Continuing Education Activity

Microsporidial keratitis is a relatively less talked about and underreported disease entity. Thorough knowledge of disease identification and treatment is essential. This activity reviews the etiology, clinical features, treatment, and prognosis of the microsporidial keratitis. The activity also emphasizes the role of the interprofessional team in disease identification and early institution of treatment.

Objectives:

• Identify the etiology and mode of transmission of microsporidial keratitis.
• Describe the various clinical spectrum of microsporidial keratitis
• Explain the clinical features of different presentations of microsporidial keratitis and facilitate early diagnosis.
• Review the treatment plans according to the presenting clinical features in different spectrums of microsporidial keratitis.

Introduction

Keratitis is inflammation of the cornea with associated infiltration of inflammatory cells. Out of various infective and non-infective causes, infective keratitis is more common. Microsporidial keratitis is one such infective keratitis caused by microsporidia species. The microsporidial keratoconjunctivitis and deep stromal keratitis are two important but relatively less common corneal manifestations of microsporidia.

This disease is often misdiagnosed, mistreated, or overtreated in view of lack of awareness among the ophthalmologists regarding the varied clinical presentation and disease course in corneal microsporidiosis. This article highlights the etiology, variable clinical manifestations, disease course, and its management.[1]

Etiology

Microsporidia is a spore-forming unicellular organism belonging to kingdom Protista and has recently been reclassified as fungi based on molecular studies.[2][3] Microsporidia has prokaryotic features like the absence of mitochondria, centrioles, and peroxisomes. They also have primitive genomes and ribosomes similar to protozoa; however, mitochondrial hsp70, mitosome, and alpha-beta tubulin have resulted in their reclassification to fungi.[4][5]

Among humans, microsporidia can affect various organs, including the eye.[6]

In the eye, the microsporidia can cause epithelial keratoconjunctivitis, deep stromal keratitis, scleritis, and endophthalmitis.[3][7][8] The microsporidial keratoconjunctivitis is said to be associated with immunosuppression and is predominantly seen in patients who are HIV positive.[9][10] However, a literature search revealed its occurrence even among immunocompetent.[11][12] On the contrary, deep stromal keratitis is largely seen among immunocompetent. Microsporidia are waterborne opportunistic pathogens and commonly spread through contaminated water, like lake water, swamp water, swimming pool water, or spring water.[13][14][15]

Epidemiology

The microsporidial keratitis is largely endemic to southeast and south Asian countries. Countries like Thailand, Taiwan, Singapore, and India are the endemic zones for microsporidial keratitis.[16][17] A rapid surge in the number of cases of microsporidial keratoconjunctivitis is seen in the rainy season.[17] However, a large epidemiological study is needed to identify endemic zones, geographical variation, and the mode of disease transmission.[18][19][20]

Pathophysiology

Low CD4 count was previously considered to be an important predisposing factor for corneal epithelial microsporidiosis.[21] Literature does not give an exact cutoff value; however, a CD4+T lymphocyte count of <100 predisposes to microsporidial infection.[22][23]

However, subsequent studies revealed the occurrence of disease even among immunocompetent.[16] 

Enterocytozoon, Encephalitozoon, Pleistophora, Trachipleistophora, Vittaforma, and Nosema are different genera of microsporidia responsible for infecting humans.[2] The microsporidia often reach the human eye through contaminated water. Minor trauma to the corneal epithelium facilitates the entry of the organism to the cornea.

Histopathology

The diagnosis of corneal epithelial microsporidiosis is mainly clinical, and corneal scraping is not done routinely. However, in suspicious cases, corneal scraping helps reach the diagnosis. The scraping on Gram staining shows intra-epithelial gram-positive oval-shaped spores of microsporidia. Corneal button in microsporidial stromal keratitis, when subjected to histopathology, reveals PAS (periodic acid–Schiff)-positive oval-shaped spores measuring 3.5 to 5 micrometer by 2.5 to 3 micrometer.[24] 

On electron microscopy, sporoblasts with thick capsules are noted with 11 to 13 coils of filaments.[24] In histopathological samples, the stroma may show intracellular and/or extracellular spores with variable degrees of signs of inflammation.[25]

History and Physical

In patients with corneal epithelial microsporidiosis, the patient gives a typical history of exposure to contaminated water, mainly in the rainy season. The history of exposure to pool water or natural spring or indulgence in water sports can also be elicited. The patients usually come with complaints of redness, decreased visual acuity, watering, lid swelling, and foreign body sensation following exposure to contaminated water. Initially, the disease is usually unilateral but can also be bilateral.

On clinical examination, multiple fine to coarse punctate, raised epithelial corneal lesions (stuck on appearance) are seen. The lesion stains variably with fluorescein stain and can be distributed peripherally, paracentrally, or diffusely. Corneal lesions are invariably associated with significant conjunctival congestion and are so appropriately termed as keratoconjunctivitis. In some cases, anterior chamber inflammation can also be noted with/without keratic precipitates. Some cases may present with multiple subepithelial punctate infiltrates and are indistinguishable from adenoviral keratoconjunctivitis.

Depending on the presenting features, the disease can be graded from grade 1 to 4.[15] Grade I lesions are greyish white and <10 in number and largely distributed in the peripheral cornea. Grade II lesions are central 10-20 elevated epithelial lesions; when the number increases to 21 to 40, the lesions are categorized under grade III. Grade IV lesions are largely peripheral and have subepithelial infiltrates.

In stromal microsporidial keratitis, there may be a history of multiple episodes of remission and recurrence. The corneal infiltrates are multifocal, mid to deep stromal, and often with intact overlying epithelium. The stromal infiltrates are non-specific and associated with stromal edema with or without deep stromal vascularization. The infiltrate can be ulcerative at times.[26]

The disease is often treated as viral keratitis, though it can mimic fungal and bacterial keratitis also.[27] The patient may give a history of waxing and waning of the symptoms. The disease course often varies from months to years as the disease mimics more common diseases.[28][29] If a patient has a history of chronic culture-negative stromal keratitis, the differential diagnosis of microsporidial stromal keratitis should be considered.[29]

Evaluation

As already indicated, the diagnosis of corneal epithelial microsporidiosis is mainly clinical; however, corneal scraping for direct smear helps to make a definitive diagnosis. 

Considering the inability to culture the microsporidia in any culture media, the direct smear findings become more important. Microsporidia spores stain well with Gram stain, silver stain, and 10% potassium hydroxide (KOH) with 0.1% calcofluor white. For stromal microsporidial keratitis, the diagnosis is difficult to make. Because of deep stromal infiltrates, the corneal scraping may not help in this condition. A corneal biopsy or therapeutic penetrating keratoplasty in chronic culture-negative keratitis is a valid option to reach the diagnosis. The biopsy or excised corneal button is subjected to histopathological evaluation to get a tissue diagnosis. In the microsporidial keratoconjunctivitis anterior segment, optical coherence tomography (ASOCT) shows hyperreflective raised dots at the level of epithelium.[30] Similarly, in-vitro confocal microscopy (IVCM) in microsporidial keratoconjunctivitis shows epithelial rosettes with hyperreflective spores.[31] 

The microsporidia are obligate intracellular organisms and are not amenable to culture in routine media, and require cell cultures to grow. Initially, a pan microsporidian primer-guided test is done to ascertain the microsporidia; subsequently, species-specific PCR (polymerase chain reaction) is run to identify the species. Polymerase chain reaction with known sequenced pan-microsporidial primers has a sensitivity of 83% and specificity of 98%.[32]  

Blast search and DNA sequencing are also being used apart from pan-microsporidial PCR.[33]

Treatment / Management

Medical Management

The medications used frequently are:

1. Antimicrobial agents: Topical fumagillin and fluoroquinolones are important antimicrobials used frequently in the treatment regimen.[2][34] Fluoroquinolone monotherapy has been found to be effective in few studies. Fluoroquinolones target topoisomerase and DNA gyrase enzymes and are very effective. Among fluoroquinolones ciprofloxacin 0.3%, gatifloxacin 0.5%, moxifloxacin 0.5%, levofloxacin 0.5%, norfloxacin 0.3%, and ofloxacin 0.5% are effective against microsporidia in an hourly dose with or without oral albendazole. However, the results are largely anecdotal, from in-vivo and in-vitro studies, and not supported by randomized clinical trials.[34][16][11] 

Fumagillin, on the other hand, is a parasitistatic derivative of Aspergillus and affects RNA synthesis, and is thus effective against microsporidia.[2] Bi-cyclohexyl ammonium fumagillin is often used in a concentration of 0.113 mg/ml.[35][11][35] 

Molecular studies have proved that microsporidia come under fungi. Azoles are important in the treatment of fungal infection because these inhibit ergosterol synthesis in the plasma membrane of fungi. Topical fluconazole 0.3%, four times a day dosing has been found to be effective.[20] Oral Itraconazole (200 mg/day) has been shown to be effective for disseminated infections; however, the role of an oral antifungal is not proven in the treatment of microsporidial keratitis.[36]

2. Biguanides: Polyhexamethylene biguanide (PHMB) 0.02% and chlorhexidine gluconate 0.02% are the biguanides found effective in treating microsporidial keratitis when combined with debridement.[12] Both PHMB and chlorhexidine have been used as frequently as 1/2 hourly to one hourly. These are disinfectant antiseptics. However, in a randomized controlled trial, the biguanides were no better than a placebo.[37] The biguanides are reconstituted to 0.02% and are commercially not available in the given concentration.

3. Lubricants: Microsporidial keratoconjunctivitis was finally found to be a self-limiting disease.[38][26] One can prescribe topical lubricant to relieve the foreign body sensation. In a randomized controlled trial, the lubricants were as effective as biguanides.[37] Lubricants can be prescribed as per the symptoms of patients. The more symptomatic the patient, the more frequently the dosing will be.

4. Steroids: The spectrum of keratoconjunctivitis showing sub-epithelial infiltrates requires a short course of topical steroids coupled with steroid-sparing agents.[31] Apart from this, endotheliitis, limbitis, and anterior chamber reaction warrant the use of topical steroids, and it has been proven to be effective.[16][20][18][15] Tacrolimus 0.3%  and cyclosporine 0.5% are the two commonly used steroid-sparing agents.[39]

5. Anti-protozoal/helminthic: Albendazole is an anti-protozoal found to be effective in treating microsporidial keratoconjunctivitis.[40] Albendazole has an inhibitory effect on microtubules and thus inhibits cell division. This is more effective against Encephalitozoon species, and can be used in a dose of 400 mg twice daily.[34]

Surgical Management

The debridement of corneal epithelium is a viable option in microsporidial keratoconjunctivitis; however, this modality should be kept reserved for more severe cases considering the risk of superadded infection with debridement.[41] In addition, debridement does not have an additional advantage over conservative management as far as the resolution of lesions is concerned.[41] Corneal swabbing is another relatively less invasive procedure with more acceptance compared to scraping.[18] 

For corneal stromal microsporidiosis, therapeutic penetrating keratoplasty is the only viable option in the current scenario.[29] Medical management usually does not seem to work in cases of stromal microsporidial keratitis.[42]

Differential Diagnosis

The differential diagnosis for microsporidial keratoconjunctivitis are:

• Adenoviral keratoconjunctivitis
• Thygeson superficial punctate keratitis usually has coarse raised epithelial lesions predominantly located centrally, and it responds to topical steroids.
• Superficial punctate keratitis (dry eyes)
• Filamentary keratitis
• Herpes simplex virus epithelial keratitis (vesicular stage)
• Acanthamoeba keratitis
• Atypical mycobacterial keratitis[31]

The differential diagnosis for microsporidial stromal keratitis are:

• Herpetic stromal keratitis
• Fungal corneal ulcer
• Bacterial corneal ulcer

Prognosis

The microsporidial keratoconjunctivitis with typical raised epithelial punctate lesions usually resolves without any long-term sequelae or residual effect. However, there is another spectrum of the disease wherein subepithelial infiltrates appear and may leave behind sub-epithelial scars if not adequately treated with topical steroids.[18] 

Other sequelae can be limbitis, endotheliitis, and keratic precipitates.[31] These can be treated with topical steroids coupled with steroid-sparing agents like 0.5% 0r 1% cyclosporine or 0.3% tacrolimus.[39]

The microsporidial stromal keratitis often remits to recur again until removed in toto by penetrating keratoplasty.

Complications

Nummular scars can persist in cases of inadequately treated microsporidial keratoconjunctivitis. Limbitis, endotheliitis, and uveitis are other sequelae of microsporidial epithelial keratitis. In cases of stromal keratitis, corneal scars and edema usually increase with each recurring episode of inflammation.

Deterrence and Patient Education

Microsporidial keratoconjunctivitis can be avoided by avoiding contact with contaminated water sources.[13] The use of protective eye wears during swimming and water sports may help decrease the incidence of disease. 

Enhancing Healthcare Team Outcomes

An awareness among ophthalmologists is needed in correctly identifying and then treating the disease. On late diagnosis, the disease course becomes prolonged and adds to the morbidity of patients. Though the diagnosis of the disease (microsporidial keratoconjunctivitis) is mainly clinical, the role of microbiologists cannot be underestimated. In atypical presentations, direct smear examination helps establish early diagnosis and helps clinicians understand the prognosis. Considering the inability to culture microsporidia in routine culture media, the direct smear examination becomes more important. 

As far as stromal microsporidial keratitis is concerned, a pathologist plays an essential role in identifying the organisms in the histopathology slides of corneal biopsy or corneal button. A well-coordinated interplay between an ophthalmologist, microbiologist, and pathologist helps treat the disease at the earliest, thereby decreasing morbidity and improving the quality of life.