Tetrachloroethene (PCE) and trichloroethene (TCE) are significant groundwater contaminants. Microbial reductive dehalogenation at contaminated sites can produce nontoxic ethene, but often stops at toxic cis-1,2-dichloroethene (cis-DCE) or vinyl chloride (VC). The magnitude of carbon relative to chlorine isotope effects-as expressed by ΛC/Cl, the slope of δ 13 C vs. δ 37 Cl regressions-was recently recognized to reveal different reduction mechanisms with Vitamin B12 as model reactant for reductive dehalogenase activity. Large ΛC/Cl values for cis-DCE reflected cob(I)alamin addition followed by protonation, whereas smaller ΛC/Cl values for PCE evidenced cob(I)alamin addition followed by Clelimination. This study addressed dehalogenation in actual microorganisms and observed identical large ΛC/Cl values for cis-DCE (ΛC/Cl = 10.0 to 17.8) that contrasted with identical smaller ΛC/Cl for TCE and PCE (ΛC/Cl = 2.3 to 3.8). For TCE, the trend of small ΛC/Cl could even be reversed when mixed cultures were precultivated on VC or DCEs and subsequently confronted with TCE (ΛC/Cl = 9.0 to 18.2). This observation provides explicit evidence that substrate adaptation must have selected for reductive dehalogenases with different mechanistic motifs. The patterns of ΛC/Cl are consistent with practically all studies published to date, while the difference in reaction mechanisms offers a potential explanation to the long-standing question of why bioremediation frequently stalls at cis-DCE.
Purpose: To establish the sensitivity and specificity of the Bruckner reflex (comparison of the fundus red reflexes) in the detection of amblyopia or its risk factors.Methods: A single masked study using a medical student's diagnostic ability against the gold standard of a full clinical examination by a paediatric ophthalmologist. Setting:A university teaching hospital outpatient facility.Patients: Three hundred and one patients consecutively referred for ophthalmic assessment. Results:The sensitivity of the test for the detection of children with amblyopia risk factors was 86% and the specificity was 65%. Of 70 children with a definite diagnosis of amblyopia, the sensitivity of the test was 95%. Conclusions:The Bruckner reflex is a useful screening test for primary eye care personnel. However, there is a significant false-positive rate which will result in unnecessary referrals. We recommend that it form part of a battery of tests, including, for example, the modified base down prism test, which are useful in assessing the vision of preverbal children.Key words: Ambylopia, Bruckner reflex, paediatric ophthalmology.The detection of amblyopia or conditions likely to produce amblyopia such as anisometropia or strabismus is the major task to be undertaken during the examination of a child's eyes. In the younger child who is unable to provide recognition acuity there is a need for simple screening tests to detect amblyopia. In 1962 and subsequently, Bru~knerl-~ described a test using the direct ophthalmoscope which he had found useful for the detection of strabismus and amblyopia or risk factors for their development. He described several separate assessments including: (1) a comparison of the fundus red reflexes when viewed simultaneously at arms' length through the direct ophthalmoscope (the so-called Bruckner reflex4); and ( 2 ) evaluation of the fundus red reflex, pupillary action, and fixation movement with alternate illumination of each eye (the successive illumination test). The assessments together are usually referred to in the English literature as the Bruckner test. ' It is claimed that the simultaneous comparison of the fundus red reflexes (1 above) enables the detection of anisometropia as well as strabismus of three or so prism diopters or more.5 In other words, this assessment alone should detect the common causes of amblyopia. There have been few studies undertaken to determine the validity of the Bruckner test in toto or of the Bruckner reflex in particular as a screening test for ambly~pia.',~ None of the papers by Bruckner which we have cited provided evidence for the efficacy of the recommended tests.Due to an interest in photorefraction, which uses the Bruckner reflexJ4 and the need to
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