Retina Today - Microbiologic Diagnosis of Acute Endophthalmitis (May/June 2015)

摘要

At a Glance • Microbiologic diagnosis of endophthalmitis may have implications for selecting appropriate therapy and thus may influence visual and anatomic outcomes. • New methods of polymerase chain reaction (PCR) analysis are making this diagnostic modality more readily applicable in this setting. • Panbacterial PCR may be important for microbiologic diagnosis and may be complementary to standard cultures. Endophthalmitis is most commonly caused by exogenous sources. Exogenous bacterial endophthalmitis is a known postoperative complication of elective ocular surgery and after perforating ocular trauma. The severity of this type of infection and the speed with which lesions evolve requires rapid diagnostic confirmation, which, in turn, influences the therapeutic management of the patient and hence the anatomic and functional prognosis. Molecular biology techniques that can rapidly detect and identify causative microorganisms have improved the ability to correctly identify the microbiologic etiology of endophthalmitis. SAMPLING TECHNIQUES Samples for microbiologic evaluation can be collected from the aqueous humor (AH) or vitreous humor (VH). Ideally, a sample is collected in the OR after local antisepsis, under local or general anesthesia, and before any intravitreal administration of antibiotics. VH samples tend to yield more accurate results; they may be collected via transscleral puncture with a needle through the pars plana (Figure 1A) or via posterior vitrectomy performed for diagnostic and/or therapeutic purposes (Figure 1B). About 200 to 300 µL of sample can be collected by means of a vitreous puncture using a 25-gauge needle attached to a 2- or 3-cc syringe. In severe endophthalmitis, the density of intravitreal suppuration may preclude needle aspiration of the VH; in such cases, pure or diluted VH can be collected via a pars plana vitrectomy in the following manner: After connecting a 3-cc syringe to the aspiration line of the vitreotome and flushing the vitreotome tubing with air (to prevent dilution of the sample with liquid in the tubing), the surgical assistant aspirates the vitreous gel without perfusion of infusion fluid. Then the vitrectomy can be continued while infusion fluid is injected into the posterior chamber to restore intraocular pressure. The mixture of VH and infusion fluid collected is referred to as diluted VH. Panbacterial polymerase chain reaction (PCR: ie, 16SrRNA gene amplification and sequencing; more on this below) appears to have comparable sensitivity with pure and diluted VH.1 Collecting diluted VH is easier and induces less intraocular hypotonia, thereby limiting the risk of choroidal hematoma, retinal detachment, and displacement of the infusion terminal. The contents of the vitrectomy cassette can also be cultured. Microbiologic tests performed on both pure VH and the vitrectomy cassette yield a diagnosis in about 57% of cases, whereas with the pure VH or the vitrectomy cassette alone the rates are 44% and 49%, respectively.2,3 If one opts to sample the vitrectomy cassette, caution must be taken to avoid exogenous contamination of the sample. A high vitreotome cutting speed (up to 1500 cuts per minute [cpm]) does not affect the result of bacteriologic cultures.4 Transconjunctival vitrectomy systems may be useful, notably in patients with glaucoma, but the need for scleral tunneling in patients with chemosis may be a limiting factor.5 Alternatively, the endoscope is useful in cases of severe corneal opacity.6 Figure 1. Ocular samples during endophthalmitis: vitreous needle puncture (A), sample taken during pars plana vitrectomy (B), pediatric hemoculture vials and microtube for PCR (C), and aspect of vitreous infected in 2.5-ml syringes during a vitrectomy (D). The AH is technically easier to collect than the VH, this can be done through a transcorneal puncture of the anterior chamber using a 25-, 27- or 30-gauge needle for as