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Christoph Friedburg, Wadim Bowl, Birgit Lorenz; Fundus-controlled Mesopic Perimetry With A Commercial Microperimeter And An External Optical Filter. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4843.
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To develop fundus-controlled perimetry of mesopic vision with an existing device and without interfering with medical safety standards. The approach by Crossland et al (2011) of changing the light path within the microperimeter MP1 (Nidek Technologies, Padova, Italy) invalidates the CE-sign which guaranties medical safety. Therefore, their modified device is limited to research. For clinical use, an additional expensive recertification process is necessary.
Light output of the MP1 was reduced by a Schott RG780 filter outside the instrument. This can be done by either using goggles or placing the filter in front of the objective lens. Absorption of infrared light by this filter is low thereby minimizing problems with fundus observation by the built-in infrared camera system. Five normal subjects were investigated with a pattern covering 55 spots at the posterior pole of the retina. This included a central spot, 4 paracentral spots and rings of ≥8 spots each at 3, 4, 6, 8, and 10 deg radius. Thresholds were determined using a 200 ms 1/3 deg (Goldmann III) stimulus white stimuli on a background of about 0,2 cd/m2 employing the built-in 4-2-1 strategy in a dimmed room.
Median visual sensitivity was maximal at the 6 deg ring. It continuously decreased towards the periphery and towards the center where it was lower by about 6 dB. This compares well with previous profile perimetry reported e.g. by Aulhorn and Harms (1972) without positional control.
A relative central scotoma is found when retinal luminance is dropped to sufficiently low light levels so that stimulus detection becomes rod-mediated. The dip is the result of decreased rod density within the macula. The dip proofs that positional control with the MP1 is functioning, and that mesopic function can be readily measured in the central retina with our modification. Depending on the relative sensitivities of rods and cones in various pathologies, stimulus detection will be rod- or cone-mediated. Our modification does not affect the CE mark of the instrument.Literature:1. Crossland et. al. (2011): BMC Ophthalmology 11:5. Online http://www.biomedcentral.com/1471-2415/11/5.2. Aulhorn & Harms (1972). Visual perimetry. In: Jameson & Hurvich (Eds.), Handbook of sensory physiology, Vol. VII/4: Visual psychophysics (pp. 102-145). Berlin: Springer.
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