In such patients, therapeutic penetrating keratoplasty with cataract extraction alone might not be sufficient for management of such cases, and a limited pars plana vitrectomy might be needed (Kuriakose and Thomas, 1991).

Anterior chamber tap

In ocular infections confined to the anterior segment, an anterior chamber tap is a useful procedure in the management to clear a persistent infiltrate and thick hypopyon. It also allows the instillation of amphotericin B (5 μg) into the anterior chamber (Sridhar et al, 2002). Strict aseptic conditions are mandatory. If the infection involves the anterior capsule of the lens, care should be taken to avoid injury to the lens.

Duration of therapy

Medical therapy of mycotic keratitis is usually protracted since the effective concentrations achieved by most specific antifungal agents in the cornea, with the possible exception of amphotericin B, only inhibit growth of the fungus, and host defence mechanisms must eradicate the organism. Initially, topical therapy (usually natamycin 5% where available, or amphotericin B 0.15% or topical azole antifungals [econazole 2%, fluconazole 1%, itraconazole 1%] if natamycin and amphotericin B are unavailable) should be given hourly. 'Topical saturation therapy' has also been described (Jones BR, 1979). This intensive topical antifungal therapy is continued until clinical improvement is observed ('rounding up' of the ulcer margin, decreased density of infiltrate, disappearance of satellite lesions, diminished corneal oedema, and resolution of the overlying epithelial defect (Jones DB, 1981). As in other causes of microbial keratitis, the absence of progression implies clinical improvement. As the ulcer resolves, the frequency of antifungal administration is gradually decreased (once in two hours, then once in four hours, then four times per day). Clinical improvement in fungal keratitis occurs slowly and antifungal therapy may be required for up to 2 months (Polack et al, 1971). Oral therapy (ketoconazole 200 mg twice daily; itraconazole 200 mg once daily, more recently, voriconazole) may be given in addition to topical therapy if there are deep lesions in the cornea or if the ulcerated area is very large (> 50 mm2 ).

Philip A. Thomas
Department of Microbiology
Institute of Ophthalmology
Joseph Eye Hospital
Tiruchirapalli
620001 INDIA

December 2008