Expert Lecture: Microbial Keratitis in the Developing World

Dr. Prashant Garg ( Director & Kallam Anji Reddy Chair Ophthalmology, Paul Dubord Chair of Cornea, LVPEI, India )
Dr. Aravind Roy ( LVPEI, India )

Introduction

Microbial keratitis is a potentially blinding disorder in the developing world. It is defined as a breach in the continuity of corneal epithelium with an underlying stromal infiltration, and associated tissue necrosis. The causative factors for microbial keratitis vary by the region. In the developed world, the majority of corneal infections are caused by bacteria, while the preponderance of fungal keratitis is equal to or more than bacterial keratitis in the developing world (Lalitha et al. 2017; Xie et al. 2006). Acanthamoeba infection ranges from 1-3% and is commonly associated with exposure to muddy water or ocular trauma (Gopinathan et al. 2009). Therefore it is important to understand regional variances of the etiological factors. Recent studies on the use of steroids for microbial keratitis and mycotic ulcer treatment trial help shed new information on the management of microbial keratitis. We will be discussing in this article the types of infectious keratitis, patterns of epidemiology, clinical features, methods for diagnosis, referral guidelines and treatment paradigms.

Extent of the problem epidemiology and burden of disease

 Corneal blindness is a significant cause of ocular morbidity in Africa and Asia. In a study from South India the reported incidence of corneal ulceration was 113 per 10000 population (Gonzales et al. 1996). When these estimates are extrapolated to all of India, 840 000 new cases of corneal ulceration are expected to develop annually and the projections approach 1.5 to 2 million for Africa and Asia though the actual numbers may be considerably more (Whitcher, Srinivasan, and Upadhyay 2001). In comparison, the incidence of corneal ulcerations in the developed world range from 11-27.6/100000 in the United States, 3.6-40.3/100000 in the United Kingdom and 6.3/100000 in the developed city of Hong Kong (Ung et al. 2019). Hence the magnitude of this disease is much more severe in the developing world and demands urgent attention of policy makers and treating physicians.

Diagnosis of common causes of microbial keratitis

 A large series of infectious keratitis from L V Prasad Eye Institute, studied between 1991 to 2001, reported outcomes of 5897 cases of corneal ulceration. Bacterial keratitis accounted for 51.9%, fungal keratitis was 38.2%, polymicrobial keratitis 7.5% and Acanthamoeba was 2.4% (Gopinathan et al. 2009). The clinical history is significant in microbial keratitis with ocular trauma having a 5.33 times risk and agriculture related activities 1.33 times risk of developing corneal ulceration (Gopinathan et al. 2009). Although clinical signs such as breach in the continuity of corneal epithelium, underlying corneal stromal infiltration and anterior chamber reaction is suggestive of a underlying infection; clinical history, careful slit lamp examination may provide pointers specific for the particular organisms. The components of a good slit lamp evaluation includes examining the size of the epithelial defect and infiltrate, nature, depth and edges of infiltrate, extent of thinning, appearance of surrounding cornea and anterior chamber reaction.

Classic patterns for Bacterial, fungal, parasitic disease

Corneal ulcers may have a rapid fulminant course or an indolent progressive clinical course.

Fig – 1

Gram positive cocci typically present with a short clinical history, presence of an ulceration with a well defined/distinct margin and minimal surrounding stromal edema and haze (Prashant Garg and Rao 1999). Fig – 1

Fig – 2

Gram negative bacterial keratitis typically heralds more tissue destruction and corneal stromal melting with thinning and rapid progression of the ulceration with perforation. Fig – 2

Fig – 3

Atypical organisms such as Nocardia may present as pin head like corneal infiltrates that are arranged in a wreath like pattern (Sridhar et al. 2001). Fig – 3

Fig – 4

Fungal keratitis Fig – 4 is marked by a dry raised plaque, the margins have a feathery and ill defined margins with satellite lesions and endothelial exudates. The hypopyon is often thick and convex. In a study of severe keratitis Fusarium ulcers were more likely to have feathery margins or a non yellow infiltrate as apposed to Aspergillus which may have a raised surface or a hypopyon (Chidambaram et al. 2018). A dry plaque like lesion with brownish or black pigmentation on the surface is suggestive of dematiaceous fungi, are commonly caused by Curvularia spp Fig – 5. These organisms are a common causes of fungal keratitis in the tropics after Fusarium and Aspergillus (P. Garg et al. 2000).

Fig – 5

Acanthamoeba keratitis can present as ring shaped infiltrate. Pain out of proportion to the lesion and radial keratoneuritis may not be significant for non contact lens associated Acanthamoeba keratitis (Prashant Garg and Rao 1999). Fig – 5

Fig – 6

An insight into Viral keratitis 

Herpes simplex virus(HSV)

Viral keratitis constitutes an important cause of corneal morbidity. Viruses of the herpetic family are primarily responsible for the blinding corneal disorders, especially herpes simplex virus(HSV). Most of the adult population show serologic evidence of herpetic infections by middle age. Primary HSV infection occurs by a direct exposure to infected lesions of the skin, mucosa and genitalia or their secretions. The virus remains latent in the nerve root ganglia; HSV 1 gets sequestered in the trigeminal nerve ganglia, whereas HSV 2 may remain sequestered in the genital ganglia (Guess, Stone, and Chodosh 2007). The clinical manifestations are similar in both. Epithelial keratitis typically presents as punctate, linear, dendritic or geographic ulcerations of the epithelium. The disease is easily diagnosed with the characteristic presentation of dendrites with terminal bulbs that stain with Rose Bengal, the bed stains with fluorescein. Ocular sensations are greatly reduced. Treatment consists of topical antiviral agents such as topical acyclovir 3% five times a day or ganciclovir 0.15% three times a day for two weeks (Colin 2007).

HSV stromal disease is the leading cause of ocular blindness. It can manifest as necrotising or immune stromal keratitis. The necrotising variety is rare and causes rapid corneal stromal lysis with corneal melt. The active virus and immune mediated mechanisms play a role in tissue destruction. Non-necrotising stromal keratitis is the commoner variety, it is immune mediated primarily by CD4 and CD8 T lymphocytes. Stromal keratitis may present as interstitial keratitis or disciform keratitis. Interstitial keratitis presents as stromal edema or haze, without ulceration of the overlying epithelium, repeated episodes may lead to corneal stromal vascularisation. Disciform variety has deep stromal edema and endothelial keratic precipitates localised to the area of edema. The herpetic eye disease study (HEDS) recommended definitive guidelines for the management of herpetic stromal disease. The most definitive evidence is on the role of topical prednisolone 1% in addition to topical trifluridine 1%; this leads to a significant reduction in the severity and progression of HSV stromal keratitis. There was no additional benefit of adding oral acyclovir to this regimen. Prophylaxis with oral acyclovir 400mg twice a day led to 50% reduction of recurrence rates of HSV stromal disease. The protective effect was noted to be as long as oral prophylaxis was continued and was more effective in patients who had previous episodes of recurrence (K. R. Wilhelmus et al. 1994; Kirk R. Wilhelmus et al. 1998; Barron et al. 1994; “Oral Acyclovir for Herpes Simplex Virus Eye Disease: Effect on Prevention of Epithelial Keratitis and Stromal Keratitis. Herpetic Eye Disease Study Group” 2000).

Herpes Zoster Ophthalmicus(HZO)

Herpes zoster ophthalmicus (HZO) is caused by the varicella zoster virus (VZV) which is a neurotrophic virus. It can affect all dorsal sensory ganglia; trigeminal ganglion affliction may lead to ophthalmic manifestations. The disease usually is acquired in childhood as an exanthematous fever-chickenpox, following which the virus is latent in the dorsal sensory ganglia. It reactivates later in life when the natural immunity is reduced or in immunosuppressive diseases such as HIV/AIDS. HZO has a brief prodrome of fever, chills and malaise. There may be dermatomal involvement of the skin of the nose (Hutchison’s sign) skin of the forehead, lid edema and crops of papules and pustules. HZO can affect all the ocular tissues, corneal lesions may typically appear as punctate, stuck on lesions or broad, blunt, pseudo-dendrites that stain poorly with fluorescein and Rose Bengal. The other corneal manifestations include interstitial keratitis with corneal stromal edema, vascularization, lipid keratopathy and gross dimunition of corneal sensations. Cornea is prone to neurotrophic keratitis, dryness, filamentary keratopathy, corneal melt and perforation. Sclerokeratitis with cresentic peripheral corneal involvement may lead to a peripheral ulcerative keratitis like picture. HZO keratouveitis is a severe clinical manifestation involving the anterior chamber and trabecular meshwork. It manifests as an anterior chamber reaction, dirty appearing hypopyon and increase of the intraocular pressure(Liesegang 2008). The long term complications of HZO includes post herpetic neuralgia that is characterized as severe lancinating dermatomal pain. Treatment involves use of oral Acyclovir 800mg 5 times a day for 14 days initiated within 72 hours of manifestation (Cobo et al. 1986). Famciclovir and Valaciclovir may also be used. Topical corticosteroids are used for control of anterior segment inflammation. A slow taper is required as there may be rebound increase in inflammation on stopping corticosteroids. They are often prescribed with cycloplegics and anti glaucoma medications. As the long term complications includes neurotrophic keratitis and sicca syndrome, lubricating eye drops, punctal plugs, tarsorraphy and conjunctival flaps may be necessary. The treatment of post herpetic neuralgia involves treatment in coordination with an internal medicine specialist. The drugs prescribed include antidepressants such as amitriptyline and nortriptyline, pregabalin and gabapentin. Opiods such as oxycodone and capsaicin cream may be used for pain relief (“Herpetic Corneal Infections: Herpes Zoster Ophthalmicus” n.d.).

New organisms- Microsporidia and Pythium

Microsporidia keratitis may present in immunocompetent individuals. It usually presents as a keratoconjunctivitis an stromal keratitis. Keratoconjunctivitis presents as multifocal punctate coarse epithelial lesions. They are usually diagnosed by a corneal scraping and subjecting the 10% KOH wet mount to calcofluor white stains (S. Sharma et al. 2011). Microspsoridia stromal keratitis has a very unusual clinical presentation: it may either present as a diffuse anterior to deep multifocal stromal infiltrates with endothelial exudates without overlying epithelial defect or as a full thickness, monofocal infiltrate with overlying epithelial defect with thinning descemetocele and/or perforation (Sabhapandit et al. 2016).

Pythium is a fungus like microbe that is emerging as cause of non resolving microbial keratitis as it may not be recognized early on in its clinical course due to its similarity with fungal keratitis. The ulcer typically has a tentacle like or dot like corneal infiltrates, peripheral guttering ill defined margins and overlying epithelial defect. Due to its clinical resemblance to fungal keratitis and lack of response to conventional antifungal medications and consistent poor outcomes to surgical management the disease assumes importance (Ramappa et al. 2017).

Laboratory diagnosis

 A specific clinical picture may not be always apparent as the patient may present early or late in the clinical course of the ulcer. There is a chance of prior treatment with multiple medications, herbal or traditional preparations and pretreatment with corticosteroids. Hence relying only on the clinical manifestation may lead to outright mismanagement or unnecessary treatment of microbial keratitis. A study comparing the clinical scoring to guide the treatment of microbial keratitis conducted simultaneously from India and Ghana urged treating ophthalmologists to scrape corneal ulcers and subject to microbiological evaluation where available(Thomas 2005). Microbiology laboratory techniques include corneal scraping and smear examination and inoculation into various media to allow growth of colonies for subsequent identification. The technique of corneal scrapings that we routinely perform includes instillation of a drop of proparacaine 0.5% in the eye followed by using a sterile No 15 Bard Parker blade on a handle to scrape the edges and base of an ulcer. The material is subjected to 10% KOH- Calcofluor white wet mount, Gram’s and Geimsa staining on a glass slide. The material is also inoculated into solid and liquid media in order to facilitate the growth of bacteria, fungi and acanthamoeba. Smear examinations provide a rapid diagnosis of causative organisms. Culture methods involve inoculation into appropriate media to allow growth of relevant organisms for a period of seven days before a negative report is generated (Prashant Garg and Rao 1999). Microbiological cultures are significant if a) growth of same organisms from two or more solid media b) microscopy consistent with confluent growth of same organism from one media c) growth of same organism on repeated scraping (Thomas 2005). Several media are used in the routine identification of causative organisms of ocular suppuration, these include- Sheep blood agar, chocolate agar, saburauds dextrose agar, potato dextrose agar, non nutrient agar with E.Coli overlay, thiolglycolate broth, brain heart infusion broth, phosphate buffered saline and viral transport media (Figure 6). A large study from south India recommended that the detection rate of fungi and acanthamoeba was higher in smears compared to bacteria. They reported the sensitivity of gram’s stain to be 89.8% for fungi and 73.3% for acanthamoeba compared to 56.6% to bacteria. The low sensitivity to bacteria was attributed to prior antibiotic usage and other patient related factors(Gopinathan et al. 2009).

Corneal biopsy is an effective technique to access the deeper layers of the corneal stroma when conventional scraping may not yield sufficient material to subject to microbiology(Lee and Green 1990). A paracentral area of cornea is access to create a flap and tissue from deeper corneal lamella is place in a filter paper for histopathology and inoculated in appropriate media for microbiological diagnosis.

Confocal microscopy is a non invasive diagnostic test for detection of fungal ad acanthamoeba infection in the deeper corneal stroma(Kaufman et al. 2004). The technique has several limitations, besides the cost of the equipment, it is difficult to acquire images in a painful inflamed eye and is dependent on the operator skill and experience(P Garg 2012).

Treatment and referral patterns

Treatment philosophy for the management of microbial keratitis would include performing a through microbiological work up and instituting a specific therapy directed at the causative microorganism. The alternative approach for this would be to direct treatment as per clinical signs and expertise of the treating physician and subject the severe or atypical ulcers to microbiology. This may be a possible way forward for community ophthalmologists who may examine a case of microbial keratitis in the early stage of its evolution. As microbial keratitis is predominantly bacterial it would be recommended to initiate therapy with broad spectrum antibiotics. This would include newer generation fluroquinolones such as Moxifloxacin, Ofloxacin or combination regimens of Fortified cefazolin with fluoroquinolone, or aminoglycosides with fluroquinolones(Prashant Garg and Rao 1999)(Table 1). This would effectively increase the spectrum of the drug against gram positive and gram negative organisms. It will be imperative to follow such cases closely in order to refer them to a cornea specialty service in case of worsening such as increase in size of the infiltrate, thinning or corneal melting, impending or actual perforation. Fungal and acanthamoeba keratitis have a long and chronic course therefore it is recommended to start treatment only upon documented evidence of the causative organism. The WHO recommended referral guidelines for microbial keratitis. This included referring cases that are worsening on empirical treatment, cases that are clinically suspected as fungus where fungi are not demonstrated, one eyed, pediatric cases and cases with impending or actual perforation(M. Srinivasan 2004). (Table 2)

The point counterpoint for the use of corticosteroids in microbial keratitis stems from the fact that corneal injury in the course of a infectious keratitis includes tissue destruction by the invading pathogen and the host inflammatory response. Corticosteroids exert their anti inflammatory effect by decreasing the output of proinflammatory molecules and thereby limiting the host response such as scarring. The concern against usage of corticosteroids is the decreased corneal stromal integrity, corneal melting and perforation. A recent Cochrane review concluded that the role of corticosteroids in managing microbial keratitis is controversial and is best avoided (Herretes, Wang, and Reyes 2014). A large multi centre randomized control trial, steroids for corneal ulcer trial (SCUT), enrolled 500 patients in order to compare the additional role of prednisolone sodium phosphate to bacterial keratitis treated 48 hours prior with topical moxifloxacin. The study did not find significant difference between both the arms in the 3 month best corrected visual acuity, time to reepithelialization, infiltrate/scar size or corneal perforation(Muthiah Srinivasan et al. 2012). At a subsequent 12 month follow up no difference was noted between steroid and antibiotic alone study arms(Muthiah Srinivasan et al. 2014). Non Nocardia keratitis had a mean 1 line improvement in the best corrected visual acuity and Nocardia keratitis had a mean 0.47mm larger scar size at 12 months of follow up. Few well designed smaller trials, Carmichael et al(Carmichael, Gelfand, and Welsh 1990), Blair et al(Blair et al. 2011) also did not observe a significant difference between antibiotics alone versus corticosteroids combined with antibiotics in the final visual acuity, healing and rate of complications in bacterial keratitis.

Fungal keratitis is usually caused by filamentous fungi in the developing world. Natamycin is the first line of treatment in such cases. Natamycin is a polyene fungistatic agent. Pharmacologically the tissue penetration and bioavailability is a concern. Therefore the relatively newer azole, Voriconazole generated considerable interest due to better bioavailability, tissue penetration and in vitro susceptibility. However well conducted trials did not find in vivo superiority of Voriconazole compared to Natamycin. The Mycotic ulcer treatment trial(MUTT) is a randomized multicenter double masked trial that compared the outcomes in 323 subjects to topical 5% Natamycin versus 1% Voriconazole(Prajna et al. 2013). Three month best corrected visual acuity was better in subjects randomized to receive Natamycin. The Voriconazole arm had a significant higher chance of perforation and need for therapeutic keratoplasty. There was no difference in the time to re epithelialization and scar/infiltrate size between the two study arms. A re appraisal of the MUTT Trial did not report clinical superiority of Voriconazole over Natamycin in the treatment of filamentous fungal keratitis(S. Sharma et al. 2015). The MUTT II a prospective randomized clinical trial did not find additional benefit of adding oral Voriconazole in filamentous fungal keratitis. There was no difference in the corneal perforation rates or need for therapeutic penetrating keratoplasty. The microbiologic cure at 6 days, rate of re epithelialization, best corrected visual acuity, infiltrate and scar size at 3 weeks and 3 months were comparable in both arms(Prajna et al. 2016). A Cochrane review on medical interventions in fungal keratitis assessed 12 clinical trials with nine antifungals. The data were underpowered to assess best corrected visual acuity and clinical cure at 2 -3 months, need for penetrating keratoplasty, compliance to treatment, adverse outcomes and quality of life as the outcome measure. The exception was MUTT trial that found conclusive evidence of Natamycin being more effective compared to Voriconazole in the treatment of filamentous fungal keratitis especially in Fusarium keratitis(FlorCruz, Peczon, and Evans 2012).

Anti acanthamoeba agents include frequent instillation of poly hexamethyl biguanide(PHMB)  0.02% and chlorhexidine 0.02%. These need to be compounded by a pharmacy and are potentially toxic to the ocular surface. Additional treatment for the management of suppurative keratitis includes the use of cycloplegics to relieve the ciliary spasm and promote mydriasis to prevent synechia formation. Analgesics are administered orally to control the ocular pain and inflammation. Debridement and debulking of the ulcer may help in decreasing the microbiological load and allow access of the drug to the deeper infiltrates. Small perforations less than <4mm may be attempted to be sealed with n butyl cyanoacrylate glue with bandage contact lens. Penetrating keratoplasty is reserved for large corneal ulcers that are refractory to medical management, threatening the integrity and tectonicity of the globe, impending or actual perforation of the cornea that is not amenable to sealing with a tissue adhesive. The role of amniotic membrane and fibrin glue in the setting of suppurative keratitis is limited though some authorities have used such modalities effectively. Several anecdotal reports have promoted the therapeutic benefits of Intrastromal and intracameral antibiotics. Recent randomized control trials have not found definitive evidence in favour of such interventions (N. Sharma et al. 2013).

Corneal collagen cross linking( CXL) may have therapeutic and palliative roles in intractable infective keratitis  Infective keratitis leads to tissue destruction due to release of matrix metalloproteinases and pro inflammatory cytokines. This causes destruction of the corneal collagen, cell death and tissue necrosis.  While the use corneal collagen cross linking is a validated measure of increasing the tensile strength of the cornea (Wollensak, Spoerl, and Seiler 2003), its use in corneal infection was first published in five patients of keratitis non responsive to systemic and topical antibiotics. The study reported reduction in size of infiltration and halting of the corneal melting(Iseli et al. 2008). Two recent randomized control trials investigated the adjuvant effect of CXL in addition to microbial keratitis, bacterial fungal and parasitic, though the effect ranged from decreased to no effect on the final infiltrate size. The control arm of antimicrobial therapy alone had more perforations in one study. The studies concluded protective effect of adjuvant CXL with photoactivated Riboflavin(Said et al. 2014; Bamdad, Malekhosseini, and Khosravi 2015). The experience from CXL in fungal keratitis has not been encouraging. A recently published large randomized control trial of collagen cross linking assisted infection reduction (CLAIR)  reported no difference in infiltrate, scar size, percentage of epithelialization and rate of adverse events between CXL and non CXL arms(Prajna et al. 2019). The  best corrected visual acuity was 2.2 lines worse in the CXL arm. There was 1.32 fold increased odds of 24 hour culture positivity to subjects with adjuvant CXL, though this was not found to be statistically significant. A previous publication from the same group reported more perforations in culture positive deep stromal fungal keratitis eyes that had received adjuvant CXL(Uddaraju et al. 2015). Although successful management of adjuvant CXL has been reported in Acanthamoeba keratitis the experience is limited and insufficient to draw definitive conclusions (Papaioannou, Miligkos, and Papathanassiou 2016).

Conclusions

 Microbial keratitis is a severe sight threatening ocular disorder. It has been described as a silent epidemic and is a major cause of visual loss in the developing world. There are several challenges in the management of this condition, it includes the lack of access to eye care, delayed presentation, lack of access to microbiology, need for cornea specialty services and antibiotic stewardship. In an agrarian population with a high likelihood of sustaining an ocular trauma with vegetative matter, it is important to create a community awareness regarding the condition, early diagnosis and institution of specific therapy and prompt referral to a cornea specialty service where indicated.

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