Limitations in tandem scanning confocal microscopy as a diagnostic tool for microbial keratitis

Diagnosis of infections caused by pathogenic free-living amoebae

Naegleria fowleri, Acanthamoeba spp., Balamuthia mandrillaris, and Sappinia sp. are pathogenic free-living amoebae. N. fowleri causes Primary Amoebic Meningoencephalitis, a rapidly fatal disease of the central nervous system, while Acanthamoeba spp. and B. mandrillaris cause chronic granulomatous encephalitis. Acanthamoeba spp. also can cause cutaneous lesions and Amoebic Keratitis, a sight-threatening infection of the cornea that is associated with contact lens use or corneal trauma. Sappinia pedata has been identified as the cause of a nonlethal case of amoebic encephalitis. In view of the potential health consequences due to infection with these amoebae, rapid diagnosis is critical for early treatment. Microscopic examination and culture of biopsy specimens, cerebral spinal fluid (CSF), and corneal scrapings have been used in the clinical laboratory. For amoebic keratitis, confocal microscopy has been used to successfully identify amoebae in corneal tissue. More recently, conventional and real-time PCR assays have been developed that are sensitive and specific for the amoebae. In addition, multiplex PCR assays are available for the rapid identification of these pathogens in biopsy tissue, CSF, and corneal specimens.

Contact lens-induced changes in the anterior eye as observed in vivo with the confocal microscope

The availability of the confocal microscope over the past decade has allowed clinicians and researchers to refine their understanding of the physiological and pathological basis of the ocular response to contact lens wear, and to discover previously unknown phenomena. Mucin balls, which form in the tear layer in patients wearing silicone hydrogel lenses, can penetrate the full thickness of the epithelium, leading to activation of keratocytes in the underlying anterior stroma. Epithelial cell size increases in response to all forms of lens wear, with lenses of higher oxygen transmissibility (Dk/t) interfering least with the normal process of epithelial desquamation. A higher density of Langerhans' cells is observed in the layer of the sub-basal nerve plexus among contact lens wearers, suggesting that contact lens wear may be altering the immune status of the cornea. Dark lines and folds are observed in the oedematous cornea in response to contact lens wear. Mechanical stimulation of the corneal surface, due to the physical presence of a contact lens, and the consequent release of inflammatory mediators, is the likely cause of reduced keratocyte density associated with lens wear. Highly reflective stromal 'microdot deposits' are observed throughout the entire stroma in higher numbers in lens wearers. 'Blebs' in the endothelium have a bright centre surrounded by a dark annular shadow; this appearance is explained with the aid of an optical model. The confocal microscope has considerable clinical utility in diagnosing Acanthamoeba and fungal keratitis. At the limbus, contact lenses can induce structural changes such as increases in basal epithelial cell size. An increased number of rolling leucocytes is observed in limbal vessels in response to low Dk/t lenses. It is concluded that the confocal microscope has considerable utility in contact lens research and practice.

Diffuse Lamellar Keratitis Associated With Recurrent Corneal Erosions After Laser in situ Keratomileusis

PURPOSE

Diffuse lamellar keratitis (DLK) is marked by the presence of diffuse or multifocal infiltrates confined to the laser in situ keratomileusis (LASIK) interface. These infiltrates are culture-negative, and the etiology is thought to be noninfectious. Most cases of DLK occur within the first week or two following surgery.

METHODS

We describe one case of diffuse lamellar keratitis that occurred 3 months after LASIK. The patient developed a spontaneous corneal erosion in one eye. Over the next 2 days while the erosion was being treated, there was rapid development of DLK. Slit-lamp biomicroscopy and in vivo scanning slit confocal microscopy were performed. The patient was treated with intensive topical corticosteroids.

RESULTS

Scanning slit confocal microscopy revealed numerous, highly-reflective round bodies consistent with a polymorphonuclear infiltrate located at the flap interface. Treatment with topical 1.0% prednisolone acetate was instituted, with rapid improvement in patient symptoms, visual acuity, and slit-lamp biomicroscopy.

CONCLUSIONS

Diffuse lamellar keratitis may occur months after LASIK as a result of a spontaneous recurrent corneal epithelial erosion.

Corneal biopsy with tissue micro-homogenisation for isolation of organisms in bacterial keratitis

PURPOSE

To evaluate a novel two-stage technique to increase yield of bacteria isolated from infected corneal ulcers.

METHODS

A new blade was designed to remove friable material from infected corneal ulcers. The new blade was used in combination with standard tissue micro-homogenisation equipment in a two-stage technique intended to distribute biopsy samples evenly between relevant agar plates. Patients with presumed-bacterial corneal ulcers underwent sequential corneal sampling using the new two-stage technique and a scalpel blade, used without micro-homogenisation (the order of sampling was varied between two groups). Bacterial isolation rates were compared using the chi-squared test.

RESULTS

Twenty-four patients with presumed-bacterial corneal ulcers were studied. The overall positive bacterial isolation rate was 88%, with identical bacterial isolation rates for the new two-stage technique and the scalpel blade (71%). The new technique isolated bacteria from three ulcers that had initially been 'sterile' when sampled with a scalpel blade. Polymicrobial infections were identified in two ulcers with the new blade where only a single organism had been identified using the scalpel blade (not significantly different).

CONCLUSIONS

The new two-stage technique shows promise for improving bacterial isolation rates from presumed-bacterial corneal ulcers.

Confocal microscopy as a tool for the investigation of the anterior part of the eye

In recent years, confocal microscopy has become a powerful tool for examining microscopic structures in the living eye. The decisive advantage of this technique is that it permits the investigation of optical sections of relatively thick (> 10 microns) specimens. Because confocal microscopy suppresses the out-of-focus blur, sharp three-dimensional images with excellent resolution can be obtained. Confocal microscopy is therefore able to provide more information than the classic methods--i.e., specular microscopy and slit-lamp biomicroscopy. This paper reviews recent applications of confocal microscopy in three fields of ophthalmology: the observation of the anatomy of the anterior parts of the eye, the investigation of these structures after local administration of drugs and, finally, the use of this technique for the diagnosis of infectious ocular diseases.