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Dylan L. Yu, George Timberlake, Jemshed Khan; A Simple Method For Accurate Measurement Of Below-the-Pupil MRD (Margin-to-Reflex Distance). Invest. Ophthalmol. Vis. Sci. 2011;52(14):729.
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Traditional penlight MRD measurement is able to measure ptosis only when the lid margin is above the pupillary center. We introduce the Angulated Illumination Method (AIM) of measuring ptosis below the pupillary center by displacing the corneal reflex inferiorly. [figure 1a]
First, a geometric model was constructed to validate AIM [figure 1b]. Next, an apparatus was constructed with a reflective sphere mounted in the center of an orbiting light source 66 cm away. The sphere had the same radius as a typical corneal curvature (7.93 mm). The sphere was painted black on the non-illuminated half to simulate eyelid ptosis. A CCD camera captured the displacement of the reflex on the sphere as the light source rotated around the sphere.[figure available] Data was then compared to that predicted by the geometric model. From angulation, we then extrapolated the position of the light source that must be held in order to produce the proper inferior displacement of corneal reflex. Clinical photos of healthy subjects were then taken, with a penlight held at the predicted positions, and the amount of corneal reflex displacement was measured using software.
There was good correlation between the predicted and measured corneal reflex displacement from the reflective sphere.[figure available] This following nomogram was established: when one examines a patient 50 cm away, a 1 mm inferior displacement of the corneal reflex can be generated by holding the penlight at the height of the examiner's chin, 2 mm when penlight is lowered to the level of examiner's mid-sternum, and 3 mm at the level of the umbilicus.
A penlight inferiorly displaced at a angle "theta" will producing a corresponding inferior displacement "d" of the corneal reflex, following the relationship d = r sin (theta). This allows the examiner to easily measure the millimeters in ptosis when the lid has fallen below the pupillary axis.
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