Baylor College of Medicine, Houston, Texas
None of the authors have any financial interest in any of the contents within this article.
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Address correspondence to Rod Foroozan, MD, Neuro-ophthalmology Service, Cullen Eye Institute, Baylor College of Medicine, 6565 Fannin NC-205, Houston, TX 77030; E-mail: firstname.lastname@example.org
Assessing pupil reactivity using the swinging flashlight technique in patients with miotic pupils or heavily pigmented irides can be difficult due to the reflection of the examination light off the cornea. We describe a modification of the standard method of detecting pupil reactivity in these difficult-to-see pupils. With the patient facing forward and eyes looking in maximal upgaze, fixed at distance, the examiner shines the light at the limbus in the straight ahead direction. Improved view of the pupil in upgaze results from reduction of the blink reflex and elimination of the Purkinje-1 image with minimal refractive distortion by the cornea. Optical calculations and multimedia demonstrations are provided to document these findings.
Pupil testing is a critical component of the ophthalmic examination. Since the swinging flashlight test to measure the relative afferent pupillary defect (RAPD) was described by Levatin (1) in 1959, several modifications have been described including use of neutral density filters (2), pupilometers (3), and slit lamps (4) to increase accuracy in detecting abnormal pupillary responses. However, these modifications do not address factors that may hinder visualization of the pupil itself. Dark irides and sluggish or miotic pupils-intrinsically difficult to assess-are further obscured by the reflection of the examiner's light source off of the cornea (4-6) (Fig. 1A). Here, we describe a modification to the swinging flashlight test that facilitates viewing these “difficult pupils” without the use of any specialized equipment.
While facing forward in a dark or dimly lit room, the patient is asked to fixate directly up at the ceiling on a specific target. The examiner then shines the light directly at the limbus (Fig. 1C). The standard swinging flashlight technique is then performed.
Healthy asymptomatic individuals with no history of ocular problems and individuals with a previously documented RAPD were invited to participate in these demonstrations. Informed consent was obtained. Using a Canon SD1100 digital camera in the digital macro setting, videos of the penlight examination were taken with the patient fixating at a distant target in both primary position and upgaze (see Videos, Supplemental Digital Content 1, http://links.lww.com/WNO/A10, and Supplemental Digital Content 2, http://links.lww.com/WNO/A11). The camera was mounted on a tripod, and multimedia recorded in a dim light setting with the patient in a fixed head position. The videos were then cropped using Windows Movie Maker 5.0.
Video 1 shows a pupil examination of a normal subject in primary position and upgaze. (The subject is a 56-year-old healthy asymptomatic man with no history of ocular problems. Both the pupil and pupil reactivity are more easily seen with the subject's eyes in upgaze than in primary position. The Purkinje reflex no longer obscures visualization of the pupil with a shift from primary position to upgaze.) Video 2 shows a pupil examination of a left RAPD in primary position and upgaze. (The subject is a 40-year-old woman with a history of left-sided traumatic optic neuropathy and previously documented left RAPD. Both the pupil and pupil reactivity are more easily seen with the subject's eyes in upgaze than in primary position. The Purkinje reflex no longer obscures visualization of the pupil with a shift from primary position to upgaze.)
Table 1 summarizes the values and dimensions of the average eye and optics formulas used in the following calculations.
Formation and Location of Purkinje Image
The cornea acts as a convex mirror (7), reflecting light rays originating from infinity (x > 2f = 7 mm = 0.23 inches), which converge behind the cornea at the focal point (3.5 mm).
Location of Entrance Pupil and Magnification in Primary Position and Upgaze
Using the lensmaker's equation, the distance of the entrance pupil when viewed in primary position is −3.05 mm with pupil magnification to 113%. This value is consistent with previous reports (8). The corneal surface is aspheric such that the corneal surface on a sagittal section of the cornea resembles an ellipse (7). The average central corneal power (Pcenter) is 42 diopters (D) with a gradual decrease of approximately 2 D in power at the peripheral cornea near the limbus (Plimbus = 40 D) (7). Thus, where θ represents the angle between the patient's and examiner's visual axes, Pθ ≈ Pcenter − (θ/90°) 2 D. Assuming θ = 40° in upgaze, Pθ = 41.1 D with pupil magnification to 116%.
Visualization of the “difficult” pupil may be facilitated in upgaze due to the following advantages:
Reduces or Eliminates Blink Reflex
One frequently described modification of the standard swinging flashlight test has the examiner hold the light source obliquely and below the visual axis beside the nose (Fig. 1B) (9-11). However, the effectiveness of the oblique method is diminished by the blink-to-threat reflex. This takes place right at the time of pupil constriction as the patient perceives the light source moving quickly toward and away from the face in close proximity to the eyes.
Having the patient look upward engages the levator palpebrae muscle, counteracting the blink reflex, and allows the patient to maintain open eyes for examination. Loewenfeld (12) alludes to this latter point in her textbook, where she describes asking patients to look “slightly up,” just enough to engage the levator muscle to better facilitate pupil photography.
Purkinje-1 Image Located at the Pupil When Examined in Primary Position
Light rays that are reflected off the cornea can obstruct the view of the pupil (12). This is due to the fact that the cornea acts as a curved mirror, and the reflected light rays form an image behind the cornea, called the Purkinje-1 image. This is 1 of the 4 Purkinje images formed as external light rays pass from media of lower to higher refractive indices (tearfilm, inner surface of the cornea, anterior surface of the lens, and posterior surface of the lens, respectively). When shining a light onto the cornea, the greatest refractive index change is the air/tearfilm surface. Thus, the Purkinje-1 image is the brightest and most visually significant of all the Purkinje images (7). Given the magnitude of the power of the cornea compared to vergence, the lensmaker's equation predicts that variation in the distance of the light source has an insignificant effect on the location of the Purkinje image (7). Therefore, with an anterior chamber depth of 3.6 mm, the Purkinje-1 image is located at the pupil, hindering the examiner's view. In contrast, with the eyes in upgaze, the examiner's view of the pupil is not obscured by the Purkinje-1 image.
Minimal Refractive Distortion of the Pupil When Viewed From Peripheral Cornea
When viewing the pupil through the cornea, the optical aberration of major significance is magnification (8). Light rays refracted by the cornea form a magnified image of the pupil anterior to the actual pupil. This “entrance pupil” is that which is measured when quantifying pupil diameter during the eye examination (8). When viewing the pupil in primary position, the entrance pupil is magnified to 113% of actual pupil size. Based on our calculations, when viewed in 40° upgaze, the pupil is magnified to 116% resulting in minimal refractive distortion.
We are aware that our study has a number of limitations. While much has been written regarding the light stimulus, ambient illumination, and interpersonal differences in testing the pupils (12-14), there is little information on the effects of light shone at various angles on pupillary contractile efficiency.
Other potential drawbacks of our upgaze technique include symmetric limitation of upgaze in elderly patients and unequal lighting that may occur in individuals with asymmetric supraduction deficits. Fortunately, the latter are rare (15).
In summary, we suggest the following steps in the assessment of pupils with the modified upgaze technique:
1. While facing forward in a dark or dimly lit room, the patient is asked to shift the eyes into upgaze and fixate on a ceiling target.
2. The examiner then shines the light “straight on” at the limbus (Fig. 1C).
3. The standard pupil swinging flashlight test is then performed.
In her textbook, The Pupil, Loewenfeld notes that “after many years of experimentation with different methods, we believe that there is no inherently ‘bad’ method. Every technique becomes misleading when its limitations are not recognized” (12). We want to emphasize that the upgaze method is not intended to replace but rather supplement the traditional technique of the swinging flashlight test.
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