The detection of viral nucleic acids in intraocular fluids and tissues by PCR has become increasingly important in clinical ophthalmology. While much attention has been directed toward minimizing false-positive reactions resulting from specimen contamination or amplicon carryover, relatively little attention has been given to the causes of false-negative PCRs. This report describes a PCR inhibitor in normal aqueous and vitreous fluids that can produce false-negative PCR results. As little as 0.5 l of vitreous fluid and 20 l of aqueous fluid can completely inhibit DNA amplification in a 100-l PCR mixture. This inhibition was not primer specific, nor was it due to chelation of Mg 2؉ ions or DNase activity in the ocular fluid. The inhibitor was completely resistant to boiling for 15 min. However, the inhibitory effects were completely removed by a single chloroform-isoamyl alcohol (24:1) extraction. The extent of PCR inhibition depended upon the type of thermostable DNA polymerase used in the reaction. Taq DNA polymerase was very sensitive to the inhibitor, while thermostable DNA polymerases from Thermus thermophilus HB-8 (Tth) and Thermus flavus (Tfl) were completely resistant. Thus, the inhibitory effects of intraocular fluids on PCRs can be removed by diluting the specimen, by chloroform extraction, or by using Tth or Tfl DNA polymerases.
Sympathetic ophthalmia can be seen following pars plana vitrectomy in patients without penetrating injuries or a history of trauma. Indeed, it may be seen after successful vitrectomy for retinal detachment. Diverse clinical presentations are possible, and persistent or atypical uveitis following vitrectomy should alert the surgeon to the development of sympathetic ophthalmia.
In previously healthy young men, flu-like symptoms associated with visual loss and retinitis should prompt questioning about hunting and raw game meat ingestion, especially when toxoplasmosis is suspected.
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