Ординатура / Офтальмология / Английские материалы / Corneal Ulcers Diagnosis and Management_Sharma, Vajpayee_2008

Introduction

Corneal ulceration occurs due to the host cellular and immunologic responses to the offending agent which may be bacterial, viral, fungal or protozoal organism. Sometimes it is sterile corneal ulceration, which may occur due to systemic dermatologic or connective tissue disease and chemical or thermal injuries.

The host cellular responses are mainly responsible for corneal destruction in infections and sterile corneal melting. In all cases, stromal melting is preceded by a corneal epithelial defect. The ulceration occurs secondary to the action of tissue collagenases. The polymorphonuclear cells (PMNs) are secreted in response to the corneal insult, which secrete various lytic enzymes such as collagenase, elastase and cathepsin causing destruction of the cornea.1 Simultaneously, reactive fibroblasts, synthesize collagen and cause repair of the cornea.

Apart from the infective processes, the immunologic mechanisms consequent to infection may also play a role. For example, in herpes simplex interstitial keratitis stromal cellular destruction occurs due to immunologic mechanisms as a consequence of acquisition of herpes antigens, thereby resulting in an influx of PMNs and phagocytes, which cause tissue destruction.

For the reparative phase of corneal ulcer, the interaction between the keratocytes and blood vessels is essential. Stromal vascularization inhibits the ulcerative process as nutrients (such as ascorbate) and antiproteases are delivered by the vessels to the ulcerated area.2

STAGES OF CORNEAL ULCER

The course of events, which occur in the process of corneal ulceration, can be divided into three stages.

Stage 1: Progressive Stage

In the progressive stage, the ulcer is usually saucer 8 shaped and is associated with gray zone of infiltration

Figure 2.1: Corneal ulcer

Figure 2.2: Histologic section shows breach in the corneal epithelium

(Courtesy: Dr Seema Sen)

(Fig. 2.1). In this stage, the microbes adhere to the epithelium, release toxins and enzymes and cause tissue destruction (Fig. 2.2).

Figure 2.3: Histologic section of corneal ulcer shows stromal inflammation (Courtesy: Dr Seema Sen)

Adhesion of the organisms is facilitated by bacterial pili and a glycocalyx envelope in bacteria such as Pseudomonas and Gonococcus. In response to this, PMNs are generated at the ulceration site. The PMNs originate from the tears initially and limbal vessels consequently in response to the corneal injury. Progressive invasion of the cornea by the PMNs and the phagocytes increases the size of the ulceration, due to release of various lytic enzymes by the microbes.

This leads to necrosis and sloughing of the epithelium, Bowman’s membrane and the involved stroma (Fig. 2.3) .The walls of the active ulcer project due to the swelling of the lamellae by imbibition of fluid. Ulceration may progress further by lateral extension leading to diffuse superficial ulceration or by deeper penetration of infection leading to descemetocele formation and possibly corneal perforation .

Stage 2: Regressive Stage

The termination of the progressive stage and the onset of the regressive stage is brought by the natural host defense mechanisms (humoral antibody response and cell mediate immune defenses) and the anti-microbial treatment. There is an improvement in the symptomatology and clinical signs. A line of demarcation forms around the ulcer so that the margin and floor of the ulcer become more smooth and transparent. The line of demarcation consists of leukocytes that neutralize and eventually phagocytose the offending organism and the necrotic cellular debris. The digestion of the necrotic material may cause an initial enlargement of the ulcer.

Pathogenesis of Corneal Ulceration 1

This process may be accompanied by superficial vascularization.

Stage 3: Healing Stage

The process of epithelialization starts to occur at this stage. The histiocytes and keratocytes convert to fibroblasts so that the scar tissue is formed. Vascularization occurs towards the ulcer site, which further promotes healing as a result of influx of fibroblasts and antibodies. When the healing is complete, the vessels regress and become “ghost vessels” which may be visualized by indirect illumination.2

The degree of scarring from healing varies according to the depth of involvement. Bowman’s membrane does not regenerate and is replaced by fibrous tissue, which over a period of time becomes less dense, especially in young patients. The process of cicatrization occurs due to regeneration of collagen and the formation of fibrous tissue. Since the newly formed fibers are not laid down in a regular manner as in normal corneal lamellae, a scar is formed which causes the light to be refracted irregularly.3

SEQUEL AND COMPLICATIONS

Corneal ulcers involving the superficial lamellae generally heal by varying degrees of scarring depending on the severity of inflammation. However, if the infection is severe, there may be thinning, formation of a descemetocele ectatic cicatrix or perforation.

Corneal Opacification

Depending on the depth of the corneal ulceration, different types of corneal opacities may occur that is, nebular, macular (> 50% involvement) or leukomatous (> 75% involvement) (Figs 2.4A to D).

Descemetocele

Some corneal ulcers, especially due to Pneumococci extend rapidly in depth so that the entire thickness of the cornea except Descemet’s membrane or few isolated corneal lamellae are spared. The Descemet’s membrane like any other elastic membrane offers resistance to the inflammatory process, but is unable to withstand the intraocular pressure and therefore herniates through the corneal ulcer as a transparent membrane called as descemetocele or a keratocele (Fig. 2.5). This is often 9

Figure 2.7: Corneo-iridic scar

surrounded by a white cicatricial ring and under the influence of a raised intraocular pressure may eventually rupture.

Perforation

Perforation of corneal ulcer occurs due to sudden exertion by the patient such as coughing, sneezing, straining or spasm of the orbicularis muscle. An increase in intraocular pressure occurs due to these maneuvers so that the weak floor of the ulcer gives way. When an ulcer perforates, the aqueous suddenly escapes and the intraocular pressure falls to the atmospheric levels. Subsequently, the lens iris diaphragm moves forward and adheres to the back of the cornea. Due to the decreased intraocular pressure, the pain is alleviated; extension of the ulcer decreases and the process of scar formation is initiated.

If the perforation is small, the iris is plugged to the back of the cornea, adhesions from the iris get organized and the scar tissue is formed which is called as “pseudocornea” (Fig. 2.6). The iris, which is plastered at the back of the cornea, allows anterior chamber to form and hence aqueous is secreted.

If the perforation is large, the iris prolapses out of the site of perforation; in cases of longstanding of iris prolapse, fibrin and exudates deposition occurs on the surface, thinning of the iris stroma occurs and the black pigmentary epithelium becomes visible. Thus any adherence of iris tissue to the back of cornea, which is subsequent to a perforated corneal ulcer, is called as a corneo-iridic scar (Fig. 2.7).

In very large perforations, only a small rim of the cornea remains and the total prolapse of the iris and the lens may occur (Fig. 2.8). If the perforation occurs

Figure 2.12: Corneal ulcer with endophthalmitis

12

References

1.Kenyon KR, Ghinelli E, Chaves HV. Morphology and pathologic response in corneal and conjunctival disease. In: Foster CS, Azar DT, Dohlman CH (Eds): Smolin and Thoft’s Cornea. Lippincott Williams and Wilkins NY; 4th edition, 2005;4:103-40.

2.Kenyon KR. Inflammatory mechanisms in corneal ulceration. Trans Am Ophthalmol Soc 1985;83:610-63.

3.Nordlund M, Pepose JS. Corneal Response to Infection. In: Krachmer JH, Mannis MJ, Holland EJ (Eds): Cornea, 2nd Edition Elseivier Co. NY. 2005;7:95-114.

The various microorganisms which can cause infectious keratitis can be classified into eukaryotic and prokaryotic organisms (Table 3.1). The eukaryotic organisms include the relatively complex cells such as protozoa and the fungi and the prokaryotic organisms are more primitive cells, which include the filamentous bacteria, true bacteria, spirochaetes, mycoplasma and rickettsiae and chlamydiae.

Protozoa

Protozoa are unicellular organisms which exist in two morphologic phases. Examples which cause ocular infections include the Acanthamoeba and the Microsporidia.

TABLE 3.1

Classification of microorganisms causing infectious keratitis

EUKARYOTES

Protozoa

Sporozoa : Toxoplasma

Amoebae: Entamoeba, Naegleria, Acanthamoeba

Microsporidia

Fungi

Mould like: Aspergillus

Yeast like: Candida

Dimorphic : Histoplasma. Blastomyces, Coccidioides True yeasts: Cryptococcus

PROKARYOTES Filamentous bacteria

Actinomyces, Nocardia, Mycobacterium, Streptomyces

True Bacteria

Gram-positive Bacilli and Cocci

Gram-negative Bacilli and Cocci

Spirochaetes

Borrelia, Treponema, Leptospira

Mycoplasma

Rickettsiae and Chlamydia

Primarily, Acanthamoeba is the protozoa, which can cause keratitis. The pathogenic species of Acanthamoeba

include Acanthamoeba castellanii, Acanthamoeba polyphaga, Acanthamoeba culbertsoni, Acanthamoeba palestinensis, Acanthamoeba astronyxis, Acanthamoeba hatchetti, Acanthamoeba rhysodes, Acanthamoeba divionesis, Acanthamoeba equina, Acanthamoeba lugdunensis, and Acanthamoeba

griffini.1

Acanthamoeba are ubiquitous organisms and have been isolated from soil, water (including natural and treated water), air, and dust. They are free living, pathogenic amoeba. Most persons are exposed to this organism during their lifetime, as 50-100 percent of healthy people have serum antibodies directed against

Acanthamoeba.

The life cycle consists of 2 stages: a trophozoite (which is 14-45 microns in diameter) and a cyst (which has a double-layered wall with a diameter of 10-25 microns). The trophozoites are motile and usually have one nucleolus and huge cytoplasmic vacuoles and feeds on bacteria (Fig. 3.1). In unfavorable conditions the trophozoites encyst.2 The cyst has a double wall, which is made of cellulose (Fig. 3.2). The cyst is resistant to alterations in temperature, pH, osmolarity and antimicrobial agents.

Microsporidia

Microsporidia are eukaryotic, spore forming obligate intracellular parasites. Microsporidial keratitis occurs in two forms keratoconjunctivitis is usually seen in immunocompromized individuals or in contact lens wearers, mostly by genus Encephalitozoon while Nosema and Microsporidium cause the stromal keratitis which is generally seen in the immunocompetent host.3 Most infections are transmitted by feco-oral route but corneal infections occur due to direct inoculation. These 13

Figure 3.1: Acanthamoeba trophozoite (phase contrast microscopy)

(Courtesy: Dr H Sheorey, Dept. of Microbiology, St Vincent Hospital,

Melbourne)

organisms stain poorly with the routine stains such as the Gram’s, Giemsa, Gomori methanamine silver, Periodic acid Schiff and immunofluorescent techniques. However, they stain well with calcoflor white stain.4

Mycology

Keratomycosis is common in tropical climatic regions and is a common cause of corneal ulcer. Fungi are opportunistic organisms and rarely affect intact cornea, but in a compromised or immunosuppressed state such as ocular surface disease, topical steroid use or trauma with vegetable matter particularly, they become pathogenic.

The cell wall of the fungi is primarily made up of polysaccharide (80 to 90%) and the remainder is protein or lipid.

Keratomycotic fungi can be broadly classified into filamentous fungi, yeasts, or dimorphic fungi (Table 3.2).

More than 105 species of fungi classified in 56 genera have been reported to cause oculomycosis. Fungi are saprophytic, and/or pathogenic organisms. Saprophytic fungi obtain their nutrients from decaying organic matter, whereas pathogenic fungi feed on living cells. Pathogenic fungi are actually saprophytic microbes, which cause disease in humans. Many fungi associated with ocular infections are saprophytic.

Fungal isolates can be classified into four groups: Moniliaceae, which are nonpigmented filamentary fungi including Fusarium spp and Aspergillus spp; Dematiaceae, which are pigmented filamentary fungi including

Curvularia spp and Lasiodiplodia spp; yeasts, which include

14 Candida spp; and other fungi.

Figure 3.2: Acanthamoeba cyst (phase contrast microscopy)

(Courtesy: Dr H Sheorey, Dept. of Microbiology, St Vincent Hospital, Melbourne)

TABLE 3.2

Fungal organisms causing keratitis

I.FILAMENTOUS

A.SEPTATED

1.Nonpigmented

Fusarium: Solani, oxysporum, moniliforme, episphaeria

Aspergillus: fumigatus, flavus

Acremonium (Cephalosporium) Paecilomyces

Penicillium

2.Pigmented (Dematiaceous)

Curvularia: Senegelensis, verruculosa, pallescens

Lasiodiplodia theobromae Alternaria Cladosporium Celletotrichum

Drechslera (Helminthosporuim)

B.NONSEPTATED

Rhizopus (mucormycosis)

II.YEAST

Candida: Albicans, parapsilosis, krusei, tropicalis

FILAMENTOUS FUNGI

Molds are filamentous fungi, which are multicellular organisms and possess hyphae and grow by apical extension or branching. They have a rigid cell wall composed of outer chitin and inner sterol containing cytoplasmic membrane. According to the presence or absence of cross walls in the hyphae the filamentous fungi are classified as septate or non-septate. Most fungal keratitis is caused by filamentous fungi which have septate hyphae.

Fungi with septate hyphae may be divided into nonpigmented fungi or pigmented fungi based upon

whether the hyphae are pigmented or non-pigmented on the culture media. Filamentous septate fungi can thus be broadly classified into:

i.Moniliaceae (light colored fungi like Fusarium and

Aspergillus species)

ii.Dematiaceae (dark colored fungi such as Alternaria and Curvularia).

YEASTS

Yeasts are unicellular fungi, represented by Candida, and are characterized by an oval or round blastoconidium. Yeasts reproduce by budding and are characterized by the presence of pseudohyphae (Fig. 3.3). The phase, which has pseudohyphae, is the most virulent phase. The cell walls of pseudohyphae have constrictions and are not parallel to each other unlike the hyphae.

Candida species especially Candida albicans commonly causes majority of the cases of keratitis. In culture they form smooth creamy white colonies resembling staphylococci in the early phases of growth. The presence of budding yeasts in corneal scrapings is diagnostic for Candida.5 This organism produces both true hyphae and pseudohyphae. Both yeast and hyphal forms can be seen in corneal scrapings. Candida colonies are white to tan and opaque with smooth, round, flat contour and pasty consistency. They have a distinctive fruity odor, which aids in easy identification.

DIMORPHIC FUNGI

Some fungi appear as yeasts in vivo at 37 oC and molds in the environment at 25oC. They are called as dimor-

phic fungi. These include Blastomyces, Coccidioides, Histoplasma and Sporothrix.

Moniliaceae

ASPERGILLUS

This is a common contaminant in hospital air. Aspergillus fumigatus is the most commonly isolated species, while

Aspergillus flavus and Aspergillus niger may also cause keratitis. The Aspergillus fungi are readily recognized by their morphology (Fig. 3.4). The conidospore with its swollen terminal end is surrounded by a flask shaped sterigmata, each of which produces long chains of conidia that radiate from the terminal end (Fig. 3.5). Their hyphae are septate and branch dichotomously.6 In infections progressing rapidly they are more uniform but in more indolent infections the hyphae may be irregular in shape.

Colonies of A. fumigatus are at first white but as spores are produced they become velvet green due to pigmentation of conidia. A. niger on the other hand, turns completely black as they undergo sporulation.

FUSARIUM

Fusarium causes localized infection after trauma in otherwise healthy patients. Fusarium solani is the most common species. It has sickle shaped macroconidia and clusters of fusiform microconidia (Fig. 3.6). Fusarium colonies are white initially and later acquire a buff coloration. A range of color pigments from yellow to red to purple is produced on the undersurface of the colony (reverse pigmentation).7

DEMATIACEAE

Dematiaceous fungi have melanin in their hyphal walls and hence are dark pigmented (olive, brown or black) when viewed under a microscope. Only 27 percent of corneal ulcers caused by dematiaceous fungi appear to be darkly pigmented macroscopically. Curvularia (Fig. 3.7), Alternaria (Fig. 3.8) and Cladosporium belong to this category.

Bacteriology

Bacteria are unicellular prokaryotic organisms. The cornea does not generally have any colonizing bacteria. Bacteria from eyelids may contaminate the ocular surface and cause corneal ulcer.

Pathogenic bacteria consist of virulence factors, which is responsible for infectious keratitis. These include specific antigens, proteolytic enzymes, hemolysins and toxins. In general non-pathogenic bacteria do not cause keratitis except in compromised conditions when they gain access to the cornea.

The classical differential system of staining the bacteria divides the bacteria into gram-positive and gram-negative bacteria depending on the presence of peptidoglycan and lipid layer of the cell wall. Grampositive bacteria are more frequently isolated as compared to the gram-negative bacteria in the ratio of approximately 60 to 40 percent. Apart from this there are other stains such as acid-fast, which may be, used to characterize the mycobacteria species.

Figure 3.9: Gram-positive pneumococci (Courtesy: Dr H Sheorey,

Dept. of Microbiology, St Vincent Hospital, Melbourne)

Specific Bacteria

GRAM-POSITIVE BACTERIA

Gram-positive bacteria have a high content of peptidoglycan and a low content of lipid compared to gramnegative bacteria. When the lipid layer is dissolved, crystal violet and iodine form a complex on the cell wall that appears blue under the microscope, depicting grampositive bacteria.

The most common organisms which infect the eye are the Staphylococcus aureus, Coagulase negative Staphy-

lococci, Streptococcus pneumoniae and Streptococcus viridans.

Staphylococcus aureus

Staphylococcus aureus are gram-positive cocci (0.5 to 1.5 μm), which are seen under the microscope in pairs or grape like clusters and grow as routine culture media within 18 to 24 hours. On blood agar, they appear as golden hemolysis (clear area). They are non-motile, nonspore forming, facultative anaerobes and are usually encapsulated. They are coagulase and catalase positive. Staphylococcus species resistant to oxacillin agents constitutes the methicillin resistant Staphylococcus aureus (MRSA).

Antibiotic susceptibility: The widespread use of aminoglycosides, penicillins, cephalosporins and fluoroquinolones (third generation) is responsible for the increasing resistance of these organisms against these

Microbiology 1

drugs. They are susceptible to, bacitracin, chloramphenicol, cefazolin and fluoroquinolones.

Coagulase Negative Staphylococci

The genus Staphylococcus consists of 32 species out of which 3 are coagulase positive and the rest are coagulase negative. Coagulase negative Staphylococci are normal inhabitants of human body especially the skin, mucous membrane and the eyelid margins.

Coagulase negative Staphylococci are microscopically similar to Staphylococcus aureus and appear in pairs or grape like clusters, but on blood agar isolation, they appear as white to grayish colonies within 24 to 48 hours of incubation. These bacteria are non-motile, non-spore forming, facultative anaerobes, usually non-encapsula- ted, catalase positive. The most common bacteria implicated in bacterial keratitis is negative coagulase

Staphylococcus.

Antibiotic susceptibility: Just like Staphylococcus aureus the resistance rate of 55 percent has been noted to fluoroquinolones in coagulase negative Staphylococcus. They are susceptible to vancomycin, bacitracin and chloramphenicol.

MICROCOCCUS SPECIES

They are gram-positive cocci, which may be present as saprophytes and inhabit the eyelid margins. They appear on blood agar as yellow distinct colonies.

Streptococcus and Related Bacteria

Streptococci appear microscopically as gram-positive cocci (Fig. 3.9), usually coccoid or coccobacilli and appear in chains of cocci on broth medium. They may be present as normal inhabitants especially in children. Streptococci are distinguished from Staphylococcus and Micrococcus species by negative catalase reaction S.pneumoniae is less virulent as compared to S.viridans. They are responsible for corneal ulcers, recalcitrant graft infections and infectious crystalline keratopathy.

Streptococcus species are differentiated by their hemolysis patterns on blood supplemented agar media into the following:

1.Alpha-hemolytic streptococci which partially lyse the red blood cells and a greenish halo appears around a colony. Examples of alpha-hemolytic streptococci include Streptococcus pneumoniae and Streptococcus viridans.